Trimerorhachis

Trimerorhachis is an extinct genus of dvinosaurian temnospondyl within the family Trimerorhachidae. It is known from the Early Permian of the southwestern United States, with most fossil specimens having been found in Texas (its fossils are very common in the north-central part of the state). The type species of Trimerorhachis, T. insignis, was named by American paleontologist Edward Drinker Cope in 1878. Cope named a second species from Texas, T. mesops, in 1896. The species T. rogersi (named in 1955) and T. greggi (named in 2013) are also from Texas, and the species T. sandovalensis (named in 1980) is from New Mexico.

Description
The length of the largest specimens of Trimerorhachis is estimated to have been almost a metre (3.3 feet) in length. Trimerorhachis has a large triangular head with upward-facing eyes positioned near the front of the skull. The trunk is long and the limbs are relatively short. The presence of a branchial apparatus indicates that Trimerorhachis had external gills in life, much like the modern axolotl.[1] The body of Trimerorhachis is also completely covered by small and very thin osteoderms, which overlap and can be up to 20 layers thick. These osteoderms act as an armor-like covering, especially around the tail. Their weight may have helped Trimerorhachis sink to the bottom of lakes and rivers where it would feed.

History
Trimerorhachis was first described by Edward Drinker Cope in 1878. Specimens are often preserved as masses of bones that are mixed together and densely packed in slabs of rock.[3] Fossils are rarely found in articulation, although a slab of rock has been found with sixteen skulls and their associated vertebrae in an intact position.[1] Most of these fossils preserve skulls and dorsal vertebrae, but rarely any other bones. Paleontologist S.W. Williston of the University of Chicago commented in 1915 that "it will only be by the fortunate discovery of a connected skeleton that the tail, ribs, and feet will be made known."[3] A nearly complete specimen was discovered the following year near Seymour, Texas, and Williston was able to describe the entire postcranial skeleton of Trimerorhachis.[4]

In 1955, paleontologist Edwin Harris Colbert described the scales of Trimerorhachis. He noted that they were oval-shaped and overlapping and that each had a base layer of longitudinal striations covered by another layer ring-like ridges, the growth rings of the scales. The scales were more similar to fish scales than they were to reptile scales.[5] In 1979, paleontologist Everett C. Olson claimed that there were no such scales in Trimerorhachis, and that Colbert was incorrect in his interpretation of the body covering of Trimerorhachis.[2]

A second species called T. sandovalensis was named from New Mexico in 1980. A nearly complete skeleton from the Abo Formation near Jemez Springs has been designated the holotype, but other fossils of the species are found throughout the state, giving it a wide distribution.