Introduction

The genus Neospathodus Mosher includes a group of species possessing segminate P₁ elements with a well-developed anterior process; a posterior process that is short or absent (Sweet, 1973). Neospathodus was classified in the family Xaniognathidae Sweet (Sweet, 1981) and then assigned to the family Gondolellidae, together with the genus Neogondolella Bender and Stoppel (Sweet, 1988).

The species of Neospathodus are used as index fossils in the Lower Triassic (e.g., Orchard, 1995). There is, however, a controversy regarding components of the Neospathodus apparatus, namely, is it a unimembrate type or a multimembrate one?

Sweet (1970, 1988) regarded the apparatus of Neospathodus as a single-element (unimembrate) type composed of segminate (neospathodiform) Pa elements. On the other hand, Kozur (1976) believed that Neospathodus exhibits the same apparatus composition as Gondolella with six types of elements. He noted that “Individual ramiform conodonts can, for the most part, be distinguished from those of Gondolella at the form species level; however, several forms cannot be separated at this level. The prioniodiniform (M) element differs only very slightly from apparatus to apparatus. Such a fact is not really remarkable because Neospathodus developed iteratively in several lineages by platform reduction from gondolellids.” He did not, however, provide any more detailed information about the apparatus.

Orchard (1995) pointed out that at least one multi-element apparatus of Neospathodus was tentatively reconstructed in his collections from Nevada, but whether that, or a different, apparatus could be recognized for other species was unknown.

I have undertaken a reconstruction of apparatuses of Neospathodus from the upper Spathian limestone in the Taho Formation exposed in Tahokamigumi, Sirokawa-cho, Higashiuwa-gun, Ehime Prefecture. The Triassic carbonate rocks of the formation are interpreted to have been deposited in an equatorial region of the Panthalassa Ocean (Ando et al., 2001) and occur as a large exotic block in Jurassic clastic rocks (Koike, 1994). I present herein the systematic description of two multielement species of Neospathodus, N. symmetricus Orchard and N. chionensis (Bender) and offer a hypothetical arrangement of their skeletal architecture.

All of the described conodont specimens are housed in the Faculty of Education and Human Sciences, Yokohama National University, Yokohama.

Apparatus reconstruction of Neospathodus symmetricus Orchard

On the basis of rich conodonts from the Taho Formation, I reconstruct multielement apparatuses of Neospathodus symmetricus and N. chionensis which are composed of eight types of elements: digyrate (cypridodelliform) M, alate S₀, digyrate (enantiognathiform) S₁, digyrate (grodelliform) S₂, bipennate S_3/4, angulate P₂, and segminate P₁ elements. The apparatus nomenclature, S₀ to S₄, and P₁ and P₂, follows a proposal of Purnell et al. (2000).

The stratigraphic occurrence of Neospathodus symmetricus is restricted to the upper Spathian limestone in the Taho Formation (Figure 1). It ranges from the level at which Icriospathodus collinsoni (Solien) disappears through the Spathian-Anisian boundary at which Chiosella timorensis (Nogami) makes its debut.

Figure 1

Stratigraphic section and vertical distribution of Neospathodus symmetricus Orchard 1995, N. chionensis (Bender, 1967) and important conodont species in the Taho Formation. Stratigraphic sections 005′, 1122–1125, 1187–1190, and 1644–1659 situated in the central part of the outcrop in Tahokamigumi. Stratigraphic section 1614–1620 situated at about 50 m east of section 1644–1659.

The P₁ element of N. symmetricus was proposed by Orchard (1995). Many specimens previously assigned to N. homeri P₁ elements by several workers were regarded as N. symmetricus by Orchard (1995). Thus, previously described ramiform elements accompanied by N. homeri are possibly of the N. symmetricus apparatus.

Mosher (1968) recognized six species accompanied by N. symmetricus P₁ elements [Mosher's N. cristagalli (Huckriede)] and “Cypridodella unialata” (S₂ element of N. symmetricus). Among the six species, “Cypridodella conflexa Mosher”, “Diplododella magnidentata (Tatge)”, “Enantiognathus ziegleri (Diebel)”, “Prioniodina latidentata Tatge”, and “Ozarkodina tortilis Tatge” are similar to the M, S₀, S₁, S_3/4, and P₂ elements of the N. symmetricus apparatus of this study, respectively. “Diplododella magnidentata”, “P. latidentata”, and “O. tortilis” were, however, originally described based on the material from the Middle Triassic Muschelkalk by Tatge (1956). Kozur (1976) and Orchard and Rieber (1999) regarded the mentioned form species as elements of the Middle Triassic Neogondolella mombergensis (Tatge) apparatus. “Enantiognathus ziegleri” is probably the S₁ element of the late Ladinian conodont apparatus, Meta-polygnathus (Budurovignathus) mungoensis (Diebel) reconstructed by Mietto (1982). The holotype of “Cypridodella conflexa Mosher” was recovered from the upper Norian Hallstätter Kalk of Austria. All of the illustrated specimens of the six form species by Mosher (1968) are of Anisian age. Thus, the ramiform elements mentioned above are not part of the N. symmetricus apparatus.

Prior to Mosher (1968), Bender (1967) reported several Spathian conodont faunules accompanied by N. homeri and some ramiform elements from the island of Chios, Greece. Among three specimens illustrated as the species “Apatognathus mitzopouli” by Bender (1967), two of them agree morphologically with the S₂ element of N. symmetricus. Bender's holotype of “A. mitzopouli” differs, however, from the S₂ element. Other species accompanied by N. homeri are “Ozarkodina tortilis”, “Cypridodella muelleri Tatge”, and “Enantiognathus ziegleri”. The illustrated specimens of these species are difficult to identify with the elements of N. symmetricus because of poor condition of the material.

Mosher (1973) proposed a multielement species Ellisonia sp. from the Spathian of Canada. Ellisonia sp. is composed of “Cypridodella unialata” (LC-element of Mosher, S₂ element of N. symmetricus), “C. conflexa” (LA), “D. magnidentata” (U), and “Hindeodella triassica Müller” (LB). Judging from Mosher's figures, “D. magnidentata” is similar to the S₀ element of N. symmetricus, but “C. conflexa” and “H. triassica” are dissimilar to the M and S_3/4 elements of the species.

The multielement species Ellisonia clarki of Sweet (1970) is similar to N. symmetricus. Acocrding to Sweet (1970), E. clarki occurs in the Spathian of the Salt Range and Trans-Indus Range in Pakistan, and is composed of four kinds of elements. The elements of E. clarki, U, LC, LA, and LB, correspond closely to the S₀, S₁, S₂, and S_3/4 elements of N. symmetricus. Sweet (1970), however, did not identify the LA element of E. clarki with “C. unialata Mosher” (S₂ element of N. symmetricus).

To sum up the above description, among eight element types of the N. symmetricus apparatus the P₁ and S₂ elements are identical to “N. symmetricus Orchard” and “C. unialata Mosher”, respectively, but the other six types of elements are not identical to any previously proposed species with the exception of three elements (U, LC, and LB) of E. clarki Sweet.

The number of elements of N. symmetricus occurring in each level of the upper Spathian limestone of the Taho Formation is shown in Table 1. The frequency of M, S₀, S₁, S₂, S_3/4, P₂, and P₁ elements is 1048, 267, 988, 922, 962, 634, and 3736, respectively, and an approximate ratio of the elements is 2.3 : 0.6 : 2.1 : 2 : 2.1 : 1.4 : 8.1.

Table 1

Occurrence of M, S₀, S₁, S₂, S_3/4, P₂, and P₁ elements of Neospathodus symmetricus Orchard obtained from 2 to 3 kg of limestone of the Taho Formation.

The ratio of the elements of M, S₀, S₁, S₂, S₃, S₄, P₂, and P₁ is 2:1:2:2:2:2:2:2 in a natural assemblage of Neogondolella sp. Rieber, 1980 (Orchard and Rieber, 1999) and in ozarkodinid apparatuses (e.g., Aldridge et al., 1987; Purnell and Donoghue, 1998; Purnell et al., 2000). Compared with the ratio of the elements in Neogondolella sp. and ozarkodinid apparatuses, P₁ elements of N. symmetricus occur about four times as frequently as S₂ elements. The high abundance of P₁ elements is probably due to their robust constitution. Similar occurrence of P₁ elements is also recognized in the case of Cratognathodus kochi (Huckriede) (revised from C. multihamatus (Huckriede) of Koike, 1999) in the Taho Formation. The frequency of S₀ and S_3/4 elements is about half of what could be expected among ramiform elements. The low abundance of S₀ and S_3/4 elements is probably due to their fragility. I regard the N. symmetricus apparatus as composed of a single unpaired S₀, single pairs of M, S₁, S₂, S₃, S₄, P₂, and P₁ elements. The S₃ and S₄ elements are not distinguished from each other, because they show various transitional forms. Thus, I treat two single pairs of S₃ and S₄ elements as S_3/4 herein.

Neospathodus symmetricus occurs in the upper Spathian strata in the Taho Formation and is accompanied by two or three other species of Neospathodus in most levels. Table 2 shows the occurrence of the segminate P₁ elements and neospathid ramiform elements associated with N. symmetricus (Table 1), except for N. chionensis (Table 3) and N. sp. B mentioned on page 134.

Table 2

Occurrence of P₁ elements of Neospathodus abruptus Orchard, N. brochus Orchard, N. curtatus Orchard, N. sp. A, and N. spp. and ramiform elements probably of N. abruptus. These elements were obtained from 2 to 3 kg of limestone of the Taho Formation. Occurrence of N. symmetricus and N. chionensis is shown in Tables 1 and Table 3, respectively.

Table 3

Occurrence of M, S₀, S₁, S₂, S_3/4, P₂ and P₁ elements of Neospathodus chionensis (Bender) obtained from 1 and 10 kg limestone of Levels 1649 and 1614, respectivery of the Taho Formation.

The ramiform elements listed in Table 2 are probably of Neospathodus abruptus Orchard (Figure 2). The S₁, S₂, and S_3/4 elements are characterized by possession of thin and broad processes, and numerous denticles on the processes. The M elements possess a relatively long outer lateral process with 5 to 6 denticles, and the P₂ elements have a relatively short posterior process with 2 to 4 denticles. I will describe N. abruptus in detail in the near future.

Figure 2

Neospathodus from the Taho Formation. 1. Neospathodus brochus Orchard, 1995, P₁element, YNUC16003 from Lev. 1616, inner view, ×50. 2. Neospathodus curtatus Orchard, 1995, P₁element, YNUC16004 from Lev. 1614, inner view, ×60. 3. Neospathodus sp. A, P₁element, YNUC16005 from Lev. 005′, inner view, ×50. 4, 5. Neospathodus sp. N. aff. symmetricus Orchard, 1995, P₁elements, YNUC16006, 16007 from Lev. 1647, inner view, ×60. 6. Neospathodus abruptus Orchard, 1995, P₁element, YNUC16008 from Lev. 1614, inner view, ×50. 7–12. Neospathodus abruptus Orchard?, 1995, from Lev. 1414, all ×50, 7: P₂element, YNUC16009, inner view,. 8: M element, inner view, YNUC16010. 9: YNUC16011, S₀element, lateral view. 10: S₁element, inner view, YNUC16012. 11: S₂element, YNUC16013, inner view. 12: S_3/4element, YNUC16014, inner view.

The P₁ element of Neospathodus sp. A is quite similar to that of N. symmetricus in outline of the unit and denticulation, but it possesses a very narrow slitlike basal cavity (Figure 2). These elements, however, probably fall within the range of intraspecific variation of N. symmetricus P₁ elements, because the degree of expansion of the basal cavity seems to be more variable than that described by Orchard (1995).

Unfortunately, it is impossible to discriminate the ramiform elements of Neospathodus brochus Orchard, N. curtatus Orchard, and N. spp. listed in Table 2, because of inadequate data in the present collection. Furthermore, another difficulty is in distinguishing the ramiform elements from those of N. symmetricus. The ramiform elements of N. symmetricus are inadequately preserved. Therefore, the frequency of ramiform elements of N. symmetricus listed in Table 1 may include a small number of elements of Neospathodus brochus Orchard, N. curtatus Orchard, and N. spp.

Apparatus reconstruction of N. chionensis (Bender)

The occurrence of Neospathodus chionensis is entirely restricted within about a 50 cm-thick ammonoid-bearing bed above the level where Icriospathodus collinsoni (Solien) disappears.

The M element is identical with “Ctenognathus chionensis” of Bender (1967). According to Bender, “C. chionensis” occurs in the upper Scythian of the island of Chios, Greece. “Ctenognathus chionensis” is accompanied by P₁ elements of Neospathodus homeri (Bender) and N. triangularis (Bender), and nine or ten types of ramiform elements. Bender (1967) illustrated one specimen of a Neospathodus homeri P₁ element (plate 5, figure 18) associated with “C. chionensis”. The illustrated specimen cannot be assigned to N. homeri but it is certainly a Spathian Neospathodus and similar to P₁ elements of a species of Neospathodus associated with N. chionensis in the Taho Formation. This species (Neospathodus sp. B) seems to possess eight types of elements that are characterized by large stout processes with isolated denticles. Neospathodus sp. B is restricted in occurrence in Level 1614 and easily distinguished from N. chionensis, N. symmetricus, N. abruptus and other Neospathodus species obtained from the level. Therefore, the frequency of elements of N. sp. B (25 specimens of P₁ elements) is not listed in Table 2.

Figure 3

Hypothetical arrangement of S₀and sinistral S₁, S₂, and S_3/4elements of (a) Neospathodus symmetricus Orchard and (b) N. chionensis (Bender). The position and arrangement of the elements are based on the Neogondolella apparatus illustrated by Orchard and Rieber (1999) and the Carboniferous ozarkodinid Idiognathodus and Gnathodus apparatuses reconstructed by Aldridge et al. (1987), and Purnell and Donoghue (1997, 1998). The arrangement of S₁and S₂elements is based on my observation of Neogondolella sp. Rieber (1980).

As far as I know, none of the seven types of elements including the neospathodiform P₁ in the N. chionensis apparatus has been described. As mentioned above, the occurrence of this apparatus is restricted within a 50 cm-thick-bed. Thus, it is impossible to increase the reliability of the reconstruction on the basis of comparison of the occurrence of elements in various levels. I regard the reconstruction of N. chionensis as reasonable on the basis of the following facts: all the eight types of elements are characterized by possession of stout process with discrete denticles. Furthermore, their ratio of occurrence is not so different from that in a neogondolellid natural assemblage, as mentioned below.

The number of elements of N. chionensis obtained from two levels is listed in Table 3. The frequency of M, S₀, S₁, S₂, S_3/4, P₂, and P₁ elements is 131, 67, 117, 124, 174, 109, and 132, respectively, and an approximate ratio of the elements is 2:1:1.7:1.9:2.6:1.6:2. Consequently, I regard N. chionensis as a multielement apparatus composed of a single unpaired S₀, single pairs of M, S₁, S₂, S₃, S₄, P₂, and P₁ elements. The S₃ and S₄ elements are also undistinguishable from each other as in the case of N. symmetricus.

The eight types of elements of N. chionensis and N. symmetricus apparatuses are very similar to each other in morphology. The presence of digyrate (enantiognathiform) S₁ and digyrate (grodelliform) S₂ elements in these apparatuses is common to Neogondolella (Orchard and Rieber, 1999) and Cratognathodus kochi (Huckriede) of Koike (1999). The apparatus structure of Neospathodus is comparable to the standard 15-element plan of ozarkodinids (Purnell and Donoghue, 1998; Purnell et al., 2000).

Systematic paleontology

Class Conodonta

Order Ozarkodinida

Superfamily Gondolellacea

Family Gondolellidae

Genus Neospathodus Mosher, 1968

Type species.—Spathognathodus cristagalli Huckriede, 1958.

Neospathodus chionensis (Bender, 1967) Figure 5. 1–25

M element Ctenognathus chionensis Bender, 1967, p. 503–504, pl. 1, figs. 13, 15, 16.

Diagnosis. — Neospathodus chionensis (Bender, 1967) is composed of single pairs of digyrate (cypridodelliform) M, digyrate (enantiognathiform) S₁, digyrate (grodelliform) S₂, bipennate S_3/4, angulate P₂, and segminate P₁ elements, and a single unpaired alate S₀ element. These elements are characterized by possession of stout processes and relatively discrete thick denticles.

Figure 4

Neospathodus chionensis (Bender). Relative positions and schematic arrangement of component elements. This figure is based on the Neogondolella apparatus illustrated by Orchard and Rieber (1999). On the basis of my observation of Neogondolella sp. Rieber (1980), I arrange the S₁ and S₂ elements in each set with their inner-lateral process and the cusp directed in almost parallel to the posterior process and the cusp of the S₃ and S₄ elements.

Figure 5

1–25. Neospathodus chionensis (Bender, 1967) from the Taho Formation. all ×50.

All from Lev. 1614 except for 11 from Lev. 1649. 1–3, S₀elements. 1: YNUC16003, posterior view. 2, 3: YNUC16004, 16005, lateral views. 4–7, M elements, inner views. 4, 5: YNUC16006, 16007, sinistral. 6, 7: YNUC16008, 16009, dextral. 8–10, S₁elements. 8: YNUC16010, sinistral, upper view. 9: YNUC16011, dextral, inner view. 10: YNUC16012, sinistral, inner view. 11–13, S₂elements, YNUC16013–16015, dextral, inner views. 14–17, S_3/4 elements. 14, 15: YNUC16016, 16017, sinistral, inner views. 16, 17: YNUC16018, 16019, dextral, inner views. 18–21, P₂elements, inner views. 18, 19: YNUC16020, 16021, sinistral. 20, 21: YNUC16022, 16023, dextral. 22– 25, P₁elements, inner views. 22–24: YNUC16024–16026, sinistral. 25: YNUC16027, dextral.

Description. — P₁ element: Unit is stout, and ranges from 480 μm to 980 μm in length and from 240 μm to 530 μm in height. Ratio of length and height is 1.5– 1.8 : 1. Denticles are 10 to 12 in number, subequal-sized, and upright in anterior and reclined in posterior portions. Upper surface of basal cup is thick on inner side. Basal cavity broadly expanded and quadrate to triangular in outline.

P₂ element: Anterior and posterior processes are almost equal in length and range from 240 μm to 370 μm and 250 μm to 380 μm in length, respectively. Both processes meet at an angle of about 140 to 160 degrees in upper view and 160 to 170 degrees in lateral view. Denticles on anterior process 5 to 7 in number, fused, and long, and tend to increase in height and inclination posteriorly. Denticles on posterior process 5 to 7 in number, discrete, short, and almost equal in size. Cusp is subequal to or much larger than largest denticles on anterior process. Basal cavity is slitlike and narrow groove extends toward anterior and posterior from basal cavity.

M element: Outer- and inner-lateral processes range from 210 μm to 470 μm and from 790 μm to 910 μm in length, respectively. Both processes meet at an angle of 120 to 150 degrees in lateral view. Outer-lateral process projects upward and flexed anteriorly and carries 1 to 5 relatively large discrete denticles. Inner-lateral process projects downward and curves posteriorly in distal portion. Denticles on inner-lateral process 12 to 15 in number, and tend to be long in middle portion and gradually increase in inclination proximally. Cusp is subequal to twice as long as largest denticle on processes. Basal cavity is small and triangular in shape with fine lip on inner side. Basal groove not observed.

S₀ element: Posterior process ranges from 510 μm to 780 μm in length. Each lateral process ranges from 270 μm to 430 μm in length. Lateral processes meet at cusp or first denticle anterior of cusp, and form an angle of 90 to 160 degrees to each other in antero-posterior view and 90 to 160 degrees on anterior side in upper view. Denticles on each lateral process 3 to 5 in number, discrete, and relatively large. Denticles on posterior process 7 to 10 in number, discrete, and increase in length and inclination posteriorly. Cusp large and may attain three times the length of the largest denticle on the lateral processes. Basal cavity indistinct and basal groove unobserved.

S₁ element: Outer-lateral process ranges from 280 μm to 710 μm in length. Inner-lateral process ranges from 290 μ to more than 630 μ in length and is slightly longer than outer-lateral process. Both processes meet at cusp with an angle of about 60 degrees on upper view. Outer-lateral process ranges from 190 μm to 390 μm in height of blade, and somewhat convex inward. Inner-lateral process very low in height of blade, thin, and projects toward posterior and then curves and extends laterally. Denticles on outer-lateral process 7 to 11 in number, discrete, and incline toward cusp and inward forming a high convex crest in distal to medial portion. Denticles on inner-lateral process 12 to 15 in number and tend to increase in length and inclination distally. Cusp is as large as largest denticle on outer lateral process. Basal cavity indistinct and basal groove unobserved.

S₂ element: Outer-lateral process ranges from 500 μm to 630 μ in length and from 210 μ to 270 μ in height of blade, and extends downward making an angle of 60 to 90 degrees with basal margin of cusp. Denticles 8 to 14 in number, discrete, and tend to be large in proximal to medial portion and increase in inclination proximally. One small denticle may be present on inner-lateral process. Zone of recessive basal margin commonly developed near basal part of cusp. Basal cavity indistinct and basal groove unobserved.

S_3/4 elements: Anterior and posterior processes range from 380 μ to 430 μ and from 740 μ to 830 μ in length, respectively. Ratio of length of anterior and posterior processes is 1 : 1–2. Anterior process bends inward with an angle of 10 to 45 degrees and projects downward with an angle of 10 to 45 degrees. Number of denticles on both anterior and posterior processes is almost the same and ranges from 5 to 10. Denticles on anterior process tend to be large in size and decrease in inclination anteriorly. Cusp is subequal to twice as large as largest denticle on posterior process. Basal cavity indistinct and basal groove unobserved.

Remarks. — The P₁ element of N. chionensis is characterized by possessing a stout blade and a cup with thick upper surface on inner side. Neospathodus crassatus Orchard is similar to the P₁ element of N. chionensis but it is distinguished from the latter by having a few inwardly bending posterior denticles above the basal cavity.

Neospathodus symmetricus Orchard, 1955

Figure 6. 1–38

Figure 6

1–38. Neospathodus symmetricus Orchard, 1995 from the Taho Formation. all ×60.

1–5, S₀elements, lateral views. 1: YNUC15965 from Lev. 1616, proximal. 2, 3: YNUC15966, 15967 from Lev. 1617. 4: YNUC15968 from Lev. 1655. 5: YNUC15969 from Lev. 1616, proximal. 6–8, M elements, inner views. 6: YNUC15970 from Lev. 1616, sinistral. 7: YNUC15971 from Lev. 1655, sinistral. 8: YNUC15972 from Lev. 1614, dextral. 9–12, S₁elements, inner views, sinistral except for 9. 9: YNUC15973 from Lev. 1614. 10: YNUC15974 from Lev. 1617. 11: YNUC15975 from Lev. 1614. 12: YNUC15976 from Lev. 1656. 13–19, S₂ elements, inner views. 13, 14, sinistral, 15–19, dextral. 13: YNUC15977 from Lev. 1619. 14: YNUC15978 from Lev. 1655. 15, 16: YNUC15979, 15980 from Lev. 1617. 17, 18: YNUC15981, 15982 from Lev. 1616. 19: YNUC15983 from Lev. 1617. 20–24, S_3/4 elements, inner views. 20–22, dextral. 23, 24, sinistral. 20: YNUC15984 from Lev. 1617. 21: YNUC15985 from Lev. 1656. 22: YNUC15986 from Lev. 1618. 23: YNUC15987 from Lev. 1616. 24: YNUC15988 from Lev. 1655. 25–33, P₂elements, inner views. 25–30, dextral. 31–33, sinistral. 25: YNUC15989 from Lev. 1656. 26: YNUC15990 from Lev. 1617. 27: YNUC15991 from Lev. 1617. 28: YNUC15992 from Lev. 1619. 29: YNUC15993 from Lev. 1653. 30: YNUC15994 from Lev. 1616. 31: YNUC15995 from Lev. 1617. 32: YNUC15996 from Lev. 1655. 33: YNUC15997 from Lev. 1656. 34–38, P₁elements, inner views. 34, 36–38, sinistral. 35, dextral. 34, 35: YNUC15998, 15999 from Lev. 1617. 36: YNUC16000 from Lev. 1614. 37: YNUC16001 from Lev. 1651. 38: YNUC16002 from Lev. 1614.

P₁element

Neospathodus symmetricus Orchard, 1995, p. 120, 121, figs. 2.6, 2.10–2.13, 2.18.

S₀element

Ellisonia clarki Sweet, 1970, p. 225, 226, pl. 4, figs. 17, 18. [U element]

S₁element

Ellisonia clarki Sweet, 1970, p. 225, 226, pl. 4, fig. 15. [LC element]

S₂element

Apatognathus mitzopouli Bender, 1967, p. 501, pl. 1, figs. 11, 14 (only)

Cypridodella unialata Mosher, 1968, p. 922, pl. 113, figs. 21, 27.

Ellisonia clarki Sweet, 1970, p. 225, 226, pl. 4, fig. 16. [LA element]

S_3/4 element

Ellisonia clarki Sweet, 1970, p. 225, 226, pl. 4, fig. 19. [LB element]

Diagnosis. — Neospathodus symmetricus Orchard, 1995 is composed of single pairs of digyrate (cypridodelliform) M, digyrate (enantiognathiform) S₁, digyrate (grodelliform) S₂, bipennate S_3/4, angulate P₂, and segminate P₁ elements, and a single unpaired alate S₀ element. These elements are characterized by having a thin blade with fused denticles.

Description. — P₁ element: Unit ranges from 260 μm to 790 μ in length and from 140 μ to 470 μ in height, respectively. Ratio of length and height is 1.5– 2 : 1. Posterior and anterior ends may slightly curve inward. Denticles 8 to 15 in number, almost equal in size, and upright in anterior portion and increasingly reclined toward posterior. Basal cavity is broadly to narrowly expanded and basal groove extends toward anterior end.

P₂ element: Anterior and posterior processes range from 130 μ to 350 μ and from 150 μ to 500 μ in length, respectively. Anterior process is commonly higher than posterior one. Both processes meet at an angle of 130 to 160 degrees in lateral view and 120 to 150 degrees in upper view. Denticles on anterior process 4 to 9 in number and increase in size and inclination posteriorly. Denticles on posterior process are 4 to 8 in number, and increase in size and inclination posteriorly. The ratio of number of denticles on anterior process to that on posterior one is 1 : 0.7–1.5. Cusp is larger than largest denticle on anterior process. Basal cavity minute and basal groove unobserved.

M element: Outer- and inner-lateral processes range from 50 μ to 210 μ and from 280 μ to 890 μ in length, respectively. Both processes meet at an angle of 60 to 120 degrees in lateral view. Outer-lateral process slightly flexed anteriorly and carries 1 to 5 short denticles. Inner-lateral process projects downward and curves posteriorly, and carries 14 to 20 equal-sized fused denticles. Cusp is twice to three times as long as denticle on inner-lateral process. Basal cavity and basal groove unobserved.

S₀ element: Posterior process ranges from 330 μm to 630 μ in length. Lateral processes meet at cusp or the first or second denticle anterior of cusp and form an angle of 90 to 120 degrees in antero-posterior view and 120 to 170 degrees on anterior side in upper view. Denticles on each lateral process 2 to 3 in number, small, and fused. Denticles on posterior process 16 to 20 in number, fused, and increase in length and inclination posteriorly. Cusp is equivalent to or slightly larger than largest denticle on posterior process. Basal cavity indistinct and basal groove unobserved.

S₁ element: Outer- and inner-lateral processes range from 190 μ to 410 μ and from 150 μ to 540 μ in length, respectively, and meet at cusp with an angle of about 60 degrees in upper view, and project downward. Outer-lateral process is deep and ranges from 280 μ to 370 μ in height of blade, and slightly convex inward. Inner-lateral process is thin, about 85 μ in height of blade, and directed toward posterior and then flexed and extending laterally. Denticles on outer-lateral process 10 to 16 in number, fused, incline proximally and inward, and form a high convex crest in distal to medial portion. Denticles on inner-lateral process up to 20 in number, small, and tend to increase in inclination and size distally. Cusp is slightly larger than largest denticle on anterior process. Basal cavity indistinct and basal groove unobserved.

S₂ element: Outer-lateral process ranges from 290 μ to 530 μ in length and from 140 μ to 220 μ in height of blade, and extends downward approximately in parallel with axis of cusp. Denticles on outer-lateral process 15 to 20 in number, fused, sub-equal in size, and tend to increase in inclination proximally. One to three small denticles may be present on inner-lateral process. Cusp is subequal to twice as long as largest denticle on outer-lateral process. Basal margin near cusp curves shallowly to deeply. Basal cavity indistinct and basal groove unobserved.

S_3/4 elements: Anterior and posterior processes range from 150 μ to 370 μ and from 420 μm to 1010 μ in length, respectively. Anterior process bends inward with an angle of 10 to 45 degrees and projects downward with an angle of 10 to 45 degrees. Denticles on anterior process range from 6 to 10 in number and tend to increase in size and decrease in inclination anteriorly. Denticles on posterior process range from 15 to 25 in number and tend to increase in size and inclination posteriorly. Cusp is subequal to twice as large as largest denticle on anterior process. Basal cavity indistinct and basal groove unobserved.

Remarks. — According to Orchard (1995), N. symmetricus is distinguished from N. homeri by having a shorter unit, fewer and more strongly reclined denticles, a more symmetrical basal cavity, and lacking a denticulate posterior process. P₁ elements of N. symmetricus of the Taho Formation are identical to N. symmetricus described by Orchard (1995) in morphological characteristics. The segminate P₁ elements of N. homeri revised by Orchard (1995) are entirely absent in the Taho Formation.

Neospathodus sp. aff. N. symmetricus Orchard, the biostratigraphic occurrence of which is lower than that of N. symmetricus (Figure 1), resembles N. symmetricus, but it is distinguished from the latter in possessing a shorter P₁ element with fewer denticles, and circular outline of the basal cup (Figure 2). The apparatus of this species probably consists of eight kinds of elements, the M element of which is distinguishable from that of N. symmetricus while the others are closely similar to those of the species.

On the basis of the morphological similarity of the P₁ elements between Neospathodus and Neogondolella, Kozur (1976) and Sweet (1981) suggested that the two genera are phylogenetically related to each other. The common presence of enantiognathi-form S₁ and grodelliform S₂ elements in the Neospathodus apparatuses and the natural assemblage of Neogondolella sp. Rieber, 1980 (Orchard and Rieber, 1999) provides an additional evidence for the view of Kozur (1976) and Sweet (1981).