MICROBIAL INFLUENCE AND ENVIRONMENTAL CONVERGENCE IN MARINE (PROTEROZOIC) AND LACUSTRINE (JURASSIC) DEPOSITIONAL SETTINGS

Jurassic lake sediments of the Hartford Basin, Massachusetts, have produced microbial structures in lacustrine fine clastics that resemble similar structures observed in Proterozoic marine strata. The microbial surfaces in both cases appear to result from sediment stabilization by biomats. Both cases are associated with treptichnid trace fossils. Proterozoic treptichnids have been implicated in the destruction of microbial surfaces and the initiation of the Cambrian marine substrate revolution. The Jurassic lacustrine strata therefore provide an interesting potential analog for marine events immediately prior to the Cambrian transition.

Fossil biomats from fine clastics in Massachusetts compare closely to similar structures from Pteridinium-bearing strata of Namibia. Hartford Basin sites have produced finely pustular lacustrine mat surfaces, some of which are associated with distinctive trace fossils. A newly excavated dinosaur track site in Holyoke has also yielded superb specimens of Treptichnus bifurcus, the first report of this ichnotaxon from the Hartford basin. Putative feather impressions associated with dinosaur tracks held at Amherst College are better interpreted (P. Olsen) as drag folds associated with scraping across a cohesive microbial surface.

Co-occurrence of treptichnids and Proterozoic-style microbial structures in Jurassic lacustrine sediments of the Hartford Basin may be more than mere coincidence. Particular environmental settings will evoke distinctive depositional environments, and these depositional environments will in turn be characterized by a somewhat predictable biota. In the same way that convergent evolution can influence the morphology of organisms, environmental convergences can influence the evolution of ecosystems. Hence, a late Proterozoic-style ecosystem in a Mesozoic lake (biomats in a siliclastic setting) can evoke similarities in the ichnofauna (non-marine treptichnids). Such environmental convergences are in accord with the concept that “certain states of the global biosphere-climate system may be more-or-less inevitable” (M. A. S. McMenamin, 2004, Climate, paleoecology and abrupt change during the late Proterozoic, pp. 226 in G. S. Jenkins et al., eds., The Extreme Proterozoic, American Geophysical Union Monograph Series 146).