Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 Unravelling the evolutionary biology of the Bivalvia: a multidisciplinary approach E. M. HARPER l, J. D. TAYLOR 2 & J.A. CRAME 3 1 Department of Earth Sciences, Downing Street, Cambridge CB2 3EQ, UK (e-mail: emh21 @cus.cam.ac.uk) 2 Department of Zoology, The Natural History Museum, London SW7 5BD, UK British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, UK Bivalves have been important members of marine taxonomic diversification of the bivalves (Pojeta communities since the early Palaeozoic, in terms of 1978) and the rostroconchs (Runnegar 1978) are both their numerical abundance and diversity. They still widely cited. However, in 1977 the Treatise are particularly prevalent in shallow shelf volumes (Cox et al. 1969; Stenzel 1971) were still sediments, but they have also conquered the very much in vogue as a reliable data source, intertidal zone as well as the deep sea, where they although even then there was a feeling that it was in are successful predators and key components of need of a comprehensive revision (Yonge 1978). some vent communities. They have also invaded This sentiment has been echoed ever since, most freshwater systems a number of times, where today strongly by Johnston & Haggart (1998) in their they are important (and costly) foulers. In terms of introduction to Bivalves: An Eon of Evolution. general community structure, bivalves are Paleobiological Studies Honoring Norman D. important as prey items for a range of different Newell. The Royal Society volume was also written predatory groups, and as major space occupiers, at a time when cladistic studies were virtually particularly on hard substrata where space may be unknown and there was not the wealth of molecular limited. techniques at hand to tackle questions of The abundance and diversity of both Recent and phylogeny. The enormous changes which have fossil bivalves have made them attractive subjects taken place in the way in which evolutionary for both zoologists and palaeontologists, and both processes are thought about has changed during the disciplines have contributed to the present system last 20 years, as have the number of taxa, both of classification and the understanding of their Recent and fossil, that have been discovered during phylogenetic relationships. However, somewhat this period, both of which have inspired this inevitably, the focus of the two groups has been volume of multidisciplinary papers from both rather different, with zoologists concentrating on palaeontologists and zoologists. anatomical characters such as those associated with the gills and stomach, whilst palaeontologists have Bivalve classification and phylogeny necessarily dwelt on hard-part characters such as dentition and shell microstructure. It is becoming Despite the long scientific interest in bivalves, the increasingly clear, however, that convergence and relationships between the higher taxa are perhaps parallelism is rife within the class and more surprisingly unresolved. Certainly the Treatise integrated approaches are necessary to unravel classification is now dated, but even more recent these. schemes are often deeply conflicting, probably The last major attempt to integrate the palaeo- because of their reliance on single or small groups ntological and zoological approaches to bivalve of characters (Starabogatov 1992; Morton 1996; evolution was more than 20 years ago, at the Royal Salvini-Plawen & Steiner 1996; Waller 1998; Society of London meeting in 1977. The resulting Amler 1999). The relatively recent boom in volume of the Philosophical Transactions of the molecular techniques is of great potential here and Royal Society of London, Volume 284, is still a two papers in this volume have used them to try to benchmark volume. In particular, an early attempt elucidate the relationships between higher taxa. at a cladistic analysis of the Pteriomorphia by Steiner & Hammer have used an analysis of 18S Waller (1978) and papers on the origin and rDNA sequences in order to tackle the phylogenetic From: HARPER,E. M., TAYLOR,J. D. & CRAME,J. A. (eds) The Evolutionao' Biology of the Bivalvia. Geological Society, London, Special Publications, 177, 1-9. 1-86239-076-2/00/$15.00 9The Geological Society of London 2000. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 2 E.M. HARPER ET AL. relationships of the pteriomorphs, both in terms of character refinement may reduce the numbers of the major pteriomorph taxa (such as the ostreoids, possible models. The paper is accompanied by full pinnoids and pterioids, etc.), and between the character descriptions and the data matrix which Pteriomorphia as a whole and the other major will allow such future re-analysis. Despite the large bivalve clades (e.g. protobranchs, heteroconchs, number of trees produced, the consensus trees do anomalodesmatans). Their researches supports the introduce a number of interesting conclusions, monophyly of the Protobranchia, Heteroconchia some of which support the conclusion of previous and Pteriomorphia, and float the idea that the studies, such as Waller (1998), on the basal position Heteroconchia and Pteriomorphia may be sister of the Mytiloidea amongst the pteriomorphs, and taxa. Within the pteriomorphs, there is strong the basal dichotomy within the palaeotaxodonts support for two major clades, ([Pinnoidea between the Nuculanoidea and the Nuculoidea + (Ostreoidea + Pterioidea)] and [(Anomioidea + Solemyoida. They also support the view of shared Plicatuloidea) + (Limioidea + Pectinoidea)]. ancestry of the Nuculoidea and the palaeo- Further molecular studies are presented by heterodonts. In other areas, however, the authors Campbell, also using the 18S gene, who come to rather different conclusions to those investigated the relationships between a wide range reached by others. The most striking of these is the of bivalve superfamilies, orders and subclasses. rejection of the popular notion that the Bivalvia Gratifyingly, he found all superfamilies, which were derived from the rostroconchs, preferring have been established on morphological grounds, instead a stenothecoid monoplacophoran origin. appeared monophyletic, as were most orders. Only Elsewhere they are at odds with Waller (1998), the Myoida appear polyphyletic, thus confirming rejecting a link between the Anomalodesmata and the suspicions of some previous workers (e.g. the Heteroconchs, preferring to place the former as Morris et al. 1991). a sister group to the pteriomorphs. Again, there are Palaeontologists face the fascinating but difficult striking anomalies between the results of this challenge of trying to test various models of bivalve analysis and those of molecular studies. Cope has phylogeny with the known fossil record. At face used a more traditional approach, based on the value the task should be relatively simple as the culmination of detailed research of a range of early bivalves have, by virtue of their robust calcareous Ordovician bivalve faunas. He bases his scheme on valves, an excellent fossil record, but this has not dentition, shell microstructure and inferred gill proved to be the case. It is particularly difficult grade. Cope identifies the evolution of the because following the good fossil record of the filibranch gill, which he infers from dental changes, earliest widely accepted bivalves (Fordilla and as providing the initial impetus for the Pojetaia), which are relatively abundant from the diversification of the bivalves in either the latest Tommotian of North America, Europe and Cambrian or earliest Ordovician. He recognizes Australasia, there is an unfortunate gap in the fossil two subclasses; (1) the Protobranchia- including record of the class that spans from the middle the Nuculoidea and chemosymbiotic forms such as Middle Cambrian-Early Ordovician, some 4% of the solemyoids; (2) the Autolamellibranchiata - the entire evolutionary history of the class (Harper from which he derives three principal stocks, the 1998). Unhappily, this gap in the fossil record Trigonioida, the Anomalodesmata and the appears to cover a period of major evolutionary Heteroconchia, which he believes then gave rise to changes within the class, for on their reappearance the Pteriomorphia. the bivalves are larger and more diverse both Two other papers in the volume deal with the taxonomically and in life habits. None the less, new phylogenetic relationships of enigmatic higher discoveries in the last decade have improved bivalve taxa. In a major new synthesis, Skelton & current knowledge of early Palaeozoic bivalve Smith present a first attempt at a comprehensive faunas. These findings have inspired two papers in phylogeny for the rudists, an extinct group of this volume which tackle the problem in two rather sessile, epifaunal bivalves that flourished in warm different ways. temperate and tropical seas from the Late Jurassic Carter et al. have undertaken an ambitious to the Late Cretaceous. Their cladistic study was cladistic analysis of 62 bivalve taxa of Palaeozoic based on 33 skeletal characters and 31 species, age, including a number of equivocal forms such as representing each of the higher taxa recognized in Tuarangia, using 117 characters derived almost previous classification schemes. One of the ten exclusively from the hard-parts. The large number most parsimonious trees obtained from their study of characters (and states) and taxa has limited this was selected as a working hypothesis. This showed study to a