Palaeoecological Aspects of Cambrian Bivalves: Conclusions from Perigondwanan Occurrences  

The palaeoecology and palaeobiology of Cambrian bivalves (=pelecypoda) is controversial due to limitations in critical taxonomic characters and poor preservation.  Here we report Perigondwanan specimens from Germany, Turkey, and Morocco.  Of the several hundred fossils, most are represented by steinkerns of articulated Pojetaia and Fordilla preserved in late-Early Cambrian to early-Middle Cambrian limestones.  They occur in two different facies realms. 

The bivalve-bearing sediments of Germany (Charlottenhof Fm.) and Turkey (Çal Tepe Fm.) are bioclastic wackestones and floatstones of a shallow subtidal, open-marine environment within a transgressive system.  The fauna is autochthonous to par-autochthonous and mainly represented by echinoderms and chancelloriids, but trilobites, hyoliths, other molluscs, poriferans, and “small shelly fossils” (hyolithelminths, cambroclaves, Rhombocorniculum, Halkieria, etc.) also occur.  In both regions, the bivalve level is immediately below nodular limestones which are interpreted as the initial drowning of the ramp or platform.  For the relatively small palaeoecological window of the bivalves, a quiet and muddy, shallow carbonate environment of rather low latitude palaeogeographic regions with somewhat reduced carbonate precipitation rate is envisaged.  In contrast, no bivalves occur in typical oligotrophic environments which are characterized by calcimicrobial carpets and archaeocyathan reef mounds. 

Moroccan bivalves come from allochthonous limestone hash layers (Jbel Wawrmast Fm.) of a storm-influenced, high-energy intertidal to very shallow subtidal environment (nearshore to shoreface) of a transgressive regime.  Along with these bivalves, reworked remains of trilobites, brachiopods, echinoderms, and various “small shelly fossils” occur.  We reinterpret the habitat of the redeposited fauna as more distal (shallow, low energy platform, occasionally agitated by storms) than the definite depositional area.  This palaeoecological window corresponds to that of the bivalve-specimens from Germany and Turkey, and supports the hypothesis that Perigondwanan bivalves are very sensitive to palaeoecological changes during early stages of transgression. 

Several arguments point to an infaunal mode of life of Pojetaia and Fordilla: (1) laterally compressed shape, (2) general articulated preservation, (3) shell symmetry (very well-developed anterior end), and (4) lack of microbial overgrowth.  The feeding strategy of the early bivalves is hard to estimate.  There is no indication of a pallial sinus or a siphon as in modern species, leading to the assumption that Pojetaia and Fordilla were deposit feeders using their ciliated body and mantle surface to collect food particles.  We assume that their early ontogenetic stage was lecitotrophic, with a relatively short planktonic larval phase.  We take this to imply that the Cambrian palaeocontinents were more closely connected than commonly assumed.  Along with poor preservation, and the absence of critical characters typical of phylogenetically younger bivalves, this makes it generally difficult to distinguish between species and variants in Cambrian taxa. 

Because of their paleoecological sensitivity and the persistent taxonomic uncertainties, Cambrian bivalves should be used only carefully as biostratigraphic and paleogeographic tools.