376 FOSSIL INVERTEBRATES/Bivalves Taxonomy and Biological Relationships There are around 100 families of living and a further 150 families of extinct bivalves Traditionally, these are split into six subclasses which have been defined on the basis of a mosaic of characters, few of which are diagnostic in their own right Table gives the names and basic attributes of each of these subclasses, providing examples of each Most modern bivalves belong to one of two subclasses: the infaunal Heterodonta and the largely epifaunal Pteriomorphia Despite the excellence of the bivalve fossil record, the wealth of Holocene material on which to base studies, and the general familiarity of the class, our understanding of the relationships between higher taxa is extremely unclear, and the monophyly of particular subclasses has been called into question Even the relationship between the Bivalvia and the other molluscan classes is debatable The principal difficulties stem from the high degree of convergent and parallel evolution Some instances of convergence are clear; for example, the multiple evolution of byssate attachment has led to the heteromyarian ‘mussel’ form being adopted in unrelated taxa, such as the marine mytilids (e.g., Mytilus) and the freshwater driessenids In these instances, close study of other anatomical characters clearly demonstrates that overall morphological similarity is superficial The problem is more acute where convergence has affected much smaller groups, for example where the cemented habit has evolved repeatedly (five times) within the single family Pectinidae Most attempts to classify the bivalves have used either a single or very few characters Zoologists and palaeontologists have invented different schemes because, not unnaturally, they have placed different importances on anatomical (e.g., gill or stomach architecture) and shell characters, such as hinge structure and shell microstructure, respectively These are all subject to convergence Modern computer-driven cladistic analyses which utilize large numbers of different characters offer a better hope of establishing phylogenetic relationships and of identifying convergent characters Most informative of all are the new battery of molecular techniques; in particular, studies of the 18S rRNA gene have proved to be illuminating Superimposing morphological data onto molecular phylogenies provides perhaps the ultimate means of establishing convergent characters However, both the use of data sets combining shell and anatomical characters and the use of molecular methodologies are clearly of little help when dealing with extinct taxa The fossil record has a critical role Table Six subclasses of bivalves Subclass Examples Characteristics Palaeotaxodonta Nucula Cryptodonta Solemya, Cardiola Pteriomorphia Mytilus, Modiolus, Lithophaga, Coralliodomus, Pinna, Ostrea, Gryphaea, Pecten, Spondylus, Isognomon, Imoceramus, Arca Palaeoheterodonta Modiolopsis, Unio, Trigonia Heterodonta Lucina, Mya, Corbula, Venus, Mercenaria, Ensis, Wholly aragonitic shells of nacre and prisms Taxodont dentition Equivalve Deposit feeders with small protobranch gills All infaunal Wholly aragonitic shells of nacre and prisms Toothless Infaunal Protobranch gills, specialized to house symbiotic bacteria Active burrowers All include some aragonite within the shell (nacre, crossed lamellar microstructures), but many also have calcitic layers (prisms, foliae) A range of different hinge architectures and musculature (iso and heteromyarian) Eulamellibranch and filibranch gills Many inequivalve Pteriomorphs are primitively byssate, but have repeatedly evolved cementing, boring, and free living habits Wholly aragonitic nacre and prisms Equivalve and a small number of heterodont teeth Burrowers Includes freshwater forms Wholly aragonitic, mainly crossed lamellar and complex crossed lamellar microstructures Mostly large heterodont teeth Eulamellibranch gills Equivalve and inequivalve forms Majority are shallow and deep burrowers, but the extinct rudists were either cementing or free living Wholly aragonitic (largely prisms and nacre) Largely toothless Mainly isomyarian musculatures Variety of morphologies reflecting very diverse life habits (burrowing, cementing, byssate) extinct rudists Anomalodesmata Cuspidaria, Poromya