Did this massive, barrel-shaped Permian herbivore spend its life on land or underwater?

Nature · Published 3 October 2026 Swipe more like this →

In short: Cotylorhynchus was a giant, plant-eating synapsid with a massive, barrel-shaped body and powerful claws. While traditionally viewed as a terrestrial creature, its unique bone structure and limited rib mobility have sparked a fierce debate among paleontologists, with some now arguing it was actually a semi-aquatic animal.

Living during the late Lower Permian, Cotylorhynchus is the best-known genus of caseid synapsids. Its anatomy is striking: a tiny head perched on a massive, barrel-shaped body that likely housed a large digestive tract. This setup was an evolutionary necessity, allowing the animal to process vast quantities of low-nutrient vegetation. With limbs that were short, robust, and tipped with powerful claws, it was long assumed to be a strictly terrestrial creature, perhaps using its strength to dig for tubers or manipulate large plants.

However, this conventional view has been challenged. In 2016, researchers led by Lambertz analyzed the bone microstructure of Cotylorhynchus specimens and found a highly spongy interior with an extremely thin cortex—features more typical of aquatic animals than land-dwellers. They argued that such low bone density would be a significant handicap for a heavy, terrestrial animal. Furthermore, the limited movement of its rib cage suggested that breathing on land would have been difficult, leading to the hypothesis that these creatures were primarily aquatic, venturing onto dry land only for thermoregulation or reproduction.

The taxonomy of the genus itself remains in flux. While three species—C. romeri, C. hancocki, and C. bransoni—have historically been grouped together, recent phylogenetic studies suggest the genus might be paraphyletic. A 2022 analysis by Werneburg and colleagues proposed that C. hancocki and C. bransoni may not belong in the same genus as the type species, C. romeri. As paleontologists continue to refine these relationships, the mystery of how these giants lived persists, complicated by the fact that their arid fossil environments were dotted with rivers and lakes, leaving open the question of whether they were masters of the land or the water.

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