Teraterpeton

Teraterpeton had an unusual appearance compared to other early archosauromorphs. Members of the genus had a long skull with no teeth at the ends of the upper and lower jaws. Over each eye socket is a bony projection formed by the lacrimal and prefrontal bones. At the back of the jaws are a set of small, sharp, closely spaced teeth. They continue below the level of the eye, an unusual trait among early archosauromorphs. The upper tooth row does not run along the edge of the jaw, but is inset closer to mouth. An additional tooth row on the palate runs alongside the maxillary tooth row of the upper jaw. The teeth of the upper jaws fit closely, or occlude, with the teeth of the lower jaw. The upper jaw teeth have sharp cusps with indentations in front of them, while the lower jaw teeth have cusps with indentations behind them. The cusps of the upper teeth fit into the indentations of the lower teeth, while the cusps of the lower teeth fit into the indentations of the upper teeth.[2]

Teraterpeton has an uncommon feature compared to most archosauromorphs: a euryapsid-type skull. Euryapsids have a single hole at the back of the skull called the supratemporal fenestra, which is located toward the top of the head. Teraterpeton evolved from diapsid reptiles with two holes at the back of their skull, the supratemporal fenestra and an infratemporal fenestra below it. Although it lacks an infratemporal fenestra, Teraterpeton still belongs to Diapsida because it is a descendant of true diapsids. Most other euryapsids such as the marine plesiosaurs and ichthyosaurs are not closely related to Teraterpeton. However, the euryapsid archosauromorph Trilophosaurus has been identified as a close relative.[2][1]

Another unusual feature of Teraterpeton is the large size of its narial fenestra, a hole in the skull that serves as the opening for the nostril. This hole is positioned directly in front of the eyes and extends to the level of the toothless portion of the snout. It is positioned where the antorbital fenestra would normally be. In fact, Hans-Dieter Sues, the original describer of Teraterpeton, first considered the hole to be an antorbital fenestra before revising his hypothesis. The narial fenestra of Teraterpeton is approximately 1.5 times longer than the eye socket.[2]

A string of eight cervical (neck) vertebrae are preserved in the holotype. They have large neural spines, and the second to fourth vertebrae have additional blade-like structures directly behind the neural spines. Overall, the neck is similar to that of the early rhynchosaur Mesosuchus. The cervical ribs are completely fused to the large rib facets of the vertebrae. Isolated dorsal (back), sacral (hip), and caudal (tail) vertebrae are also preserved. Caudal vertebrae from a referred specimen had very long transverse processes (column-like rib facets).[3] The scapula (shoulder blade) is long and narrow. The unguals (claws) of both the hands and feet are deep and blade-like, a morphology described as "trenchant".[2]

The preacetabular process (i.e. the front blade of the ilium bone of the hip) was large, thin and had a convex upper edge. This is in contrast to the small, bump-like preacetabular process of Trilophosaurus, and more similar to that of advanced rhynchosaurs like Hyperodapedon. The pubis and ischium bones of the hip were vertically oriented and separated by a tall, rectangular gap likely homologous with the thyroid fenestra of lepidosauromorphs and tanystropheids, rather than the lack of separation present in other early archosauromorphs. Also like tanystropheids, the fifth metatarsal of the foot was thick, short, and straight.[3]

The simple femur and compact hip supports the interpretation that Teraterpeton had a sprawling gait. However, the modified and expanded ilium suggests that Teraterpeton's hindlimb musculature (as well as that of rhynchosaurs) was well-adapted for protracting and retracting during more erect forms of locomotion. Some paleontologists have argued that expanded leg musculature in rhynchosaurs was an adaptation for scratch-digging rather than an upright posture. While this may apply to thick-legged rhynchosaurs, digging was less likely for Teraterpeton, which had rather slender hindlimbs by comparison.[3]