MY, LOOK HOW YOU'VE GROWN: BRACHIOPOD SIZE TRENDS IN THE EARLY PALEOZOIC

Global marine diversity increased notably through the Lower Paleozoic in spite of a temporary drop in diversity during the Ordovician mass extinction. Possible consequences of the increase in biomass are the passive increase in maximum body size and overall size range in marine organisms. Conversely, when diversity is reduced, size range and maximum size would decrease. Here we investigate body size trends of Early Paleozoic brachiopods and compare these trends to the concurrent changes in marine diversity. Brachiopod size was estimated by measuring the length and width of pedicle valves for all specimens included on photographs published in taxonomic monographs. Resulting size estimates were recorded along with stratigraphic, lithologic, and geographic data. Both, calcitic (rhynchonelliform) and phosphatic (linguliform) brachiopods were included in this study to assess if these two higher taxa, with different shell mineralogy and body plan, differ in body size trends.

Preliminary data, acquired from 12 taxonomic monographs, include 208 specimens in 67 linguliform genera and 421 specimens in 77 rhynchonelliform genera ranging in age from the Early Cambrian to the Early Silurian. Despite the fact that the data include a wide variety of unrelated taxa, length and width correlate tightly suggesting that bivariate estimates may be a reliable and consistent measure of size in brachiopods. There is a remarkably tight correspondence of body trends between the two brachiopod taxa. The similarity is surprising considering the major mineralogical and anatomical differences of the two brachiopod subphyla. Comparison of rhynchonelliform brachiopods grouped by geological periods indicate that "allometric" growth coefficients remain stable through time, (k=0.86 and r^2=0.92 for Ordovician, k=0.82 and r^2=0.90 for Silurian). In both rhyncholliforms and linguiliforms mean and maximum size increased in the Early Paleozoic, in concert with the concurrent diversity increase. However, when diversity was reduced temporarily during the Ordovician mass extinction, brachiopod size was not affected, suggesting size trends of brachiopods are not just a passive consequence of diversity changes.