And a recent discovery adds a little bit more evidence to that hypothesis.
A new specimen from the 309-million-year-old Late CarboniferousMazon 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.)
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.
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.)
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: