A preliminary phylogeny for rudist bivalves: sifting clades from grades
Peter W. Skelton, Andrew B. Smith, 2000. "A preliminary phylogeny for rudist bivalves: sifting clades from grades", The Evolutionary Biology of the Bivalvia, E. M. Harper, J. D. Taylor, J. A. Crame
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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.
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The Evolutionary Biology of the Bivalvia
Bivalves are key components of Recent marine and freshwater ecosystems and have been so for most of the Phanerozoic. Their rich and long fossil record, combined with their abundance and diversity in modern seas, has made bivalves the ideal subject of palaeobiological and evolutionary studies. Despite this, however, topics such as the early evolution of the class, relationships between various taxa and the life habits of some key extinct forms have remained remarkably unclear.
In the last few years there has been enormous expansion in the range of techniques available to both palaeontologists and zoologists and key discoveries of new faunas which shed new light on the evolutionary biology of this important class.
This volume integrates palaeontological and zoological approaches and sheds new light on the course of bivalve evolution. This series of 32 original papers tackles key issues including: up to date molecular phylogenies of major groups; new hard and soft tissue morphological cladistic analyses; reassessments of the early Palaeozoic radiation; important new observations on form and functional morphology; analyses of biogeography and biodiversity; novel (palaeo)ecological studies