AMERICAN MUSEUM Novltates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 3216, 14 pp., 4 figures, 2 tables December 31, 1997

A Reappraisal of Trigoniacean Families (Bivalvia) and a Description of Two New Early Triassic Species

DONALD W. BOYD' AND NORMAN D. NEWELL2

ABSTRACT Our 1975 proposal that the complex trigonian By contrast, the tight-fitting teeth and sockets of dentition was independently acquired by three the strongly buttressed trigonian hinge interlock separate but related families assumed that external along conspicuously striated faces, and tooth 2 is morphology was more stable than hinge structure broad and biramous. during evolution of the Trigoniacea. This hypoth- In returning to the conventional distinction be- esis is not supported by our further work and the tween Myophoriidae and Trigoniidae, we abandon experience of other workers, so we have reinter- Costatoriidae and demote Eoastartidae to a sub- family of Schizodidae. preted the Schizodidae, Myophoriidae, and Tri- Two additions to the sparse Early Triassic rec- goniidae emphasizing hinge grades as family ord of the Trigoniacea, Neoschizodus elongatus n. characteristics. sp. and Lyriomyophoria paullorum n. sp., are de- The myophorian grade is distinguished by my- scribed from the Spathian Virgin Limestone of ophorous buttresses and a posterior wing on tooth southern Nevada. The hinges of these new species 2, both lacking in the schizodian hinge. Tooth stri- represent a transitional state between myophorian ation is either absent or sporadically developed. and trigonian grades.

INTRODUCTION Love the organisms for themselves first, then strain for general explanations, and, with good fortune, discoveries will follow; If they don't, the love and the pleasure will have been enough. E. 0. WILSON

I Research Associate, Department of Invertebrates, American Museum of Natural History; Professor Emeritus, Department of Geology and Geophysics, University of Wyoming, PO Box 3006, Laramie, WY 82071. 2 Curator Emeritus, Department of Invertebrates, American Museum of Natural History.

Copyright © American Museum of Natural History 1997 ISSN 0003-0082 / Price $2.10 2 AMERICAN MUSEUM NOVITATES NO. 3216

Two decades have passed since we pro- are rare, and we have nothing to add to our posed a classification of the Trigoniacea previous discussion of the Devonian and Up- based on the hypothesis that shell ornamen- per Silurian record (Newell and Boyd, 1975). tation is more important than hinge mor- Subsequent to that writing, Lower Silurian phology for recognizing the main lines of schizodian-like hinges were illustrated by phyletic divergence in the superfamily (New- Harrison and Harrison (1975: pl. 2, figs. 14- ell and Boyd, 1975). Our subsequent apprais- 17). These tiny valves are juveniles of Ly- al of this concept results in a different con- rodesma, and several specialists have noted clusion. the possibility that late Paleozoic trigoni- We had thought that the distinctive trigon- aceans evolved by neotenous retention of this ian dentition was independently acquired by juvenile hinge (e.g., Pojeta et al., 1986). three separate but related lineages. These The myophorian hinge, intermediate be- three, the mainly unornamented Myophori- tween schizodian and trigonian grades, pos- idae, the annulate Trigoniidae, and the radi- sesses an anterior buttress, a third (posterior) ally ribbed Costatoriidae, branched from the tooth on the left valve, and a distinctive pos- ancestral line (Schizodidae) early in the terior elongation of that valve's major tooth. Permian Period. In each branch, the primitive The elongation, a parallel-sided ridge, and hinge condition (schizodian grade) under- the elongated posterior tooth of the right went similar modifications recognized suc- valve resemble the lateral teeth of hetero- cessively as myophorian and trigonian donts. Tooth striation is either absent or spo- grades. radically developed. This interpretation of parallel hinge evo- In the trigonian hinge, tight-fitting teeth lution in several families was a striking de- and sockets interlock along conspicuously parture from the concept employed in the striated occluding faces. The central tooth of Treatise on Invertebrate Paleontology (Cox, the left valve is broad and biramous, and the 1969). There, most trigoniaceans with schi- anterior end of the hinge is strengthened by zodian and myophorian hinges are grouped a prominent buttress in each valve. in the family Myophoriidae (Paleozoic and All three hinge grades include a parivin- early Mesozoic) and numerous younger gen- cular ligament. Whereas the ligament, as ev- era with trigonian hinges constitute the fam- idenced by insertion groove and nymph, has ily Trigoniidae, conventionally thought to be varied only in length through trigoniacean monophyletic. history, trigonian dentition is the last in a Since 1975, several important papers, es- chronologic sequence of hinge grades exhib- pecially Fleming (1987), on post-Paleozoic ited by the superfamily Trigoniacea. trigoniaceans have disagreed with our 1975 phylogenetic hypothesis. It is not supported THE TRIGONIAN HINGE by the experience of these authors, so we re- vert to the traditional emphasis on hinge The trigonian hinge (fig. 1) is the distin- grade as a family characteristic. guishing character of the important family Here we discuss the trigonian hinge grade, Trigoniidae. In his diagnosis of this family, and reappraise our families, describing two Poulton (1979: 14) provided a succinct de- new species that supplement the sparse Low- scription of the dentition: er Triassic trigoniacean record. Triangular tooth in left valve with ventrally diverging sockets on each side; sockets bear regular slightly curving striae. Subordinate elongate teeth anterior THE THREE TRIGONIACEAN HINGE and posterior to the sockets. Two grooved teeth in GRADES right valve match the sockets of the left. The typical schizodian hinge, present in Two other characteristics of the trigonian most Paleozoic trigoniaceans, consists of two hinge not specified in Poulton's statement are teeth, one major and one minor, on each the bifid aspect of the triangular tooth on the valve. The teeth are not striated and a my- left valve and a corresponding gap (hiatus) ophorous buttress is absent. in the right-valve hinge plate. Johnston Undoubted pre-Carboniferous examples (1993: 107-111) discussed the functional 1997 BOYD AND NEWELL: TRIGONIACEAN FAMILIES 3

1D 4a ppr/

/pa

A B Fig. 1. Trigonia sp. Diagram of hinge notation used herein. A. LV; B. RV, modified from Cox, 1969: N472. significance of this combination in allowing groups might be a derived paedomorphic fea- unobstructed passage of pedal elevator mus- ture. cles from dorsal insertion points to an ex- We have not observed striae in Paleozoic tended, trunklike foot. representatives of the families discussed It is clear from the literature on Mesozoic here, and we have found nothing in the lit- trigoniaceans that the terms "trigonian" and erature to contradict our experience. Naka- "trigoniid" are applied to a somewhat broad- zawa (letter of July 9, 1996) did not observe er spectrum of hinges than that described by striae in Japanese Permian species of Neo- Poulton. The one feature implied by all schizodus and Costatoria. workers who use these terms is the presence Striae are few and coarse in the earliest of tooth striation. The striae are parallel (Triassic) trigoniacean species to exhibit grooves on the anterior or posterior face of them (Newell and Boyd, 1975: 76), but they a tooth. Just as adjacent synclines and anti- are numerous and regularly arranged in the clines share a limb, so a given striated sur- typical trigonian hinge. Cox (1952: 46) face is shared by a socket and the adjacent called attention to the consistent asymmetry tooth. Thus, some authors refer to striated of individual ridges in an array of curved sockets whereas others describe striated striae. The cross-sectional profile of any one teeth. Although we prefer the term "striated" ridge is gently sloping on the concave (dor- for this condition, various authors use sal) side and steeper, even slightly undercut, "ridged," "grooved," or "secondary denti- on the convex (ventral) side. tion" instead. These terms are easily misin- Poulton's description, quoted above, im- terpreted since they are applicable to other plies striation on the three left-valve teeth as aspects of tooth form. well as on two prominent teeth of the right Stanley (1978) provided a functional in- valve. This situation is illustrated in our fig- terpretation of tooth striation. Ubiquitous in ure 1. The figured hinge represents the max- trigoniids, the feature also typifies adult imum elaboration of the trigonian hinge valves of the lower Paleozoic Lyrodesma grade, with three teeth and four striated faces (Cox, 1969) and occurs as an intraspecific in each valve. This condition is exemplified variation in Permian Astartella (Boyd and by the extant Neotrigonia and the Upper Cre- Newell, 1968). Striated larval teeth are pres- taceous Pteritrigonia (Scabrotrigonia), al- ent in diverse living bivalve species (Waller, though their minor teeth (4b and 5a) are in- 1981: 46). This widespread taxonomic oc- conspicuous. currence of a functional feature suggested to We have argued on two occasions (Boyd Waller (letter of April 15, 1997) the likeli- and Newell, 1968; Newell and Boyd, 1975) hood of its multiple origins in the Trigoni- against use of the Bernard system of hinge acea. Alternatively, he noted that absence of notation because of its implication of ho- striation in adults of some trigoniacean mology with specific aspects of the ontogeny 4 AMERICAN MUSEUM NOVITATES NO. 3216 of heterodont bivalves. However, seeking the New Zealand specimens of Neoschizodus cf. advantage of easy reference to particular N. laevigatus show "a few coarse transverse teeth (not permitted by objective Steinmann striations" on tooth 2. notation) and recognizing that the Bernard If the family Trigoniidae is to be defined scheme is well represented in trigoniacean by dentition, Poulton's (1979) description of literature, we employ the tooth designations the trigonian hinge should be sharpened in of figure 1 in the subsequent discussion. two respects. First, regarding striation, we It is not surprising, given the common oc- suggest that the minimum requirement for currence of mosaic evolution, that the vari- the trigonian hinge should be striae on both ous components of the trigonian hinge ap- sides of tooth 2 and on both sides of tooth pear at different times in trigoniacean histo- 3a. It follows that the opposing sockets will ry. Many genera with hinges cited as trigo- also be striated on both sides but, in this min- niid or trigonian don't possess all the features imal condition, tooth 4b and its correspond- in figure 1, which leads to the question of the ing socket are smooth. Second, the posterior minimum requirements necessary for a hinge corner of the triangular tooth 2 should be at to be called trigonian. This has taxonomic the convergence of two sloping surfaces, the importance if the trigonian condition is to be ventral and posterior sides of the tooth. This diagnostic for a family. excludes the parallel-sided posterior limb Incipient tooth striation, variable in degree characteristic of the myophorian hinge. and in location on the hinge, is present in The minimum requirements are met by several Triassic species (e.g., Newell and some Triassic genera (e.g., Middle Triassic Boyd, 1975: 144). The literature on Meso- Praegonia and Upper Triassic Maoritrigon- zoic trigoniaceans contains numerous de- ia) but not others (e.g., Neoschizodus; Ly- scriptions of taxa in which the hinge is des- riomyophoria; Gruenewaldia). Our admit- ignated as trigonian on the basis of less ex- tedly limited knowledge of post-Triassic tri- tensive tooth striation than that shown in fig- goniaceans suggests that they are typified by ure 1. In describing the new Middle Triassic the trigonian hinge, although it is not ade- genus Agonisca, Fleming (1963) character- quately documented in many genera (e.g., ized the hinge as "fully trigoniid" but with Lower Jurassic Liotrigonia). At least one "limited ridging on the teeth." species with only minimal development of Early evidence of tooth striation in the tri- the trigonian hinge is present as high strati- goniacean record is found in Neoschizodus graphically as Upper Cretaceous (e.g., Yaa- thaynesianus from the upper Lower Triassic dia hemphilli as illustrated in Saul, 1978: pl. of Utah (Newell and Boyd, 1975). In our col- 10, fig. 6). lections of that species, some hinges lack striae whereas others have a few coarse striae PROBLEMS WITH THE TAXONOMY on either the anterior face or both faces of OF NEWELL AND BOYD (1975) tooth 2, and on complementary faces of right of Several specialists have raised serious ob- valves. We have found incipient striation jections to specific aspects of our 1975 fam- similar type in Middle Triassic species of ily-level taxonomy for the Trigoniacea. Crit- Gruenewaldia and Lyriomyophoria. icism has centered on our revision of two The variable aspect of tooth striation in families, Myophoriidae and Trigoniidae, and Triassic Neoschizodus has been observed by our erection of the Costatoriidae. As Fleming others. Fleming (1987: 18) characterized the (1987) pointed out, the last name is a junior genus as having "myophorian to trigonian" synonym of Kobayashi's (1954) Minetrigo- grade hinge, with the "sides of teeth and niinae. A more basic problem, however, in- sockets smooth or transversely striated." He volves our weighting of shell sculpture more noted that Ichikawa (1954) erected Okunom- than dentition in inetania for Neoschizodus-like shells but heavily family diagnoses. with striated teeth, and that Farsan (1972) separated Afghanistan species of Neoschizo- COSTATORIIDAE dus into two subgenera based on presence or Our grouping of all trigoniaceans with ra- absence of striated teeth. Fleming stated that dial ribs in the Costatoriidae resulted in a 1997 BOYD AND NEWELL: TRIGONIACEAN FAMILIES 5 family that ranges from Permian to Recent Until our 1975 paper, no one had chal- and includes representatives of the three ma- lenged Cox's view, and our alternative has jor hinge grades known in the superfamily. not found support from experts on post-Pa- Fleming (1987: 14, 24) granted the pragmat- leozoic trigoniaceans (e.g., Poulton, 1979; ic advantage of radial ornament as a family Fleming, 1987; Cooper, 1991; Leanza, 1993). character and utilized the Minetrigoniidae for These specialists believe that the complex tri- radially ribbed Triassic trigoniaceans. How- gonian hinge is a conservative feature un- ever, he doubted the credibility of a lineage likely to have developed by parallel evolu- (our Costatoriidae) in which sculpture re- tion in separate lineages. Although arguably mained constant through several geologic pe- true for the overall hinge, one aspect of the riods while dentition changed. character complex described by Cox does The evolutionary continuity implied in our seem to have developed independently at concept becomes suspect in light of the rarity various times. The gap in the right valve of radial sculpture during the Jurassic-Cre- hinge plate thought by Cox (1952) and Flem- taceous heyday of trigoniacean diversity, a ing (1964) to be a unique trigoniid feature is fact emphasized by Saul (1978), Fleming also found in some Paleozoic trigoniaceans (1987), and Cooper (1991). Fleming, noting of schizodian hinge grade (e.g., Newell and the sporadic development of radial sculpture Boyd, 1975: fig. 12E; Johnston, 1993: fig. in early Cretaceous Australasian genera, 78) and, less commonly, in myophoriids thought it likely that the ornament originated (e.g., Newell and Boyd, 1975: fig. 83C). independently in different lineages. While concluding that this hiatus evolved in- Regarding the radially ribbed Neotrigonia, dependently at different times, Fleming Fleming (1987: 14) pointed out that the ju- (1987: 12) reaffirmed its universality in the venile ornament suggests an ancestor with Trigoniidae and pointed out our mistake in Trigonia-type sculpture rather than one with labeling as a platform the umbonal mound purely radial ornament. This supports his be- beneath the hiatus in a photograph of Neo- lief that the extant genus descended from trigonia (Newell and Boyd, 1975, fig. 12A). Eotrigonia, not from older radially orna- Our decision to recognize commarginal or- mented forms. nament as a diagnostic feature of the Trigo- niidae represented a notable departure from MYOPHORIIDAE AND TRIGONIIDAE the prevailing concept of that family. Annu- lar shells comprising the Trigoniidae of our In a seminal paper, Cox (1952) distin- revision include representatives of all three guished between Myophoriidae and Trigoni- hinge grades and the group ranges from idae on the basis of the hinge. For the latter, Permian to Cretaceous. The argument noted he emphasized the prominence of tooth 2, its above against independent origin of a trigon- conspicuous transverse ridges (striation), and ian-grade hinge is as applicable to the Tri- the absence of a hinge plate between teeth goniidae as it is in regard to the Costatori- 3a and 3b. This hiatus was thought to be ab- idae. However, our decision to abandon our sent in myophoriids. Striation was recog- hypothesis is also motivated by the ambigu- nized in advanced myophoriids, but never to ous ornament of many taxa. the conspicuous degree characteristic of tri- The Lower Jurassic Liotrigonia is de- goniids. scribed by Fleming (1987: 20) as having The two families have been widely adopt- "purely concentric ornament" whereas Cox ed, together with the implication that the tri- (1969: N473) characterized it as smooth "ex- goniids are a monophyletic group descended cept for narrow, irregular subconcentric from the Myophoriidae. Although Cox wrinkles mostly confined to anterior part." (1952: 48) had toyed with the idea of poly- We (1975: 67) stated that it is "nearly phyletic origin for the trigoniid hinge, his smooth," and admitted our inability to tell treatment of the superfamily in the Treatise whether that condition is primary or second- on Invertebrate Paleontology ignored the ary. topic and retained the two families as pre- Use of commarginal ornament as a crite- viously established. rion for family assignment becomes difficult 6 AMERICAN MUSEUM NOVITATES NO. 3216 when shells combine two types of surface to- grade hinges are grouped in Trigoniidae (Tri- pography. Fleming (1987: 15) noted this assic-Recent). Those with myophorian-grade problem in regard to Upper Triassic trigoni- hinges are assigned to either Myophoriidae aceans of Canada, in which the diverse or- (Permian-Triassic) or Pachycardiidae (Trias- nament styles include combinations that blur sic), depending on size and shape of the ma- the distinction between radial and commar- jor teeth (Newell and Boyd, 1975: 136). The ginal sculpture. exclusively Paleozoic schizodian-grade hinge A comparable problem is encountered in characterizes the specialized, monogeneric taxa where commarginal ornament is present Schapellinidae (Permian) as well as the di- on part of the flank whereas the other part is verse Schizodidae (Silurian-Permian). Within smooth. Fleming's (1987: 22) description of the latter family, the Eoastartinae consists of the subgenus Trigonia (Heslingtonia) states the schizodian-grade members of our former that the commarginal costae of the anterior eoastartid group. The one myophorian-grade flank stop abruptly near its midline. He noted genus of the Eoastartidae, Heminajas, is the similarity of this condition in his New reassigned to the Pachycardiidae. As we not- Zealand Triassic material and the Jurassic- ed in 1975 (p. 136), Heminajas shares sev- Cretaceous Eurasian genus Psilotrigonia eral characters with other pachycardiids. As Cox. for the Eoschizodidae, the taxon continues to The great diversity of surface sculpture exist for one unique Devonian species. found in post-Paleozoic trigoniaceans en- We recognize that our conclusions are courages the intuitive judgment that the or- strongly influenced by our concentration on nament has adaptive significance. This view pre-Jurassic trigoniaceans. Workers dealing gains support from experiments by Stanley with Jurassic and Cretaceous material are (1977) and Saul (1978). Stanley concluded confronted with a bewildering diversity of that many patterns of trigoniid ornamentation external morphology produced during the aided burrowing capability. Saul's experi- late Mesozoic radiation of the group. Al- ments convinced her that similarities among though the typical practice has been to group several Cretaceous groups of knobby trigo- genera into an expanding array of subfami- niids reflect convergent or parallel adapta- lies within the Trigoniidae (e.g., Leanza, tions to similar habitats. 1993), a markedly different approach is rep- These results support Poulton's (1979) resented by Cooper's (1991) revision of the opinion that external morphology is evolu- order Trigonioida. tionally labile because of its adaptive nature, His classification recognizes four super- and thus of lesser taxonomic importance than families, two of which include more than one hinge structure. hinge grade. These two, Myophoriacea and Trigoniacea, are distinguished by differences REVISION OF OUR 1975 WORK in shape, flank/area relationship, and promi- nence of myophorous buttress. Although Given the present state of knowledge of most of Cooper's myophoriacean taxa have trigoniacean fossils and the need for a clas- schizodian hinges and most of the trigoni- sification acceptable to most workers, we acean taxa have trigonian hinges, the my- conclude that hinge structure should have ophorian hinge is represented in both super- priority over surface sculpture as a family families. Cooper's other two superfamilies, criterion. In reappraising the classification Myophorellacea and Megatrigoniacea, are utilized in our trigoniacean monograph uniformly trigonian in hinge grade. Whereas (Newell and Boyd, 1975), we abandon Cos- we follow Cox (1969) and many other work- tatoriidae, return to Cox's (1952) distinction ers in accepting the trigonian hinge as the between Myophoriidae and Trigoniidae, de- definitive character for the Trigoniidae, Coo- mote Eoastartidae to a subfamily of Schizo- per's revision includes nine families with that didae, and retain that family as well as Eos- hinge. chizodidae, Scaphellinidae, and Pachycardi- The great diversity of Jurassic and Creta- idae. ceous trigonioids may well involve numer- In this plan, all genera with trigonian- ous clades that share the trigonian-grade 1997 BOYD AND NEWELL: TRIGONIACEAN FAMILIES 7 hinge. Recognition and taxonomic organiza- profile unlike the undercut one typical of ad- tion of such lineages will require sophisti- vanced trigoniaceans (Cox, 1952: 46). cated methodology. The trigonioids offer Although hinges of the two Nevada spe- great potential for morphometric study and cies are very similar in their mixture of my- cladistic analysis, and we look forward to the ophorian and trigonian traits, the hinge of the results of such research by workers skilled in ornamented form is slightly more advanced those techniques. than that of the smooth form. The former has equally strong striae on both sides of tooth 2 NEW SPECIES FROM THE LOWER whereas striae are weaker on the posterior NEVADA side of that tooth in the smooth species. Fur- TRIASSIC OF thermore, the posterior limb of tooth 2 of the In the following section, we describe two ornamented species is less prominent than its new trigoniacean species, one smooth (Neo- counterpart in the smooth form. schizodus) and the other ornamented (Lyrio- Morphologic terms employed in the taxo- myophoria), from a silicified fossil assem- nomic descriptions that follow are defined ei- blage in the Virgin Limestone (Spathian) ther in Cox (1969: 102-109) or in Newell southwest of Las Vegas. Together with the and Boyd (1975: 66-82). Detailed informa- essentially contemporaneous Neoschizodus tion pertinent to cited locality numbers is in thaynesianus from the upper Thaynes For- Newell and Boyd (1995: 85-86). mation of northeastern Utah (Newell and DESCRIPTIONS Boyd, 1975), they represent the oldest Me- sozoic trigoniaceans known to us. FAMILY MYOPHORIIDAE BRONN, 1849 The new species are of particular interest DIAGNOSIS: Small to moderate size shells as evidence of the early Mesozoic transition with prosogyrous to orthogyrous umbones from myophorian to trigonian hinge grades. and subangular to rounded posterior ridge. Furthermore, the ornamented form (Lyrio- Surface smooth or (less commonly) with myophoria) represents an intermediate con- commarginal or radial ornament. Myophori- dition between the simple commarginal or- an hinge. nament found in some Paleozoic trigoni- DISTRIBUTION: Lower Permian-Upper Tri- aceans and the disparate flank/corselet sculp- assic. ture typical of post-Triassic members of the superfamily. Genus Neoschizodus Giebel, 1855 The remarkably similar hinges of the two TYPE SPECIES: Lyrodon laevigatum Gold- Nevada species exhibit characteristics of fuss, 1837, SD Stoliczka, 1871; Middle Tri- both myophorian and trigonian grades. We assic of Germany. classify them as myophorian because tooth 2 DIAGNOSIS: Unornamented orthogyrous to has the distinctive posterior extension char- moderately prosogyrous shells with slightly acteristic of that grade. By contrast, they pos- incurved beaks. Respiratory margin oblique- sess the typical, albeit not unique, trigonian ly truncate and pointed. Posterior ridge an- feature of a hiatus in the right valve hinge gular to subangular in transverse profile. plate. Furthermore, their tooth striation meets Teeth smooth or incompletely striated. the minimum requirement for trigonian grade DISTRIBUTION: Middle and Upper Permian, as defined in a previous section of this report. Kitakami and Gujo formations, Japan; Lower Both faces of teeth 2 and 3a are consistently Triassic, upper Thaynes Formation, Utah, striated. This is a notable advance over the and Virgin Limestone, Nevada; Middle Tri- situation in Neoschizodus thaynesianus, assic, Muschelkalk Series, Germany. where striation is not only less extensive but also variable within a population (Newell Neoschizodus elongatus Boyd and Newell, and Boyd, 1975: 144). As in younger trigon- new species iaceans, striae of the two new species are Figure 2 bold, regular, and concave dorsally. Howev- ETYMOLOGY: elongatus, from the Latin, er, each ridge has a symmetrical transverse long. 8 AMERICAN MUSEUM NOVITATES NO. 3216

.j

Fig. 2. Neoschizodus elongatus Boyd and Newell, n. sp., silicified shells, Lower Triassic, Moenkopi Fm. (Virgin Limestone), Nevada, AMNH loc. B7979. A. Holotype, RV, exterior and interior views each X2, AMNH cat. 44075; B. RV, X 12, showing deep striae on anterior and posterior faces of tooth 3a (left arrow), weak striae on anterior face of tooth 3b (middle arrow), unstriated socket (right arrow) for reception of tooth 4b, and nymph (upper arrow), SEM stub AMNH cat. 44076; C. LV, X2 and X3 respectively, showing posteroventral extension (arrow) of tooth 2 characteristic of myophorian hinge grade, AMNH cat. 44077. 1997 BOYD AND NEWELL: TRIGONIACEAN FAMILIES 9

TABLE 1 Neoschizodus elongatus Boyd and Newell, New Species AMNH loc. B7979; measurements in millimeters L H L/H C H/C PL' L/PL T2 Le3 Left valves 31.1 23.3 1.33 7.6 3.07 9.3 3.34 2.5 28.4 20.5 1.39 5.6 3.66 8.5 3.34 2.5 3.7 24.4 18.2 1.34 5.2 3.50 8.7 2.80 2.5 21.1 16.3 1.29 4.9 3.33 6.7 3.15 20.7 15.6 1.33 5.0 3.12 6.6 3.14 15.5 12.2 1.27 3.9 3.13 4.7 3.30 14.3 10.8 1.32 3.2 3.38 4.7 3.04 1.5 2.1 12.6 9.7 1.30 2.8 3.46 3.8 3.32 1.6 11.0 8.2 1.34 2.6 3.15 2.9 3.79 9.7 8.0 1.21 2.2 3.64 3.7 2.62 1.6 Right valves 33.5 22.7 1.48 7.3 3.11 9.2 3.64 3.4 5.7 29.0 20.3 1.43 6.1 3.33 9.9 2.93 2.9 23.7 18.2 1.30 5.6 3.25 7.4 3.20 2.3 3.5 21.6 16.1 1.34 4.5 3.58 6.2 3.48 2.3 20.7 16.3 1.27 4.1 3.98 7.3 2.84 2.0 18.7 13.5 1.39 3.9 3.46 6.3 2.97 2.0 3.0 16.4 12.9 1.27 3.8 3.39 4.9 3.35 1.9 14.5 10.8 1.34 3.4 3.18 4.8 3.02 2.2 12.3 9.5 1.29 3.1 3.06 4.1 3.00 10.2+ 8.6 2.8 3.07 3.3 1.7 2.0 1 PL = linear extension of valve ahead of beak. 2 T = distance from beak to anterior corner of major tooth. 3Le = length of ligament groove.

DIAGNOSIS: Umbo slightly prosogyrous, vexity; PL = linear extension of valve an- beak less than one-third of valve length be- terior to beak. hind anterior extremity. Anterior and ventral DISTRIBUTION: Lower Triassic (Spathian) margins form continuous, smooth curve from of southern Nevada (Virgin Limestone Mem- beak to posterior extremity; respiratory mar- ber, Moenkopi Formation). AMNH loc. gin broad, gently sloping, meeting dorsal B7977, B7979, B8517. margin with little or no angularity. Posterior DIscusSION: The study collection includes ridge angular to carinate initially, becoming 450 silicified valves from Cottonwood Pass, broadly rounded beyond valve height of 9 between Las Vegas and Goodsprings, Neva- mm. Corselet slightly concave, lacking dis- da. The two collections involved, B7979 and tinct escutcheon. Small myophorian hinge are to be from the same stra- plate; major tooth on each valve striated on B8517, thought both sides; major socket of right valve un- tum, offset by a fault. All valves are disar- floored. Anterior adductor scar with dorsal ticulated, many are broken, and most have extension for pedal retractor; posterior re- the interior concealed by a coarse silica crust tractor scar isolated above adductor at end of, and/or silicified matrix. Although most ex- respectively, posterior tooth on RV and teriors are well preserved, examination of equivalent socket on LV; pedal elevator scar many outer surfaces provided no evidence of at apex of umbonal cavity. Pallial line pos- original microstructure. The same species is teriorly truncate, rarely distinct. L/H = 1.34 represented by more than 150 internal molds (n = 39); H/C = 3.30 (n = 26); L/PL = 3.20 in our collection from loc. B7979 east of Las (n = 39). L = length; H = height; C = con- Vegas. Here, silica coated the inner surface 10 AMERICAN MUSEUM NOVITATES NO. 3216 of the valve and replaced adjacent matrix, Commarginal ridges in front of posterior but the shell was not silicified. ridge larger and more regular in form and Although most shells of N. elongatus are spacing than those of corselet; escutcheon unornamented, rare valves exhibit indistinct narrow and inconspicuous. Myophorian commarginal ridges on the anterior slope. hinge with prominent striae on both sides of The ligament groove length is about equal to the major tooth on each valve; major socket that of the right-valve posterior tooth. A pos- of right valve unfloored. L/H = 1.08 (n = terior tooth is weakly developed on large left 20); H/C = 3.66 (n = 20); L/PL = 2.57 (n valves but is unrecognizable on small valves. = 20). The corresponding socket on large right DISTRIBUTION: Virgin Limestone Member valves is an unstriated furrow separating a (Spathian), Moenkopi Formation, near Las slender posterior tooth from the narrow Vegas, Nevada. AMNH loc. B7977, B7979, nymph. B8517. The new species is distinguished from the DISCUSSION: Lyriomyophoria paullorum approximately coeval Neoschizodus thayne- occurs with the smooth trigoniacean Neo- sianus by its more elongate form, less an- schizodus elongatus in our samples of Virgin gular posterior margin, smaller hinge plate, Limestone from the Las Vegas area. Our and more thoroughly striated teeth. In the study collection consists of several hundred last-mentioned character, N. elongatus differs disarticulated valves, most of them fragmen- from the minimal trigonian condition only in tal and imperfectly silicified. The RV/LV ra- the inequality between striae of the two sides tio is approximately 3/2. Fewer well-pre- of tooth 2. In all but the largest left valves, served interiors are present than in the N. the posterior face of that tooth is only weakly elongatus collection, but the number and po- striated. sition of muscle scars appear to be the same in both species. Genus Lyriomyophoria Kobayashi, 1954 The prominent commarginal ridges in front of the posterior ridge are narrower than TYPE SPECIES: Lyriodon elegans Dunker, the valleys between them, in contrast to the 1849, OD; Middle Triassic of Germany. finer, more numerus ones of the corselet DIAGNOSIS: Prosogyrous shells with sub- where discontinuous and anastomosing ridg- dued umbones; truncate respiratory margin es are common. The corselet is divided into greater than half valve height. Posterior ridge anterior and posterior parts by an arcuate sharply defined. Prominent commarginal groove, most prominent as a midline in the ridges on both corselet and flank; corselet in- markedly concave dorsal part of the corselet. terrupted by radial groove. Myophorian A diminutive escutcheon is bordered by a hinge with striae on some teeth. threadlike carina. On one well-preserved DISTRIBUTION: Triassic; Eurasia, North Af- valve, 11.6 mm long, the escutcheon extends rica, Nevada. 4.3 mm behind the beak and is 0.7 mm in maximum width. Lyriomyophoria paullorum Boyd and The myophorian hinge is close to the tri- Newell, new species gonian grade in that both sides of the major Figures 3, 4 tooth in each valve are thoroughly striated, ETYMOLOGY: paullorum, for Drs. Rachel the posterior limb of the left-valve major and Richard Paull, experts on Lower Triassic tooth is short, and a hiatus is present in the stratigraphy of the Rocky Mountains and right-valve hinge plate. The left-valve major Great Basin. tooth is relatively narrow; its two sides di- DIAGNOSIS: Slightly prosogyrous shells verge at an average angle of 510 (n = 6). The with height nearly equal to length; beaks notably short ligament groove is about half slightly forward of midlength. Anterior mar- the length of the right-valve posterior tooth. gin strongly convex; respiratory margin high L. paullorum differs from the type species and rectilinear, meeting dorsal margin at ob- of the genus in that commarginal ridges do tuse angle. Posterior ridge marked by prom- not cross the carina on the posterior ridge, inent carina but without adjacent trough. and there is no trough bordering the ridge. 1997 BOYD AND NEWELL: TRIGONIACEAN FAMILIES I11

Fig. 3. Lyriomyophoria paullorum Boyd and Newell, n. sp., silicified shells, Moenkopi Fm. (Virgin Limestone), Nevada. A. Holotype, LV, X3, showing carinate ridge not crossed by commarginal orna- ment, AMNH loc. B8517, cat. 44078; B. LV, X3, posterodorsal view showing contrast between ornament on both sides of carina, narrow escutcheon (upper arrow), and shallow furrow (lower arrow) bisecting corselet, AMNH loc. B8517, cat. 44079; C. RV, X3, AMNH loc. B8517, AMNH cat. 44081; D. LV, X 12, showing posteroventral wing (arrow) of tooth 2. Pitted surface and lack of striae are effects of imperfect fossilization, AMNH loc. B8517, SEM stub AMNH cat. 44080; E. RV, X 12, AMNH loc. B7979, SEM stub AMNH cat. 44082; El. downward convex striae on posterior face of tooth 3a (left arrow) and nonstriated socket (right arrow) for reception of tooth 4b; E2. same specimen tilted to expose striated anterior faces of tooth 3a (left arrow) and tooth 3b (right arrow). 12 AMERICAN MUSEUM NOVITATES NO. 3216

Fig. 4. Lyriomyophoria paullorum Boyd and Newell, n. sp., RV, X9 and X3, shows myophorous buttress supporting major tooth 3a, bordering ventral side of anterior adductor scar, AMNH loc. B8517, AMNH cat. 44083.

TABLE 2 Lyriomyophoria paullorum Boyd and Newell, New Species AMNH loc. B7979; measurements in millimeters L H L/H C H/C PL1 L/PL T2 Left valves 14.2 13.2 1.08 3.7 3.57 5.7 2.49 2.8 13.6 12.9 1.05 3.2 4.03 5.6 2.43 2.5 13.2 12.2 1.08 3.2 3.81 4.2 3.14 2.3 12.8 12.0 1.07 3.3 3.64 5.4 2.37 2.3 11.1 10.5 1.06 2.8 3.75 5.0 2.22 2.0 10.8 10.4 1.04 2.8 3.71 4.3 2.51 2.0 10.5 9.8 1.07 2.8 3.50 3.9 2.69 1.9 10.3 9.1 1.13 2.7 3.37 3.9 2.64 2.1 9.4 8.8 1.07 2.4 3.67 3.8 2.47 1.6 9.4 8.9 1.06 2.2 4.05 4.0 2.35 1.6 Right valves 14.1 13.6 1.04 3.8 3.58 5.6 2.52 3.0 13.8 11.5 1.20 3.0 3.83 5.1 2.71 2.9 13.2 11.5 1.15 3.3 3.48 4.7 2.81 2.5 12.6 11.2 1.13 2.9 3.86 5.0 2.52 2.3 12.4 11.5 1.08 3.2 3.59 4.5 2.76 2.4 11.1 11.1 1.00 3.0 3.70 5.0 2.22 2.2 11.0 10.8 1.02 3.1 3.48 4.4 2.50 2.3 10.3 9.8 1.05 2.6 3.77 3.6 2.86 2.1 10.1 8.9 1.13 2.5 3.56 4.1 2.46 1.9 8.1 7.6 1.07 2.3 3.30 3.0 2.70 ' PL = linear extension of valve ahead of beak. 2 T = distance from beak to anterior corner of major tooth. 1997 BOYD AND NEWELL: TRIGONIACEAN FAMILIES 13

Furthermore, the new species has a very Drs. Knut Andersson and Xunhong Chen, short ligament groove and prominently stri- then graduate students at the University of ated major teeth. Wyoming, provided excellent field assis- In overall valve shape and in the disparate tance, and Chen did much of the laboratory condition of commarginal ornament between acid processing of the limestone. Gillian flank and corselet, L. paullorum resembles Newell participated fully in the fieldwork the Permian schizodian-grade Lyroschizodus and in production of the final manuscript. orbicularis. The latter species lacks myopho- Walter Elvers has volunteered his services in rous buttresses and carinate posterior ridge. the organization of the collections, and has Similar inequality of commarginal orna- assisted enthusiastically in many other ways. ment between flank and corselet character- Our respective institutions, the University izes the Upper Triassic Prosogyrotrigonia of Wyoming and the American Museum of and the Jurassic-Cretaceous Trigonia (Fren- guelliella). The former lacks a posterior ca- Natural History, provided office, laboratory, rina and a truncate respiratory margin. The and other facilities. Scanning electron mi- latter is opisthogyrous and its corselet ridges croscopy was done at the University of Wy- originate in a sulcus in front of the posterior oming by Keith Krugh. Andrew Simon and ridge. Portia Rollings, under Newell's guidance, did the photography at the American Museum of ACKNOWLEDGMENTS Natural History. Although not every suggestion from re- The new species described here were dis- viewers Terrence Poulton, Steven Stanley, covered during fieldwork supported by Na- and Thomas Waller was followed, the paper tional Science Foundation grants EMS 74- is much better as a result of their work, and 12372, DEB 77-01558, and BSR 88-06186. we thank them for their efforts.

REFERENCES Boyd, D. W., and N. D. Newell stan). Palaeontogr., Abt. A., 140: 131- 1968. Hinge grades in the evolution of cras- 191. satellacean bivalves as revealed by Fleming, C. A. Permian genera. Am. Mus. Novitates 1963. A new genus and species of Trigoni- 2328: 52 pp. idae from the Middle Triassic of New Cooper, M. R. Zealand. New Zealand J. Geol. Geo- 1991. Lower Cretaceous Trigonioida (Mollus- phys. 6(5): 843-846. ca, Bivalvia) from the Algoa Basin, 1964. History of the bivalve family Trigoni- with a revised classification of the or- idae in the south-west Pacific. Austra- der. Ann. S. African Mus. 100(1): 52 lian J. Sci. 26(7): 196-204. PP. 1987. New Zealand Mesozoic bivalves of the Cox, L. R. Superfamily Trigoniacea. New Zealand 1952. Notes on the Trigoniidae with outlines Geol. Surv. Paleontol. Bull. 53: 74 pp. of a classification of the family. Proc. Harrison, W. B. III, and L. K. Harrison Malacol. Soc. London 29: 45-70. 1975. A Maquoketa-like molluscan commu- 1969. Superfamily Trigoniacea, Family My- nity in the Brassfield Formation (Early ophoriidae Bronn, 1849 and Family Silurian) of Adams County, Ohio. Bull. Trigoniidae Lamarck, 1819. In R. C. Am. Paleontol. 67: 193-234. Moore (ed.), Treatise on invertebrate Ichikawa, K. paleontology N(1), Mollusca 6: 471- 1954. Late Triassic Pelecypods from the Ko- 488. Lawrence: Geol. Soc. Am. and chigatani Group in the Sakurandani and Kansas Univ. Kito areas, Tokushimo Prefecture. Shi- Farsan, N. M. koku, Japan, part 11. J. Inst. Polytech, 1972. Stratigraphische und palaogeograph- Osaka City Univ., Ser. G, 2: 53-72. ische Stellung der Khenjan-Serie und Johnston, P. A. deren Pelecypoden (Trias, Afghani- 1993. Lower Devonian Pelecypoda from 14 AMERICAN MUSEUM NOVITATES NO. 3216

southeastern Australia. Assoc. Austral- U.S. Geol. Surv. Prof. Pap. 1394-A-G: asian Palaeontol. Mem. 14: 134 pp. 57-107. Kobayashi, T Poulton, T P. 1954. Studies on the Jurassic Trigonians in 1979. Jurassic trigoniid bivalves from Canada Japan, part 1. Preliminary notes. Japa- and western United States of America. nese J. Geol. Geogr. 25(1-2): 61-80. Geol. Surv. Can. Bull. 282: 82 pp. Leanza, H. A. Saul, L. R. 1993. Jurassic and Cretaceous bivalves from 1978. The North Pacific Cretaceous trigoniid west-central Argentina. Bull. Am. Pa- genus Yaadia. Univ. California Publ. leontol. 105(343): 95 pp. Geol. Sci. 119: 65 pp. Newell, N. D., and D. W. Boyd Stanley, S. M. 1975. Parallel evolution in early trigoniacean 1977. Coadaptation in the Trigoniidae, a re- bivalves. Bull. Am. Mus. Nat. Hist. markable family of burrowing bivalves. 154(2): 162 pp. Palaeontology 20(4): 869-899. 1995. Pectinoid bivalves of the Permian-Tri- 1978. Aspects of the adaptive morphology assic crisis. Ibid. 227: 95 pp. and evolution of the Trigoniidae. Phi- Pojeta, J. Jr., R. Zhang, and Z. Yang los. Trans. R. Soc. London B(284): 1986. Chapter G. Systematic paleontology of 247-258. Devonian pelecypods of Guangxi and Waller, T. R. Michigan. In J. Pojeta, Jr. (ed.), Devo- 1981. Functional morphology and develop- nian rocks and Lower and Middle De- ment of veliger larvae of the European vonian pelecypods of Guangxi, China, oyster, Ostrea edulis Linne. Smithson. and the Traverse Group of Michigan. Contrib. Zool. 328: 70 pp.

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