amniote – Nix Illustration

Anthracodromeus

Anthracodromeus longipes was an early reptile* that lived in what is now Ohio, USA, during the late Carbonifeorus about 307-305 million years ago.

(*or possibly a very reptile-like stem–amniote)

Around 20cm in total length (~8″), it had a rather lizard-like shape with a long body and a short tail. The digits on all four of its limbs were highly elongated with hooked claws, which appears to have been an adaptation for climbing.

It inhabited a coal forest dominated by lycopsid trees and seed ferns, and as one of the earliest known tetrapods to develop climbing behavior it would have had some ecological advantages over its relatives, being better able to escape from predators and access new food sources.

Araeoscelis

Araeoscelis gracilis was a superficially lizard-like animal that lived during the mid-Permian, around 275 million years ago, in what is now Texas, USA. About 60cm long (~2′), it had a slender body, proportionally long legs, and a solidly-built skull with strong teeth, suggesting that it was a fast runner that specialized in cracking open the carapaces of thick-shelled prey.

It was one of the last known members of a lineage known as araeoscelidians, which are usually considered to be very early members of the diapsid reptiles – but a recent study has proposed they might have even more ancient roots than that, possibly being a branch of stem–amniotes instead.

Cabarzia

Cabarzia trostheidei here lived during the early Permian in what is now Germany, about 295 million years ago.

Despite its very lizard-like appearance it was actually part of the varanopid lineage, a group of scaly amniotes traditionally classified as early synapsids (distant relatives of modern mammals), but which more recently have been proposed to instead be sauropsid reptiles closer related to early diapsids.

It was around 50cm long (1’8″), and its short arms, long legs, slender body, and long tail suggest it was capable of shifting into a bipedal posture when running at high speeds, similarly to some modern lizards – probably mainly to escape from larger predators, but possibly also used to pursue fast-moving prey like flying insects.

And whether varanopids were actually synapsids or sauropsids, this makes Cabarzia the earliest known example of an animal running on two legs.

Gordodon

Some of the earliest large terrestrial herbivores on Earth were the edaphosaurids – a very early-branching group of synapsids, the evolutionary lineage whose only modern surviving members are mammals. Like their more famous cousin Dimetrodon these animals sported huge elaborate sails on their backs formed from highly elongated vertebral spines, but despite the similarity in appearance they actually seem to have evolved these structures completely independently.

Known from a single partial skeleton discovered in southern New Mexico, USA, the edaphosaurid Gordodon kraineri dates to around the very end of the Carboniferous or the very earliest Permian, about 299 million years ago.

It was fairly small for an edaphosaurid at about 1.5m long (~5′), and seems to have had transitional anatomy between earlier and later members of the group. Its sail spines were thicker than those of earlier species but still less heavyset than those of later forms, and while each spine had numerous side projections these structures were small, thorn-like, and randomly distributed, unlike the more organized thick crossbars seen in Edaphosaurus.

Its head was proportionally small compared to its body, but still relatively large for an edaphosaurid, and it had an unusually long neck for an early synapsid. But its most distinctive features were its jaws and teeth – it had a narrow snout with a pair of large incisor-like teeth at the front of both its upper and lower jaws, followed by a large toothless gap (a diastema) and then a short row of small peg-like teeth. Like Edaphosaurus it also would have had batteries of grinding tooth plates inside its upper and lower jaws, but probably not as extensively.

Overall its tooth arrangment looked more like a modern herbivorous mammal than an early synapsid, much more highly specialized than anything else known to be alive at the time – the next synapsid known to convergently evolve similar teeth lived around 90 million years later!

It probably had a very different diet to its relatives, with its specialized teeth and fairly slender body suggesting it may have been a selective feeder, cropping the softer more nutritious parts of plants like the fleshy seeds and cones of gymnosperm plants.

Its discovery also hints that herbivorous edaphosaurids in general were much more diverse than we previously thought, and there may be even more surprising forms out there still to be discovered.

Diasparactus

A group of early tetrapods known as diadectids were some of the first land animals to become specialized herbivores, developing adaptations for the bulk processing of tough fibrous vegetation. They had powerful jaws, chisel-like front teeth, and grinding cheek teeth, and they grew to relatively large sizes for their time with bulky bodies supporting voluminous plant-fermenting guts.

Although usually considered to be reptilomorphs – “amphibian-grade” animals more closely related to amniotes than to modern amphibians – some studies have instead placed these early plant-eaters as being true amniotes related to the synapsids. Fossil trackways show they may have had amniote-like claws on their feet, and that their highly flexible lizard-like ankle joints allowed them to walk much more efficiently than other early tetrapods, possibly using a semi-upright gait, but these may be convergently evolved features. Since we don’t know whether they laid amniote-like eggs or if they instead spawned amphibian-style in water, it’s currently hard to tell for certain just what they really were.

Diasparactus zenos (sometimes alternately known as Diadectes zenos) was a diadectid that lived during the early Permian in New Mexico, USA, about 296 million years ago. Around 1.3m long (4’3”), it was only about half the size of its largest relatives, but it’s notable for having unusually high neural spines on its vertebrae – not quite long enough to be considered a sail, but more of a “high back” that may have supported powerful musculature or fatty deposits.

Brachydectes

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”.

Echinerpeton

Echinerpeton intermedium here was one of the earliest known members of the synapsids, the lineage that includes all mammals along with other “reptile-like” stem-mammals such as the famous sailbacked Dimetrodon.

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.

But a newly described fossil has completely changed what we know about this animal.

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.

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.

Ascendonanus

The recently-described Ascendonanus nestleri from the Early Permian of Germany (~290 mya). This 40cm long (1′4″) animal was a member of a group called varanopids – which may have been an early branch of the synapsid lineage and distantly related to modern mammals*.

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.)

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.