Ecology and evolution of Jurassic trigoniid bivalves in Europe

A modern palaeobiological approach to the taxonomy, making full allowance for a wide range of variation, has allowed the distinction of 22 Jurassic trigoniid species in Europe, grouped into 7 genera. Most species were confined to shallow marine habitats ranging up to a few tens of metres in depth but were adapted to low energy environments as well as environments of moderate to high energy. The presence of external ornament on the shell flanks was probably an adaptation to facilitate burrowing, but in the absence of useful external ornament other adaptations to increase burrowing efficiency may have been realized by the development of an elongate shape. Because their occurrence in Jurassic strata is only intermittent, confident inferences on evolutionary patterns are limited to the two commonest genera, Trigonia and Myophorella. Phyletic size increase has been recognized in the latter but not the former genus. The calculated species longevities correspond fairly closely with those established for Jurassic bivalve species in general. The mode of speciation is dominantly punctuational, but only one likely example of species splitting is recognized.