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.)

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”.