SIZING UP LEVIATHAN: A STUDY ON ISOMETRIC AND ALLOMETRIC SCALING IN THE VERTEBRAL COLUMNS OF PHYTOSAURS AND CROCODYLIANS

Phytosaurs are extinct reptiles commonly found as fossils in Upper Triassic deposits. Their resemblance to extant crocodylians has led to the common perception of them as ecological equivalents. We demonstrated previously that vertebral morphology strongly differs between phytosaurs and crocodylians. In particular, the centrum in phytosaurs is taller relative to width than in crocodylians, though it is similar to non-gharial crocodylians in length relative to width. In terrestrial mammals, as body size increases the height of the centrum scales with positive allometry, adding greater dorsoventral rigidity to the column. This supports the body against increased gravitational loads. This study tested the hypothesis that the difference in centrum dimensions between phytosaurs and crocodylians is a function of increased body size in phytosaurs. Our results suggest that centrum height scales with isometry against centrum width in phytosaurs and crocodylians. Both measures scale with positive allometry relative to centrum length, consistent with what has been observed in terrestrial mammals. Thus, phytosaurs likely needed to traverse the terrestrial realm throughout their lives, like crocodylians. Despite having similar scaling patterns, phytosaur and crocodylian vertebrae have differences that are irrespective of absolute body size. At comparable sizes, phytosaurs had narrower, taller centra than crocodylians. Also, centrum length in phytosaurs was less than crocodylids and gavialids in the cervical regional and less than alligatorids and crocodylids in the dorsal region. This suggests that phytosaur vertebral morphology is determined more by conformity to underlying structural patterns than by convergence with crocodylians. It may also suggest that phytosaurs and crocodylians display alternate solutions to the problem of overcoming the gravitational load on the vertebral column. In future studies, we would like to compare phytosaur vertebrae with those of other non-archosaurian archosauromorphs and non-eusuchian crocodyliforms to determine whether the construction of the phytosaur bracing system corresponds to the former pattern or to the latter, which presents an alternative solution to the mechanical challenges associated with life out of the water from that of extant crocodylians.