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.
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.)
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.
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”.
2023 Update: …But the discovery of Funcusvermis suggests Chinlestegophis isn’t a caecilian at all, but instead an unrelated case of convergent evolution!