BODY SIZE VARIATION IN EARLY PALEOZOIC BIVALVES AND BRACHIOPODS: ASSESSING ENVIRONMENTAL EFFECTS

Analyses of body size in Early Paleozoic brachiopods and bivalves show that bivalves attained a greater size than brachiopods early in their history. The larger mean size of bivalves mirrors size relationships observed for these groups in modern environments, despite brachiopod ecological dominance in many shallow shelf settings during the Early Paleozoic. These consistent taxonomic differences suggest that factors extrinsic to the organisms, such as the physical environments in which they lived, may have had little influence on the patterns observed.

To conduct an initial, direct assessment of environmental influences on body size in bivalves and brachiopods, Paleozoic occurrences were parsed into two broad environmental categories—siliciclastic or carbonate—based on the lithology in which each specimen was collected. Body sizes of brachiopods and bivalves from the Ordovician and Silurian were determined based on linear shell dimensions measured from photographs in taxonomic monographs. This initial analysis was conducted at the family level, and was limited to families in which sample sizes were deemed adequate (n>20) and siliciclastic and carbonate representation was nearly equal. Each family was analyzed separately in a 5000 iteration randomization procedure to determine whether mean sizes of representatives from the two settings were significantly different.

Four of the seven brachiopod families analyzed had significantly larger mean sizes in siliciclastic environments (p<<0.005). Two of the six bivalve families analyzed were significantly larger in one setting than they were in the other, (p<< 0.005): one was larger in carbonates and the other in siliciclastics. Thus, six of thirteen families analyzed showed significant size differences between the two sedimentary regimes. These data therefore suggest that body sizes among Early Paleozoic taxa may, in at least some instances, have been affected significantly by environmental factors. Work that is ongoing will augment the database substantially, thereby permitting a definitive assessment at the genus level of any such tendencies among brachiopods or bivalves.