Scientists estimate the weight of two giant extinct amphibians

The last of the temnospondyls – amphibians that look more like crocodiles – became extinct during the Cretaceous period, about 120 million years ago, after thriving on Earth for more than 200 million years.

Now a team of scientists led by Lachlan Hart, a palaeontologist and PhD candidate in the School of Biological, Earth & Environmental Sciences at UNSW Sydney, has assessed various methods of estimating the weight of these unique extinct animals. The team’s study is published in Palaeontology.

“Estimating mass in extinct animals presents a challenge, because we can’t just weigh them like we could with a living thing,” said Mr Hart. “We only have the fossils to tell us what an animal looked like, so we often need to look at living animals to get an idea about soft tissues, such as fat and skin.”

Temnospondyls as case studies

Mr Hart said temnospondyls were “very strange animals”.

“Some grew to enormous sizes, six or seven metres long. They went through a larval (tadpole) stage just like living amphibians. Some had very broad and round heads – such as Australia’s Koolasuchus, recently named as the Victorian State Fossil Emblem – and others, like the temnospondyls we used in this study, had heads that were more croc-like.”

3D models of the animals featured in the study. Image by Lachlan Hart.

The 1.8 metre-long Eryops megacephalus lived during the Permian period in what is now the USA, while the slightly longer Paracyclotosaurus davidi is known from the Triassic of Australia. The more aquatically inclined Paracyclotosaurus was the heftier of the two, tipping the scales at roughly 260 kilograms, where Eryops was a more modest 160 kilograms.

“The size of an animal is important for many aspects of their life,” said Mr Hart. “It impacts what they feed on, how they move and even how they handle cold temperatures. So naturally, palaeontologists are interested in calculating the body mass of extinct creatures so we can learn more about how they lived.

“There have been several studies on body mass estimation in other groups of extinct animals, such as dinosaurs, but not extensively on temnospondyls.

“They survived two of Earth’s Big Five mass extinction events which makes them a very interesting case study on how animals adapted following these global catastrophes,” Mr Hart said.

Because temnospondyls have no direct living relatives, the team of scientists had to assemble a selection of five modern ‘analogues’ (such as the Chinese Giant Salamander and the Saltwater Crocodile) to test a total of 19 different body mass estimation techniques to determine their suitability for use in temnospondyls.

“We found several methods which gave us consistently accurate body mass estimations in our five living animals, which included using mathematical equations and 3-dimensional digital models of the animals,” said Dr Nicolas Campione from the University of New England, Armidale, an authority on body mass estimation who was also involved in the study. “We hypothesised that as these methods are accurate for animals which lived and looked like temnospondyls, they would also be appropriate for use with temnospondyls.”

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Dr Matthew McCurry, Senior Lecturer in Earth Science at UNSW, and co-author on the study said, “This work has shown there are multiple methods for estimating mass in temnospondyls.

“We don’t need the whole skeleton to do this, as some methods involve using the width of the skull or the circumference of the legs. The work will be useful for palaeontologists because many fossils we find are only of one or two parts of the skeleton.”