THE END-PERMIAN MASS EXTINCTION AND RELATIVE SHIFTS IN BODY SIZE BETWEEN HERBIVOROUS AND CARNIVOROUS SYNAPSIDS

In modern mammalian terrestrial ecosystems, herbivores tend to be larger than carnivores. We explore the history of the relationships between diet and body size by analyzing trends in body size and diet for Permian and Triassic Synapsida, the dominant terrestrial vertebrates during that time. We were interested in the role of the Permian-Triassic (P-T) Mass Extinction and recovery in changing the relationships between body size and diet.

To assess the relationship between body size and diet across the P-T interval, we compiled a dataset of 267 synapsid genera. For each genus we used skull length as a proxy for body size and coded their diet as herbivore or carnivore. These data were compiled from the literature and times of origination and extinction were downloaded from the Paleobiology Database. We computed mean body size over time for each diet type and used logistic regression to measure the selectivity of origination and extinction as related to body size and diet.

Our results show that prior to the P-T extinction, herbivorous and carnivorous body sizes mirrored each other in trajectory and magnitude. At the P-T boundary, there was a sharp decline in the average size of carnivores, while the average herbivore size increased. During the Triassic recovery, both experienced increases in body size—with carnivores consistently smaller than herbivores by a factor of about 1.8. This sustained separation of herbivores and carnivores in mean body size is reminiscent of modern mammalian ecosystems. The logistic regressions show that during the P-T interval there were no strong origination or extinction selectivity patterns for diet. There was a shift in body size selectivity from extinction of larger genera in the Early Permian to smaller genera by the end of the period; this trend was largely reversed during the Triassic. Strong origination selectivity for body size was only present in the Early Triassic recovery—newly evolved genera originate at a smaller size.

This work exhibits the ability of mass extinctions to disrupt size distributions of herbivores and carnivores, which impacts ecosystem function. The lack of extinction selectivity during the P-T extinction followed by origination selectivity highlights the importance of recovery intervals in determining how ecological structure rebounds after mass extinction events.