BRACHIOPOD AND BIVALVE SIZE DURING THE ORDOVICIAN: INTERPRETING GENERAL TRENDS

The Ordovician is one of most dynamic periods in earth history, characterized by a rapid diversification (a three fold increase in the number of marine families) and a major marine mass extinction. Yet, little is known about what effect these evolutionary transitions had on global biomass or the sizes of individual organisms. This study is part of an ongoing effort to investigate possible correlations between body size and the Ordovician diversification of bivalves and brachiopods.

To estimate body size of mature fossil invertebrate specimens (>1200 specimens; 121 brachiopod and 81 bivalve genera) we used linear shell dimensions measured from photographs in taxonomic monographs. Similar dimensions were used for both taxa despite obvious differences in the relationship between shell size and body mass in these groups. As a result, their size trends must be evaluated and interpreted separately.

Overall, the median bivalve length exhibits an increasing trend from the Arenig (2.40 mm) through the Caradoc (3.95mm). However, size fluctuates greatly at stage-level resolution, especially in the Late Ordovician. This volatility may be attributed to the presence of large Pterioida and Actinodontoida that exhibit a large size range and are highly variable in size from site to site. These groups tend to obscure patterns that are present in smaller-sized clades. For example, a slight increase in body size is observed in Nuculoida when that group is treated alone. This suggests that rapid temporal fluctuations in body size observed in bivalves may be driven by changes in dominant bivalve groups rather than concurrent responses of all bivalve faunas to some extrinsic factors.

Brachiopods are characterized by smaller median sizes than bivalves in every stage- despite their ecological dominance in some environments at this time. All brachiopod genera that were measured declined continuously in size, with only minimal fluctuations, from the Early to Middle Ordovician. Orthids and Strophomenids exhibit an abrupt size increase in the Soudleyan stage (Late Ordovician), but this is not attributable to the introduction of new large-sized genera. This suggests the influence of environmental or other extrinsic factors.