Diabloroter

The exact evolutionary relationships between the earliest amphibians and amniotes is rather murky, and the recently-discovered Diabloroter bolti here is a member of a group in the middle of this uncertain classification.

It was part of a lineage known as the recumbirostrans – small burrowing aquatic salamander-like creatures, many of which had elongated bodies and short tails. Although traditionally considered to be lepospondyl “amphibians”, more recent studies have suggested that these animals might instead have been very early true amniotes related to early reptiles.

Measuring only about 6cm long (2.4″), Diabloroter is known from a single fossil  from Illinois, USA, dating to the Late Carboniferous about 309-307 million years ago. Its anatomy indicates it was probably a herbivore – making it one of the earliest known plant-eating tetrapods – with teeth adapted for scraping at algae-covered surfaces and a rather rotund body that would have housed a large gut region.

It also had fairly well-developed limbs, which were probably used for burrowing like many of its close recumbirostran relatives, but may also suggest it spent a lot of time walking around on land.

Westlothiana

Westlothiana lizziae from the Early Carboniferous of Scotland (~338 mya).

About 20cm in length (8″), this superficially lizard-like creature (nicknamed “Lizzie” by its discoverer) had a long slender body with relatively small legs, which may have been adaptations for burrowing similar to modern skinks.

Its anatomy shows a mixture of both amphibian and reptilian characteristics, suggesting it may have been a close relative of the first true amniotes. But exactly where it fits in that area of the evolutionary tree is still uncertain, with different paleontologists classifying it as either an early amniote, a reptilomorph, or a lepospondyl.