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.