DIFFERENCES IN SIZE OF EARLY PALEOZOIC BIVALVES AND BRACHIOPODS: THE INFLUENCE OF INTRINSIC AND EXTRINSIC FACTORS ON BODY SIZE EVOLUTION

Body size is commonly recognized as one of the most important biological characteristics of organisms. Trends in body size can offer key insights into evolutionary and long-term ecological processes, yet temporal patterns in size of organisms through the Phanerozoic remain largely unexplored. This preliminary study focuses on a comparative analysis of body size in Early Paleozoic brachiopods and bivalve mollusks.

Body size of mature specimens was estimated with linear shell dimensions measured using photographs in taxonomic monographs. The data compiled so far are focused primarily in the Ordovician period (>1000 specimens; ~150 genera of brachiopods and bivalves).

Results indicate that, in the Ordovician, the maximum linear dimension of an average bivalve (mean=1.8cm) was more than two times that of an average brachiopod (mean=0.8cm). This pattern is statistically significant (p<<0.05) and persists when data are analyzed separately for the Early, Middle, and Late Ordovician and when calcitic and phosphatic brachiopods are analyzed separately.

The results mirror present day patterns in body size for these groups. On average, present-day brachiopods, often confined to cryptic, high-latitude, and low-nutrient habitats, tend to be smaller than present-day bivalves. This parallel pattern is somewhat surprising given that, unlike today, Early Paleozoic brachiopods were not confined to low-nutrient settings and, in fact, were predominant compared to bivalves in terms of niche occupation, diversity, and overall abundance in many environments. Thus, for at least these two groups, extrinsic environmental factors such as nutrient supply may not be as important in dictating typical body size as intrinsic physiological characteristics, such as metabolic rates. This study also suggests that the average body size of a higher taxon need not be strongly correlated with factors that affect evolutionary and ecological success.