Introduction

Among the global biotic crises of the Phanerozoic, the Permian-Triassic (P/Tr) mass extinction was the largest and eliminated more than 90% of marine genera (Erwin, 1994). Besides the magnitude of the extinction, one other striking, yet enigmatic, aspect of the P/Tr event is its aftermath. Low-diversity, depauperate marine ecosystems continued well into the Early Triassic. Unlike other major mass extinction events, substantial ecological recovery is not observed among marine communities until the end of the Early Triassic (some 10 < Ma; Erwin, 1994; Schubert and Bottjer, 1995; Twitchett, 1999; Bottjer, 2001). This apparently unusual situation has been attributed to various causes including the intensity of the extinction, prolonged environmental stresses, and preservational bias (Hallam, 1989; Erwin, 1993; Schubert and Bottjer, 1995; Erwin, 1996; Erwin and Pan, 1996; Twitchett, 1999). Twitchett (1999) studied the benthic macro-fauna and the ichnofossils across the P/Tr boundary of northern Italy (Werfen Formation) and showed that the delayed recovery of the fossil community in the Lower Triassic of northern Italy is partly explained by prolonged low-oxygen conditions. To fully answer this question, however, further paleoecologic studies are required so that the temporal and geographic patterns of ecological recovery become better illustrated.

Early Triassic shallow marine benthic faunas have been reported from several localities in Japan (summarized in Hayami, 1975). It has been previously noted that these localities are grouped into two types: the first type is carbonates that are sporadically distributed in the Outer Zone of Southwest Japan; the second type is siliciclastic formations that are located in the Inner Zone of Southeast Japan (Maizuru Terrane) and Southern Kitakami Mountains (here-after, S. Kitakami; Nakazawa, 1971; Nakazawaet al., 1994).

We studied the fossil record of the Hiraiso Formation: a siliciclastic Lower Triassic unit from S. Kitakami. The benthic invertebrates were described and the sedimentology and taphonomy were assessed in order to estimate the probable habitats of the assemblage. Finally, the fossil community was compared to those of other Lower Triassic units in nearby localities.

Geologic settings

In northwestern Japan, the Lower and Middle Triassic are represented by thick siliciclastic shelf deposits of the Inai Group, exposed in the southeast to southern region of S. Kitakami (Figure 2). The Inai Group unconformably overlies the Upper Permian and is itself unconformably overlain by the Upper Triassic Saragai Group or the Jurassic Shizugawa and Hashiura Groups. It consists of two major transgressive sequences; the Hiraiso Formation and the overlying Osawa Formation represent the first sequence.

Figure 1

Stages from the Late Permian through the Middle Triassic and the stratigraphy of the South Kitakami Mountains.

Figure 2

Locality map and map of the studied area showing facies distribution. “Section” with numbers indicating routes for stratigraphic sections appearing in Figure 4.

The Hiraiso Formation unconformably overlies the uppermost Permian Toyoma Formation (Shiida, 1940; Kambe, 1963) with slight angular unconformity (Kamada, 1993). The base of the Hiraiso Formation consists of a conglomerate facies, which is overlain by alternating laminated sandstone and bioturbated silt-stone. Finally, it grades up to a finely laminated silt-stone facies of the Osawa Formation.

S. Kitakami is regarded as an allochthonous terrane (the South Kitakami Terrane; Kobayashi, 1999). Paleomagnetic and paleobiogeographic data have set the paleolatitude of S. Kitakami in Permian-Triassic time within 0°∼S20° (Saito and Hashimoto, 1982; Ozawa, 1987; Maruyamaet al., 1989; Davydovet al., 1996). Based on the micropaleontologic data, Kobayashi (1999) reconstructed S. Kitakami together with Kurosegawa Terrane as an arc-trench system and concluded that the South Kitakami-Kurosegawa Arc was located near South China during the Late Permian (Figure 3).

Figure 3

Late Permian paleogeography after Kobayashi (1999). Reconstructed locations of S. Kitakami and Maizuru are shadowed. S. Primorye, which is now located next to the Amuria, was probably located rather close to Maizuru or S. Kitakami (see discussion in text).

The sedimentary basin of the Inai group was thus probably located in an active margin (Kamada, 1989). Kamada (1989) inferred a high rate of sedimentation due to tectonic instability at the source area – large-scale strike-slip faults that were active in Permian-Triassic time.

Locality

The field survey was conducted on the Hiraiso Coast in Motoyoshi-cho, Miyagi Pref., Japan (Figure 2), the type locality of the Hiraiso Formation. The rocky coast between Maekawara and Akaushi fishing ports provides a continuous 800 m outcrop that embraces the entire Hiraiso Formation.

Previous works

Biostratigraphy. The age of the Hiraiso Formation is currently considered as Smithian along the Hiraiso Coast (Bando, 1970). Bando (1970) originally assigned the Hiraiso Formation to the Griesbachian based on a single specimen of the ammonite Glyptophiceras cf. gracile. However, later it was assigned to the Smithian by Nakazawaet al. (1994) who reidentified the ammonite as G. aequicostatus. This new correlation seems reasonable because the overlaying Osawa Formation ranges from the Smithian to the uppermost Spathian, assigned also by ammonite fossils (Bando, 1968; Bando and Shimoyama, 1974; Nakazawaet al., 1994).

Paleontology. Ichikawa (1951) presented the first list of fossils from the Hiraiso Formation. Later, this study was summarized by Bando (1964) with minor additional information. Nonetheless, no systematic description of these fossils was made. In addition to the fossil collections gathered in this study, we also reexamined some of the Ichikawa (1951) collection that is in the University Museum, University of Tokyo.

Sedimentology. Kamada and Kawamura (1988) and Kamada (1992) studied the sedimentary fabrics in the Hiraiso Formation in the Kamiwarizaki area, 20 km south of our study area. They recognized four types of storm-generated deposits, including amalgamated hummocky-cross-stratification (Storm Deposit Type A), hummocky-cross-stratification (Storm Deposit Type B and B′), sheet-shaped thin sandstones (Storm Deposit Type C), and turbidites (Storm Deposit Type D). Corresponding sedimentary settings for these lithofacies were then interpreted in terms of basin depths relative to normal and storm wave bases (Kamada and Kawamura, 1988).

Facies analysis and reconstruction of environmental gradient

We identified nine facies (Facies Cs, Sm, S, Ha, Hb, Hc, Hd, He, Ms) within the Hiraiso Formation (Table 1; Figure 4). Sedimentary structures indicative of storm-related deposition appear in Facies Ha–He and Ms. The thick hummocky-laminated sandstone facies (Facies Ha) is overlain by the facies consisting of discrete HCS sequences (Facies Hb and Hc). Such discrete sets of the HCS sequences become unclear upward; the lamination is destroyed by bioturbation (Facies Hd); then, each sequence becomes thinner with nearly parallel lamination (Facies He). These facies (Facies Hc–He) repeatedly appear until the undisturbed, thinly bedded siltstone facies (Facies Ms) of the basal Osawa Formation.

Table 1

Descriptions of facies recognized in this study. Facies Cs, Sm, S, and Ha∼He are found in the Hiraiso Formation. Facies Ms is found in the base of the Osawa Formation. Storm deposit types are after Kamada and Kawamura (1988). For depositional environment, refer to Figure 5.

Figure 4

Correlated stratigraphic sections of the Hiraiso Formation of the Hiraiso Coast. The base of the Hiraiso Formation is composed of a conglomerate layer (Facies Cs) with immediately overlying sandstone layers (Facies Sm and Facies S) that probably correspond to shoreface or shallower environments. The middle to the top of the Hiraiso Formation comprises various storm-depositional facies (Facies Ha–He and Ms), grading from shallower facies (Facies Ha) to deeper facies (Facies He) upward. Finally, it grades into further offshore facies of the base of the Osawa Formation (Facies Ms). Figures in the right-top corner are schematic diagrams of Facies Ha and Hb.

Storm deposits are excellent bathymetric indicators. Thus, bathymetric interpretations are made by identifying the storm deposit types (Storm Deposit Type A∼D; see the previous section) of Kamada and Kawamura (1988) in our storm-generated facies (Facies Ha∼He and Ms; Table 1). Facies Ha is dominated by the Storm Deposit Type A, i.e., amalgamated hummocky cross stratification (amalgamated HCS; Dott and Bourgeois, 1982); similarly, Facies Hb and Facies Hc are dominated by Storm Deposit Type B (HCS type) and Storm Deposit Type B′(HCS type without lag deposits, which we call herein “basal shelly deposits”), respectively (Figure 4); Facies He and Facies Ms are alternations of thin siltstone layers and Storm Deposit Type C (sand-sheet type) or Storm Deposit Type D (turbidite type), respectively. Therefore, judging from the interpretation by Kamada and Kawamura (1988), Facies Ha is correlated to the upper inner shelf, and Facies Hb, Hc, He, and Ms are correlated to successively deeper environments, respectively (i.e., lower inner shelf to outer shelf; Figure 5). Thus, the facies analysis clearly illustrates that the entire Hiraiso Formation and the base of the Osawa Formation constitute one major deepening-upward sequence with some minor deepening-shallowing fluctuations (Figure 4).

Figure 5

Schematic diagram showing transect of the continental shelf. Estimated bathymetric ranges of facies S, Ha–He, and Ms are shown. Diagram at the bottom schematically illustrates the gradual change in composition of fossil assemblages along the depth gradient.

The occurrences of fossils are limited mostly to Facies Ha and Hb (Figure 4). Facies Ha consists of hummocky-cross-laminated fine to very fine sandstone with basal shelly deposits, in which siltstone layers are only occasionally intercalated (Figure 4). Basal shelly deposits comprise either densely or sparsely concentrated shells, occasional rip-up clasts, and sandy matrix with calcareous cement (Figure 6); they are preserved without any apparent later disturbance (e.g., bioturbation). Basal shelly deposits and siltstone layers pinch out laterally within several meters. Therefore, single discrete depositional events cannot be identified in this facies (i.e., HCS sequences are amalgamated; Dott and Bourgeois, 1982). Such amalgamation as seen in Facies Ha is thought to be the consequence of little accumulation of background sediment and/or considerable erosion of silt prior to deposition of storm sand (Dott and Bourgeois, 1982; Kamada, 1992). In contrast, Facies Hb comprises alternation of hummocky- to parallel-laminated sandstone and partially or entirely bioturbated siltstone. Therefore, discrete (idealized) HCS sequences (Dott and Bourgeois, 1982) are identified, where basal shelly deposits constitute the base of such sequences (Figure 4). Thus, Facies Ha represents a higher energy setting than Facies Hb, or a shallower (and more proximal) sedimentary environment is inferred (Kamada, 1992).

Figure 6

Cross-section of basal shelly deposits (acetate peels photographically printed onto developing papers). Typical basal shelly deposits of: a Facies Ha; and b Facies Hb. Scale bar 1 cm.

Mode of fossil occurrence

Fossils from the Hiraiso Formation are found in two types of shell beds: 1) dense concentrations in basal shelly deposits and 2) sparse concentrations within hummocky-cross laminated sand/siltstone in HCS layers. The former type contains disarticulated bivalves, a few small gastropods, and rare crinoid ossicles. On the other hand, the latter comprises crinoid ossicles only. Shelly fossils are nearly absent in intervening siltstone layers with a few minor exceptions.

Methods of fossil study

The majority of fossil specimens of bivalves and crinoid ossicles were collected from moderately weathered rock samples as natural external molds from which silicone rubber casts were obtained; few specimens could be successfully prepared from the highly consolidated and cemented fresh sandstone samples. Although such shelly deposits are ubiquitous, relatively small numbers of fossil specimens were collected due to limited exposure of moderately weathered rocks in the studied area. In addition, casts of fine shells (less than several millimeters in diameter) and small gastropods of adequate quality for species identification could not be obtained due to the fragility of the weathered sandstone. On the other hand, crinoid ossicles are preserved in finer substrates (i.e., very fine sand to silt); thus, fine details of their surface ornamentation were recovered in the silicone rubber cast.

To supplement the in situ observation of shell beds, sections of unweathered shelly deposits were slabbed and polished; details of the fabric of the shell beds were then observed in acetate peels that were photographically printed onto photographic papers (i.e., peels were treated like negative films; Figure 6).

Shell bed analysis

Observation of the polished slab sections showed that nearly all bioclasts in basal shelly deposits are bivalves with minor small gastropod shells and probable crinoid ossicles. All shells are very thin, at most 1 mm, but are unfragmented. Each shell deposit contains bivalve shells of wide size variation. The size of bivalves of Facies Ha ranges from submillimeter up to 60+mm in cross-section, with a mode of 2 mm. Those of Facies Hb range from submillimeter to 10 mm in cross-section, with a mode of 2 mm. Although size distribution varies among the shelly deposits, minute bivalves (< 4 mm) were revealed to be considerably abundant (> 90%) in most of the beds. Shells are loosely to densely packed, with no obvious internal size sorting or grading. There was a tendency among large shells to be stacked concave up, indicative of repeated resuspension and fallout during turbulent storm currents (Kidwell and Bosence, 1991). All of these fabrics suggest rapid deposition as is expected for basal deposits of HCS (Dott and Bourgeois, 1982).

The nature of the basal shelly deposits varies between facies, that is, along the bathymetric depth gradient. Both abundance and thickness of shelly deposits are high in Facies Ha but significantly decrease toward deeper facies (e.g., Facies Hb). Basal shelly deposits in Facies Hb typically lack large shells (over a few centimeters) and contain occasional articulated valves. In contrast, the majority of crinoid accumulations are observed in Facies Hb. In the deeper facies, such fossil concentrations are nearly nonexistent, except for three thin sheets of shell beds within Facies Hc and Hd.

Discussion

Taxonomic diversity

Very low diversity at both higher taxonomic levels and the species level apparently characterizes the Smithian shallow marine benthic fauna of S. Kitakami, even though the number of individuals preserved is remarkably large. The fossil taxa recognized from the Hiraiso Formation only include five species of bivalves, two types of gastropods (genera unknown), one crinoid, and one ammonoid (Table 2). Observations of the cross-sections are adequate to determine the higher taxonomy of the skeletal fossils (Figure 6), which affirmed that, indeed, only bivalves, gastropods, and crinoids are present.

Table 2

List of fossils identified in this study. The table in the middle shows occurrences of these species in three proximal localities discussed in this paper (⊚: abundant; ○: common; △: rare). Fossil genera listed by Ichikawa (1951) are correlated in the right column.

We could only recognize five species of bivalves (one for each of five genera), although Ichikawa (1951) listed nine genera (Table 2). We suspect over-splitting and, indeed, many of these are synonymized in this study. However, species richness is probably underestimated due to sampling bias because numerous small shells (< 10 mm) could not be identified because of their poor preservation, although low variability in cross-sectional morphology among such minute valves implies that the bias might not be very significant. In addition, some of the minute valves are possibly juvenile forms. However, cross-sectional views are not reliable for species identification.

Such low species richness of bivalves is unusual in a normal marine environment, provided that the assemblage represents a wide range of environments (i.e., lower shoreface to upper inner shelf or even shallower environments). For a more conclusive discussion of thelowness of the fossil diversity, however, it should be compared to the fossil faunas of pre-event time (i.e., Late Permian) and the post-event time (after the end of the recovery period; i.e., Middle and Upper Triassic). Since no other example of an assemblage in shallow-marine siliciclastic settings has been well documented from immediately preceding and following times, we could only compare the taxonomic diversity of the S. Kitakami fauna to those of carbonate settings. Schubert and Bottjer (1995; p. 29) compared species richness and number of higher taxa in faunas from Permian and Early, Middle, and Late Triassic times (Table 3). Average species richness of the Permian and the Middle to the Late Triassic are 46 and 44, respectively, which is six to seven times larger than that of the S. Kitakami fauna. In addition, the number of higher taxa of the Permian and the Middle to the Late Triassic (9 and 8, respectively) also doubles that of the S. Kitakami fauna.

Table 3

Comparison of species richness and number of higher taxa; modified after Schubert and Bottjer (1995).

Diversity of the S. Kitakami fauna is similar to or even lower than those of the same time in other localities. The S. Kitakami fauna is comparable to the low-diversity Smithian fauna from the western USA (Schubert and Bottjer, 1995) and northern Italy (Twitchett and Wignall, 1996) where the faunas consist of only bivalves and gastropods. The Smithian fauna of the western USA consists of slightly different bivalve genera from those of Hiraiso and has more diverse gastropods. Significantly, however, crinoids have not been reported from the Smithian of the western USA and northern Italy only from the Spathian (Schubertet al., 1992; Schubert and Bottjer, 1995; Twitchett, 1999).

Although comparison of uniquely siliciclastic S. Kitakami to other carbonate settings makes the argument less conclusive, the siliciclastic setting certainly could not be the primary factor causing the apparent low diversity of the assemblage. Many earlier Paleozoic and later Cenozoic examples assure that storm-dominated siliciclastic shallow marine environments have obviously been supportive of rich benthic faunas, and significantly diverse fossil assemblages are thus potentially preserved in the corresponding storm-generated facies. For example, the Lower Cretaceous Miyako Group of northeast Japan preserves a total of 245 species that fall into nine higher-taxonomic groups, including foraminifers, sponge, corals, brachiopods, bivalves, gastropods, cephalopods, crinoids, echinoid, and crustacean (Hanaiet al., 1968). Considering the high abundance of the fossils preserved, we conclude that the observed lowness of diversity in the Hiraiso assemblage most likely reflects genuine low faunal diversity of the Smithian S. Kitakami fauna.

Low faunal diversity may be attributed to instability of the environmental conditions of the shallow shelf settings that was caused by intermittent invasion of an oxygen-depleted water mass from a deeper, but immediately adjacent, setting. Increasingly stressful conditions are inferred from the decreased intensity of bioturbation in deeper facies (e.g., Facies Hc and He) where body fossils are rare or absent; bioturbation is nearly nonexistent and laminae are preserved undisturbed in the deepest facies (i.e., Facies Ms), indicating anoxic conditions (Davis and Byers, 1989; Wignall and Hallam, 1992). In addition, the majority of bivalves from deeper facies than Facies Hb are less than 1 cm in size; there are abundant minute intact valves as small as 0.5 mm (Figure 6). Such diminished size of bivalves may be explained by oxygen depletion of the living habitat. This observation is thus concordant with the hypothesis of prolonged stressful conditions in the Lower Triassic (Hallam, 1989; Twitchett, 1999) and the superanoxic event (Isozaki, 1997; Wignall and Twitchett, 2002).

Conclusion

The Hiraiso Formation comprises a gradation of facies that represent storm-dominated siliciclastic shelf environments of varying bathymetry. Shelly fossil assemblages are found in basal deposits of storm beds, but their occurrence is limited to those facies that represent upper inner shelf (Facies Ha and Hb). These assemblages are probably derived from a range of shallow environments and, as a whole, are time-averaged over a period of relative environmental stability. It also implies that the habitat for benthic organisms was largely restricted to settings shallower than the upper inner shelf.

Observed diversity of the shelly assemblage in both species and higher taxonomic levels is very low; it consists almost exclusively of bivalves with simple morphology. Judging from the wide range of environments that the assemblage probably represents, this apparent low diversity should indicate a genuine anomaly in the shallow marine benthic community at the time of deposition. In deeper facies, where fossils are nearly absent, bioturbation gradually diminishes in the lower inner shelf and finally disappears in the outer shelf environment. Such patterns are most likely explained by the presence of anoxic bottom water in deeper environments. Therefore, low diversity may also have resulted from the influence of oxygen-depleted deeper water that could have occasionally invaded shallower environments, leading to fluctuations in living conditions in these shallower environments.

On the regional scale, the assemblages from similar facies of different localities resemble each other, indicating little endemism of communities. However, we recognized species-level specialization among the siliciclastic localities compared to carbonate settings.

There is only limited evidence of ecological recovery in the shallow marine benthic community of these tropical, siliciclastic settings in the Smithian. Although the occurrence of the crinoid genus Holocrinus may reflect the onset of the ecological recovery phase, their geographically restricted occurrence may imply that recovery was geographically limited in the Smithian. Such relative delays of ecological recovery in tropical shallow marine environments may be a consequence of low diffusivity of oxygen in tropical, hotsea surface water that maximized the chronic impact of the superanoxic event (Isozaki, 1997; Wignall and Twitchett, 2002).

Systematic Paleontology of Bivalves (by Y. Kashiyama)

Order Pterioida

Suborder Pteriina

Superfamily Pteriacea

Family Bakevelliidae

Genus Bakevellia

Subgenus Maizuria Nakazawa, 1959

Bakevellia (Maizuria) sp. cf. kambei Nakazawa, 1959

Figure 8a–c

cf. Gervilleia cf. exporrecta Lepsius: Bittner, 1899, p. 16, pl. 3, fig 1–16.

cf. Gervilleia exporrecta Lepsius: Kiparisova, 1938, p. 249, pl. 5, figs. 22, 23.

cf. Bakevellia (Maizuria) kambei Nakazawa: Nakazawa, 1959, p. 204, pl. 2, figs. 1–12, textfig. 7.

Materials.—Two nearly complete external casts of left valves and two nearly complete external casts of right valves.

Description.—The shell is small in size (shell height 10–16 mm), strongly inequilateral, somewhat inequivalved, moderately to somewhat strongly inflated, posteroventrally elongated, and prosocline. The left valve is more broadly inflated than the right valve. The umbo is not very prominent, located in 30– 40% along hinge line from the anterior end; beak salient above hinge line; hinge line straight; anterior auricle moderate to large, but only very poorly separated from the umbonal flank by a weak depression. The posterior auricle is larger and fairly defined from the disc by a broad depression along the flank of the beak elongated posteroventrally. Growth lines are smoothly continuous from the disc to the auricle.

The umbo is narrower in right valves than in left ones, with the beak salient above the hinge line. The posterior auricle is less clearly separated from the disc by a broader depression. Growth lines are less prominent or less well preserved in these specimens.

Discussion.—Although similar small fossils are abundant in sample rocks, only a few specimens are actually distinguishable. These external molds lack information of the cardinal area, including dentition. However, identification based solely on external morphology should be adequate with respect to the Early Triassic Bakevellia, of which dentition is more variable than the external shape (Nakazawa, 1959). Right valves of the specimens are somewhat more inflated than those of Bakevellia (Maizuria) kambei Nakazawa. The posterior auricles of the Hiraiso specimens are thus slightly more prominent, which invokes the form of Gervilleia exporrecta Lepsius decribed from S. Primorye by Kiparisova (1938). It may also be related to Bakevellia (Maizuria) oluyamensis Nakazawa described by Nakazawa (1959).

Occurrence.—Common in Facies Ha and Hb of the Hiraiso Formation in Hiraiso Coast.

Superfamily Pectinacea

Family Aviculopectinidae

Subfamily Aviculopectininae

Genus Eumorphotis

Eumorphotis iwanowi (Bittner), 1899

Figure 7

Figure 7

Eumorphotis iwanowi (Bittner); a∼k, left valves, models made by compound from the external casts (a [MM28618], b [MM28619], d [MM28621], g [MM28624], i [MM28626], j [MM28627], and k [MM28628]), and naturally occurred external molds (c [MM28620], e [MM28622], f [MM28623], and h [MM28625]); right valves, models made by compound from the external casts (m [MM28630] and n [MM28631]), and naturally occurred external molds (l [MM28629]). c and l are collections by K. Ichikawa. All figures ×1.0.

Figure 8

a∼c, Bakevellia (Maizuria) sp. cf. kambei Nakazawa; a [MM28633] and b [MM28634], left valves; c [MM28635], right valve; external molds. d and e, Entolium ussuricus (Bittner); e [MM28637], left valve; d [MM28636], right valve; external molds. f [MM28638],Unionites aff. canalensis (Catullo) var. fassaensis (?); left valve; external molds. g–k, Neoschizodus cf. laevigatus (Ziethen): Nakazawa; g [MM28639], h [MM28639], and i [MM28640], left valves, h side view of g; j [MM28641] and k [MM28642], right valve. a–j are external molds; k is a model made by compound from the external casts. b, c, g (h), and i were collected by K. Ichikawa. Scale: a and c ×1.5, b ×2.0, d∼k ×1.0.

Pseudomonotis iwanowi Bittner, 1899, p. 8, pl. 1, figs 1–9.

Eumorphotis maritima Kiparisova, 1938, p. 223, 286, pl. 2, figs. 3, 6–8, 13.

Eumorphotis aff. maritima Kiparisova: Nakazawa, 1961, p. 259, pl. 12, figs. 25–28.

Materials.—Two nearly complete and two incomplete external casts of left valves, four nearly complete and four incomplete external molds of left valves, one external cast of a right valve, and two complete and one incomplete external molds of right valves.

Description.—The shell is relatively large in size (shell height 34–71 mm), strongly inequivalved, and inequilateral. The left valve is strongly inflated. The umbo is prominent and rounded. The beak is salient above the hinge line. The disc is strongly convex both anterodosally and dosally, but its posterior margin is slightly concave. The ventral margin is well rounded. The anterior auricle sulcus is fairly arched. The hinge line is straight and nearly equal to the shell length. The anterior auricle is trigonal, sharply separated from the disc by a steep sulcus and a moderate sinus, and convex moderately shaping a vertical section of a cone. The posterior auricle is a little larger than the anterior auricle, is not clearly separated from the umbonal flank but separated from the disc by a moderate sinus, and is nearly flat but convex close to the beak. Both auricles have straight grooves along the hinge line about 1 mm from the edge. The surface is covered with crowded and very fine radial costae and growth lines. Very fine radial costae are mostly faint due to poor preservation, are intercalated in two orders, and are commonly offset at growth lines. Growth lines are prominent and arranged in irregular intervals, forming a radially very undulant surface. The growth lines of the disc continue into the auricles.

The right valve is nearly flat. The umbo is insignifi-cant. The disc is faintly convex around the beak but slightly concave near the margin. The anterior auricle is elongated and fan-shaped, slightly convex, and sharply separated from the disk by a narrow and very deep byssal notch. The posterior auricle is narrowly trigonal, flat, and separated from the disc by a moderate sulus and sinus. The surface is not covered by any radial ornament but has growth lines.

Discussion.—Ornamentation among specimens varys, which is probably due to postmortem abrasion. The ornamentation of molds tends to be less clear than that of casts.

The external shape and the dimension match well Eumorphotis iwanowi Bittner (Bittner, 1899). However, the left valve, their ornamentation in particular, more closely resembles that of Eu. maritima Kiparisova (Kiparisova, 1938). On the other hand, the flat-shaped right valve that lacks radial ornamentation certainly resembles that of Eu. iwanowi Bittner.

Although all occurrences of the genus Eumorphotis that were previously reported from other Japanese localities are represented only by Eu. multiformis (Hayami, 1975), the Hiraiso species clearly belongs to Eu. iwanowi and is distinguished from Eu. multiformis by less prominent and low-ordering radial costae, a nearly completely flat right valve, the outlines of both auricles of the right valve, and its larger size distribution.

Occurrence.—Very common in Facies Ha of the Hiraiso Formation at Hiraiso Coast.

Family Entoliidae

Genus Entolium

Entolium ussuricus (Bittner), 1899

Figure 8d, e

Pecten ussuricus Bittner, 1899, p. 4, pl. 1, fig 11.

Pecten sichoticus Bittner, 1899, p. 5, pl. 1, fig 10.

Aequipcten ussuricus Kiparisova, 1938, p. 251, pl. 4, figs. 14–16.

“Pecten” ussuricus Bittner: Nakazawa, 1961, p. 257, pl. 12, figs. 10–13.

Materials.—Three nearly complete external casts of left valves and one nearly complete external cast of right valve.

Description.—The shell is moderate in size (shell height 26–28 mm), nearly equivalved, inequilateral, ovate, somewhat elongated, and prosocline. The left valve is moderately inflated. The umbo is broad but is not very prominent or rounded. The beak is not salient above the hinge line. Both anterodosal and dosal flanks are moderately convex, but both anterior and posterior flanks are rather flat. The hinge line is straight and far shorter than the shell length. The posterior auricle is a little larger than the anterior one. Both auricles are nearly flat and distinctly separated from the disc. The byssal notch is absent. The surface ornamentation is hardly observable due to poor preservation except for faint radial costae. Growth lines are not prominent.

The right valve is nearly as inflated as the left valve or slightly less inflated. Its umbo is less prominent than that of the left valve. The posterior auricle is a little larger than the anterior auricle. The surface ornamentation is not preserved.

Discussion.—Preservation of surface ornamentation of external molds is very poor, and specimens are more or less distorted. Their morphology perfectly resembles that of En. ussuricus from the Maizuru Terrane described by Nakazawa (1961). They also resemble En. ussuricus from S. Primorye by Kiparisova (1938), except for their more prominent ornamentation, which is, however, probably a matter of preservation.

This species was assigned to Aequipecten Kiparisova (1938) and was later reassigned to Eupecten by Kiparisova (1954). Nakazawa (1961) thought that it might be a new genus and assigned it tentatively toPecten. He suggested its close relationship to Entoliidae, noting that the pair of internal ridges and the smooth left valve resemble those of Entolium., but also a possible relationship to Amussiidae. Here, it is tentatively assigned to Entolium by considering the characters discussed in Nakazawa (1961).

Occurrence.—Common in Facies Ha of the Hiraiso Formation at Hiraiso Coast.

Order Unionoida

Superfamily Unionacea

Family Pachycardiidae

Genus Unionites

Unionites aff. canalensis (Catullo) var. fassaensis (?)

Figure 8f

aff. Anodontophora (Myacites) canalensis Catullo: Bittner, 1899, p. 23, pl. 3, fig 34–38.

aff. Anodontophora canalensis Catullo: Kambe, 1963, p. 48, pl. 5, figs. 24, 25.

aff. Unionites canalensis (Catullo): Nakazawa, 1971, p. 126, pl. 24, figs. 14, 15.

Materials.—One complete, relatively well preserved external cast of left valve.

Description.—The shell is small (shell height 9 mm), relatively smooth, roughly subtrigonally outlined, weakly to moderately inflated, inequilateral, and remarkably anteroposteriorly elongated with L/H ratio of 2.2 (distorted?). The umbo is somewhat prominent, which is located in one third from the anterior end. The surface is very smooth with faint growth lines. The dentition is not observed.

Discussion.— Unionites canalensis is usually distinguished from U. fassaensis by its trapezoidal outline and more elongated form with L/H ratio of about 1.6 to 2.0 (Kambe, 1963). This specimen has a L/H ratio of 2.2, which is significantly elongated and suggests assignment to U. canalensis. However, its rather smooth outline is somewhat closer to that of U. fassaensis. Here, it is tentatively assigned to U. canalensis due to very high value of L/H ratio.

Occurrence.—Facies Ha (?) and Hb of the Hiraiso Formation at Hiraiso Coast.

Order Trigonioida

Superfamily Trigoniacea

Family Myophoriidae

Genus Neoschizodus

Neoschizodus cf. laevigatus (Ziethen): Nakazawa, 1960

Figure 8g–k

cf. Myophoria cfr. laevigata Alb.: Bittner, 1899, p. 20, pl. 3, fig 17–26.

cf. Neoschizodus cf. laevigatus (Ziethen): Nakazawa, 1960, p. 56, pl. 6, fig 21–32.

Materials.—Three complete external casts of left valves, one nearly complete external cast of a right valve, and one complete external mold of a right valve.

Description.—The shell is small (shell height 13∼18 mm), equivalved, inequilateral, subtrigonal, somewhat elongated, prosocline, and moderately inflated. The umbo is prominent. The beak is orthogy-rate. A diagonal ridge is developed well but less steep and sharp posteroventrally, and is strongly elongated posteriorly. The corselet is relatively narrow and depressed in the middle range. The escutcheon is narrow and is not separated well. The surface is smooth and has faint growth lines in some specimens. The dentition is not observed.

Discussion.—External morphology is similar to Neoschizodus of the Maizuru Terrane, described by Nakazawa (1961), which exhibit various outlines due to deformation (Nakazawa, 1961). This may also explains the variable outlines of the specimens of Hiraiso. It probably better resembles Neoschizodus of the Southern Primorie (Bittner, 1899). This is, however, hard to determine because dentition was not preserved here, which is critical for classification of Myophoriidae.

Occurrence.—Common in Facies Ha and Hb of the Hiraiso Formation at the Hiraiso Coast.

Systematic Paleontology of Crinoid (by T. Oji)

Order Isocrinida

Family Holocrinidae

Genus Holocrinus

Holocrinus sp.

Figure 9

Figure 9

Holocrinus sp. a and b, Articular facet of internodal (symplexy), silicone rubber casts made out of the natural external molds, scale 1 mm; c, Lateral view of nodal and internodals, silicone rubber casts, scale 1 mm. Note that the upper surface of nodal (distal facet of nodal) shows relatively smooth, symplectial articulation.

Isocrinus sp.: Kobayashi and Ichikawa, 1951, p. 6.

Isocrinus sp.: Tamura, 1982, p. 23, pl. 1, figs. 20 and 21.

Description.—Only columnals and cirrals were collected, and no calyces or arms have been hitherto found from the Hiraiso Formation. The columnals are small in size, ranging from 2.0 to 2.5 mm in diameter. On the articular facet there are four to five well developed adradial and marginal ridges around the petal. The cross-section is usually pentagonal but sometimes stellate. The lateral surface of the inter-nodal is somewhat inflated, and the suture is straight and depressed. The nodals have five well developed cirral scars. The height of the nodals is approximately 1.7 times the height of the internodals. The distal facet of the nodal has a faint articulation pattern, and is categorized as cryptosymplexic.

Remarks.—The articular facet of the columnals shows a typical“Isocrinus”-type articulation. Triassic genera that have such articulation are Isocrinus and Holocrinus. The genus Holocrinus is characterized by possessing well developed high infrabasals that can be seen from the outer surface. From Hiraiso, only columnals and cirrals were available, thus the assignment to the genus Holocrinus is rather tentative from a morphologic viewpoint. However, from the Early Triassic, only Holocrinus has been found and no reliable record of Isocrinus has been reported. Thus, here we assigned the material to the genus Holocrinus.

Previous studies onHolocrinus? smithi from the western United States (Schubertet al., 1992) showed that the distal articular facet of nodals has relatively well developed symplexy. However, the present material from Hiraiso has cryptosymplectial articulation on the distal facet of nodals, as in most species of Isocrinus. Such existence of cryptosymplexy indicates that there was already differentiation of cryptosymplexy from the usual symplexy, and there was also some functional differentiation of the stalk at the distal articulation of the nodal. As early as in the Smithian, the present Holocrinus sp. possessed the ability to autotomize its distal stalk. This ability to discard the distal stalk enabled Holocrinus to relocate themselves to different places for anchorage, in the same manner as the later Isocrinidae.

The occurrence of Holocrinus from the Hiraiso Formation is the oldest in the world. The previous oldest Holocrinus was from the Virgin Limestone (Spathian) of western United States (Schubertet al., 1992).

Occurrence.—Molds of Holocrinus skeletons are commonly found in sandstone from the middle to the upper part of the formation (Facies Ha, Hb, and Hc). The best preservation shown here came from the silt-stone of the middle part of the formation (Facies Hc).