Drivers of the Body Size Distributions in Phanerozoic Tetrapods

Body size has long been recognized as an important aspect of animal biology1, which influences not only an individual's physiology and energy requirements2, but also broader trends of global species distribution3-5, extinction risk3, and evolution5-8. One of the most important ways body size has been assessed at these broader spatial scales is with the body size frequency distribution (BSFD)4: a histogram of the number of species binned by body size (commonly body mass or body volume) for a given clade.

Understanding what controls this distribution has been of particular interest for modern vertebrates at the continental scale, with early investigations identifying a common, unimodal and right-skewed (very few large species) BSFD shape for both North American mammals and birds, postulated to result from reproductive energetics2,9 (see fig. 1a). More recently, examination of the BSFD for fossil mammals has found that the apparent unimodality of this modern North American data was spurious, resulting from a loss of megafaunal taxa at the end of the Pleistocene10-11, and that there is no decisive evidence for a single driver of the distribution's shape.

Instead, the "true" Cenozoic mammal BSFD is multi-modal, with individual peaks driven by taxonomic membership or ecology10,12 (e.g. locomotor style; see fig. 1b). A similar multi-modal pattern has also been recognized in a BSFD for non-avian dinosaurs, suggesting the pattern may be more widespread throughout Tetrapoda (though interestingly of opposite skew-very few small species present)13, but little work has gone into explaining what controls the modes or the shape of that distribution, or indeed into the BSFD of other Mesozoic tetrapod groups

This project will expand the study of the geometry and controls of the BSFD into a wider array of fossil tetrapod clades. Given the massive scope of Tetrapoda, study will focus on several well-studied reptilian tetrapod groups with good fossil records, namely non-avian dinosaurs, as well as other Mesozoic clades such as plesiosaurs, ichthyosaurs, and pseudosuchians, and potentially Permian/Triassic synapsids.