Changing community dynamics during the Mesozoic Marine Revolution (MMR)

Roxanne M.W. Banker and Peter D. Roopnarine, Department of Invertebrate Zoology and Geology, California Academy of Sciences, San Francisco, CA, Ashley A. Dineen, University of California Museum of Paleontology, Berkeley, CA, Carrie L. Tyler, Geology and Environmental Earth Science, Miami University, Oxford, OH

Background/Question/Methods
The Mesozoic Marine Revolution (MMR), which occurred from 251 million years ago (mya) to 65 mya, represented a dramatic increase in the proportion of active predators and diversity of anti-predation strategies (e.g., shell sculpture, infaunality). These patterns of escalating functional diversity and energy use by marine taxa have been attributed to both bottom-up enabling factors (i.e., enhanced primary production) and top-down agents of selection (i.e., predation). Although scientists have developed a strong understanding of ecospace expansion and functional diversity patterns during this time, how precisely these changes affected community structure and dynamics remains uncharacterized. This project aims to test whether observed escalatory trends correspond with changes in community structure, trophic organization, and energetics (i.e., body size) in Mesozoic marine ecosystems from the Tethys Sea. Here we use fossil data from the Paleobiology Database (PBDB), supplemented with material from museum collections and published literature, to reconstruct food web networks of paleocommunities and model community structure and dynamics. Fossil taxa were grouped based on common ecological traits (e.g., life mode and diet) into functional groups (e.g., Mobile Benthic Carnivores), which were further subdivided on the basis of body size bins into guilds. We compare distributions of mean guild network trophic position (NTP) and mean body size amongst guilds that were present in paleocommunities from the Anisian, Carnian, and Bathonian Stages of the Mesozoic.
Results/Conclusions
We demonstrate that during the time between the Anisian and Carnian, selection occurred against high NTP guilds that were not replaced in the Carnian. In contrast, new guilds that appeared during the Bathonian had significantly higher mean NTPs than guilds that persisted from the Carnian. Interestingly, guilds that were new in the Bathonian only had significantly higher mean NTPs if new herbivores (NTP ≈ 2) were excluded. This indicates that there was a notable expansion of herbivores as well as higher level consumers. Analyses of mean body size of guilds yielded similar patterns. For example, from the Anisian to Carnian, guilds that persisted had larger mean body size than guilds that were new to the Carnian. Guilds that went extinct during this time also had higher mean body size than new guilds. Though neither of these comparisons were statistically significant. These results illustrate how community level dynamics developed during the MMR, and deepen our understanding of community evolution over geological timescales.