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BIVALVE SYSTEMATICS DURING THE 20TH CENTURYAuthor(s): JAY A. SCHNEIDERSource: Journal of Paleontology, 75(6):1119-1127. 2001.Published By: The Paleontological SocietyDOI: http://dx.doi.org/10.1666/0022-3360(2001)075<1119:BSDTC>2.0.CO;2URL: http://www.bioone.org/doi/full/10.1666/0022-3360%282001%29075%3C1119%3ABSDTC%3E2.0.CO%3B2

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1119

J. Paleont., 75(6), 2001, pp. 1119–1127Copyright q 2001, The Paleontological Society0022-3360/01/0075-1119$03.00

BIVALVE SYSTEMATICS DURING THE 20TH CENTURYJAY A. SCHNEIDER

Department of Geology and Geophysics, University of Wisconsin, Madison 53706, ,jaschnei@geology.wisc.edu.

ABSTRACT—Over the past 75 years, the higher-level taxonomy of bivalves has received less attention than that of their fellow molluscs,gastropods. The publication of the bivalve volumes of the Treatise on Invertebrate Paleontology in 1969 was not followed by anexplosion of study into the evolution of bivalves; rather, with only one or two exceptions, bivalve workers were noticeably absent fromthe cladistic and molecular revolutions that were taking place during the 1970s and 1980s, even as gastropods received considerableattention. Over the past ten years, cladistics and molecular systematics have begun to be applied to solve problems of bivalve evolu-tionary biology. These studies, most of which have been undertaken by paleontologists, have halted the stagnation in bivalve systematics.Bivalve systematics looks to have an exciting future, as the excellent fossil record of the Bivalvia will be used in conjunction withcladistics and molecular systematics to solve problems in not just bivalve evolution but evolutionary biology in general.

INTRODUCTION

ALTHOUGH BIVALVES are an important component of the Re-cent marine and non-marine fauna—and have been since at

least the Triassic—their taxonomy and systematics have receivedconsiderably less attention than most other invertebrate groupswith substantial Recent and fossil diversity. This is especially truewhen the state of bivalve systematics is compared to that of theirfellow molluscs, the gastropods. Bieler (1992; see Figs. 1–4 there-in) graphically illustrated the dynamic changes in our understand-ing of gastropod taxonomy and systematics during much of the20th century. However, comparing the figures in Bieler’s reviewto those in this present review of bivalve taxonomy and system-atics (Figs. 1–3), we can see that it is much easier to follow thechanges in higher-level taxonomy of bivalves through the decadesthan it is for the gastropods, even if we take into account thegreater diversity of gastropods (see Boss, 1982). I imagine it wasconsiderably easier for this author to put together Figures 1–3than for Bieler to construct his diagrams.

Although taxonomic stability may be a desideratum, in realitytaxonomic stability is a manifestation of scientific stagnation. Thesecond half of the 20th century has seen the advent of two revo-lutions in systematics: cladistics and molecular phylogenetics. Theapplication of these two methods has altered not only taxonomicclassifications but even how we view not only the practice of sys-tematics, but the importance of the fossil record. Not only havecladistics and molecular phylogenetics altered our view of the evo-lution of such a group as the gastropods (Bieler, 1992), but in justthe past few years these methods have even drastically altered ourview on mammalian evolution (see van Dijk et al., 2001; Liu etal., 2000; Murphy et al., 2001; and references therein), the cladewhich contains we humans and has arguably been the subject ofmore systematic work than any clade even remotely comparable.

It is unfortunate that a group with such an extensive fossilrecord as the bivalves had been neglected by modern advances insystematics. Perhaps it was even because of the excellent bivalvefossil record that few bivalve workers attempted cladistic and mo-lecular analyses, fearful that the results would drastically disagreewith the sequence of first appearances of taxa in the fossil record.For if even the bivalve fossil record failed to reflect phylogeneticrelationships as hypothesized by cladistics and molecules, thenperhaps the fossil record really is not useful in reconstructingevolutionary history.

In just the past several years, word of these revolutions in sys-tematics has finally made its way to the land of bivalve system-atics. Whereas for most of the late 1970s and 1980s there wereonly one or two bivalve systematists using cladistics and/or mo-lecular phylogenetics, there are now roughly a dozen or moreindividuals using these techniques to estimate bivalve phyloge-netic history and evolutionary patterns.

THE TREATISE AND BEFORE

Newell (1969), in the bivalve volume of the Treatise on Inver-tebrate Paleontology, used a table to summarize the last sixty orso years of higher-level bivalve taxonomy (see Fig. 1), and New-ell (1965, 1969) and Cox (1960) should be consulted for thoroughreviews of bivalve taxonomy up until the publication of the bi-valve volume of the Treatise. Throughout the late 19th centuryand well into the 20th century, bivalve classification epitomizedthe use of single character or single organ systems, to the exclu-sion (or nearly so) of other characters (see Cox, 1960; Morton,1996; Salvini-Plawen and Steiner, 1996). Although this fact isindeed pointed out by numerous authors, few of these taxonomicschemes gained any adherents beyond their immediate propo-nents. Only a few of these proposed taxonomic schemes provedpopular, and they are summarized below and in Figure 1.

Hinge teeth were not used in higher-level bivalve taxonomyuntil Neumayr (1884), who was also arguably the first worker toattempt to reconstruct bivalve phylogeny. This focus on hingestructure also influenced the work of Dall (1889, 1895, 1913),who considered the edentulous anomalodesmatans the most prim-itive of the bivalves. The prionodonts (mostly forms with taxo-dont, schizodont, isodont or dysodont dentition) were intermediatebetween the anomalodesmatans and the advanced teleodesma-ceans with heterodont dentition (see Cox, 1969, for descriptionof bivalve hinge types). By the time of Zittel (1900), it had be-come accepted that the Prionodesmacea were the most primitivebivalves, and that the edentulous condition in the Anomalodes-macea was a derived condition.

Meanwhile, Pelseneer (1889, 1891) had come up with a clas-sification of bivalves based on the structure of the ctenidia. Al-though Pelseneer placed comparatively less emphasis on the fossilrecord and shell morphology, at least one of his followers, Ride-wood (1903) did state that much of the problem in bivalve sys-tematics lay with workers who base their classifications on singleorgan systems [this view would be echoed by Boss (1978), whoexempted paleontologists from erecting bivalve taxonomicschemes based on single organ systems]. Indeed, Pelseneer (1911)had come to realize that the structure of the ctenidia was but oneset of characters that needed to be used to reconstruct bivalveevolutionary history, and that parallel and convergent evolutionof ctenidial morphologies could and did occur.

This syncretic approach to bivalve taxonomy was selected inthe classification system of Thiele (1935), which was heavilybased upon that of Cossmann and Peyrot (1909). There were threemain orders of bivalves: 1) Taxodonta, including all forms withtaxodont dentition; 2) Anisomyaria, which included forms withone adductor muscle much larger than the other, or with but asingle adductor muscle; and 3) Eulamellibranchia, which includedall forms with lamellibranch ctenidia.

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FIGURE 1—Bivalve taxonomy from Dall (1913) to the Treatise (Newell, 1969). Newell (1965) used the term Poromyoida for the septibranchs; in1969, Newell relegated the septibranchs to a subgroup of the Pholadomyoida. Black boxes indicate that a fossil taxon not considered by the authorin question. Compare Figures 1–3 to Figures 1–4 in Bieler (1992). Modified from Newell (1969).

By the 1960s (Franc, 1960; Cox, 1960; Newell, 1965, 1969) itwas apparent that the arcoid bivalves, although they displayedtaxodont hinges, were not allied with other such bivalves withtaxodont hinges. The Arcoida (5Taxodonta of Franc, 1960 andEutaxodontida of Cox, 1960) were allied with scallops, oysters,marine mussels and their relatives in the taxon Pteriomorphia(5Filibranchia of Franc, 1960). The problem now was whetherthe taxodont hinge of arcoids was evolved in parallel with othertaxodont bivalves, or whether the arcoid hinge retained primitivecharacters. The corallary to this problem was whether or not thearcoids were the most basal pteriomorphs.

Although by 1960 agreement had been reached regarding theaffinities of the arcoids, there were still major disagreements aboutthe higher-level taxonomy of bivalves. For one, Franc (1960) hadremoved the carnivorous septibranchs from the Anomalodesmata.In addition, the affinities of the extinct Paleozoic groups Modi-omorphoida (5Pantodontida of Cox, 1960) and Praecardioida(5Cryptodontida of Cox, 1960) would remain controversial. Fi-nally, there was disagreement over how many major subdivisionswere recognized within the bivalves. Franc (1960) and Cox(1960) followed Thiele (1935) and more broadly Dall (1913) inrecognizing three major groups within Bivalvia. Both Franc andCox recognized Protobranchia, for forms which had protobranchctenidia; both authors also accepted a taxon for forms with usually

filibranch ctenidia and with a tendency toward reduction or elim-ination of one of the sets of adductor muscles (Filibranchia ofFranc, Pteriomorphia of Cox). The remaining taxa were placedby Cox in the Heteroconchia, a taxon broadly equivalent to Thie-le’s Eulamellibranchia. As stated above, Franc excluded the sep-tibranchs from his version of the Eulamellibranchia. Franc’s andCox’s (mostly) tripartite schemes stood in contrast to that offeredby Newell (1969) in the Treatise. Newell considered the Proto-branchia to be one of those taxa erected simply on the basis of asingle character, namely ctenidial morphology, and he posited twotaxa of protobranchiate bivalves of equal rank, the taxodont Pa-laeotaxodonta and the usually edentulous Cryptodonta. Newell’s(1969) conception of the Pteriomorphia was nearly the same asthat of Franc (1960) and Cox (1960). However, Newell questionedthe monophyly of Thiele’s (1935) Eulamellibranchia, which mostauthors, including Franc and Cox, had usually followed. Newellallied the extinct modiomorphoids with the schizodont-hingebearing unionoids (freshwater mussels) and trigonioids in Palaeo-heterodonta. Like Dall (1913) and Thiele (1935), Newell (1969)placed the pholadomyoids and septibranchs (Poromyoida of New-ell, 1965) together in the Anomalodesmata. However, unlike Thie-le, Newell considered this group of equal rank with the Palaeo-heterodonta and the remaining eulamellibranch bivalves, for

1121SCHNEIDER—BIVALVE SYSTEMATICS

FIGURE 2—Bivalve taxonomy post-Treatise to just before the introduction of cladistics into the study of bivalve phylogenetics. Purchon (1987)classified the Leptonacea as a taxon of Unionoida and the Lucinacea as a taxon of Veneroida; in 1990 Purchon did the opposite. Black boxesindicate that a fossil taxon not considered by the author in question.

which Newell used the name Heterodonta. The heterodont/het-eroconch controversy continues into the present.

THE ORIGIN OF THE BIVALVES

At the time of the Treatise, bivalves were thought to be derivedfrom the univalved monoplacophorans (Cox, 1969), with the ear-liest bivalve known being the lower Middle Cambrian Lamello-donta. Pojeta (1975) and Havlıcek and Krız (1978) showed thatLamellodonta was simply a distorted obollelid brachiopod. Withthe exclusion of Lamellodonta from the Bivalvia, progress couldfinally be made on understanding the relationship of bivalves toother molluscs, and of the identity, morphology, and evolution ofthe earliest bivalves.

Pojeta (1975) concluded that the Early Cambrian Fordilla troy-ensis Barrande, 1881 was the only undoubted Cambrian bivalve.Subsequently, Jell (1980) described another Early Cambrian bi-valve, Pojetaia runnegari, and placed it together with Fordilla inthe family Fordillidae and order Fordilloida. Runnegar and Bent-ley (1983), Pojeta and Runnegar (1985) and Pojeta (1987) con-sidered Pojetaia a palaeotaxodont and Fordilla an isofilibranch(i.e., mytiloid pteriomorph). After further investigations of shellmicrostructure, Runnegar and Pojeta (1992) did put these twogenera in one taxon, Fordillidae, which they consider as possibly

‘‘ancestral’’ to the palaeotaxodonts but certainly not pteriomor-phian. Cope (1996) and Geyer and Streng (1998) recently treatedPojetaia as a palaeotaxodont. Carter et al. (2000) classified Po-jetaia, Fordilla and Tuarangia as plesions basal to Palaeotaxo-donta.

Runnegar and Pojeta (1992) concluded that Fordilla is knownfrom Tommotian rocks in Siberia; its range lasts until the end ofthe Botomian (Pojeta, 2000). Pojetaia is known from the Tom-motian to the middle Middle Cambrian (Pojeta, 2000).

Meanwhile, shortly after the publication of the Treatise, thelatest Cambrian to latest Permian concocardioids were removedfrom the Bivalvia and placed in their own class, Rostroconchia,along with the ribeirioids, which hitherto had usually been con-sidered bivalved arthropods (Pojeta et al., 1972). Cox (1960) hadconsidered the conocardioids as an order (Rostronconchida) ofpteriomorphs. The conocardioids had later been placed as ‘‘Sub-class Uncertain’’ in the Treatise (Newell, 1969).

Runnegar and Pojeta (1974) suggested that bivalves evolvedfrom Early Cambrian rostroconchs, which were in turn derivedfrom monoplacophorans such as Anabarella and/or Watsonella.Waller (1998) regarded Watsonella as a rostroconch. However,Carter et al. (2000) showed that Watsonella is more closely re-lated to the Bivalvia than to the Middle Cambrian rostroconch

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FIGURE 3—Bivalve taxonomy at present. Carter (1990) and Campbell et al.’s (1998) schemes are identical except that Campbell et al. remove thepen shells (‘‘Pinnoida’’) from the wing-shells (‘‘Pterioida’’) and give them equal rank. Prezant (1998) and Coan et al. (2000) have identicaltaxonomic schemes except that Coan et al. consider the Septibranchia as equal in rank to the Pholadomyoida, not a subgroup of Pholadomyoidaas does Prezant. Black boxes indicate a fossil taxon not considered by the author in question.

Ribeiria. Becaused Early Cambrian Watsonella antedates all un-doubted rostroconchs, Carter et al. (2000) classified Watsonellaas a monoplacophoran, placing it in Stenothecidae along withAnabarella. MacKinnon (1982) subsequently described Tuaran-gia paparua from the Middle Cambrian. MacKinnon (1982) ten-tatively placed Tuarangia in the Pteriomorphia on the basis of hisinterpretation of its shell microstructure; he grouped Tuarangiawith the problemmatic middle Cambrian mollusc Pseudomyonain the pteriomorphian order Tuarangiida. Runnegar (1983) con-sidered these two taxa as bivalved monoplacophorans. Mac-Kinnon (1985), this time without reservations, placed Tuarangiain the Pteriomorphia, but rejected any relationship of Tuarangiawith Pseudomyona. Runnegar and Pojeta (1992) maintained aclose relationship of the two genera, but emphasized that theywere not particularly closely related to Fordilla and Pojetaia.Carter et al. (2000) abandoned the order Tuarangida, and regardedPseudomyona as a monoplacophoran of uncertain family.

The Cambrian bivalve fossil record has been thoroughly re-viewed by Runnegar and Pojeta (1992) and Pojeta (2000), whoalso do an excellent job of debunking numerous cases of allegedbivalves which turn out to be brachiopods, bivalved arthropods,and pseudofossils. In addition, Carter et al. (2000) suggested thatthe Middle Cambrian Arhouiella is an ostracode, not a bivalve.

THE CLADISTIC REVOLUTION

During the 1970s and 1980s the discipline of systematic biol-ogy was revolutionized by the rapidly increasing use of the meth-odology of phylogenetic systematics or cladistics, generally con-sidered to have been invented by Hennig (1950). However, witha very few exceptions, it took until the late 1990s for word ofthis methodological revolution to reach the world of bivalve sys-tematics.

For the most part unaware of Hennig’s (1950) book, many sys-tematists took up the methodology of numerical taxonomy (alsocalled phenetics or overall similarity, see Wiley, 1981) to studyphylogenetic relationships. Numerical taxonomy has been definedas the ‘‘numerical evaluation of the affinity or similarity betweentaxonomic units and the ordering of these units into taxa on thebasis of their affinities’’ (Sokal and Rohlf, 1981).

Bretsky (1971) attempted to use numerical taxonomic methodsto study the relationships of Recent and fossil lucinid bivalves.She found these methods unable to find well-accepted higher taxaof bivalves, and concluded that ‘‘phylogenetic classifica-tions. . . are more likely than phenetic ones to be productive offuture investigations.’’ Bretsky, like numerous others workers,had found numerical taxonomy to be of extremely limited use in

1123SCHNEIDER—BIVALVE SYSTEMATICS

reconstructing phylogenetic history. One major drawback (amongmany others) of numerical taxonomy was its inability to distin-guish primitive character states from derived character states (Wi-ley, 1981).

Purchon (1956, 1957, 1958, 1960a, 1960b) had publishedgroundbreaking work on the anatomy of the bivalve stomach.Although he did use stomach morphology as a key character com-plex in his taxonomic system (Fig. 2), it is clear that he under-stood the concepts of primitive and derived character states (es-pecially see Purchon, 1960b, p. 69). However, in his later work(Purchon, 1978, 1987, 1990) he used numerical taxonomic meth-ods to reconstruct bivalve phylogeny, and seemingly abandonedany appreciation for the concept of primitive versus derived. OnlyPurchon’s 1978 paper used computer-assisted methods. Not sur-prisingly, Purchon produced some results which have failed tohave been replicated in any other study of bivalve phylogeneticrelationships. Specifically, Purchon (1978, 1987, 1990) removedmajor groups (including Carditacea and Crassatellacea) which hadlong been considered heterodonts (Thiele, 1935; Cox, 1960; New-ell, 1969) and united them with the freshwater mussels (Union-oida) on the basis of stomach morphology. Using a methodologywhich took neither character polarity nor monophyly into account,Purchon mistook a few characters shared between unionoids andcertain veneroids as evidence of close relationship, and did notconsider the possibility that these were either shared primitivecharacters or convergently evolved characters.

The first cladistic analyses of bivalves (Boss, 1978; Waller,1978) were the results of a 1977 symposium on bivalve system-atics. These were apparently the first cladistic analyses of anymolluscan group (but see Bieler, 1992). Boss (1978) used cladistictechniques to propose a hypothesis of phylogenetic relationshipsbetween six families of pandoroid anomalodesmatans. However,Boss’s study lacked explicit character and character state descrip-tions; it is not even entirely clear how many characters were con-sidered in the analysis. Waller (1978), on the other hand, used amuch more explicit set of data (22 characters, 63 states; explicituse of ontogeny and outgroup analysis to polarize characters) toestimate the phylogenetic relationships of Pteriomorphia. Wallerfound that the mytiloids were the most basal pteriormorphians, aresult that would prove crucial to future studies of not only pter-iomorphian relationships, but bivalve phylogenetics as a whole.

Davis and Fuller (1981) presented a cladogram of Recentunionoidean bivalves. This ‘‘cladogram’’ was constructed not onthe basis of discrete characters, but ‘‘on the basis of . . . immu-nological results, morphology, the fossil record, and zoogeogra-phy.’’ Davis (1983, 1984) employed discrete characters as but oneset of data to construct cladograms used to estimate the phylo-genetic relationships of unionids. Regarding molluscs, Davis(1983) was of the opinion that there were ‘‘insufficient detailedanatomical studies to pursue modern cladistic analyses for mostgroups within any superfamily.’’ Even if detailed studies of mol-luscan anatomy were forthcoming, the shell of molluscs would beof little use in cladistic analysis because of rampant convergence(Davis, 1979, 1982), a view echoed by Seilacher (1984) and Mor-ton (1996). Furthermore, Skelton et al. (1990) and Skelton andBenton (1993) suggested that bivalves lacked sufficient numbersof shell characters to make cladistic analyses practicable. The im-plication was that if shells were not useful for reconstructing phy-logenetic history, then the fossil record could certainly not be usedfor such a purpose.

Ironically, the only other bivalve cladistic analysis during thistime was that of an extinct group whose shell characters wereused to illustrate a new cladistic method called transformationseries analysis (Miyazaki and Mickevich, 1982). Not only werethe results in broad agreement with the stratigraphic record, butcases of peramorphosis and paedomorphosis were detected.

Computer-assisted cladistic analyses of gastropods began to bepublished in 1984 (Davis et al., 1984; Houbrick, 1984; Harasew-ych, 1984) and soon became commonplace. A symposium onprosobranch phylogeny was held in 1986 (Ponder, 1988a) whichresulted in several papers using cladistic analysis to produce phy-logenetic hypotheses of various gastropod groups (Ponder, 1988b;Lindberg, 1988; Houbrick, 1988; Bieler, 1988).

So what were bivalve systematists doing during the 1980s, ifnot using the valuable new tool of cladistics to study phylogeneticrelationships? The following two examples may be used for il-lumination. In an attempt to study the evolution of tellinoids, Poh-lo (1982) used a variety of characters to ‘‘search for tellinaceanancestors’’; one of the section headings is ‘‘A Search for Ances-tors.’’ Newell and Boyd (1989) favored the Treatise classificationof pteriomorphians over that presented by Waller (1978). Newelland Boyd criticized Waller’s ‘‘emphasis on differences’’ whereasNewell and Boyd were ‘‘more impressed by mutual resemblanc-es.’’ It is unclear whether Newell and Boyd attempted to discernwhether such ‘‘resemblances’’ were shared derived characters orshared primitive characters, for it would normally take a cladisticanalysis to tell these two types of ‘‘resemblances’’ apart.

This resistance to cladistic analyses of bivalves because of 1)a perceived lack of characters, especially of the shell; and 2) ram-pant convergence of those few characters which may even bediscerned. However, this resistance was unwarranted. Further an-atomical and conchological investigations of bivalves continuedto prove useful for finding more characters. Two such exampleswere sperm morphology (Healy, 1989, 1995, 1996; Healy et al.,2000; Keys and Healy, 2000) and shell microstructure (Carter,1990). As Schneider (1995) pointed out, when cladistics is un-dertaken in concert with rigorous character analysis, cladistics isnot hampered by what a priori has been considered homoplasy,but actually has the ability to discern theretofore unknown orunder-appreciated cases of convergence and parallelism. It isthrough cladistic analysis that we are able to tell homoplasy fromsynapomorphy.

A symposium on marine bivalves held in 1991 resulted in thefirst two computer-assisted cladistic analyses of bivalves (Bielerand Mikkelsen, 1992; Schneider, 1992). Numerous studies of phy-logenetic relationships of bivalves using cladistic methodologyapplied to morphologic characters followed these initial studies(Waller, 1993, 1998; Huelsenbeck, 1994; Schneider, 1995, 1998a,1998b; Schneider and Carter, 2001; Roopnarine, 1996, 2001; Ada-mkewicz and Harasewych, 1996; Salvini-Plawen and Steiner,1996; Taylor and Glover, 1997; Simoes et al., 1997; Cope, 1997;Harte, 1998; Carter et al., 2000; Harper et al., 2000; Skelton andSmith, 2000); all of these studies save those of Waller and Copeused computer-assisted methods. Furthermore, over the past de-cade there has been considerable use of molecular characters toproduce hypotheses of phylogenetic relationships amongst bi-valves (Rice et al., 1993; Kenchington et al., 1994; Adamkewiczand Harasewych, 1994; Canapa et al., 1996, 1999, 2000; Steinerand Muller, 1996; Winnepenninckx et al., 1994, 1996; Adamke-wicz et al., 1997; Peek et al., 1997; Rosenberg et al., 1997; Hoehet al., 1998; Campbell et al., 1998; Frischer et al., 1998; Roe andLydeard, 1998; Schneider and O Foighil, 1999; O Foighil andJozefowicz, 1999; Baco et al., 1999; Campbell, 2000; Cooley andO Foighil, 2000; Graf and O Foighil, 2000; Park and O Foighil,2000; Steiner and Hammer, 2000; Lydeard et al., 2000; Boganand Hoeh, 2000; O Foighil et al., 2001; numerous others).

RESULTS

As a result of this intense study over the past decade, a con-siderable amount of progress has been made regarding the inter-relationships and evolutionary patterns of bivalves. This activityhas been illustrated graphically by comparing the complexity of

1124 JOURNAL OF PALEONTOLOGY, V. 75, NO. 6, 2001

Figure 3 to that of Figures 1 and 2. However, it is granted thatmy choice of taxonomic schemes to illustrate—as well as mydecision to focus on higher-level taxonomy—is indeed subjective.

There is still no consensus regarding the sister group of bi-valves. Using morphologic data, Waller (1998) concluded thatrostroconchs are the bivalve’s sister group, and that stenothecid‘‘monoplacophorans’’ are the sister group to Bivalvia 1 Rostro-conchia. Using a larger and more explicit data set, Carter et al.(2000) found the stenothecids to be paraphyletic, with the rostro-conchs and the bivalves separately derived from within ‘‘Stenoth-ecidae.’’

Molecular analyses of bivalves still prove problemmatic. Mostsuch analyses produce results which indicate that the Bivalvia isnot monophyletic (Kenchington et al., 1994; Steiner and Muller,1996; Winnepenninckx et al., 1996; Rosenberg et al., 1997; Ada-mkewicz et al., 1997; Hoeh et al., 1998; Campbell et al., 1998;Steiner and Hammer, 2000). It is acknowledged by most of theabove authors that sampling within the Bivalvia, as well as abetter sampling of outgroups [none of these analyses have in-cluded a member of the Tryblidiida (5modern ‘‘monoplacophor-ans’’)] will produce results that indicate monophyly of the Biv-alvia.

There is a virtual consensus regarding a fundamental dichoto-my within the Bivalvia, namely, that extant bivalves are repre-sented by the two monophyletic lineages Palaeotaxodonta (5Pro-tobranchia) and Autolamellibranchiata (5Autobranchia). Theonly dissent from this view seems to come from Nevesskaia etal. (1971) and Starobogatov (1992), who consider the carnivorousseptibranch bivalves to be living rostroconchs (and all rostro-conchs therefore to be bivalves), and this clade (Conocardiformiiof Starobogatov, 1992), which also contains the Cambrian fordil-loids, to be the sister taxon to the remaining non-palaeotaxodontbivalves.

Morris and Fortey (1976) suggested that their new genus Ti-ronucula was intermediate between non-taxodont bivalves and thePalaeotaxodonta (taxodonty being a derived condition), and Wal-ler (1990, 1998) considered Tironucula as the sister taxon to theAutobranchia. Cope (1997, 2000) considered the Palaeotaxodontato be a paraphyletic group from within which rose the remainingmembers of Bivalvia. However, in the most detailed phylogeneticanalysis to date of bivalves, Carter et al. (2000) found Tironuculato occupy an intermediate position along the cladogram axis with-in the Palaeotaxodonta. This may be yet another case of a moredetailed character analysis coupled with a cladistic study usingnumerous well-defined characters and outgroups that is able touncover previously unknown instances of homoplasy.

Progress has been made on the early diversification of the Au-tolamellibranchiata. Cox (1960), Newell (1965, 1969), Morris(1978) and Cope (1996, 1997) had traced the ancestry of thepteriomorphians, anomalodesmatans and the heterodonts to agroup of bivalves known as the Modiomorphoida; Newell (1969)placed them in the Palaeoheterodonta in the Treatise even as heacknowledged that they ‘‘lead in various lines to the Pteriomor-phia, Heterodonta, and possibly the Mytiloida. . . ’’ Carter (1990)and Morton (1996) considered the modiomorphoids to be basal(to) mytiloids. Carter et al.’s (2000) cladistic analysis of earlybivalves makes clear that the Modiomorphoida was a paraphyleticgroup which contained early members of the various autolamel-libranchs. Carter et al. found: Cycloconcha and Actinodonta tobelong to a clade within the Heteroconchia that is the sister taxonto the veneroids; Redonia and Nyassa to be heteroconchs; Col-pomya to be a basal pteriomorphian; Matheria and Modiolodonto be basal non-mytiloid pteriomorphians; Modiomorpha itself abasal anomalodesmatan; Anomalodesmata the sister taxon to Pter-iomorphia; and mytiloids to be relatively basal pteriomorphians.

Lucinoids had usually been considered to be basal heterodonts

(Thiele, 1935; Newell, 1969). However, Morris (1978) and Cope(1996) removed the lucinoids from the Heterodonta; Cope (1996)considered the Ordovician lucinoid Babinka to be a palaeoheter-odont. Carter et al. (2000) found that the lucinoids in his analysisdid not belong to the heterodont clade, and in majority-rule con-sensus trees were the sister taxon to palaeoheterodonts. Lucinoidshave only been included in one of the molecular analyses of bi-valve systematics (Steiner and Hammer, 2000) and their positionwas unsettled. That hypothesis that lucinoids are basal palaeohet-erodonts—and that their morphologic similarities to heterodontsare symplesiomorphies—deserves to be further investigated.

Another advance that has been made regarding the palaeohet-erodonts has been the work of biologists. Many workers have longaccepted that the two schizodont groups of bivalves—unionoidansand trigonioidans—form a monophyletic group (Dall, 1895; Thie-le, 1935; Franc, 1960; Newell, 1969; Nevesskaia et al., 1971).However, modern Neotrigonia does not share many anatomicalcharacters with unionoids (Morton, 1987; Salvini-Plawen andSteiner, 1996) and this has led these workers to posit that trigo-nioidans are most closely related to pteriormorphians. Ultrastruc-tural studies of sperm (Healy, 1989) and COI data (Hoeh et al.,1998) indicate that trigonioidans and unionoidans are indeed sistertaxa. Morton (1996) subsequently therefore considered the Pa-laeoheterodonta to be a subclade of Pteriomorphia. Molecularanalyses (Campbell, 2000; Steiner and Hammer, 2000) and mor-phologic cladistic analyses (Carter et al., 2000) have found thePalaeoheterodonta to be either the sister taxon to the Heterodonta,or relatively basal members of the Heterodonta, and not closelyrelated at all to the Pteriomorphia.

Finally, there are the arcoids. Thought to be related to nuculoidprotobranchs because they both possessed taxodont dentition(Dall, 1913; Thiele, 1935), enough anatomical and conchologicaldata (Neumayr, 1884; Pelseneer, 1889; Ridewood, 1903) were dis-seminated that it was eventually accepted that the arcoids werepteriomorphians with taxodont dentition (Franc, 1960; Cox, 1960;Newell, 1965, 1969). Since then, the controversy has been wheth-er arcoids are relatively basal (Cope, 1996; Waller, 1998) or de-rived (Carter et al., 2000) pteriomorphians.

CONCLUSIONS

The publication of the bivalve volumes of the Treatise on In-vertebrate Paleontology in 1969 seemingly did not stimulate sys-tematic research. Instead, that compendium’s publication preceed-ed a period of twenty years of little progress, even as the cladisticand molecular revolutions were going on. It is interesting to notethat gastropod systematists—the bulk of whose taxa were not yetcovered in any Treatise volume—were at the forefront of thecladistic revolution during the 1980s.

The past ten years have seen an exciting reinvigoration of bi-valve systematics, and an end to taxonomic stagnation. Paleon-tologists have played a leading role in this renewal in the studyof bivalve phylogenetics.

ACKNOWLEDGMENTS

I would like to thank the editors of the Journal of Paleontologyfor inviting me to write this paper. The manuscript was improvedby comments from J. G. Carter.

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