Keraterpeton galvani here was part of a group of amphibian-like early tetrapods called lepospondyls.
Living in what is now southern Ireland during the Late Carboniferous, about 318-314 million years ago, this 30-40cm long (~1′-1’4″) fully aquatic animal was the earliest known member of the diplocaulid lineage (although its skull was much less elaborately modified than its famous boomerang-headed relative Diplocaulus).
It had a broad short-snouted head with eyes set far forward, and a pair of backwards-pointing bony “horns” at the back of its skull. Its forelimbs were smaller than its hindlimbs, and unlike most other diplocaulids it had five fingers on its hands instead of four.
Its vertically flattened paddle-like tail was also around twice as long as the rest of its body, and was probably its main source of propulsion in the water.
Keraterpeton seems to have been quite numerous in the coal swamps it inhabited, representing the most common species preserved in the Irish Jarrow Assemblage site – a location where fossil specimens were uniquely “cooked” and partially replaced with coal during the fossilization process.
With its bizarre boomerang-shaped skull, Diplocaulus is probably the most iconic ancient amphibian. (It even inspired the design of the pokémon Dragapult!) It was part of the lepospondyls, a diverse collection of early tetrapods mainly found in Europe and North America between the Early Carboniferous and the mid-Permian, about 350-270 million years ago.
But one species hung on a bit longer into the late Permian, about 259-254 million years ago, and this late-surviving lepospondyl was perhaps the oddest of them all.
Diplocaulus minimus was the only lepospondyl known from the supercontinent of Gondwana, found in what is now Morocco in northern Africa. About 70cm long, around half of which was its long tail, it had the distinctive elongated skull of a diplocaulid – but in a bizarrely asymmetrical shape.
The left prong of its skull was long and tapering, but the right was shorter and more rounded. This doesn’t seem to have been due to individual deformity or distortion of the fossil material, since more than one skull has been found with the same features, but the reason for such a striking amount of asymmetry in this species is unknown.
Diplocaulids’ head shapes are thought to have acted as hydrofoils, providing lift while they were swimming. Perhaps Diplocaulus minimus‘ much more wonky skull means this species wasn’t relying on that hydrodynamic function as much as its relatives, and something else was going on with its ecology.
…Although, that weird head does bear a surprising resemblance to a proposed asymmetric “flying wing” aircraft design from the 1950s, so it might have worked better for underwater flight than it seems at first glance.
Named after a legendary Scandinavian serpent, Joermungandr bolti here was a recumbisrostran “microsaur” – part of a group of animals that were traditionally considered to be lepospondyl amphibians, but more recently have been proposed to in fact be a lineage of early reptiles.
Discovered in the Mazon Creek fossil beds in Illinois, USA, this species dates to the late Carboniferous period around 310 million years ago. A single near-complete specimen about 5cm long (~2″) preserves impressions of the body outline and numerous tiny scales, giving us a pretty good idea of what it looked like in life.
Joermungandr had a long streamlined tubular body with small limbs and a short tapering tail, and a stubby snout with fused bones heavily reinforcing its skull. Along with microscopic ridges on its body scales that resemble the dirt-repelling scales of some modern reptiles, this combination of features suggest it was a headfirst burrower that wriggled its way through soil with snakelike motions.
The lysorophians were a group of Carboniferous and Permian tetrapods with highly elongated bodies and tiny limbs, usually thought to be part of the lepospondyl amphibians – but some recent studies have instead placed them and some of their close relatives as possibly being very early members of the reptile branch of amniotes.
Brachydectes newberryi here is one of the best-known lysorophians, represented by a good amount of fossil material compared to many of its relatives. Living in the Midwestern United States during the late Carboniferous and early Permian, around 310-290 million years ago, it had a proportionally tiny head and reached lengths of around 60-70cm (2′-2’4″).
Its wide shovel-shaped snout and thickened reinforced bones around its braincase suggest it was adapted for headfirst digging, and some specimens have actually been found preserved inside their burrows. The roof of its skull also developed extensive “sculpturing” as individuals aged, with juveniles having smooth bone surfaces and larger adults having a distinct rough bumpy texture.
So I’ve depicted it here with a speculative keratinous “head shield”.
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 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.