Month of Mesozoic Mammals #02: Swimming Cousins

Kayentatherium

Known from the Early Jurassic of Arizona (196-183 mya), Kayentatherium was part of a group of cynodonts called tritylodontids – very close cousins of the true mammals, specialized for herbivory. They had strong jaw muscles, large incisors, and grinding cheek teeth, an arrangement convergently similar to modern rodents, and were some of the latest-surviving non-mammalian synapsids, persisting into the Early Cretaceous.

Kayentatherium wellesi skull by 5of7 || CC BY 2.0

Kayentatherium was one of the larger tritylodontids at just over 1m long (3′3″), and appears to have been semi-aquatic, with oar-shaped hindlimbs and a flattened beaver-like tail. Although not the first non-mammalian synapsid to be interpreted as a swimmer, it was the earliest close relative of the true mammals to develop these sorts of adaptations.

Thoatherium

While this animal might look like some sort of deer or horse, it was actually only distantly related to any modern hoofed mammals.

This is Thoatherium from the Early Miocene (~17-16 mya) of Argentina. About 70cm long (2′3″), it was related to the weird llama-like Macrauchenia and was part of an extinct group of ungulates (the Meridiungulata) which evolved during South America’s time as an isolated island continent.

It was adapted for fast running, with long legs and only a single horse-like hoof on each foot – but it was even more one-toed than modern horses are, having no remaining “splint bones” from vestigial side toes.

Shringasaurus

Shringasaurus indicus, an archosauromorph reptile from the Middle Triassic of India (~247-242 mya). About 3-4m long (9′10″-13′1″), it was part of a group known as the allokotosaurs, specialized herbivores with lizard-like bodies, and was related to the strange long-beaked Teraterpeton.

Partial remains of about seven different Shringasaurus individuals of varying ages have been found, with several of them having large curving horns over their eyes convergently similar to those of the later ceratopsids. A couple of adult specimens lack horns entirely, suggesting the feature was sexually dimorphic with only the males developing ornamentation – a very rare arrangement among archosauromorphs, but similar to some horned mammals.

Potanichthys

Potanichthys xingyiensis, a fish from the Middle Triassic of China, living around 235-242 million years ago.Measuring about 15cm long (6″), it was one of the oldest known fish capable of aerial gliding – possessing a “four-winged” body plan with enlarged pectoral and pelvic fins, and an asymmetrical tail with a long lower lobe. It was also almost completely scale-less, which may have helped to reduce drag and make it more aerodynamic.

Despite the similar appearance it had no close relation to modern flyingfishes, and was instead a result of convergent evolution in a completely different lineage of the ray-finned fishes.

Bathornis

Bathornis grallator, a flightless bird about 75cm tall (2′6″) from the Late Eocene and Early Oligocene of Midwestern USA (~37-34 mya).

It was originally mistaken for a long-legged vulture (under the name Neocathartes) when first discovered in the 1940s, but later studies have shown it was actually one of the smaller members of the bathornithids – close cousins of the more well-known South American “terror birds”, successfully occupying terrestrial predator niches alongside large carnivorous mammals.