Meeting Abstract
Extant crocodylians are characterized by slow growth and low metabolic rates, historically considered the plesiomorphic condition for Archosauria. Recent histological studies have established that some stem archosaurs had bone tissue more similar to that of birds and mammals than to living crocodylians (i.e., osteohistological correlates of faster growth and higher metabolism), which strongly suggests that the slow growth physiology of living crocodylians is secondarily derived. The origin of this physiological slow-down among croc-line archosaurs is unknown. In order to determine the point of origin of crocodylian metabolism, I histologically sampled representative taxa from several extinct lineages outside Crocodylia (phytosaurs, aetosaurs, shuvosaurids, rauisuchians, and early crocodylomorphs). I included individuals from several localities when possible to test how differences in paleoenvironment influenced interpretations of growth rate. I analyzed mid-diaphyseal femoral microstructure for characters known to vary with growth rate in living tetrapods (density and organization of blood vessels, bone cells, and collagen fibers; number of annual growth lines; annual bone deposition rate). In nearly every case, taxa varied in their histology and inferred capacity for faster growth among localities. Clades hypothesized to grow slowly based on 1-2 individuals were actually capable of much faster growth (e.g., bone deposition rates in some phytosaurs were ~10 times faster than extant Alligator). My results establish that the ancestral archosaurian condition of faster growth was still present in early crocodylomorphs. Because individual specimens or localities cannot capture the true range of histological or growth variation, greater caution should be taken when inferring the capacity for fast growth in fossil animals, and in reconstructing the evolutionary history of growth rates.
