How do you weigh an extinct amphibian?

Estimating the mass of a long-extinct animal is a challenging endeavour – and there is often no “one best method” of doing so. But Australian Museum, UNSW and UNE scientists have just helped us answer the question, in a newly published case study: how do you weigh an extinct amphibian?

Understanding the weight, or body mass, of living animals is vital, as an animal’s weight influences how much it eats, how it moves, its metabolism and its ability to regulate its body temperature. So, it is of great interest to scientists to know the mass of animals. In most living animals, finding out their mass is easy – you just need to put them on a scale.

But for extinct animals, it’s a little harder. We can’t exactly put the fossil of a T. rex on a scale and presume that is the correct weight of the animal in life. So, palaeontologists have developed a range of methods to estimate the mass of extinct animals.

Why do we need a range of methods? Wouldn’t one “best” method be enough? Well, the answer to that is in the fossils. Fossil skeletons of animals are often incompletely preserved. This means that they are missing bones, or even whole regions of the body. There are effective methods of body mass estimation that create 3D shapes out of the skeletons of the animals – but you really do need the entire skeleton to do this.

As such, scientists have developed a swathe of other methods that use dimensions of parts of the body, such as the width of the skull, the circumference of the legs, or the head to tail length. These methods have, in the past, been applied to a range of extinct animals, from dinosaurs to dodos, but there has been one major group that has been left out: temnospondyls.

Temnospondyls are a weird group of often very large amphibians, which look a bit like a cross between a crocodile and a salamander. They have a fossil record which spans about 210 million years – which is longer than all non-avian dinosaurs.

In a new study, scientists from the Australian Museum, University of New South Wales and the University of New England have applied a total of 19 different body mass estimation techniques to two temnospondyls; a lumbering land-walker called Eryops and a swimming croc-like fish eater called Paracylotosaurus. They have also applied the techniques to five living animals which either live or look like them, such as crocodiles and salamanders. They found that 5 of these 19 methods were accurate in estimating the mass of the living animals (whose weight we already know), which tells us that these same five methods would be appropriate for use in temnospondyls.

Why is this important? Across their 210-million-year history on Earth, temnospondyls survived two of Earth’s “Big 5” mass extinction events (at the ends of the Permian and Triassic periods). We are currently living in the midst of the sixth. Most of these mass extinctions feature climate change to some degree. Temnospondyls provide an opportunity to analyse how the body mass of animals change through time, especially on either side of these catastrophic events, and through this we can see how animals physically react to climate change. By knowing this, perhaps we can help slow down the rate at which our living species are going extinct, or at the very least learn to read the warning signs, so they don’t end up like dinosaurs, dodos or temnospondyls!

Lachlan Hart, PhD Candidate, UNSW & Technical Officer (Palaeontology), Australian Museum.