Thrinaxodon liorhinus

Thrinaxodon liorhinus from Wikipedia

Name: Thrinaxodon liorhinus [Smooth-nosed trident-tooth]

When: Early Triassic Period, about 249 million years ago

Where: Various sites in South Africa and the Trans-Antarctic Mountains of Antarctica.

Claim to fame: Thrinaxodon was the first fossil vertebrate extensively to be studied using high-resolution CT scanning. These historic scans, and others, can be seen in Dr. Timothy Rowe's Digimorph site. Thrinaxodon was a relatively early and primitive member of the cynodonts, which include mammals and their closer non-mammalian fossil relatives. Its semi-erect stance, evidence of a muscular diaphragm to aid breathing, and evidence of rapid growth suggest something resembling warm-blooded metabolism. This, together with its small size, makes it probable that it had some kind of furry coat, though perhaps not in a full mammalian form. Unlike other fossil cynodonts, Thrinaxodon's thoracic ribs are broad and overlapping, a feature associated with animals who dig strongly with their forelimbs. At least one Thrinaxodon fossil was found inside a burrow that it probably dug.

Although Thrinaxodon and other fossil cynodonts shared many evolved features with mammals, there are mammalian traits that it lacked. For example, the bones that in mammals contribute to the middle-ear and lining of the ear-drum, the malleus and ectotympanic, are still part of Thrinaxodon's lower jaw, although highly specialized for hearing. Thus, Thrinaxodon's ears were in its jaw. Thrinaxodon's brain was very small compared to that of any mammal of similar size, and it still possessed a pineal eye in the roof of its skull, like living lizards do, functioning as a light-meter to help coordinate daily cycles. For fun, note that the pineal eye communicated with the brain through the parietal fontanelle, an opening in the top of the skull that in mammals closes shortly after birth. Oddly, the gene that promotes the closure of the fontanelle also promotes development of mammals' rich fur coat. Could the presence of the pineal eye indicate that Thrinaxodon's coat, if it had one, was poorly developed?

Thrinaxodon is also famous because one specimen was part to the Triassic Odd-Couple - an unusual pairing of a Thrinaxodon and a small amphibian, Broomistega, whose bodies were fossilized inside the same burrow.