The Upper Cretaceous (Coniacian-Santonian) of the Western Canada Foreland Basin, contains a rich record of scaphitid ammonites (scaphites). We describe four species: Scaphites (Scaphites) preventricosus Cobban, 1952, Scaphites (S.) ventricosus Meek and Hayden, 1862, Scaphites (S.) depressus Reeside, 1927, and Clioscaphites saxitonianus (McLearn, 1929). These are widespread index fossils that demarcate the upper lower-middle, middle, and upper Coniacian, and the lower Santonian, respectively. They occur in the lower part of the Wapiabi Formation, Alberta. The Coniacian part of the section has been divided into 24 informal allomembers based on the recognition of marine flooding surfaces, most of which can be traced through the >750 km extent of the study area. The most distinctive feature in the ontogenetic development of scaphites is the change in coiling during ontogeny. At the approach of maturity, the shell uncoils slightly, forming a shaft, which then recurves backward approaching the earlier secreted phragmocone. However, this sequence of scaphites shows an evolutionary trend toward recoiling, accompanied by an increase in size and degree of depression. These changes occurred against a background of changing environmental conditions resulting from the expansion of the Western Interior Seaway during the Niobrara transgression. This resulted in an increase in the area of offshore habitats, which may have promoted the appearance of larger species with more depressed whorl sections. Scaphites probably lived at depths of less than 100 m, and may have fed on small organisms in the water column.
INTRODUCTION
The Western Canada Foreland Basin (WCFB) comprises a stratigraphic succession of mudstones and sandstones approximately 5 km thick (Wright et al., 1994). The basin extends approximately 1000 km to the east of the Rocky Mountain Foothills. This succession yields a rich record of Late Cretaceous scaphitid ammonites (hereafter referred to as scaphites) ranging from the Turonian to the early Maastrichtian. In this paper, we document the stratigraphic and geographic distribution of the early Coniacian and lower Santonian scaphites, which, together with the co-occurring inoceramid bivalves, permit a subdivision of the Canadian succession into biostratigraphic units (Walaszczyk et al., this issue). Examination of the distribution of the scaphites also allows an evaluation of their biogeography and evolutionary patterns through time.
GEOLOGIC BACKGROUND
The fossils described in this report were collected at 18 localities in the Rocky Mountain Foothills of Alberta. The localities are plotted in figure 1. They cover the western part of Alberta and extend from approximately 49°N to 54°N. The outcrop sections were correlated using a large database of publicly available wireline logs that permitted both lithostratigraphic and allostratigraphic relationships to be established, as described in more detail by Plint et al. (this issue).
The Wapiabi Formation consists of Coniacian to lower Campanian rocks and is divided into seven members, each with a distinctive succession of lithologies (Stott, 1963, 1967). The lowest unit is the Muskiki Member, which is dominated by mudstones and rests uncomformably on the Cardium Formation. The Muskiki Member is overlain by the Marshybank Member, which is represented by offshore facies in the south and nearshore sandstones and terrestrial deposits in the north. Beneath the Peace River Plains in the Northeast, the Marshybank Member is uncon-formably overlain by an upper Coniacian unit called the Bad Heart Formation, which consists of very fine-grained sandstones and ooidal ironstones and is exposed along the Smoky River (Plint et al., 1990; Donaldson et al., 1998, 1999). The Cardium and Wapiabi formations in Alberta correlate with the Ferdig and Kevin members of the Marias River Formation in Montana (Cobban et al., 1976; Shank, 2012; Shank and Plint, 2013; Walaszczyk et al., 2014).
Plint et al. (this issue) divided the Coniacian and lowermost Santonian rocks of the Wapiabi Formation into 24 informal allomembers based on the recognition of marine flooding surfaces. These surfaces are identified by an abrupt transition from coarser- to finer-grained sediments, and in many instances, the surfaces are marked by a veneer of intra- or extrabasinal pebbles that imply an episode of shallowing and possibly subaerial emergence. Plint et al. (this issue) interpreted these flooding surfaces as approximate time lines that allow a reconstruction of the subsidence history of the basin. The flooding surfaces in Coniacian rocks are designated CS1 (Coniacian Surface 1) to CS23. The allostratigraphically defined units of rock bounded by these flooding surfaces are designated CA1 (Coniacian Allomember 1) to CA24. In addition, the boundary between the Cardium Formation and the overlying Muskiki Member is designated E7 (after Plint et al. 1986), and the basal surface of the Santonian strata is designated SS0 (Santonian Surface 0).
TERMINOLOGY
Landman et al. (2010) reviewed the terms used to describe scaphites of the genus Hoploscaphites. These terms equally apply to the species described herein: Scaphites (S.) preventricosus Cobban, 1952, S. (S.) ventricosus Meek and Hayden, 1862, S. (S.) depressus Reeside, 1927, and Clioscaphites saxitonianus (McLearn, 1929). The adult shell consists of two parts, a closely coiled phragmocone and a slightly to strongly uncoiled body chamber (fig. 2). The part of the phragmocone that is exposed in the adult shell (as compared to the part that is concealed inside) is called the adult phragmocone. The most adapical point of the adult phragmocone is called the point of exposure. The body chamber consists of the shaft, beginning near the last septum, and a hook terminating at the aperture. The point at which the hook curves backward is called the point of recurvature.
Measurements of the adult shell are the same as those described and illustrated in Landman et al. (2013: 9, fig. 3). All measurements were made using electronic calipers on actual specimens, rather than on photos, with the exception of the apertural and septal angles. The following sequence follows the order of the measurements such that Hp is linked to Wp, etc.
LMAX = maximum length from the venter of the phragmocone to the venter of the hook
UD = umbilical diameter through the center of the umbilicus parallel to the line of maximum length
WP = whorl width of the phragmocone along the line of maximum length
HP = whorl height of the phragmocone along the line of maximum length
WS = whorl width of the body chamber at midshaft
HS = whorl height of the body chamber at midshaft
WH = whorl width of the hook at the point of recurvature
HH = whorl height of the hook at the point of recurvature
AA = in macroconchs, the angle of intersection (in degrees) between two lines (a line drawn along the umbilical shoulder and another line drawn along the apertural margin), extending from approximately the point of recurvature to the aperture
SA = in macroconchs, the angle of intersection (in degrees) between two lines (a line drawn along the umbilical shoulder coinciding with the line of maximum length and a line drawn through the position of the last septum); negative values are defined as above (adapical of) the line of maximum length; positive values are defined as below (adoral of) the line of maximum length
Several ratios were calculated to describe the shape of the adult shell and facilitate comparisons among specimens:
WP/HP = the ratio of whorl width to whorl height of the phragmocone along the line of maximum length
WS/HS = the ratio of whorl width to whorl height of the body chamber at midshaft
WH/HH = the ratio of whorl width to whorl height of the hook at the point of recurvature
LMAX/HP = the ratio of maximum length to whorl height of the phragmocone along the line of maximum length
LMAX/HS = in macroconchs, the ratio of maximum length to whorl height of the body chamber at midshaft
FIG. 2.
Scaphite terminology. A, B, D. Macroconch, left (A, B) and right (D) lateral views. The position of the last septum marks the base of the body chamber. The umbilical seam of the shaft is straight in macroconchs. C. Close-up of the umbilicus; the umbilical diameter is measured parallel to the long axis of the specimen. D. Angle of orientation of the aperture with respect to the vertical (orientation). Abbreviations: HP = whorl height of the phragmocone along the long axis; HS = whorl height at midshaft; HH = whorl height of the hook at the point of recurvature; LMAX = maximum length along the long axis; apt. 
= apertural angle; SA = septal angle; UD = umbilical diameter; X= center of buoyancy; • = center of mass.
FIG. 3.
Comparison of macroconch (C, D) and microconch (A, B) of Scaphites (S.) depressus Reeside, 1927, in lateral view (A, C) and median cross section (B, D). The tick marks indicate the base of the body chamber. A, B. ×1.5; C, D. ×1.
A number of terms are used to describe ornamentation:
primary ribs = ribs that originate near the umbilicus
secondary ribs = ribs that originate on the flanks or venter, either by branching or intercalation
rib density = number of ribs/cm on the venter as measured on the adapical and adoral parts of the phragmocone, the midshaft, and the hook
tubercles = small conical swellings
Photographs of adult shells are natural size. Small black tick marks on the photos mark the base of the body chamber, where visible. The base of the body chamber is defined as the position of the median saddle in the ventral lobe. Specimens are photographed in lateral, apertural, and ventral views. The suture terminology is that of Wedekind (1916), as reviewed by Kullmann and Wiedmann (1970).
TAXONOMIC BACKGROUND
Cobban (1952) revised the taxonomy of the Turonian-Santonian scaphites of the U.S. Western Interior in his classic monograph in which he described 27 new species. He placed most of these species in the genus Scaphites but also introduced the genera Clioscaphites for species that are closely coiled at maturity with trifid lateral lobes and Desmoscaphites for species that develop constrictions in their early ontogeny. These species were subsequently studied by Birkelund (1965), Crick (1978), Landman (1987, 1989), Kennedy and Cobban (1991), Cooper (1994), Braunberger (1994), Braunberger and Hall (2001), Collom (2001), Landman and Cobban (2007), and Cobban et al. (2006). Cooper (1994: 176) introduced the new genera Anascaphites and Billcobbanoceras and reassigned several of the species that Cobban (1952) had previously included in Scaphites and Clioscaphites to these new genera; these changes have not been followed by any subsequent workers (e.g., Braunberger and Hall, 2001; Cobban et al., 2006; Landman and Cobban, 2007). Until a thorough phylogenetic revision of all these species is undertaken, we prefer to follow the simplified taxonomy outlined by Kennedy and Cobban (1991) in their recent treatment of these forms.
The taxonomic discrimination of scaphite species mostly relies on features of the mature shell. Throughout most of ontogeny, the shell is closely coiled, similar to so-called normal ammonites. However, at the approach of maturity, the shell uncoils slightly, forming a shaft, which then recurves backward approaching the earlier secreted phragmocone. This change in shape is usually accompanied by a change in ornamentation affecting the coarseness and spacing of ribs and the appearance of ventrolateral tubercles. Internally, these changes are associated with a reduction in spacing of the last few septa, known as septal approximation.
Dimorphism is present in all scaphites, but usually is apparent only at maturity (fig. 3). It is generally interpreted as sexual in nature (Cobban, 1969; Landman and Waage, 1993; Davis et al., 1996). The dimorphs are referred to as the macroconch (M), presumably the female, and the microconch (m), presumably the male. In the scaphite species described in this report, dimorphism is expressed by size, robustness, and degree of uncoiling, with macroconchs larger, more robust, and more tightly coiled. The body chamber is more inflated in macroconchs than in microconchs, possibly due to the expansion of the female reproductive organs. In both dimorphs, the whorl width and height decrease at the end of ontogeny, so that the adult aperture is smaller than that at midshaft (Collom, 2001: pl. 45, figs. 4–8).
As documented in other scaphites, macroconchs are larger than microconchs (Makowski, 1962; Cobban, 1969; Machalski, 2005). Dimorphs of the same species overlap in size, but the largest macroconch is usually larger than the largest microconch. For example, in our sample of Scaphites (S.) depressus, the extent of size overlap is 14% of the total combined size range of both dimorphs. The average size of microconchs is 72% that of macroconchs (or conversely, the average size of macroconchs is 139% that of microconchs). The difference in size between the two dimorphs in this species amounts to less than one whorl (Collom, 2001).
The dimorphs of the Turonian-Santonian scaphites were previously described as separate species or varieties of the same species (Cobban, 1952). In Scaphites (Scaphites) preventricosus, the microconch was originally designated as S. (S.) preventricosus var. sweetgrassensis, in S. (S.) ventricosus, it was originally designated as S. (S.) tetonensis, in S. (S.) depressus, it was originally designated as S. (S.) depressus. var. stantoni, and in Clioscaphites saxitonianus, it was originally designated as C. saxitonianus var. keytei. We follow the current systematic practice of combining the dimorphs (to the extent that we can recognize them) into the same species.
GEOGRAPHIC DISTRIBUTION
The Coniacian and Santonian scaphites described in this report are widely distributed in the Western Interior of North America (fig. 1). In addition, Birkelund (1965) described several of these species from western Greenland, supporting a connection between this area and the Interior Seaway. However, none of these species has been reported from either the Gulf of Mexico and Atlantic Coastal Plains or Europe. Their absence there may represent a taphonomic bias, but more likely, it reflects their preference for boreal seas.
Scaphites (S.) preventricosus is present in the Cardium Formation at Highwood River, Alberta, and in the Wapiabi Formation at Mill Creek, Cutpick Creek, Oldfort Creek, Wapiabi Creek, and Bighorn Dam, Alberta (fig. 1). It is present in the United States in the Kevin Member of the Marias River Shale in north-central Montana and in the uppermost part of the Frontier Formation in Wyoming. Outside North America, it has been reported from Umivik, Svartenhuk, western Greenland (Birkelund, 1965). Scaphites (S.) ventricosus is present in the Wapiabi Formation at Ram River, East Thistle Creek, James River, Blackstone River, Chungo Creek, Mill Creek, Sheep River, Bighorn Dam, and Bighorn River, Alberta. It is present in the United States in the Kevin Member of the Marias River Shale in north-central Montana, the Cody Shale in western Wyoming, and the Mancos Shale in New Mexico. Outside North America, it has been reported from Alianaitsúnguaq, Nugssuaq, western Greenland (Birkelund, 1965). Scaphites (S.) depressus is present in the Wapiabi Formation at Ram River, East Thistle Creek, West Thistle Creek, James River, Sheep River, Bighorn Dam, Bighorn River, Cardinal River, and Mill Creek, Alberta. In the United States, it is present in the Kevin Member of the Marias River Shale in north-central Montana and the Cody Shale in western Wyoming. It is rare in the Smoky Hill Chalk Member of the Niobrara Formation in southwestern Colorado and the Mancos Shale in western Colorado and eastern Utah. Clioscaphites saxitonianus is present in the Wapiabi Formation at James River, West Thistle Creek, Cardinal River, Cripple Creek, Lynx Creek, and above the measured section at Ram River, Alberta. In the United States, it is present in the Apishapa Shale of southeastern Colorado and the Kevin Member of the Marias River Shale on the east flank of the Sweetgrass Arch of north-central Montana. Outside North America, it has been reported from western Greenland (Birkelund, 1965).
FIG. 4.
Ammonite zonation of the Coniacian and Santonian of the U.S. Western Interior (modified from Cobban et al., 2006).
STRATIGRAPHIC DISTRIBUTION
Based on the extensive biostratigraphic research summarized in Cobban et al. (2006), the Upper Cretaceous of the U.S. Western Interior was subdivided into 66 zones using ammonites and co-occurring inoceramid bivalves (fig. 4). According to this scheme, the Coniacian was subdivided, in ascending order, into the lower Coniacian Scaphites (S.). preventricosus Zone, the middle Coniacian S. (S.) ventricosus Zone, and the upper Coniacian S. (S.) depressus Zone. This succession is confirmed by the Canadian material studied here. However, the base of the Scaphites (S.) ventricosus Zone is lower down in the section than previously thought and occurs in the upper part of the lower Coniacian. The base of the middle Coniacian is defined by the lowest occurrence of the inoceramid genus Volviceramus. The apparent coincidence of the lowest occurrences of S. (S.) ventricosus and Volviceramus is probably the result of a pan-regional stratigraphic gap (e.g., Walaszczyk and Cobban 2000, 2006). The overlying Santonian consists, in ascending order, of the lower Santonian Clioscaphites saxitonianus Zone, the middle Santonian C. vermiformis Zone, and the upper Santonian C. choteauensis, Desmoscaphites erdmanni, and D. bassleri Zones. The S. (S.) preventricosus Zone is dated at 88.55 ± 0.59 Ma, the S. (S.) depressus Zone at 87.14 ± 0.39 Ma, and the D. bassleri Zone at 84.30 ± 0.34 Ma, so that our study interval spans approximately 4 Myr (Sageman et al., 2014).
Scaphites are present in all outcrop sections examined in our study (fig. 1). They occur as isolated specimens preserved as internal molds composed of siderite. The biostratigraphic distribution of the various species conforms to the established zonation. The stratigraphic position of each species is recorded in table 1 (for more details about this stratigraphic distribution, see Plint et al., this issue). The ranges of several of the species overlap. For example, at Bighorn Dam, a specimen of Scaphites (S.) ventricosus occurs in the lower part of the S. (S.) depressus Zone at a height of 107.5 m. At Cardinal River, two specimens of S. (S.) depressus occur in the lower part of the Clioscaphites saxitonianus Zone at a height of 141.5–143.5 m. Similarly, at West Thistle Creek, two specimens of S. (S). depressus occur in the lower part of the C. saxitonianus Zone at a height of 121.0 and 123.6 m. Cobban et al. (2005) also noted the co-occurrence of S. (S.) depressus and C. saxitonianus. These overlaps in stratigraphic range imply evolutionary episodes of cladogenesis rather than anagenesis, which was the pattern previously envisioned by Cobban (1952).
TABLE 1
Height (m) of species in the measured section at each locality. X = present, but height unknown.
The lowest occurrences of each scaphite species can be interpreted in the context of the relative sea-level curve (fig. 5) based on the allostratigraphic scheme developed by Plint et al. (this issue). The lowest occurrence of Scaphites (S.) preventricosus is just above erosional surface E5.5, which marks the beginning of a major transgression that commenced in the very latest Turonian (Walaszczyk et al., 2014). The lowest occurrence of S. (S.) ventricosus is immediately above surface CS2 in allomember CA3 just below an interpreted highstand and prior to a major regression that culminated at surface CS4, which marks the boundary between the lower and middle Coniacian. It is just below the lowest occurrence of Volviceramus, which marks the base of the middle Coniacian. The lowest occurrence of S. (S.) depressus is in allomember CA15, immediately above surface CS14 in an overall regressive succession, which marks the base of the upper Coniacian. The increase in abundance of this species above surface CS14 is possibly coupled with a change in water chemistry (oxygen level?) that is expressed by an abrupt increase in the intensity of bioturbation. The lowest occurrence of Clioscaphites saxitonianus is at the base of the Santonian (surface SS0), and coincides with a major transgression that is expressed by a gradual change from bioturbated sandy siltstones to dark mudstones with a very low level of bioturbation, indicative of a transition to a more offshore setting with more oxygen-deficient bottom water.
FIG. 5.
Stratigraphic ranges of Scaphites (S.) preventricosus, S. (S.) ventricosus, S. (S.) depressus, and Clioscaphites saxitonianus (plotted on the left side) next to a curve of the changes in relative sea level during the Turonian-Santonian based on analysis of stratigraphic sections in the Western Foreland Basin (Plint et al., this issue). The evolutionary relationships proposed by Cobban (1952) are presented as a cladogram showing the trend toward recoiling at maturity.
PALEOECOLOGY
One of the most important clues about the mode of life of ammonites is the angle of orientation of the aperture with respect to the vertical. Assuming that scaphites maintained near neutral buoyancy during life, this parameter can be calculated based on the positions of the centers of mass and buoyancy, using the same approach as Trueman (1941). We used photos of specimens in lateral view and “cut out” the whole specimen and body chamber separately. In incomplete specimens, we filled in the missing portions of the shell. Using a computer program (Image J), the centers of the areas of the whole specimen and of the body chamber were determined and treated as proxies for the centers of buoyancy and mass, respectively. We drew two lines: one through the center of mass and buoyancy constituting the vertical and another line through the apertural margin. The angle of orientation of the aperture is defined as the angle between the apertural margin and the vertical.
TABLE 2
Angle of orientation of the aperture with respect to the vertical in Turonian-Santonian scaphites.
See figure 2 for description of measurements ;angles are reported to the nearest 0.5°.
The above method involves as least three simplifying assumptions: (1) the phragmocone is completely filled with air, with no cameral liquid present; (2) the soft tissues are uniformly distributed in the body chamber, thus neglecting the weight of the aptychus (jaws) at the adoral end; and (3) the thickness of the shell is uniform (for more details about these assumptions, see Landman et al., 2010).
We examined the angle of orientation of the aperture in four specimens of Scaphites (S.) preventricosus (one macroconch and three microconchs), five specimens of S. (S.) ventricosus (four macroconchs and one microconch), 10 specimens of S. (S.) depressus (seven macroconchs and three microconchs), and one specimen of Clioscaphites saxitonianus (macroconch). The angle of orientation is nearly the same in all four species (table 2). It averages 127.0° and ranges from 114.0° to 141.5°. It averages 128.0° in S. (S.) preventricosus, 132.0° in S. (S.) ventricosus, 123.5° in S. (S.) depressus, and 116.0° in C. saxitonianus (table 2). In addition, the angle of orientation of the aperture is approximately the same in dimorphs within the same species. In S. (S.) preventricosus, it averages 125.0° in macroconchs and 129.0° in microconchs; in S. (S.) ventricosus, it averages 134.5° in macroconchs and 121.0° in microconchs; and in S. (S.) depressus, it averages 123.5° in macroconchs and 123.0° in microconchs.
FIG. 6.
Hypothetical reconstruction of a macroconch (female) of Scaphites (S.) preventricosus with small delicate arms joined together by a thin web. The shell is covered by a thin periostracum. Artist: Mariah Slovacek (AMNH).
These high values indicate that the aperture of these scaphites faced upward during life. This orientation is incompatible with feeding on the bottom, as in modern Nautilus. In addition, the aperture at maturity is smaller in size relative to that at midshaft, especially in macroconchs, further impeding access to the bottom. Instead, these scaphites may have fed on small organisms in the water column. To facilitate the capture of small prey, the soft body may have extended weblike out of the aperture with the jaws protruding forward (fig. 6).
Collom (2001) reached a similar conclusion about the mode of life of scaphites based on the orientation of the aperture. However, he argued that these animals lacked jaws altogether based on the absence of jaws in the large collection of Scaphites (S.) depressus at his disposal. This is extremely unlikely given the presence of jaws in closely related species such as S. (S.) mariasensis, as documented by Landman et al. (2012). Indeed, the preservation of jaws is relatively rare and depends on many factors including whether the shells floated into the site after death or immediately fell to the sea floor in the same area in which they lived (Wani et al., 2005). It also depends on whether the animals were buried rapidly or remained at the sediment water interface for some time. As a clue to the circumstances surrounding the burial of the specimens in our study, two macroconchs of S. (S.) depressus in our collection bear bryozoan colonies that cover the inside surfaces of the body chambers (exposed on the outside surfaces of the internal molds), suggesting that these specimens remained on the sea floor long enough for epizoans to colonize them.
The preservation of jaws also depends on the length and shape of the body chamber. The short, depressed body chambers of Scaphites (S.) depressus do not seem to favor in situ jaw preservation. Landman et al. (2017) reported similar findings for fossil nautilids with short, depressed body chambers. They studied a collection of approximately 300 specimens belonging to the genus Eutrephoceras, and despite the abundance and excellent preservation of the shells, they did not find a single in situ jaw. Further studies could shed additional light on the relationship between body chamber size and shape and the incidence of in situ jaw preservation (for more details about the taphonomy of jaws in externally shelled cephalopods, see Wani, 2007).
Most investigations of the habitat of scaphites suggest that they lived a few meters above the bottom. This inference is based on several lines of evidence, including incidence and kinds of injuries on the shells, facies and faunal associations, and isotopic analyses (Landman et al., 2012; Landman and Klofak, 2012). For example, in the Campanian Pierre Shale of the U.S. Western Interior, scaphites are associated with a rich benthic and nektic fauna (Tsujita and Westermann, 1998; Landman et al., 2010). Such associations suggest that the same factors (elevated levels of oxygen?) that favored the development of benthic communities also promoted an increase in the number of scaphites. The habitat depths of scaphites have been estimated based on investigations of the mechanical strength of the shell and septa. According to Hewitt (1996), the implosion depth of Scaphites (S.) whitifieldi, S. (S.) preventricosus, and Clioscaphites vermiformis averages 170 m. In analogy with modern Nautilus, the ammonites would probably have lived at shallower depths, “to be on the safe side.” These values are consistent with depth estimates of the Alberta portion of the Western Interior Seaway during the Coniacian (Plint et al., this issue).
EVOLUTION
The evolutionary relationships among Coniacian and Santonian scaphites in the Western Interior of North America have not yet been resolved through rigorous phylogenetic analysis. However, based on his studies, Cobban (1952) assembled a detailed record of the stratigraphic succession of species in the U.S. Western Interior. This succession provides a biostratigraphic framework as well as a hypothesis of evolutionary relationships from more primitive to more advanced species (fig. 5). Cobban (1952) noted a succession of endemic species starting in the late Turonian and extending to the end of the Coniacian. In ascending biostratigraphic order, they are Scaphites (S.) whitfieldi Cobban, 1952, S. (S.) nigricollensis Cobban, 1952, S. (S.) corvensis Cobban, 1952, S. (S.) preventricosus and the closely related species S. (S.) mariasensis Cobban, 1952, S. (S.) ventricosus, and S. (S.) depressus. He also noted two micromorph species, S. (Pteroscaphites) pisinnus (Cobban, 1952) and S. (P.) auriculatus (Cobban, 1952), which co-occur with S. (S.) whitfieldi and S. (S.) preventricosus, respectively. Since then, a “macromorph” species, S. (S.) borealis Cobban and Kennedy, 1991, has also been described, which co-occurs with S. (S.) whitfieldi.
The evolutionary relationships proposed by Cobban (1952) are presented as a phylogenetic tree in figure 5. If these relationships are valid, they suggest a directional change in morphology from more loosely to more closely coiled shells at maturity, with the most marked change occurring between Scaphites (S.) preventricosus and S. (S.) ventricosus. The degree of uncoiling of macroconchs at maturity is expressed by the ratio of LMAX/HP (the ratio of maximum length to whorl height of the phragmocone along the line of maximum length, fig. 2). The value of this ratio is lower in more tightly coiled shells (Landman, 1987: fig. 74). In the present study, the value of this ratio averages 3.07 in Scaphites (S.) preventricosus, 2.55 in S. (S.) ventricosus, and 2.55 in S. (S.) depressus. The degree of uncoiling of macroconchs at maturity is also expressed by the apertural angle (fig. 2), not to be confused with the angle of orientation of the aperture with respect to the vertical during the lifetime of the animal. The value of the apertural angle is lower in more tightly coiled shells. In the present study, the value of this angle averages 100.0° in S. (S.) preventricosus, 82.5° in S. (S.) ventricosus, and 70.2° in S. (S.) depressus.
The change in the degree of uncoiling of the shell correlates with three other variables: position of the last septum, degree of whorl compression, and adult size (as observed in macroconchs). The position of the last septum relative to the line of maximum length of the shell is expressed by the septal angle (fig. 2). Negative angles are defined as above (adapical of) the line of maximum length whereas positive angles are defined as below (adoral of) the line of maximum length. The position of the last septum shifts toward the apex in more closely coiled shells. In the present study, the value of the septal angle averages 52.0° in Scaphites (S.) preventricosus, 12.0° in S. (S.) ventricosus, and -2.5° in S. (S.) depressus. The degree of whorl depression is also higher in more closely coiled shells. The degree of whorl depression at maturity is expressed by the ratio of WS/HS (the ratio of whorl width to height at midshaft). In the present study, the value of this ratio averages 1.27 in S. (S.) preventricosus, 1.31 in S. (S.) ventricosus, and 1.39 in S. (S.) depressus. Finally, the adult size (LMAX) is correlated with the degree of shell coiling, with larger shells more tightly coiled. Based on the measurements in the present study, the average value of LMAX equals 70.1 mm in S. (S.) preventricosus, 85.4 mm in S. (S.) ventricosus, and 90.6 mm in S. (S.) depressus.
The degree of uncoiling, the size at maturity, and the degree of whorl compression are also linked together within species. For example, in Scaphites (S.) depressus, adult macroconchs are larger, more depressed, and more closely coiled than adult microconchs (fig.3; Collom, 2001: pl. 7, figs. 5, 6). The adult body chamber in macroconchs remains in contact with the phragmocone whereas in microconchs, it uncoils slightly, so that a space develops between the body chamber and the phragmocone.
The position of the last septum and the apertural angle are also linked together within species. Landman et al. (2010) documented this relationship in adults of Hoploscaphites nodosus (Owen, 1851) and H. brevis (Meek, 1876) from the upper Campanian Pierre Shale. They discovered that the apertural angle is higher in specimens in which the last septum occurs below the line of maximum length. Based on calculations of the centers of mass and buoyancy in these scaphites, they argued that the covariation between the position of the last septum and the apertural angle guarantees that the angle of orientation of the aperture with respect to the vertical remains nearly the same.
The fact that the degree of shell uncoiling, position of the last septum, degree of whorl compression, and adult size vary within species suggests that this variation was available for evolutionary modification. However, the additional fact that these features covary within species further implies that they formed an integrated whole, as expressed by the concept of morphological integration (Olson and Miller, 1958; for another example of covariation between morphological features in ammonites, see Yacobucci, 2004). This suggests that the same selective forces that acted on individuals may also have played a role in fashioning the shape of new species; any changes during evolution were constrained by interdependent interactions, so that, for example, the development of a larger shell invariably entailed a reduction in the degree of uncoiling with associated changes in the apertural angle and position of the last septum.
The reduction in the degree of uncoiling also affected the hydrostatic and hydrodynamic properties of the shell. It decreased the distance between the centers of mass and buoyancy, thus reducing stability. However, it improved the efficiency of horizontal swimming and maneuverability while maintaining the same orientation of the aperture (Landman et al., 2012).
The driving force for all these trends may have been an evolutionary increase in adult size. This increase was accommodated by the secretion of additional whorls rather than an increase in the degree of whorl expansion (Landman, 1987). If the rate of shell secretion and chamber formation remained the same in all species, this increase in size implies a delay in the timing of maturation or, in terms of heterochrony, hypermorphosis (McKinney and McNamara, 1991). Thus, the evolutionary sequence from Scaphites (S.) whitfieldi to S. (S.) depressus represents a peramorphocline in which more derived species were larger and longer lived than more primitive species. However, maturation entails its own set of morphological modifications starting at the point at which the shell departs from the spiral coil and develops into the shaft (Landman, 1989).
This evolutionary increase in adult size and associated reduction in the degree of uncoiling occurred against a backdrop of changing environmental conditions due to changes in the extent of the Western Interior Seaway, as reflected in transgressive-regressive curves (fig. 5) and paleogeographic reconstructions (Williams and Stelck, 1975; Nielsen et al., 2008; Schröder-Adams et al., 2012). The overall expansion of the seaway during this time is associated with the Niobrara transgression (Kauffman and Caldwell, 1993). In the late Turonian, the Scaphites (S.) whitfieldi Zone was deposited in the middle of an overall regressive succession (fig. 5). The seaway was a narrow strait several hundred kilometers wide (Nielsen et al., 2008: fig. 10C). In the northern United States, it extended from central Wyoming to eastern South Dakota and, in Canada, it covered most of Alberta. It was characterized by an axial belt of calcareous muds flanked on either side by noncalcareous muds to muddy silts with sands along the margins. In contrast, in the middle Coniacian, the S. (S.) ventricosus Zone was deposited during a major transgression, and, as a result, the seaway was much wider. In the northern United States, it extended from western Wyoming to central Minnesota and, in Canada, it extended from eastern British Columbia to eastern Manitoba. The seaway was characterized by broad swatches of calcareous muds and noncalcareous muds to muddy silts (Nielsen et al., 2008: fig. 13A).
Because of the much larger size and depth of the seaway (but never exceeding the depth limits of scaphites), it may have been less susceptible to environmental perturbations affecting the habitat where the scaphites lived. According to current ecological theory, more stable environments promote the evolution of longer-lived species (e.g., Hone and Benton, 2005). In addition, the increase in the areal extent of muddy facies associated with quieter water conditions in the middle Coniacian may have favored the evolution of species with more depressed whorl shapes. Such shapes permit more efficient lower speed locomotion in lower energy, more offshore environments (Jacobs et al., 1994). In addition, such lower-energy environments make fewer demands on the shell shapes of ammonites, so that streamlining is not as critical (Chamberlain, personal commun., 2017). Nevertheless, even in the most globose forms such as Scaphites (S.) depressus, the flanks are relatively flat to broadly rounded, suggesting good horizontal mobility.
Interestingly, once the maximum size and close degree of coiling were attained in Scaphites (S.) depressus, more derived species did not become more loosely coiled again even though the adult size decreased, as in Clioscaphites saxitonianus. This pattern of irreversible shape change follows the predictions of “Dollo's Law,” as recently elaborated on by Collin and Cipriani (2003). Evidently, once a size threshold was attained, a return to more openly coiled shells was not possible. This evolutionary tendency to irreversibly reduce the degree of uncoiling in scaphites has also been noted in other lineages. In the U.S. Western Interior, Landman et al. (2010) documented that the most primitive (geologically oldest) members of Hoploscaphites such as H. nodosus from the upper Campanian are more loosely coiled than the most derived (geologically youngest) members of this genus such as H. nebrascensis from the upper Maastrichtian. This same evolutionary tendency has also been documented during the geologic history of a single scaphite species from the middle Turonian of Japan (Tanabe, 1975).
REPOSITORIES
The repository of specimens described in the text is indicated by a prefix: AMNH, Division of Paleontology (Invertebrates), American Museum of Natural History, New York; BMNH, British Museum (Natural History), London; NMC, Canadian Museum of Nature, Ottawa, Ontario; TMP, Royal Tyrell Museum, Drumheller, Alberta, Canada; YPM, Yale Peabody Museum, New Haven, Connecticut; and USNM, U.S. National Museum, Washington, D.C. The localities of the specimens from Alberta are shown in figure 1 (modified from Plint et al., this issue).
SYSTEMATIC PALEONTOLOGY
Class Cephalopoda Cuvier, 1797
Order Ammonoidea Zittel, 1884
Suborder Ancyloceratina Wiedmann, 1966
Superfamily Scaphitoidea Gill, 1871
Family Scaphitidae Gill, 1871
Subfamily Scaphitinae Gill, 1871
Scaphites (Scaphites) preventricosus Cobban, 1952
Figures 7–9A
1952. Scaphites preventricosus Cobban: 26, pl. 9, figs. 1–16.
1952. Scaphites preventricosus var. sweetgrassensis Cobban: 27, pl. 10, figs. 18–25.
non 1952. Scaphites preventricosus var. artilobus Cobban: 27, pl. 8, figs. 1–6 (= S. (S.) mariasensis Cobban, 1952).
1955. Scaphites preventricosus Cobban. Cobban: 201, pl. 1, fig. 9; pl. 2, fig. 5.
1965. Scaphites (Scaphites) preventricosus svartenhukensis Birkelund: 83, pl. 16, fig. 3; pl. 18, figs. 2, 3; pl. 19, fig. 1; text-figs. 75–77.
1968. Scaphites cf. preventricosus Cobban. Cobban: L3, pl. 1, fig. 15.
1970. Scaphites preventricosus Cobban. Casanova: fig. 79.
1976. Scaphites preventricosus Cobban. Cobban: 124, pl. 1, figs. 8, 9.
1977. Scaphites preventricosus Cobban. Kauffman: pl. 24, figs. 1, 2.
1983. Scaphites preventricosus Cobban. Cobban: 10, pl. 8, figs. 11–13.
1987. Scaphites preventricosus Cobban. Landman: 227.
1989. Scaphites preventricosus Cobban. Landman: fig. 1 (6a, b).
1991. Scaphites (Scaphites) preventricosus Cobban, 1952. Kennedy and Cobban: 84, textfigs. 28C, D.
1994. Anascaphites preventricosus (Cobban). Cooper: 176.
1994. Scaphites preventricosus Cobban. Braunberger: 107–110, pl. 1, figs. 9–12; pl. 2, figs. 1–5; pl. 3, figs. 1–6; pl. 4, figs. 1–9; pl. 5, figs. 1–5; pl. 6, figs. 1–3.
2001. Scaphites preventricosus Cobban, 1951. Braunberger and Hall: 340–342, pl. 2, figs. 8–12; pl. 2, figs. 1–10.
Diagnosis: Macroconchs large and stout with an oval outline in lateral view; body chamber loosely uncoiled with a reduced aperture; apertural angle approximately 100°; ornamented by fairly straight primary and secondary ribs that are uniformly spaced on the body chamber; microconchs smaller, more slender, with a more loosely uncoiled body chamber; suture complex with asymmetrically bifid lateral lobes.
Type: The holotype is USNM 106675 from a bed of calcareous concretions in the Kevin Member of the Marias River Shale, 514 to 525 feet below the top, in the north bank of the Marias River, 5.5 miles south of Shelby, Toole County, Montana.
Material: The collection consists of 15 specimens, all of which are incomplete. They are divided into nine macroconchs and six microconchs.
Macroconch Description: LMAX averages 70.1 mm (table 3). Adults are robust with an oval outline in side view. The exposed phragmocone occupies approximately one whorl and terminates below the line of maximum length. The septal angle averages 52°. The umbilical diameter of the phragmocone is small; it averages 4.8 mm (table 3). The body chamber consists of a shaft and recurved hook. The umbilical shoulder of the shaft is straight in side view. In TMP2016.041.0038, LMAX/HS and LMAX/HP equal 2.02 and 3.18, respectively. The body chamber is slightly uncoiled producing a small gap between the phragmocone and hook, with a constricted aperture. The apertural angle equals 104° in TMP2016.041.0038.
The whorl section of the phragmocone along the line of maximum length, as shown in TMP2016.041.0207, is depressed and subovoid with maximum whorl width at one-third whorl height. The umbilical wall is steep and subvertical, the flanks are sharply rounded, and the venter is broadly rounded. WP/HP equals 1.49 in this specimen. As the shell passes from the phragmocone into the body chamber in this specimen, both the whorl width and height increase slightly, and the whorl section at midshaft is nearly the same as that along the line of maximum length. It is depressed and subovoid with maximum whorl width at one-quarter whorl height. The umbilical wall is steep and subvertical, the flanks are sharply rounded, and the venter is broadly rounded. WS/HS equals 1.30 in TMP2016.041.0207. In contrast, in TMP2016.041.0038, the whorl section at midshaft is nearly equidimensional with broadly rounded flanks; WS/HS equals 1.03. Adoral of the midshaft, as shown in this specimen, both the whorl width and especially the whorl height abruptly decrease. As a result, the whorl section at the point of recurvature is more depressed than that at midshaft. The umbilical wall is flat and slopes outward, the flanks are sharply rounded, and the venter is broadly rounded. WH/HH equals 1.64 in TMP2016.041.0038. The shell culminates in a constricted aperture with a dorsal lappet.
The ornamentation is well preserved in TMP2016.041.0038. On the phragmocone, primary ribs emerge at the umbilical seam and are slightly rursiradiate on the umbilical wall and shoulder. They develop into broad, elongate swellings that swing gently forward and then backward again before subdividing into three thin ribs, with another two thin ribs in between. They are sharp and uniformly strong on the venter, which they cross with a slight adoral projection. The ribs are equally and closely spaced, with a rib density of 6 ribs/cm on the adoral part of the phragmocone.
The same pattern of ribbing persists onto the body chamber. Primary ribs develop into broad elongate swellings on the flanks that swing gently forward and then backward again, before subdividing into three thin secondary ribs, with as many as four thin ribs in between. Ribs are uniformly strong and closely spaced on the venter of the shaft, with a rib density of 6 ribs/cm. They are equally closely spaced on the venter of the hook, but exhibit a stronger adoral projection.
The suture is complex with asymmetrically bifid lateral lobes (fig. 9A).
Microconch Description: Microconchs are elongate in lateral view. Because all of our specimens are incomplete, it is difficult to estimate LMAX. However, the body chambers of all of the microconchs are smaller than those of the macroconchs. The body chambers are also more loosely uncoiled leaving a larger gap between the phragmocone and hook. In addition, the umbilical shoulder of the shaft is slightly more concave in microconchs than in macroconchs.
The whorl section at the base of the body chamber (we measured the whorl section at the base of the body chamber because the phragmocone was missing) is depressed and subovoid. WP/HP averages 1.17 and ranges from 1.09 to 1.24 (table 4). The umbilical wall is steep and nearly vertical, the flanks are broadly to sharply rounded, and the venter is broadly rounded. Whorl width increases gradually from the phragmocone into the body chamber and reaches its maximum value at the point of recurvature. Whorl height, on the other hand, decreases such that, together, the whorl section at midshaft is much more depressed than that at the base of the body chamber. The umbilical wall slopes outward and the flanks are broadly rounded. WS/HS averages 1.34 and ranges from 1.28 to 1.38. The whorl section at the point of recurvature is even more depressed than that at midshaft. WH/HH averages 1.62 and ranges from 1.49 to 1.73.
Primary ribs are prorsiradiate on the umbilical wall and shoulder of the shaft. They develop into broad straight or slightly concave swellings on the flanks, which reach their maximum strength at two-thirds whorl height. In TMP2016.041.0034, which is a coarsely ornamented specimen, each primary rib subdivides into two thin ribs, with another one or two thin ribs intercalating between them. In contrast, in TMP2016.041.0036, which is a more finely ornamented specimen, each primary rib subdivides into three thin ribs, with as many as four thin ribs intercalating between them. Ribs are sharp and uniformly strong on the venter of the shaft, which they cross with a slight adoral projection. The density of ribs on the venter of the shaft ranges from 4.5 to 6 ribs/cm among the specimens in our sample. Ribs are equally closely spaced on the venter of the hook, which they cross with a stronger adoral projection. The density of ribs on the venter of the hook ranges from 5 to 6 ribs/cm.
The suture of the microconchs is the same as that of the macroconchs.
Remarks: Dimorphism is present in Scaphites (S.) preventricosus. Cobban (1952) initially segregated out microconchs as the variety sweetgrassensis. Macroconchs are larger and more robust, with a more closely coiled body chamber. In the present collection, all the microconchs are incomplete, but even so, the body chambers of the microconchs are smaller than those of the macroconchs.
Scaphites (S.) preventricosus can be distinguished from the overlying species S. (S.) ventricosus by its smaller size, more closely spaced ribbing, and more loosely uncoiled body chamber. The degree of uncoiling of the body chamber in macroconchs is expressed by the apertural angle. Based on our data, the apertural angle ranges from 92.0° to 104° in S. (S.) preventricosus whereas it ranges from 72° to 89° in S. (S.) ventricosus. The degree of uncoiling of the body chamber in macroconchs is also expressed by the ratio LMAX/HP. Based on our data, this value averages 3.07 in S. (S.) preventricosus whereas it averages 2.55 in S. (S.) ventricosus.
Occurrence: In the Upper Cretaceous of the Western Interior of North America, this species demarcates the lower Coniacian Scaphites (S.) preventricosus Zone. In the study area, the lowest occurrence of this species is just above erosional surface E5.5, which marks the beginning of a major transgression just above the base of the lower Coniacian (Walaszczyk et al., 2014). It is present in the Cardium Formation at Highwood River (TMP2016.041.0136–.0138) and in the Wapiabi Formation at Mill Creek (TMP2016.041.0034, .0036–.0039), Cutpick Creek (TMP 2016.041.0207 and .0208), Oldfort Creek (TMP2016.041.0473), Wapiabi Creek (TMP2016.041.0085 and .0087), and Bighorn Dam (TMP2016.041.0365), Alberta. Elsewhere, this species is abundant in the Kevin Member of the Marias River Shale in northcentral Montana and the uppermost part of the Frontier Formation in Wyoming. Outside North America, it has been reported from Umivik, Svartenhuk, Greenland (Birkelund, 1965).
TABLE 3
Measurements of Scaphites (S.) preventricosus macroconchs.
See figure 2 for description of measurements. All measurements are in mm, except for apertural angle (AA) and septal angle (SA), which are in degrees. Rib density is reported to the nearest 0.25 ribs/cm on the adapical and adoral parts of the phragmocone, the midshaft, and the hook, depending upon the preservation of the specimen. The specimens from Highwood River are from the Cardium Formation. Height (m) is the height in the measured stratigraphic section.
FIG. 7.
Scaphites (S.) preventricosus Cobban, 1952, macroconch, TMP2016.041.0038, 115 m, Wapiabi Formation, Mill Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 8.
Scaphites (S.) preventricosus Cobban, 1952, microconchs. A–C. TMP2016.041.0036, 139 m, Wapiabi Formation, Mill Creek, Alberta. A. Right lateral; B. ventral; C. left lateral. D, E. TMP2016.041.0034, 141 m, Wapiabi Formation, Mill Creek, Alberta. D. Right lateral; E. ventral. F, G. TMP2016.041.0039, 70 m, Wapiabi Formation, Mill Creek, Alberta. F. Right lateral; G. ventral.
FIG. 9.
Sutures (except for G, reproduced from Cobban, 1952). A. Scaphites (S.) preventricosus Cobban, 1952, sixth from last suture (flipped), USNM 106675, Marias River Shale, Toole County, Montana. B. Scaphites (S.) ventricosus Meek and Hayden, 1862, USNM 106698, Marias River Shale, Toole County, Montana. C. Scaphites (S.) ventricosus Meek and Hayden, 1862 (formerly Scaphites (S.) tetonensis Cobban, 1952), last suture, USNM 106707, Cody Shale, Teton County, Wyoming. D. Scaphites (S.) depressus Reeside, 1927, last suture, USNM 106695, Cody Shale, Park County, Wyoming. E. Scaphites (S.) depressus Reeside, 1927, second from last suture, USNM 106693, Cody Shale, Park County, Wyoming. F. Clioscaphites saxitonianus (McLearn, 1929), last suture, USNM 106739a, Marias River Shale, Toole County, Montana. G. Clioscaphites saxitonianus (McLearn, 1929), first lateral saddle, third from last suture, TMP2016.041.0229, Wapiabi Formation, West Thistle Creek, Alberta.
TABLE 4
Measurements of Scaphites (S.) preventricosus microconchs.
See figure 2 for description of measurements. All measurements are in mm. Rib density is reported to the nearest 0.25 ribs/cm on the adapical and adoral parts of the phragmocone, the midshaft, and the hook, depending upon the preservation of the specimen. * = measurements were taken at the base of the body chamber because the phragmocone was missing. The specimens from Highwood River are from the Cardium Formation. Height (m) is the height in the measured stratigraphic section.
Scaphites (Scaphites) ventricosus
Meek and Hayden, 1862
Figures 9B, C, 10–16
1862. Scaphites ventricosus Meek and Hayden, 1862: 22.
1876. Scaphites (Scaphites) ventricosus Meek and Hayden. Meek: 425, pl. 6, figs. 7, 8.
1894. Scaphites ventricosus Meek and Hayden. Stanton: 44, figs. 8, 9; pl. 43, fig. 1; non pl. 44, fig. 10.
1898. Scaphites ventricosus Meek and Hayden. Logan: 476, pl. 104, figs. 8, 9; pl. 105, fig. 1; non pl.104, fig.10.
1900 .Scaphites ventricosus Meek and Hayden. Herrick and Johnson: pl. 45, figs. 8–10; pl.46, fig.1.
1927a. Scaphites ventricosus Meek and Hayden. Reeside: 6, pl. 3, figs. 11–18; pl. 4, figs. 1–4.
non 1927a. Scaphites ventricosus var. depressus. Reeside: 7, pl. 5, figs. 6–10 [= S. (S.) depressus].
non 1927a. Scaphites ventricosus var. interjectus. Reeside: 7, pl. 5, figs. 1–5 [= Clioscaphites interjectus].
non 1927a. Scaphites ventricosus var. oregonensis. Reeside: 7, pl. 6, figs. 11–15 [= S. (S.) depressus].
non 1927a. Scaphites ventricosus var. stantoni. Reeside: 7, pl. 3, figs. 19, 20; pl. 4, figs. 5–10 [= S. (S.) depressus].
1927b. Scaphites ventricosus Meek and Hayden. Reeside: 35, pl. 10, figs 1, 2.
non 1929. Scaphites ventricosus var. saxitonianus. McLearn: 77, pl. 18, figs. 1–3; pl. 19, figs. 1, 2 [= Clioscaphites saxitonianus].
1942. Scaphites cf. ventricosus Meek and Hayden. Rosenkrantz: 38, pl. 1.
1952. Scaphites ventricosus Meek and Hayden. Cobban: 31, pl. 12, figs. 1–10; pl. 13, figs. 11–13.
1952. Scaphites tetonensis Cobban, 1952: 31, pl. 14, figs. 1–10.
1952. Scaphites ventricosus Meek and Hayden. Basse: 607, fig. 14.
1955. Scaphites ventricosus Meek and Hayden. Cobban: 201, pl. 1, fig. 6.
1960. Scaphites ventricosus Meek and Hayden. Easton: fig. 11.28–3a, b.
1965. Scaphites (Scaphites) ventricosus Meek and Hayden. Birkelund: 87, pl. 19, figs. 2, 3; text-fig. 78.
1976. Scaphites ventricosus Meek and Hayden. Kennedy and Cobban: text-fig. 17 (part).
1977. Scaphites ventricosus Meek and Hayden. Kauffman: pl. 24, figs. 3, 4.
1989. Scaphites ventricosus Meek and Hayden. Landman: fig. 1 (7a, b).
1991. Scaphites (Scaphites) ventricosus Meek and Hayden, 1862. Kennedy and Cobban: 85, text-fig. 28A, B.
1991. Scaphites (Scaphites) tetonensis Cobban, 1952. Kennedy and Cobban: 87, text-fig. 30A–C.
1994. Scaphites ventricosus Meek and Hayden. Braunberger: 115–117, pl. 13, figs. 1–4.
1994. Anascaphites ventricosus (Meek and Hayden). Cooper: 176, fig. 1A, B.
Diagnosis: Macroconchs large and stout with slightly uncoiled body chamber producing a small gap between the phragmocone and hook; cross section of shaft depressed and subovoid; apertural angle averaging 82.5°; ribbing coarse and widely spaced; microconchs smaller with more loosely uncoiled body chamber; suture complex with asymmetrically bifid lateral lobes.
Types: The holotype is USNM 1903 from the upper part of the Colorado Shale, about 20 miles northeast of Fort Benton, Montana.
Material: A total of 24 specimens, all of which are incomplete. They consist mostly of the body chamber without the phragmocone. All of them are adult, comprising 16 macroconchs and 8 microconchs. The collection also contains fragments of body chambers and phragmocones, but they are difficult to identify to species level.
Macroconch Description: In the measured sample, LMAX averages 85.4 mm and ranges from 70.4 to 101.1 mm (table 5). The ratio of the size of the largest specimen to that of the smallest is 1.44. Adults are robust with an oval outline in side view. The exposed phragmocone occupies approximately one whorl and terminates slightly below the line of maximum length. The septal angle averages 12.0°. The umbilical diameter of the phragmocone is small and averages 4.3 mm (table 5). The body chamber consists of a shaft and recurved hook. The umbilical shoulder of the shaft is straight or slightly concave in side view. LMAX/HS averages 2.16 and ranges from 1.97 to 2.34. The body chamber is slightly uncoiled. LMAX/HP averages 2.55 and ranges from 2.26 to 2.95. As a result, a small gap appears between the phragmocone and hook, and is usually filled with sediment. The apertural angle averages 82.5° and ranges from to 72.0° to 89.0°.
The whorl section of the phragmocone along the line of maximum length is depressed and subovoid with maximum whorl width at onethird whorl height. The umbilical wall is steep and subvertical, the flanks are sharply rounded, and the venter is broadly rounded. WP/HP aver ages 1.34 and ranges from 1.18 to 1.46. As the shell passes from the phragmocone into the body chamber, both the whorl width and height increase slightly, and the shape of the whorl section at midshaft is nearly the same as that along the line of maximum length. It is depressed and subovoid with maximum whorl width at onequarter whorl height. The umbilical wall is steep and subvertical, the flanks are sharply rounded, and the venter is broadly rounded. WS/HS averages 1.31 and ranges from 1.11 to 1.51. Adoral of the midshaft, both the whorl width and especially whorl height abruptly decrease. As a result, the whorl section at the point of recurvature is more depressed than that at midshaft. The umbilical wall is flat and slopes outward, the flanks are sharply rounded, and the venter is broadly rounded. WH/HH averages 1.64 and ranges from 1.45 to 1.80. The shell culminates in a constricted aperture with a dorsal lappet.
Because none of our specimens preserves the adapical part of the phragmocone, our observations are restricted to the adoral part. Primary ribs emerge at the umbilical seam and are straight and rectiradiate on the umbilical wall and shoulder. They develop into broad elongate swellings that reach their maximum strength at one-half whorl height, approximately coinciding with the ventrolateral shoulder. They each subdivide into two thin ribs, with another one or two thin ribs intercalated between them. Ribs are sharp and uniformly strong on the venter, which they cross with a slight adapical or adoral projection. They are equally and widely spaced. Rib density ranges from 2 to 4.25 ribs/cm among the specimens in our sample.
The same pattern of ribbing persists onto the body chamber. Primary ribs are rectiradiate on the umbilical wall and shoulder. They develop into broad elongate swellings that reach their maximum strength at one-half whorl height. They are prominent, rectiradiate, and equally spaced on the shaft, becoming weaker, prorsiradiate, and closely spaced on the hook. Each rib subdivides into two thin secondary ribs, with one or two thin ribs intercalating between them. Ribs are uniformly strong and widely spaced on the venter of the shaft, which they cross with a slight adoral projection. The density of ribs on the shaft ranges from 2.75 to 4 ribs/cm among the specimens in our sample. Ribs are more closely spaced on the venter of the hook, which they cross with a strong adoral projection. For example, in TMP2016.041.0379, the rib density is 2.75 ribs/cm at midshaft versus 4 ribs/cm on the hook.
A suture is not well enough preserved in any of the specimens in our study, but according to Cobban (1952), it is complex with asymmetrically bifid lateral lobes (fig. 9B, C).
Microconch Description: Microconchs are elongate in lateral view. The most notable features of the microconchs relative to the macroconchs are their smaller size and more loosely uncoiled body chamber, leaving a larger gap between the phragmocone and hook. In addition, the umbilical shoulder of the shaft is concave in microconchs whereas it is straight in macroconchs.
LMAX averages 55.2 mm and ranges from 41.8 to 67.1 mm (table 6). The whorl section of the phragmocone along the line of maximum length is depressed and subovoid. WP/HP averages 1.44 and ranges from 1.38 to 1.49. The umbilical wall is steep and nearly vertical, the flanks are sharply rounded, and the venter is broadly rounded. Whorl width increases gradually from the phragmocone into the body chamber and reaches its maximum value at midshaft. Whorl height also increases gradually from the phragmocone into the body chamber and attains its maximum value at the point of recurvature. The whorl section at midshaft is nearly the same as that along the line of maximum length. It is depressed and subovoid with maximum whorl width at onehalf whorl height. The umbilical wall is steep and subvertical, the flanks are sharply rounded, and the venter is broadly rounded. The whorl section at the point of recurvature is more depressed than that at midshaft. The umbilical wall is flat and slopes outward, the flanks are sharply rounded, and the venter is broadly rounded.
At the base of the body chamber, primary ribs emerge at the umbilical seam and are rectiradiate on the umbilical wall and shoulder. They develop into broad, straight or slightly concave swellings on the flanks, which reach their maximum strength at one-half whorl height. They each subdivide into two thin ribs, with another one or two thin ribs intercalating between them. Ribs are sharp and uniformly strong on the venter, which they cross with a slight adoral projection. The ribs are equally and widely spaced, with, for example, 5 ribs/cm on the adoral part of the phragmocone in TMP2016.041.0161.
The same pattern of ribbing persists onto the body chamber. Primary ribs are prominent and equally spaced on the shaft, becoming weaker and more closely spaced on the hook. They are straight or weakly flexuous on the flanks, swinging slightly backward on the inner flanks, slightly forward on the midflanks, and slightly backward again on the outer flanks. Each rib subdivides into two thin secondary ribs, with one or two thin ribs intercalating between them. Ribs are uniformly strong and widely spaced on the venter of the shaft, with, for example, 4.5 ribs/cm in TMP2016.041.0372. Ribs are more closely spaced on the venter of the hook, which they cross with a stronger adoral projection. The rib density ranges from 3.75 to 9 ribs/cm.
The suture of the microconchs is the same as that of the macroconchs.
Remarks: Although Scaphites (S.) ventricosus is well established in the literature, it is rare to find complete specimens. Most specimens lack the phragmocone although, interestingly, the holotype retains the phragmocone but not the hook. The specimens in our collection closely match those from the U.S. Western Interior. For example, YPM 26721 is a macroconch illustrated by Reeside (1927b: pl. 4, fig. 1–4) from the Cody Shale of Wyoming. It is approximately the same size as the macroconchs in our collection (LMAX = 83 mm). It also displays the same pattern of ribs, with more widely spaced ribs on the midshaft than on the phragmocone (5 ribs/cm on the phragmocone versus 3 ribs/cm on the midshaft).
Dimorphs in this species are distinguished on the basis of size. The larger microconchs correspond to the macroconchs in our collection. However, two smaller microconchs (TMP2016.041.0161 and .0163) would previously have been referred to as Scaphites (Scaphites) tetonensis. This form occurs in the same beds as S. (S.) ventricosus, and we argue that it simply represents a small microconch of this species. Other than size, dimorphs are distinguished by the outline of the umbilical shoulder of the shaft in side view. It is straight or slightly concave in macroconchs whereas it is markedly concave in microconchs. This is related to the fact that the body chamber is slightly more tightly coiled in macroconchs than in microconchs, although a small gap is present between the phragmocone and hook in both dimorphs.
Several closely related scaphite species occur in the Coniacian of the Western Interior of North America. Scaphites (S.) ventricosus is distinguished from the underlying species S. (S.) preventricosus by its larger size, more tightly coiled shell, and more widely spaced ribs. It is distinguished from the overlying species S. (S.) depressus by its less tightly coiled shell and more widely spaced ribs.
Occurrence: In the Upper Cretaceous of the Western Interior of North America, this species demarcates the upper lower and middle Coniacian Scaphites (S.) ventricosus Zone. In the study area, the lowest occurrence of this species is immediately above surface CS2 in allomember CA3, just below an interpreted highstand and prior to a major regression that culminates at surface CS4, which marks the boundary between the lower and middle Coniacian. It is present in the Wapiabi Formation at Ram River (TMP2016.041.0021), East Thistle Creek (TMP2016.041.0066 and .0067), James River (TMP2016.041.0155), Blackstone River (TMP2016.041.0106), Chungo Creek (TMP2016.041.0161–.0168), Sheep River (TMP2016.041.0296), Bighorn Dam (TMP2016.041.0366–.0368, .0370–.0374, and .0379), Mill Creek (TMP2016.041.0035), and Bighorn River (TMP2016.041.0349), Alberta. Elsewhere, it is abundant in the Kevin Member of the Marias River Shale in north-central Montana, the Cody Shale in western Wyoming, and the Mancos Shale in New Mexico. Outside North America, it has been reported from Alianaitsúnguaq, Nugssuaq, Greenland (Birkelund, 1965).
TABLE 5
Measurements of Scaphites (S.) ventricosus macroconchs.
See figure 2 for description of measurements. All measurements are in mm, except for apertural angle (AA) and septal angle (SA), which are in degrees. Rib density is reported to the nearest 0.25 ribs/cm on the adapical and adoral parts of the phragmocone, the midshaft, and the hook, depending upon the preservation of the specimen. Height (m) is the height in the measured stratigraphic section.
TABLE 6
Measurements of Scaphites (S.) ventricosus microconchs.
See figure 2 for description of measurements. All measurements are in mm. Rib density is reported to the nearest 0.25 ribs/cm on the adapical and adoral parts of the phragmocone, the midshaft, and the hook, depending upon the preservation of the specimen. Height (m) is the height in the measured stratigraphic section.
FIG. 10.
Scaphites (S.) ventricosus Meek and Hayden, 1862, macroconch, TMP2016.041.0021, 108.7 m, Wapiabi Formation, Ram River, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 11.
Scaphites (S.) ventricosus Meek and Hayden, 1862, macroconch, TMP2016.041.0164, 48.9 m, Wapiabi Formation, Chungo Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 12.
Scaphites (S.) ventricosus Meek and Hayden, 1862, macroconch, TMP2016.041.0167, 51.3 m, Wapiabi Formation, Chungo Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 13.
Scaphites (S.) ventricosus Meek and Hayden, 1862, macroconch, TMP2016.041.0379, 107.5 m, Wapiabi Formation, Bighorn Dam, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 14.
Scaphites (S.) ventricosus Meek and Hayden, 1862, macroconch, TMP2016.041.0035, 191.0 m, Wapiabi Formation, Mill Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 15.
Scaphites (S.) ventricosus Meek and Hayden, 1862, macroconchs, Montana. A, B. AMNH 108451, Kevin Member, Marias River Shale, Toole County, Montana. A. Left lateral; B. apertural. C, D. AMNH 91921, Kevin Member, Marias River Shale, Toole County, Montana. C. Left lateral; D. apertural.
FIG. 16.
Scaphites (S.) ventricosus Meek and Hayden, 1862, microconchs. A–C. TMP2016.041.0367, Wapiabi Formation, Bighorn Dam, Alberta. A. Right lateral; B. ventral; C. apertural. D–F. AMNH 108452, 50–60 m, Kevin Member, Marias River Shale, Toole County, Montana. D. Right lateral; E. apertural; F. ventral. G, H. TMP2016.041.0163, 41.4 m, Wapiabi Formation, Chungo Creek, Alberta. G. Right lateral; H. ventral. I, J. TMP2016.041.0161, 41.4 m, Chungo Creek, Alberta. I. Right lateral; J. ventral.
FIG. 17.
Size-frequency histogram of Scaphites (S.) depressus Reeside, 1927, based on the samples in tables 7 and 8.
Scaphites (Scaphites) depressus Reeside, 1927
Figures 9D, E, 17–32
1894. Scaphites ventricosus Meek and Hayden. Stanton: 186 (pars), pl. 44, fig. 10 only.
1898. Scaphites ventricosus Meek and Hayden. Logan: 476, pl. 104, fig. 10 only.
1927a. Scaphites ventricosus Meek and Hayden var. depressus Reeside: 7, pl. 5, figs. 6–10.
1927a. Scaphites ventricosus Meek and Hayden var. stantoni Reeside: 7, pl. 3, figs. 19, 20; pl. 4, figs. 5–10.
1927a. Scaphites ventricosus Meek and Hayden var. oregonensis Reeside: 7, pl. 6, figs. 11–15.
1952. Scaphites depressus Reeside. Cobban: 32, pl. 15, figs. 6–8.
1952. Scaphites depressus Reeside var. stantoni Reeside. Cobban: 33, pl. 15, figs. 1–5.
1952. Scaphites depressus Reeside var. oregonensis Reeside. Cobban: 33.
1964. Scaphites depressus var. stantoni Reeside. Scott and Cobban, pl. 5, fig. 2.
1970. Scaphites depressus Reeside. Jeletzky, pl. 26, fig. 2.
non 1976 Scaphites ex. gr. ventricosus Meek and Hayden. Szász: 204, pl. 3, fig. 2.
1976. Scaphites depressus Reeside. Kennedy and Cobban, pl. 7, fig. 4.
1977. Scaphites depressus Reeside. Kauffman: 263, pl. 24, figs. 9, 10.
1994. Scaphites depressus Reeside. Braunberger: 117–118, pl. 14, figs. 1–4; pl. 15, figs. 1–3; pl. 16, figs. 1–3.
1994. Scaphites depressus Reeside. Hills et al.: 735, pl. 1, figs. 2, 4.
Diagnosis: Macroconchs globular and massive, with closely coiled body chamber and broadly rounded to flattened flanks, with a reduced aperture; apertural angle averaging 70°; ornament consisting of numerous, straight, closely spaced primary and secondary ribs; microconchs smaller with more loosely uncoiled body chamber; suture complex with asymmetrically bifid first lateral lobes.
Types: Holotype YPM 6417 from 244 m above the base of the Cody Shale on the Oregon Basin Oil Field in sec. 6, T.51N., R.100W., Park County, Wyoming.
Material: Approximately 77 specimens, all of which are adult specimens, comprising 45 macroconchs and 32 microconchs.
Macroconch Description: In the measured sample, LMAX averages 90.6 mm and ranges from 74.3 to 122.7 mm (table 7). The ratio of the size of the largest specimen to that of the smallest is 1.65. The size distribution is unimodal with a peak between 90 and 95 mm (fig. 17). Adults are robust with a circular outline in side view. The exposed phragmocone occupies approximately one whorl and terminates slightly below or slightly above the line of maximum length. The septal angle averages -2.5°. The umbilical diameter of the phragmocone is small and averages 5.4 mm. The body chamber consists of a short shaft and recurved hook. The umbilical shoulder of the shaft is straight in side view. LMAX/HS averages 2.23 and ranges from 2.01 to 2.51. The body chamber is tightly coiled leaving hardly any gap between the phragmocone and hook. LMAX/HP averages 2.55 and ranges from 2.34 to 2.85. TMP 2016.041.0298–.0300 from Sheep River, which occur in the lower part of the Scaphites (S.) depressus Zone, are slightly more loosely uncoiled than most specimens of this species, and are reminiscent of S. (S.) ventricosus. In all of our specimens, the aperture is reduced in size relative to that at midshaft. The apertural angle averages 70.2° and ranges from 61° to 81°.
The whorl section of the phragmocone along the line of maximum length is depressed and subquadrate with maximum whorl width at onethird whorl height. The umbilical wall is steep and subvertical; the flanks are broadly rounded and nearly parallel; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WP/HP averages 1.46 and ranges from 1.32 to 1.74. As the shell passes from the phragmocone into the body chamber, the whorl width and height increase only slightly, so that the whorl section at midshaft is nearly the same as that along the line of maximum length. It is depressed and subquadrate with maximum whorl width at one-third whorl height. The umbilical wall is steep and subvertical; the flanks are broadly rounded and nearly parallel; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WS/HS averages 1.39 and ranges from 1.27 to 1.58. Adoral of the midshaft, the whorl width and, especially, the whorl height abruptly decrease. As a result, the whorl section at the point of recurvature is much more depressed than that at midshaft. WH/HH averages 1.54 and ranges from 1.32 to 1.87. The umbilical wall is flat and slopes outward, the flanks are sharply rounded, and the venter is broadly rounded. The shell culminates in a constricted aperture with a dorsal lappet.
On the exposed phragmocone, primary ribs emerge at the umbilical seam and are straight and rectiradiate on the umbilical wall and shoulder. They develop into broad elongate swellings that gradually reach their maximum strength at one-third whorl height, but never form nodes. They each subdivide into two or three secondary ribs, with another one or two longer secondary ribs intercalating between them. Ribs are sharp and uniformly strong on the broadly rounded venter, which they cross with a slight adoral projection. They are closely spaced on the adapical end of the phragmocone with a rib density of 3.75 to 6 ribs/cm. They become more widely spaced on the adoral end of the phragmocone with a rib density of 3 to 5 ribs/cm.
The same pattern of ornamentation persists onto the body chamber. Primary ribs are rectiradiate on the umbilical wall and shoulder. They develop into elongate swellings that follow the curvature of the flanks and never culminate in nodes. They are prominent, rectiradiate, and equally spaced on the shaft, becoming weaker, prorsiradiate, and more closely spaced on the hook. At onehalf whorl height coincident with the ventrolateral shoulder, each rib subdivides into two secondary ribs, with two to four longer secondary ribs intercalating between them. Ribs are uniformly strong and wirelike on the venter of the shaft, which they cross with a slight adoral projection. They are widely spaced with a rib density of 2 to 4.5 ribs/cm, becoming more closely spaced on the venter of the hook, with a rib density of 3.25 to 5.25 ribs/cm. They cross the venter of the hook with a slight to strong adoral projection.
A suture is not well enough preserved in any of the specimens in our study, but according to Cobban (1952), it is complex with asymmetrically bifid first lateral lobes (fig. 9D, E).
Microconch Description: Many of the microconchs are simply miniatures of the macroconchs. For example, TMP2016.041.0278 is simply a scaled-down version of TMP2016.041.0069. Other microconchs such as TMP2016.041.0221 are not only smaller than the macroconchs, but are also more elongate with a more concave umbilical shoulder in lateral view. LMAX averages 65.3 mm and ranges from 55.8 to 77.5 mm (table 8). The size distribution is unimodal with a peak between 60 and 65 mm (fig. 17).
The shell proportions of microconchs are similar to those of macroconchs. The whorl section of the phragmocone along the line of maximum length is depressed and subquadrate with maximum whorl width at one-third whorl height. The umbilical wall is steep and subvertical; the flanks are broadly rounded and nearly parallel; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WP/HP averages 1.42 and ranges from 1.22 to 1.69. As the shell passes from the phragmocone into the body chamber, the whorl width and height increase only slightly, so that the shape of the whorl section at midshaft is nearly the same as that along the line of maximum length. It is depressed subquadrate with maximum whorl width at one-third whorl height. The umbilical wall is steep and subvertical; the flanks are broadly rounded and nearly parallel; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WS/HS averages 1.38 and ranges from 1.13 to 1.58. Adoral of the midshaft, the whorl width and height decrease slightly, resulting in a slightly more depressed whorl section at the point of recurvature. WH/HH averages 1.52 and ranges from 1.29 to 1.77. The umbilical wall is flat and slopes outward, the flanks are sharply rounded, and the venter is broadly rounded. The shell culminates in a constricted aperture.
The pattern of ornamentation on microconchs is the same as that on macroconchs. On the exposed phragmocone, primary ribs emerge at the umbilical seam and are straight and rectiradiate on the umbilical wall and shoulder. They develop into broad elongate swellings that are straight or weakly concave or convex. They branch into two secondary ribs, with another longer secondary rib intercalating between them. Ribs are sharp and uniformly strong on the broadly rounded venter, which they cross with a slight adoral projection. They are closely spaced on the adapical end of the phragmocone with a rib density of 4.5 to 6 ribs/cm. They become slightly more widely spaced on the adoral end of the phragmocone with a rib density of 4 to 5.5 ribs/cm.
The same pattern of ribbing persists onto the body chamber. Primary ribs are rectiradiate on the umbilical wall and shoulder. They form elongate swellings that are widely spaced on the shaft, becoming more closely spaced on the hook. In a few, more compressed specimens, these ribs develop into bullae at the ventrolateral shoulder before branching into two or three secondary ribs, with one longer secondary rib intercalating between them. Ribs are uniformly strong and wirelike on the venter of the shaft, which they cross with a slight adoral projection. The rib density ranges from 3.5 to 5 ribs/cm. The ribs become more closely spaced on the venter of the hook, with a rib density of 4 to 6 ribs/cm. They cross the venter of the hook with a slight adoral projection.
The suture of the microconchs is the same as that of the macroconchs.
Remarks: Dimorphism is present in Scaphites (S.) depressus. The microconchs have previously been referred to as the co-occurring variety Scaphites (S.) depressus var. stantoni by Reeside (1927a) but we argue that they are simply microconchs of the typical form. The size distribution of microconchs and macroconchs is each unimodal with a peak between 60 and 65 mm and 90 and 95 mm, respectively (fig. 17). The average size of microconchs is 72.1% that of macroconchs (or conversely, the average size of macroconchs is 138.7% that of microconchs).
Two macroconchs of Scaphites (S.) depressus are encrusted with cheilostome bryozoans belonging to the genus Conopeum (fig. 32). The colonies are sheetlike and cover an area of approximately 2 cm2. They occur on the internal molds of the ammonites near the apertural margin, indicating that they must have encrusted the inside surfaces of the body chambers after the ammonites died. This suggests that the shells must have rested on the sea floor for at least several months.
Outside the study area, Scaphites (S.) depressus is widely distributed in the Kevin Member of the Marias River Shale in north-central Montana and the Cody Shale in western Wyoming. It is rare in the Smoky Hill Chalk Member of the Niobrara Formation in southwestern Colorado and in the Mancos Shale in western Colorado and eastern Utah.
Scaphites (S.) depressus co-occurs with Clioscaphites saxitonianus at several sites in Alberta and Montana. At Cardinal River, Alberta, the two species co-occur at a height of 141.5–143.5 m and at West Thistle Creek, Alberta, they co-occur at a height of 121.0 and 123.6 m. Cobban et al. (2005) also noted the co-occurrence of these two species in the Bad Heart Sandstone in central western Alberta and in the Virgelle Sandstone in southwestern Montana.
TABLE 7
Measurements of Scaphites (Scaphites) depressus macroconchs.
See figure 2 for description of measurements. All measurements are in mm, except for apertural angle (AA) and septal angle (SA), which are in degrees. Rib density is reported to the nearest 0.25 ribs/cm on the adapical and adoral parts of the phragmocone, the midshaft, and the hook, depending upon the preservation of the specimen. Height (m) is the height in the measured stratigraphic section.
Continued
FIG. 18.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0011, 132.5 m, Wapiabi Formation, Ram River, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 19.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0012, 132.5 m, Wapiabi Formation, Ram River, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 20.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0025, 127.3 m, Wapiabi Formation, Ram River, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 21.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0069, 83.3 m, Wapiabi Formation, E. Thistle Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 22.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0212, 102.0 m, Wapiabi Formation, W. Thistle Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 23.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0213, 106.7 m, Wapiabi Formation, W. Thistle Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 24.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0216, 107.0 m, Wapiabi Formation, W. Thistle Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 25.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0219, 114.9 m, Wapiabi Formation, W. Thistle Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 26.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0303, 158.8 m, Wapiabi Formation, Sheep River, Alberta. A. Right lateral; B. apertural; C. ventral.
TABLE 8
Measurements of Scaphites (Scaphites) depressus microconchs. See figure 2 for description of measurements.
All measurements are in mm. Rib density is reported to the nearest 0.25 ribs/cm on the adapical and adoral parts of the phragmocone, the midshaft, and the hook, depending upon the preservation of the specimen. Height (m) is the height in the measured stratigraphic section.* = above measured section.
Continued
FIG. 27.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0350, 109.4 m, Wapiabi Formation, Bighorn River, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 28.
Scaphites (S.) depressus Reeside, 1927, macroconch, TMP2016.041.0388, 133.0 m, Wapiabi Formation, Bighorn Dam, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 29.
Scaphites (S.) depressus Reeside, 1927, microconch, TMP2016.041.0382, 115.8 m, Wapiabi Formation, Bighorn Dam, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.
FIG. 30.
Scaphites (S.) depressus Reeside, 1927, microconchs. A–D. TMP2016.041.0278, 141.5–143.5 m, Wapiabi Formation, Cardinal River, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral. E–G. TMP2016.041.0223, 121.0 m, Wapiabi Formation, W. Thistle Creek, Alberta. E. Right lateral; F. ventral; G. left lateral.
FIG. 31.
Scaphites (S.) depressus Reeside, 1927, microconchs. A, B. TMP2016.041.0221, 121.0 m, Wapiabi Formation, West Thistle Creek, Alberta. A. Right lateral; B. ventral. C–F. TMP2016.041.0005, 132.5 m, Wapiabi Formation, Ram River, Alberta. C. Right lateral; D, ventral; E. apertural, F. left lateral. G–I. TMP2016.041.0302, 149.0 m, Wapiabi Formation, Sheep River, Alberta. G. Right lateral; H. apertural. I. ventral.
Clioscaphites saxitonianus (McLearn, 1929)
Figures 9F, G, 33–36
1929. Scaphites ventricosus Meek and Hayden var. saxitonianus McLearn: 77, pl. 18, figs. 1–3; pl. 19, figs. 1, 2.
1952. Clioscaphites saxitonianus (McLearn). Cobban: 36, pl. 13, figs. 1–10.
1952. Clioscaphites saxitonianus (McLearn) var. keytei Cobban: 37, pl. 20, figs. 5–7.
1965. Clioscaphites saxitonianus (McLearn, 1929) var. septentrionalis Birkelund: 132, pl. 45, figs. 2–5; pl. 46, figs. 1–3; text-figs. 115, 116.
1965. Clioscaphites sp. aff. saxitonianus (McLearn, 1929). Birkelund: 135, pl. 46, figs. 4–7; pl. 47, figs. 1, 2; text-fig. 117.
1977. Clioscaphites saxitonianus (McLearn). Kauffman: pl. 24, figs. 5, 6.
1994. Billcobbanoceras saxitonianum (McLearn). Cooper: 179.
Diagnosis: Macroconchs large and stout with closely coiled shell, with a reduced aperture; apertural angle averages 66.5°; whorl cross section of the shaft depressed with nearly flat flanks and broadly curved venter; ribs finely and closely spaced on the exposed phragmocone, coarser and more widely spaced on the shaft, and finer and more closely spaced again on the hook; primary ribs strong on the shaft attaining their maximum height as incipient nodes at the ventrolateral shoulder; microconchs smaller and more slender; suture moderately complex with asymmetrically bifid first lateral lobes.
Types: The holotype is NMC 9041a from the Alberta Shale of the Crowsnest River area of southwestern Alberta. The paratype is NMC 9041a; plesiotype is USNM 106739a, b. The holotype of the subspecies keytei, which is synonymized here with the typical form, is USNM 106727 from a calcareous concretion in the Apishapa Shale, 16 miles east of Trinidad, in sec. 1, T. 32 S., R. 62 W., Las Animas County, Colorado.
Material: The collection consists of 16 specimens, all of which are adult, comprising 13 macroconchs and 3 microconchs.
Macroconch Description: In the measured sample, LMAX averages 83.0 mm and ranges from 79.8 to 88.8 mm (table 9). The ratio of the size of the largest specimen to that of the smallest is 1.11. Adults are massive with a nearly circular outline in side view. The exposed phragmocone occupies approximately one whorl and terminates slightly above or below the line of maximum length. The umbilical diameter of the phragmocone is tiny; it averages 3.7 mm and ranges from 2.7 to 4.6 mm (table 9). The body chamber consists of a shaft and recurved hook. In side view, the umbilical shoulder of the shaft is straight and the venter of the shaft is broadly curved. LMAX/HS averages 2.38 and ranges from 2.37 to 2.38. The shell is tightly coiled. LMAX/HP equals 2.84 in TMP2016.041.0229. The aperture is reduced in size relative to that at midshaft. The apertural angle equals 66.5° and ranges from 55.5° in TMP2016.041.0279 to 84.0° in TMP2016.041.0229.
The whorl section of the phragmocone along the line of maximum length is depressed and subovoid with maximum whorl width at one-half whorl height. The umbilical wall is steep and subvertical; the flanks are broadly rounded and slope outward; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WP/HP equals 1.61 in TMP2016.041.0279. As the shell passes from the phragmocone into the body chamber, the whorl width remains nearly the same but the whorl height increases slightly, so that the whorl section at midshaft is slightly less depressed than that along the line of maximum length. The umbilical wall is steep and subvertical; the flanks are broadly rounded and slope outward; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WS/HS averages 1.41 and ranges from 1.39 to 1.43. Adoral of the midshaft, both the whorl width and especially whorl height abruptly decrease. As a result, the whorl section at the point of recurvature is more depressed than that at midshaft. The umbilical wall is flat and slopes outward; the flanks are broadly rounded; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WH/HH averages 1.65 and ranges from 1.56 to 1.75. The shell culminates in a constricted aperture with a dorsal lappet.
On the exposed phragmocone, primary ribs emerge at the umbilical seam and are straight and rectiradiate on the umbilical wall and shoulder. They develop into massive, elongate swellings that reach their maximum strength at the ventrolateral shoulder. On the adapical end of the phragmocone, the primary ribs split into bundles of two or three thinner ribs, with two or three ribs intercalating between them. The ribs are closely spaced on the venter, with a rib density of 5 ribs/cm in TMP2016.041.0017. They are sharp and uniformly strong on the venter, which they cross with a slight adoral projection. The primary ribs become more prominent and widely spaced on the adoral part of the phragmocone. They split into bundles of two or three thinner ribs with one rib interca lating between them. They are widely spaced on the venter with a rib density of 2.5–4 ribs/cm.
The rib pattern on the adoral part of the phragmocone becomes even more pronounced on the shaft. The primary ribs emerge at the umbilical seam and swing slightly forward and then backward again before developing into straight, massive, elongate swellings that attain their maximum strength at the ventrolateral shoulder in the form of incipient nodes. The swellings are widely and equally spaced. At the ventrolateral shoulder, they split into bundles of two secondary ribs with another secondary rib intercalating between them. Ribs are widely and equally spaced on the venter, with a rib density of 1.25–4 ribs/cm. They are uniformly strong and cross the venter with at most a slight adoral projection. The primary ribs become weaker and more closely spaced on the hook. Each rib subdivides into two or three secondary ribs, with another secondary rib intercalating between them. Ribs cross the venter of the hook with a slight adoral projection. They are closely and equally spaced, with a rib density of 4–6 ribs/cm.
The sutures are not generally preserved. However, in TMP2016.041.0229, the first lateral lobe is slightly asymmetrically bifid (fig. 9G).
Microconch Description: Microconchs are smaller and more slender than macroconchs. In addition, the umbilical shoulder of the shaft is concave in microconchs whereas it is straight in macroconchs. LMAX equals 58.2 mm in TPM2016.041.0230 (table 10).
The whorl section of the phragmocone along the line of maximum length is partly visible in TPM2016.041.0230. It is depressed and subovoid with maximum whorl width at one-half whorl height. The flanks are broadly rounded and slope outward; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. As the shell passes from the phragmocone into the body chamber, both the whorl width and height increase slightly, so that the whorl section at midshaft is nearly the same as that of the phragmocone along the line of maximum length. The inner flanks of the phragmocone are broadly rounded and slope outward; the outer flanks are nearly flat; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. Adoral of the midshaft, both the whorl width and height decrease. As a result, the whorl section at the point of recurvature is more depressed than that at midshaft. The umbilical wall is flat and slopes outward; the flanks are broadly rounded; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. The shell culminates in a constricted aperture with a dorsal lappet.
At the base of the body chamber, primary ribs emerge at the umbilical seam and are straight and rectiradiate on the umbilical wall and shoulder. They develop into massive, elongate swellings that reach their maximum strength at the ventrolateral shoulder. The primary ribs split into bundles of two thinner ribs, with one rib intercalating between them. They are sharp and uniformly strong on the venter, which they cross with a slight adoral projection.
The ribbing pattern is similar on the shaft. The primary ribs emerge at the umbilical seam and develop into straight, massive, elongate swellings that attain their maximum strength at the ventrolateral shoulder. The swellings are widely and equally spaced. At the ventrolateral shoulder, they split into bundles of two secondary ribs with another one or two secondary ribs intercalating between them. The ribs are widely and equally spaced on the venter, with a rib density of 3.75–4 ribs/cm. They are uniformly strong and cross the venter with at most a slight adoral projection. The primary ribs become weaker and more closely spaced on the hook. Each rib subdivides into two secondary ribs, with another one or two secondary ribs intercalating between them. Ribs cross the venter of the hook with a slight adoral projection. They are closely and equally spaced, with a rib density of 4.25 ribs/cm in TMP2016.041.0226.
The suture is not preserved in any of our specimens.
Remarks: Dimorphism is present in Clioscaphites saxitonianus. The microconch was initially designated by Cobban (1952) as the variety keytei. Microconchs are smaller and more slender than macroconchs. In addition, the umbilical shoulder of the shaft is concave in microconchs whereas it is straight in macroconchs.
Clioscaphites saxitonianus is distinguished from the underlying species Scaphites (S.) depressus by its less globose shape, flatter flanks, and coarser ornamentation on the body chamber. It is distinguished from the overlying species Clioscaphites vermiformis (Meek and Hayden, 1862) by having incipient nodes rather than pointed tubercles on the body chamber and rarely having the first lateral lobe of the suture trifid.
Cooper (1994) established the genus Billcobbanoceras and included Clioscaphites saxitonianus as one of its species. While subsequent workers have acknowledged this reassignment (e.g., Cobban et al., 2006), none of them has ever followed it. We continue to assign this species to Clioscaphites as originally described by Cobban (1952), in anticipation of a thorough taxonomic revision of these Coniacian and Santonian scaphites in the future.
Occurrence: In the Upper Cretaceous of the Western Interior of North America, this species demarcates the lower Santonian Clioscaphites saxitonianus Zone (Scott and Cobban, 1962). In the study area, the lowest occurrence of this species is at the base of the Santonian (surface SS0), coinciding with a major transgression and a marked change in facies to deeper-water, more offshore mudstone. This species is present in the Wapiabi Formation, Alberta, at James River (TMP2016.041.0157), West Thistle Creek (TMP2016.041.0222, .0224, and .0226–.0232), Cardinal River (TMP2016.041.0279), Cripple Creek (TMP2016.041.0148 and .0149), Lynx Creek (TMP2016.041.0339), and above the measured section at Ram River (TMP2016.041.0017 and .0018). In the U.S., this species is present in the Apishapa Shale of southeastern Colorado and in the Kevin Member of the Marias River Shale on the east flank of the Sweetgrass Arch of north-central Montana. Outside North America, it has been reported from west Greenland (Birkelund, 1965).
FIG. 32.
Close-ups of cheliostome bryozoans of the genus Conopeum encrusting two macroconchs of Scaphites (S.) depressus Reeside, 1927. The bryozoans occur on the internal molds of the body chamber near the apertural margin. A. TMP2016.041.0382, 115.8 m, Wapiabi Formation, Bighorn Dam, Alberta. B. TMP2016.041.0009, 132.5 m, Wapiabi Formation, Ram River, Alberta.
TABLE 9
Measurements of Clioscaphites saxitonianus, macroconchs.
See figure 2 for description of measurements. All measurements are in mm, except for apertural angle (AA), which is in degrees. Rib density is reported to the nearest 0.25 ribs/cm on the adapical and adoral parts of the phragmocone, the midshaft, and the hook, depending upon the preservation of the specimen. Height (m) is the height in the measured stratigraphic section.* = above measured section.
TABLE 10
Measurements of Clioscaphites saxitonianus, microconchs.
See figure 2 for description of measurements. All measurements are in mm. Rib density is reported to the nearest 0.25 ribs/cm on the adapical and adoral parts of the phragmocone, the midshaft, and the hook, depending upon the preservation of the specimen. Height (m) is the height in the measured stratigraphic section.
FIG. 33.
Clioscaphites saxitonianus (McLearn, 1929), macroconchs. A, B. TMP2016.041.0017, above measured section, Wapiabi Formation, Ram River, Alberta. A. Right lateral; B. ventral. C, D. TMP2016.041.0148, 47.0 m, Wapiabi Formation, Cripple Creek, Alberta. C. Left lateral; D. ventral.
FIG. 34.
Clioscaphites saxitonianus (McLearn, 1929), macroconchs. A. TMP2016.041.0018, above measured section, Wapiabi Formation, Ram River, Alberta, right lateral. B, C. TMP2016.041.0222, 121.0 m, Wapiabi Formation, W. Thistle Creek, Alberta. B. Right lateral; C. ventral. D, E. TMP2016.041.0279, 141.5–143.5 m, Wapiabi Formation, Cardinal River, Alberta. D. Right lateral; E. apertural.
FIG. 35.
Clioscaphites saxitonianus (McLearn, 1929), macroconchs. A, B. TMP2016.041.0229, 125.9 m, Wapiabi Formation, W. Thistle Creek, Alberta. A. Right lateral; B. ventral. C, D. TMP2016.041.0228, 125.9 m, Wapiabi Formation, W. Thistle Creek, Alberta. C. Right lateral; D. ventral.
FIG. 36.
Clioscaphites saxitonianus (McLearn, 1929), microconchs. A, B. TMP2016.041.0149, 48.6 m, Wapiabi Formation, Cripple Creek, Alberta. A. Right lateral; B. ventral. C, D. TMP2016.041.0226, 123.6 m, Wapiabi Formation, W. Thistle Creek, Alberta. C. Right lateral; D. ventral. E–G. TMP2016.041.0230, 125.9 m, Wapiabi Formation, W. Thistle Creek, Alberta. E. Left lateral; F. ventral; G. ventral hook.
ACKNOWLEDGMENTS
We thank Mary Conway and Kathy Sarg (AMNH) for curation of the specimens, Stephen Thurston (AMNH) for photographing the specimens and preparing the figures, Mariah Slovacek (AMNH) for preparation of the specimens and illustrations, Mark Wilson (College of Wooster, Wooster, Ohio) for identification of the bryozoans, Neal Larson (Larson Paleontology Unlimited, Keystone, South Dakota) for preparation of the specimens, Susan Butts (Yale Peabody Museum, New Haven, Connecticut) for access to collections in her care, and Brandon Strilisky (TMP) for facilitating the accession of the specimens into the Tyrell Museum of Paleontology. We also thank Marcin Machalski (Institute of Paleobiology, War saw, Poland), John A. Chamberlain, Jr. (Brooklyn College, Brooklyn, New York), and Royal H. Mapes (North Carolina Museum of Natural History, Raleigh, North Carolina) for reviewing an earlier draft of this manuscript and making many helpful suggestions. This research was funded in part by NSF Grant DEB-1353510 to N.H.L. and NCN Grant UMO-2015/17/B/ST10/03228 to I.W., and through multiple cycles of Discovery Grant funding from the Natural Sciences and Engineering Research Council of Canada to A.G. Plint.
