Ecology and evolution of Jurassic trigoniid bivalves in Europe

Abstract

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 con®ned 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¯anks was probably an adaptation to facilitate burrowing, but in the absence of useful external ornament other adaptations to increase burrowing ef®ciency may have been realized by the development of an elongate shape. Because their occurrence in Jurassic strata is only intermittent, con®dent 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.

Ontogeny and autecology of an Early Cretaceous trigoniide bivalve from Neuquén Basin, Argentina

“…5F 2 , I) together with the environmental characterization from the sedimentological analysis (i.e., a low energy offshore transition environment; Echevarría 2012) suggests that the posterior margin was leveled at the water-sediment interface. A comparable life position was found on M. lusitanica (Sharpe, 1850), although in this species there is evidence of exposure of the posterior end of the shell on some specimens (Fürsich 1980); a similar situation was presumed for other Jurassic species of Myophorella (Francis and Hallam 2003) and for the related genus Steinmanella Crickmay, 1930a(Villamil et al 1998Lazo 2003). Although some exposure of the posterior end on M. garatei cannot be ruled out, there is no evidence supporting it either, and it must be stressed that all the mentioned taxa are larger (in some cases much larger) than the species analyzed here.…”

A conch with a collar: early ontogeny of the enigmatic fossil bivalveMyoconcha

“…5F 2 , I) together with the environmental characterization from the sedimentological analysis (i.e., a low energy offshore transition environment; Echevarría 2012) suggests that the posterior margin was leveled at the water-sediment interface. A comparable life position was found on M. lusitanica (Sharpe, 1850), although in this species there is evidence of exposure of the posterior end of the shell on some specimens (Fürsich 1980); a similar situation was presumed for other Jurassic species of Myophorella (Francis and Hallam 2003) and for the related genus Steinmanella Crickmay, 1930a(Villamil et al 1998Lazo 2003). Although some exposure of the posterior end on M. garatei cannot be ruled out, there is no evidence supporting it either, and it must be stressed that all the mentioned taxa are larger (in some cases much larger) than the species analyzed here.…”

“…Stanley (1977) interpreted the presence of knobs on some trigoniide shells as an adaptation to burrowing in cohesive sediments, which would pack against a row of knobs that would thus act as a rib. In contrast, Francis and Hallam (2003) interpreted the tubercles of many Myophorella species as an adaptation offering a great surface in which to grip the mud, and helping in this way to stabilize the shell. The presence of smooth wide costae on the flanks on M. garatei may be interpreted in the same way, providing a more developed surface for cohesive sediments to pack against and stabilizing the shell in its life position.…”

“…2, 5, 9;Leanza 1993: pl. 2: 17;Francis and Hallam 2003;Pérez et al 2008: pls. 11, 12) is probably similar to the one described for M. garatei, at least for the general shell shape on lateral view.…”

“…Much work has been done on the systematics of the group either on a regional scale (Hoepen 1929;Kobayashi 1954;Poulton 1979;Kelly 1984;Fleming 1987;Leanza 1993;Pérez et al 2008) or on a global one (Crickmay 1932;Cox 1952), including some papers trying to establish the phylogenetic relationships among genera (Saveliev 1958;Newell and Boyd 1975;Cooper 1991;Boyd and Newell 1997). All genera have been usually interpreted as shallow burrowers (Newell and Boyd 1975), with shell morphology and ornamentation strongly correlated to their life habits and habitat (Stanley 1977; see also Francis and Hallam 2003 for a more recent review on the topic). The schizodont hinge is one of the most distinctive features of trigoniide shells, and it was interpreted as a coadaptation to the evolution of a strong muscular "T" shaped foot (Stanley 1977(Stanley , 1978.…”