At the dawn of the Permian Period, vertebrates had completed the transition from life in water to land. During the previous 100 million years, a group of lobe-finned fishes began a process that saw their fins transform into limbs, their gills into lungs, and their bodies into vessels capable of terrestrial locomotion. By the end of the Carboniferous Period, the first amniotes – or animals that lay hard-shelled eggs – had emerged, setting the stage for a massive radiation during the Permian some 300 million years ago.
While Amniota contains two central clades – Synapsida and Reptilia – the Synapsids were the ones that took control during the Permian. Synapsids are defined by their cranial anatomy, possessing a single temporal fenestra (or hole behind the eye socket) that lies beneath the connection of the postorbital and squamosal bones. Every time you look in the mirror, you can see the visage of a derived Synapsid, as mammals are the headlining members of this illustrious family. But in the Permian, mammals as we know them were many millions of years away from evolving. Their ancestors – the Therapsids – would make their mark in the Permian’s later stages. But at the onset, another strange group of synapsids dominated: the Pelycosaurs.
Despite being lumped together in most analyses, Pelycosaurs aren’t a single family. Instead, Pelycosaurs are what’s known as a polyphyletic group, an association of lineages that aren’t all descended from the same ancestor. In a modern context, a polyphyletic group would be whales, seals, and sea otters. They are all mammals living in oceans but have different ancestors that evolved this lifestyle independently. Pelycosaurs may all be primitive synapsids, but each Pelycosaur subgroup – of which there are six – all evolved independently of each other and do not share a common ancestor.
Some Pelycosaur subgroups may be familiar to you. Though the name Sphenacodonts may not ring any bells, their most famous member probably does: Dimetrodon, the sail-backed carnivore often mistaken to be a dinosaur. Though Dimetrodon’s fame has been well-earned, there is another Pelycosaur lineage whose unique appearance and important evolutionary innovations should garner far more recognition than presently allocated. These were the Caseids, an enigmatic group spearheaded by one of the most unusual animals to call prehistory home: Cotylorhynchus.
As you can see, the Caseid body plan wasn’t like modern terrestrial organisms. The most striking feature was their proportionally tiny skulls, which in Cotylorhynchus romeri accounted for a mere 20 centimetres of a 3.5-meter-long body[i]. Despite the size, Caseid skulls are broad above their eyes, possess a curved snout, and have large nasal openings[ii]. More interesting are their teeth, leaf-shaped in the jaw margins and present on their palates in broader forms. When considering the overall cranial anatomy, it becomes apparent that the Caseids were herbivores.
I know what you may think: why is this a big deal? Herbivorous vertebrates are far more common in terrestrial ecosystems than predators! You, dear viewer, are very correct – but this wasn’t always the case. In the early Permian glory days of the Caseids, herbivory was an alien concept. Other Pelycosaurs and tetrapodal amphibians were predators, including some close relatives of the Caesids known as Eothyridids. While plants were abundant in these ecosystems, no lineage took advantage of them until Caseids like Cotylorhynchus first evolved. (I should note that another Pelycosaur lineage – the Edaphosaurids – evolved herbivory around this time too, but they are the subject of another article).
The relative novelty of herbivory is present in their anatomy. The combination of palatal and marginal teeth, while helpful in segregating the duties of chewing and plucking vegetation, was primitive and became more streamlined in future herbivores. Even more primitive is the anatomy of the rest of their bodies. In Cotylorhynchus, the tiny skull precedes a massive, rotund body that would have contained an expansive gastrointestinal system for plant digestion. While Cotylorhynchus was herbivorous, it’s clear from its anatomy that it wasn’t perfectly adapted to do so.
Speaking of imperfect adaptations, the long tail of Cotylorhynchus is, in many ways, a relic of previous tetrapods. While the tails of future synapsids would shrink and shift their hind-limb musculature into the pelvis, Cotylorhynchus was not so fortunate. This configuration meant that the long, dragging tail of Cotylorhynchus was ultimately essential for hind limb movement, becoming a locomotory paradox for these enigmatic beasts.
This musculature doesn’t mean their limbs were weak, as each was heavily built and well-supported by the vertebral column. The end of the limbs featured large claws, which may have helped dig up plant matter. These stocky limbs could support a lot of weight; Cotylorhynchus romeri could weigh up to 300kg (600 lbs), and the 6-meter-long Cotylorhynchus hancocki weighed up to half a tonne[iii]. Some online sources claim that Cotylorhynchus could reach two tonnes, but no literature supports this. Regardless, Cotylorhynchus and other Caseids were the largest animals in their ecosystems at the dawn of the Permian.
One hypothesis proposes that Caseids were not terrestrial, instead engaging in an amphibious, manatee-like lifestyle. This assertion stems from Caseids having hollow and spongy bones equated to modern aquatic mammals[iv]. Many in the field have disputed this notion, citing that the sponginess of Caseid bones isn’t equivalent to aquatic mammals and the lack of adaptations for such a lifestyle. While debate persists, I would lean towards terrestrial life. I question how well-suited their limbs would be for aquatic propulsion, and just because their bones may be spongy doesn’t mean it was an adaptation for amphibious behaviours.
It is funny how we have another ‘Were they aquatic?’ debate in the field of paleontology. Looking at you, my spined friend…
By the mid-Permian, the Caseids were beginning to disappear. While the early Permian saw the lineage spread across North America and Europe, they soon found themselves replaced by more derived Therapsids. The time of these strange Pelycosaurs may have been brief, but their development of herbivory marked a crucial stage in the evolution of terrestrial ecosystems. Future herbivores would improve on the basic plan of the Caseids, leaving the barrel-chested, tiny-headed body an unfortunate relic of the past.
Thank you for reading this article! Though Cotylorhynchus isn’t a mystery, it is a Paleozoic animal with some questions surrounding it – just like the Tully Monster. Was it a lamprey? A slug? A squid? Read “What was the Tully Monster?” to find out!
I do not take credit for any images found in this article. All images come courtesy of the artists noted above.
Header image courtesy of Dinopedia, found here.
Works Cited:
[i] Reisz, Robert R., et al. “Cranial Anatomy of the Caseid Synapsid Cotylorhynchus Romeri, a Large Terrestrial Herbivore From the Lower Permian of Oklahoma, U.S.A.” Frontiers in Earth Science, vol. 10, Frontiers Media SA, Mar. 2022. Crossref, https://doi.org/10.3389/feart.2022.847560.
[ii] Benton, Michael. Vertebrate Palaeontology. John Wiley and Sons, 2014.
[iii] Reisz, Robert R., and Jörg Fröbisch. “The Oldest Caseid Synapsid From the Late Pennsylvanian of Kansas, and the Evolution of Herbivory in Terrestrial Vertebrates.” PLoS ONE, edited by Andrew A. Farke, vol. 9, no. 4, Public Library of Science (PLoS), Apr. 2014, p. e94518. Crossref, https://doi.org/10.1371/journal.pone.0094518.
[iv] Lambertz, Markus, et al. “A Caseian Point for the Evolution of a Diaphragm Homologue Among the Earliest Synapsids.” Annals of the New York Academy of Sciences, vol. 1385, no. 1, Wiley, Nov. 2016, pp. 3–20. Crossref, https://doi.org/10.1111/nyas.13264.
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