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“.
Sometimes the really weird thing about a head isn’t any sort of ridiculous ornamentation.
Sometimes it’s just the wrong size.
That’s what was going on with Cotylorhynchus romeri from the early Permian of North America, living about 280-272 million years ago. Despite looking like a big fat lizard this creature was actually a very early synapsid, closer related to modern mammals than to reptiles, and it was a distant cousin of other stem-mammals like the famous Dimetrodon.
Around 3.5m long (11’6″), it was one of the largest herbivores of the early Permian, with a very wide barrel-shaped body, chunky limbs, and a comically small head. Such a tiny head isn’t necessarily unique – another synapsid Edaphosaurus also had a fairly small skull compared to its body, and dinosaurs like stegosaurs, sauropods, and moa had heads even more disproportional. But something about Cotylorhynchus in particular just looks… incredibly odd.
It also had some surprisingly sizeable nostril openings in that little skull, and it had may have had a very good sense of smell or perhaps some sort of specialized breathing system like the modern saiga’s “air conditioning” nose.
Although usually depicted as a fully terrestrial animal, the structure of Cotylorhynchus‘ bones and its flattened paddle-like hands and feet have recently been used to suggest that it may have been semi-aquatic, more of a Permian hippo than a cow. But such a lifestyle would have required it to have a much more efficient method of breathing than previously thought – suggesting it had a mammal-like diaphragm, and possibly also explaining that weird nose.