How do you weigh an extinct amphibian?

Lachlan Hart

PhD Candidate, University of NSW

Supervisor: Dr Matthew McCurry (AM, UNSW)

Body mass is strongly associated with physiology, ecology, and locomotory style in extant taxa, making it a key proxy for inferring the biology of extinct species. A variety of extinct animals have been subject to studies of body mass estimation, including dinosaurs and mammals. Temnospondyls, a group of amphibious tetrapods from the Carboniferous–Mesozoic, have not been included in most studies of body mass estimation. This is because as there are no direct descendants of temnospondyls to serve as a direct modern analogue, and temnospondyl fossils are often incompletely preserved. Established body mass estimation methods near-complete skeletons. These challenges mean that many current body mass estimation methods are nearly impossible to apply across the range of known temnospondyl fossils. Two exceptions to these preservational issues are Eryops megacephalus from the Permian of North America and Paracyclotosaurus davidi from the Triassic of Australia, known from complete, 3-dimensionally preserved skeletons. Accordingly, we apply a wide range of established body mass estimation methods to these animals.

To evaluate the accuracy of these methods, the masses of five extant analogues were estimated with the same methods, including giant salamanders and the saltwater crocodile. We find that five of the 19 tested methods can provide accurate mass estimations across this range of living taxa, suggesting their suitability for estimating the body masses of temnospondyls. Based on these results, we propose that Eryops weighed between 102 and 222 kg, and Paracyclotosaurus between 159 and 365 kg. We propose that our work will provide a basis for accurately estimating the body mass of temnospondyls and present an opportunity to analyse evolutionary patterns in body mass over time.