#stem-tetrapod

Tanyka amnicola was a tetrapodomorph closely related to both the baphetoids and true tetrapods, living in what is now northeastern Brazil during the Permian around 278 million years ago — making it one of the latest-surviving known stem-tetrapods.

Although only known from lower jaw remains it was likely about 1m long (~3'3"), and would have resembled a large chunky salamander.

Those jaws were particularly unusual, having a distinct twist in the bone that made the teeth towards the front stick out sideways when its mouth was closed. However, this twisting brought a different set of teeth into alignment with the upper jaw — "pavements" of many small denticles on the inner side of the lower jaw, which may have been used to rasp up aquatic plants or grind small invertebrates.

Tanyka's highly specialized feeding adaptations suggest that late-surviving stem-tetrapods in Permian Gondwana weren't just evolutionary relics but were instead actively exploring new niches, and that the extinction of these animals wasn't quite as simple as just being outcompeted by more "advanced" tetrapods.

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March’s Fossil of the Month - Acanthostega (Acanthostega gunnari)

Family: Acanthostega Family (Acanthostegidae)

Time Period: 365 Million Years Ago (Late Devonian)

In life, Acanthostega gunnari would have likely resembled a cross between a lungfish and a salamander, and this isn’t dramatically different to what it really was; Acanthostega was a stem-tetrapod, an early member of the lineage of animals that now contains all reptiles, birds, mammals and amphibians, and is believed to be an example of a key stage in the transition between fleshy-finned fishes and the earliest terrestrial vertebrates. Although it possessed 4 short limbs ending in wide, 8-toed feet, the lack of any clear wrist or ankle joints suggests that Acanthostega likely couldn’t support its weight on land, and this combined with its well-developed pelvic bones implies that it was likely a fully-aquatic animal that primarily relied on a paddle-like fin on its tail to propel it forwards while its limbs were used to steer or possibly to grasp aquatic vegetation. During the late Devonian much of the world (including the area of what is now Greenland where Acanthostega fossils were first discovered) was covered in humid, swampy deciduous forests, and this combined with Acanthostega’s anatomy suggests that it likely inhabited warm, oxygen-poor forest pools, which would also explain one of its more unusual characteristics; in addition to possessing fish-like internal gills (as suggested by the presence of gill arch like structures at the base of its skull), a rudimentary rib cage implies that Acanthostega likely had lungs, allowing it to extract oxygen from water as well as air and thereby survive in shallow, oxygen-starved pools that fishes and larger stem-tetrapods would have struggled to breathe in. The teeth of Acanthostega (which were arranged in two rows and were short and sharp, with two larger fangs on the lower jaw) implies that it was likely carnivorous (possibly feeding on terrestrial arthropods caught from above-water beds of vegetation or the banks of its home pools), and comparisons of the anatomy and mineral makeup of fossils of smaller individuals (believed to be juveniles) with those of larger individuals (which are generally believed to be adults) implies that it grew slowly, possibly taking up to 6 years to reach full maturity (at which point most individuals were around 60cm/23.6 inches long, although the difference in the length of seemingly mature individuals suggests that, as with many fish, adverse environmental conditions could considerably limit Acanthostega’s growth.) Although it is unlikely that Acanthostega or its descendants ever succeeded in colonizing land, it is generally accepted that (having become so well-suited to life in the oxygen-poor pools they inhabited) they had little need to, and as several of Acanthostega’s fellow stem-tetrapods (such as the significantly larger Ichthyostega, which had jointed, six-toed limbs and a more developed rib-cage that likely allowed it to haul itself onto land for prolonged periods like modern mudskippers or seals) are known to have done so, the study of the anatomy and lifestyle of this strange little swamp-dweller can still help to shed light on how the variety of land-dwelling vertebrates seen today came to be. 

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Image Sources

Fossil: https://commons.wikimedia.org/wiki/File:Acanthostega_gunnari.jpg#/media/File:Acanthostega_gunnari.jpg

fish out of water

Gaiasia jennyae was a tetrapodomorph – an amphibian-like relative of early tetrapods – that lived about 280 million years ago during the early Permian in what is now Namibia.

Although it's only known from incomplete skull and vertebral column material it probably looked quite similar to the colosteids, a closely-related group of tetrapodomorphs with elongated bodies and small limbs. If it had the same sort of body proportions as these relatives it would have been huge, the largest known stem-tetrapod at potentially around 4m long (~13').

It had a wide flat head with a short boxy snout, and large interlocking fangs on the roof of its mouth and at the front of its lower jaw. It would have been fully aquatic and probably not a particularly fast swimmer, instead likely being an ambush predator using suction from rapidly opening its jaws to pull prey into its mouth before clamping down with its fangs.

It's also notable for living considerably later than most other stem-tetrapods, and in an unexpected part of the world. While its close relatives are all known from the tropics of the Carboniferous, Gaiasia was in a location that was much closer to the South Pole during the early Permian (~55° S), inhabiting an immense freshwater lake in a rift valley with a cold-temperate climate.

Its presence in this habitat may suggest that other stem-tetrapod lineages survived and thrived in high latitudes for much longer than previously thought, while the true tetrapods were all diversifying nearer the equator – or it might represent a Paleozoic equivalent of Koolasuchus, an isolated straggler lurking in a cold refugium.

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Crassigyrinus scoticus was an early tetrapod from the early Carboniferous Period, known from ancient coal swamps of Scotland, Nova Scotia, and West Virginia between about 350 and 330 million years ago.

Around 2m long (6'6"), it had an elongated streamlined body with tiny vestigial-looking forelimbs, and a pelvis that wasn't well-connected to its spine – features that suggest it had re-evolved a fully aquatic lifestyle at a time when its other early tetrapod relatives were specializing more and more for life on land.

Fossils of its skull are all rather crushed, and traditionally its head shape has been reconstructed as unusually tall and narrow. But a more recent study using CT scanning has instead come up with a wider flatter shape more in line with other early tetrapods.

Its mouth also had a very wide gape and a strong bite, and it may have occupied an ecological role similar to that of modern crocodilians, lurking in wait to ambush passing prey.

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Spathicephalus mirus here was part of a group of amphibian-like animals called the baphetoids, a lineage that weren't quite true tetrapods themselves but were still very closely related to them.

Living in Scotland during the mid-Carboniferous period, about 326 million years ago, this 1.5m long (~5') stem-tetrapod had an incredibly unusual head compared to its relatives – wide and flat, almost square in shape, with its jaws lined with hundreds of tiny chisel-like teeth.

Most other stem-tetrapods had deep skulls with large teeth, adapted for fish-eating, so clearly Spathicephalus was specialized for a very different diet. Some comparisons have been made to flat-headed ambush predator plagiosaurid temnospondyls like Gerrothorax, but a better ecological comparison might actually be filter-feeders like "pancake crocs".

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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.

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Eucritta melanolimnetes, an amphibian-like creature from the Early Carboniferous of Scotland (~335 mya). About 25cm long (10″), it had a mixture of anatomical characteristics similar to baphetid stem-tetrapods, temnospondyls, and reptile-like amphibians, making its exact classification difficult. It’s currently considered to be a close relative of both the baphetids and Crassigyrinus, and it was probably close in appearance to what the common ancestor of all later tetrapods would have looked like.