Ever since the earliest tetrapods crawled onto land and developed limbs and digits, some lineages have just… decided the whole “legs” thing was overrated and lost them entirely.
And the earliest known group to do this were the aïstopods. These highly elongated amphibian-like animals had specialized lightly-built skulls with large jaw muscles, and they may have filled a similar ecological niche to modern snakes, hunting small terrestrial invertebrates.
Lethiscus stocki was one of the first members of this snake-like group, living in Scotland during the Early Carboniferous about 340 million years ago. Growing to at least 50cm long (~20″), it was already a very specialized animal despite its basal position among the aïstopods, with eyes set far forward on its face and no trace of vestigial limbs.
CT scans of its skull have shown some surprisingly fish-like anatomy, especially in its braincase, features that were lost very early in tetrapod evolution. This suggests that aïstopods weren’t part of the lepospondyl amphibians like previously thought, but actually originated much much earlier in the tetrapod evolutionary tree – potentially placing them somewhere among the “fishapods” between Ichthyostega and Crassigyrinus.
Living during the Late Carboniferous in Nova Scotia, Canada, this 60-70cm long (2′-2’4″) distant cousin to modern mammals was previously known only from the fossilized remains of juveniles – with all known specimens showing slightly elongated spines on their vertebrae that gave it a sort of high-backed “proto-sail” appearance.
A single vertebrae identified as belonging to Echinerpeton shows a much much longer spine than anything we’ve ever seen before, and confirms that this species actually had a large elaborate true sailback – making it the earliest known tetrapod to experiment with this type of anatomy.
This individual seems to have been older than the other known specimens, but still not fully grown, leaving the possibility that fully mature Echinerpeton may have had even larger sails than this.
And a recent discovery adds a little bit more evidence to that hypothesis.
A new specimen from the 309-million-year-old Late CarboniferousMazon Creek fossil deposits in Illinois, USA, shows some soft-tissue impressions around the body of a terrestrial amphibamiform* — most notably showing its toes, with chunky rounded fleshy pads at the end like those seen in many modern amphibians.
Fossil trackways already suggested that some terrestrial temnospondyls had chunky toes, but up until now all known soft-tissue impressions only showed the slender tapering toes of aquatic forms. This is the first direct fossil evidence of toe pads, and hints that a lot of modern amphibians’ soft-tissue features may have actually had a very ancient origin.
(*A more precise identification couldn’t be made, but it shows some similarities to both Doleserpeton and Pasawioops.)
Sometimes the really weird thing about a head isn’t any sort of ridiculous ornamentation.
Sometimes it’s just the wrong size.
That’s what was going on with Cotylorhynchus romeri from the early Permian of North America, living about 280-272 million years ago. Despite looking like a big fat lizard this creature was actually a very early synapsid, closer related to modern mammals than to reptiles, and it was a distant cousin of other stem-mammals like the famous Dimetrodon.
Around 3.5m long (11’6″), it was one of the largest herbivores of the early Permian, with a very wide barrel-shaped body, chunky limbs, and a comically small head. Such a tiny head isn’t necessarily unique – another synapsid Edaphosaurus also had a fairly small skull compared to its body, and dinosaurs like stegosaurs, sauropods, and moa had heads even more disproportional. But something about Cotylorhynchus in particular just looks… incredibly odd.
It also had some surprisingly sizeable nostril openings in that little skull, and it had may have had a very good sense of smell or perhaps some sort of specialized breathing system like the modern saiga’s “air conditioning” nose.
Although usually depicted as a fully terrestrial animal, the structure of Cotylorhynchus‘ bones and its flattened paddle-like hands and feet have recently been used to suggest that it may have been semi-aquatic, more of a Permian hippo than a cow. But such a lifestyle would have required it to have a much more efficient method of breathing than previously thought – suggesting it had a mammal-like diaphragm, and possibly also explaining that weird nose.
Hundreds of fossils have been found of this species, from 15cm long larvae (6″) all the way up to 1.5m long adults (5′), so we’ve got a very good idea of its life history and anatomy. Larvae had external gills and shorter blunter skulls, and as they matured they developed internal gills and lungs, and their snouts elongated into more crocodile-like shapes. Every life stage was fully aquatic, with very limited ability to venture onto land, and gut contents show their favored prey was Acanthodes fish.
But despite how much Archegosaurus looked like a salamander-croc, a detailed study of its physiology has estimated that its metabolism and body functions were actually much more similar to those of air-breathing fish like bichirs and lungfish than any modern amphibian.
Known from several near-complete fossils that include rare soft tissue impressions, it’s the first varanopid to show preserved skin details – revealing a pattern of very lizard-like rectangular scales. If it is a synapsid this is a pretty big deal, since early synapsids were previously thought to have had scale-less leathery skin.
It also had unusual mosaic-like patches of tiny osteoderms above its eyes, a feature previously known only in some temnospondyl amphibians. Whether this was the result of convergent evolution or the trait actually being ancestral to most tetrapods is unclear.
Its slender body, long digits, and highly curved claws indicate it was an agile climber. It probably mainly lived up in the treetops, feeding on insects, making it one of the earliest known tetrapods specialized for an arboreal lifestyle.
(*Maybe. There’s apparently an upcoming study that suggests varanopids might actually be sauropsids instead.)
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.