A new Carboniferous edaphosaurid and the origin of herbivory in mammal forerunners

Abstract

Herbivory evolved independently in several tetrapod lineages during the Late Carboniferous and became more widespread throughout the Permian Period, eventually leading to the basic structure of modern terrestrial ecosystems. Here we report a new taxon of edaphosaurid synapsid based on two fossils recovered from the Moscovian-age cannel coal of Linton, Ohio, which we interpret as an omnivore–low-fibre herbivore.  Melanedaphodon hovaneci gen. et sp. nov. provides the earliest record of an edaphosaurid to date and is one of the oldest known synapsids. Using high-resolution X-ray micro-computed tomography, we provide a comprehensive description of the new taxon that reveals similarities between Late Carboniferous and early Permian (Cisuralian) members of Edaphosauridae. The presence of large bulbous, cusped, marginal teeth alongside a moderately-developed palatal battery, distinguishes  Melanedaphodon from all other known species of Edaphosauridae and suggests adaptations for processing tough plant material already appeared among the earliest synapsids. Furthermore, we propose that durophagy may have provided an early pathway to exploit plant resources in terrestrial ecosystems.

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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.

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Edaphosaurids were a fairly early branch of the synapsids – the evolutionary lineage whose only surviving members are modern mammals – and were some of the earliest known tetrapods to develop into large specialized herbivores. They also had huge spiny sails on their backs resembling those seen in their cousins the sphenacodontids (including the famous Dimetrodon), but the two groups actually evolved those features completely independently of each other.

Although their fossils are known from both North America and Europe, their European remains are very rare and fragmentary. Currently the best-known specimen is made up of a recently-discovered partial spinal column and a few hand and tail bones.

Given the name Remigiomontanus robustus, this edaphosaurid lived in western Germany during the end of the Carboniferous and the start of the Permian, around 300-298 million years ago. About 1.2m long (3'11"), it seems to represent an intermediate form between small insectivorous-or-omnivorous edaphosaurids like Ianthasaurus and the huge herbivorous Edaphosaurus.

(Interestingly the paper that names Remigiomontanus also makes a brief mention that the protruding cross-bars on edaphosaurid sails may have anchored larger keratinous coverings, which could have made them look even more spectacularly spiky and suggests their sails may have served an anti-predator function. Hopefully if this is true we'll see further information get officially published about it sometime!)

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Weird Backs Month #05 -- Edaphosaurus

The namesake of the edaphosaur family, and the largest, Edaphosaurus lived during the Late Carboniferous and Early Permian (~300-280 mya) of the USA, with possible fragmentary fossils also known from Central Europe. It was one of the earliest large herbivorous tetrapods -- some species reaching up to 3.5m in length (11′5″) -- with dense “pavements” of vegetation-grinding teeth on the roof of its mouth and lower jaws.

Its huge sail featured numerous crossbars, giving the bones a particularly bristly appearance. Whether they supported any additional soft tissue is unknown, although the trend among Edaphosaurus species over time was to have fewer but more elaborate bars.

Weird Backs Month #04 -- Lupeosaurus

Lupeosaurus was another edaphosaur, known from the Early Permian of Texas, USA (~298-295 mya). Only a few fragmentary fossils have been found, but it was probably an insectivorous species, around 60cm long (2′), about 3m long (9′10″), and seems to have had unusually beefy limbs -- suggesting it was a relatively powerful predator.

Unusually for an edaphosaur, its sail completely lacked the characteristic cross-bars, which led to confusion over its classification when it was first discovered. With so much of its skeleton still unknown, however, it could still turn out to be even more unusual if other fossils are ever found.

Weird Backs Month #03 -- Ianthasaurus

The synapsids diversified rapidly following their first appearance in the geologic record, and two particular families developed large showy sailbacks completely independently of each other: the edaphosaurs and the sphenacodonts.

Ianthasaurus here was a basal edaphosaur, known from the the Late Carboniferous of Kansas, USA (~302 mya). It was smaller than many of its more famous relatives, only about 75cm long (2′5″), and seems to have been an insect-hunting predator.