BRACHIOPOD BODY SIZE THROUGH THE PHANEROZOIC: MASS EXTINCTIONS PUNCTUATE LONG-TERM TRENDS AND ALTER EVOLUTIONARY MODE

Body size is an important organismal trait that relates to almost every ecological, physiological, and life history process and can vary predictably with environmental conditions. For most fossil organisms, size is more reliably preserved than ecology, physiology, or life history and is easier to measure than environmental parameters. Biological expectations for size change over multiple scales of space and time and provide a framework for evaluating otherwise unknowable traits of extinct animals.

Brachiopods are one of the most diverse, abundant, and long-ranging clades in the marine fossil record. However, most brachiopod species are extinct, limiting our ability to understand the amount of ecological, physiological, and ontogenetic variation within the phylum. We can begin to address these limitations by quantifying body size, a trait that is often as well-preserved as brachiopod shells are themselves.

In this study, we use the R package paleoTS to fit Phanerozoic trends in brachiopod body size (measured as biovolume from a previously published dataset) to common evolutionary models. Trait mean and variance are the parameters used; through model-fitting, we ask how do they vary over time? To do this, genus biovolume mean and variance were calculated within 5 Myr bins over the stratigraphic range of each brachiopod order and superfamily. Change in biovolume mean for each taxon was fit to biased random walk, unbiased random walk, stasis, Ornstein-Uhlenbeck, and punctuated models. Punctuated models included those where an abrupt change in mean coincided with a shift between an unbiased or biased random walk and stasis.

Akaike weights indicate that for most orders (81%), support is greatest for models that include a punctuation and a shift in evolutionary model. Models with a punctuation and shift are also best supported among superfamilies (40%), although an unbiased random walk is a close second (35%). Interestingly, support for a punctuated model is strongly correlated with stratigraphic range length, and punctuations tend to occur at mass extinction boundaries. Comparing patterns in body size evolution with occurrence data in a taxonomic and phylogenetic context will clarify how extinctions and recoveries impacted brachiopod diversity and evolutionary dynamics.