temnospondyl – Nix Illustration

Platycepsion

Platycepsion wilksoni was a temnospondyl amphibian that lived during the early-to-mid Triassic (~251-242 million years ago) in what is now New South Wales, Australia.

A single partial skeleton discovered in the 1880s is the only known record of this species, and represents a juvenile that would have been around 15-20cm long (6-8″). We don’t know exactly what it would have looked like as an adult, but it was probably quite similar to other closely-related members of the brachyopid family – mostly-aquatic salamander-like animals with short but wide toothy jaws, eyes set towards the front of the head, small limbs, and paddle-like tails.

A recent re-analysis of the Platycepsion specimen found evidence of soft-tissue preservation of external gills, showing that it wasn’t just a juvenile but a true larva, a sort of temnospondyl “tadpole”.

Distinct larval stages have been found in a few other types of temnospondyls, but this is the first definite example from the stereospondyls, a major Mesozoic lineage that survived all the way into the Early Cretaceous.

Temnospondyl Toes

The evolutionary origins of modern amphibians are still a bit murky, but one of the most likely possibilities is that they evolved from a group of temnospondyls known as amphibamiformes. (Or, at least, that frogs-and-salamanders evolved from them. Caecilians might be a different type of temnospondyl.)

And a recent discovery adds a little bit more evidence to that hypothesis.

A new specimen from the 309-million-year-old Late Carboniferous Mazon Creek fossil deposits in Illinois, USA, shows some soft-tissue impressions around the body of a terrestrial amphibamiform* — most notably showing its toes, with chunky rounded fleshy pads at the end like those seen in many modern amphibians.

Fossil trackways already suggested that some terrestrial temnospondyls had chunky toes, but up until now all known soft-tissue impressions only showed the slender tapering toes of aquatic forms. This is the first direct fossil evidence of toe pads, and hints that a lot of modern amphibians’ soft-tissue features may have actually had a very ancient origin.

(*A more precise identification couldn’t be made, but it shows some similarities to both Doleserpeton and Pasawioops.)

Archegosaurus

There’s something fishy about Archegosaurus decheni.

Living in the Czech Republic and Germany during the Early Permian, about 299-295 million years ago, this temnospondyl amphibian was a tropical freshwater predator occupying a similar ecological niche to modern crocodilians.

Hundreds of fossils have been found of this species, from 15cm long larvae (6″) all the way up to 1.5m long adults (5′), so we’ve got a very good idea of its life history and anatomy. Larvae had external gills and shorter blunter skulls, and as they matured they developed internal gills and lungs, and their snouts elongated into more crocodile-like shapes. Every life stage was fully aquatic, with very limited ability to venture onto land, and gut contents show their favored prey was Acanthodes fish.

But despite how much Archegosaurus looked like a salamander-croc, a detailed study of its physiology has estimated that its metabolism and body functions were actually much more similar to those of air-breathing fish like bichirs and lungfish than any modern amphibian.

This suggests that its whole evolutionary lineage had retained a lot of physiological traits from their earlier fish-like tetrapod ancestors, and many other early aquatic temnospondyls may also have been much less amphibian-like than we usually think of them.

(And since one hypothesis places modern caecilians as the descendants of this fishy lineage of amphibians, they may even still have living representatives around today!)

Sclerothorax

Sclerothorax hypselonotus was a temnospondyl amphibian that lived in Germany during the Early Triassic, around 251-247 million years ago.

Measuring about 1.2m long (3′11″), it had some unusual features for a temnospondyl – a very rectangular skull with a wide blunt snout, and elongated spines on its vertebrae that gave its body a sort of “hump-backed” shape.

It was part of a lineage of temnospondyls called capitosaurs, which mostly occupied the same sort of aquatic predator niche as modern crocodiles – but unlike its close relatives Sclerothorax’s well-developed spine and limbs suggest it spent much more time walking around on land.

(And while there was another temnospondyl known to have similar extended vertebrae – the sail-backed Platyhystrix – the two weren’t actually closely related to each other.)

Eons Roundup

This year I’ve been lucky enough to have some of my work featured in several PBS Eons videos – and I even recently got the opportunity to do some custom images for them! Since I didn’t show any of these off at the time, here they are now:

The basal temnospondyl amphibian Iberospondylus, from “When Giant Amphibians Reigned”
https://www.youtube.com/watch?v=rGthtRZl8B0

The flying paleognath bird Lithornis, from “When Birds Stopped Flying”
https://www.youtube.com/watch?v=M3h05ajJw0o

The ground sloth Nematherium, from “How Sloths Went From the Seas to the Trees”
https://www.youtube.com/watch?v=pt9tBtQoAHo

Happy new year, everybody!

Chinlestegophis

The newly-named Chinlestegophis jenkinsi, in the style of last year’s Amphibian August illustrations.

Living during the Late Triassic of Colorado, USA, (~220 mya), this 30cm long (1′) amphibian had a skull showing a mixture of features shared with both temnospondyls and modern caecilians – providing a vital “missing link” in their evolutionary history. Previously the oldest known caecilian-relative was the Jurassic-aged Eocaecilia, which already had much more modified anatomy making it harder to definitively link to other known groups.

Chinlestegophis seems to have been part of the stereospondyl branch of the temnospondyls – and an unexpected side effect of adding caecilians into this group is that many temnospondyls could now potentially also be classified as true members of Lissamphibia.

An example family tree showing this new version of amphibian relationships. If we define Lissamphibia as including all living amphibians and their extinct relatives back to their last shared common ancestor, then everything within the grey box is a lissamphibian!

Of course, this is still just one hypothesis of amphibian evolution among several other competing ideas. Maybe it’s right, maybe it isn’t – as always, we need more fossil evidence! – but it’s certainly an interesting and surprising new development in the ongoing saga of “what are lissamphibians? we just don’t know”.