Size: all it's shaped up to be? Evolution of shape through the lifespan of the Cenozoic bivalve Spissatella (Crassatellidae)
James S. Crampton, Phillip A. Maxwell, 2000. "Size: all it's shaped up to be? Evolution of shape through the lifespan of the Cenozoic bivalve Spissatella (Crassatellidae)", The Evolutionary Biology of the Bivalvia, E. M. Harper, J. D. Taylor, J. A. Crame
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Outline shape is a key morphological character of infaunal bivalves that reflects both phylogenetic history and function. A new method of Fourier shape analysis has been used here to study the evolution of shape and size in the Cenozoic genus Spissatella (Crassatellidae), using material from New Zealand. The Fourier method can be used to construct a map of morphological ‘geography’, to generate synthetic average or extreme morphologies, and to visualize marginal growth fields and allometries. The results demonstrate that growth in Spissatella was strongly allometric and that differences between individual growth stages are commonly far greater than evolutionary changes in shape spanning 20 Ma. In all taxa, allometry was dominated by the relative elongation of the posterior margin with growth, an inferred functional adaptation for life in relatively high-energy environments. Heterochrony was the dominant evolutionary mechanism and, following an initial peramorphic expansion into morphospace, both paedomorphosis and peramorphosis probably occurred in approximately equal proportions. In general, it is not possible to decouple patterns of shape and size evolution. The data reveal no directional trends in evolution of these traits. The ontogenetic pathway followed by Spissatella may be a key genus-level taxonomic character and apparently represents a developmental constraint that largely controlled evolution within the genus.
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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