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1 Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK P.W.Skelton{at}open.ac.uk
2 Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK
The taxonomic history of the late Jurassic-Cretaceous rudist bivalves (Superfamily Hippuritoidea) is reviewed and a new phylogeny based on cladistic analysis of 32 skeletal characters is proposed. Informative characters include: relative thickness and structures of the outer (calcitic) shell layer; valve asymmetry and attachment to the substrate by either the left or right valve; form of the ligament; and, in the aragonitic inner shell, the dentition and shelly supports (myophores) for the adductor muscles, as well as accessory cavities and blind-ending (pallial) canals. The myocardinal complex (teeth and myophores) is especially important for discriminating clades previously lumped in paraphyletic or even polyphyletic taxa (e.g. Caprinidae, sensu lato, in the bivalve Treatise (Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology, Part N. Mollusca 6, Bivalvia, Geological Society of America and University of Kansas).
As outgroup, a megalodontid bivalve was used, as this shares three derived traits (massive dentition, modified parivincular ligament and posterior myophores) in common with rudists. The clade of all rudists is united by possession of an outer shell layer of fibrillar prismatic calcite (albeit greatly reduced in a few taxa). Two subclades are distinguished according to the attachment of the shell — either by the left valve (all ‘diceratids’, except Diceras and Valletia, plus the monophyletic requieniids), or by the right valve (Diceras, Valletia and all other rudists). Monophyly of some previously established families is substantiated. These include Caprinidae, sensu stricto (Skelton, P. W. 1978. Philosophical Transactions of the Royal Society of London, Series B, 284, 305–318), Radiolitidae (with Agriopleura as sister taxon) and Hippuritidae (with Tepeyacia emerging as a possible sister taxon), while others are resolved into distinct clades. There is some evidence for the Polyconitidae Mac Gillavry (1937), though with negligible bootstrap support.
