Biofluorescense is the term for when living organisms “glow” under certain types of light. Although it’s not usually directly visible to human eyes, wavelengths such as ultraviolet can reveal it – and we’re still only just starting to discover how widespread it really is in nature.
This phenomenon has been found in all major groups of modern amphibians, with most of them glowing green under UV, suggesting that it originated in their ancestors at least 300 million years ago.
Living during the late Triassic (~220 million years ago), fossils of Funcusvermis were found in what is now Arizona, USA. It’s only known from fragmentary remains, but those pieces are distinctive enough to identify it as the earliest known relative of modern caecilians.
It had a caecilian-like jaw with two rows of teeth, but unlike its worm-like modern relatives it still had small legs and wasn’t as highly specialized for burrowing. The shape of its vertebrae suggest it had a tubular body, and while its exact proportions and full length are unknown it may have been comparable in size to the smallest modern caecilians, around 10cm long (~4″).
Its combination of anatomical features gives further support to the idea that all modern amphibians share a common ancestor among the dissorophoid temnospondyls. The more distantly related but also caecilian-like Chinlestegophis may be a case of convergent evolution, representing a separate branch of temnospondyls that were coincidentally exploring a similar sort of lifestyle at around the same time.
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!