Gordodon (meaning "fat tooth", referring to its large incisor-like front teeth) is an extinct genus of non-mammalian synapsid that lived during the Early Permian of what is now Otero County, New Mexico. It was a member of the herbivorous sail-backed family Edaphosauridae and contains only a single species, the type species G. kraineri. Gordodon is unusual among early synapsids for its teeth, which were arranged similarly to those of modern mammals and unlike the simple, uniform lizard-like teeth of other early herbivorous synapsids. Gordodon had large incisor-like teeth at the front, followed by a prominent gap between them and a short row of peg-like teeth at the back. Gordodon was also relatively long-necked for an early synapsid, with elongated and gracile vertebrae in its neck and back. Like other edaphosaurids, Gordodon had a tall sail on its back made from the bony neural spines of its vertebrae. The spines also had bony knobs on them, a common trait of edaphosaurids, but the knobs of Gordodon are also unique for being more slender, thorn-like and randomly arranged along the spines. It is estimated to have been rather small at in length excluding the tail and in weight.
The unique jaws and teeth of Gordodon amongst early synapsids suggest that it was one of the first herbivorous tetrapods to have specialised in selectively feeding on high nutrient, low-fibre plant fructifications (seeds and fruit-like structures). It was also one of the first tetrapods to show such specialised dentition and feeding apparatus, evolving only a few million years after the first obligate tetrapod herbivores appeared in the fossil record. Prior to the discovery of Gordodon, the earliest non-mammalian synapsid herbivores with similarly complex teeth were the mammal-like cynodonts that appeared 95 million years later during the Triassic.
Discovery and naming
The only known fossil of Gordodon was discovered in March 2013 by Ethan Schuth, a geology student of the University of Oklahoma, during a field trip. The specimen was found exposed along a road cut near the city of Alamogordo in Otero County, New Mexico, in strata identified as belonging to the base of the Bursum Formation. They contacted the New Mexico Museum of Natural History and Science (NMMNH) in Albuquerque, who collected the specimen during 2013–2014 and catalogued the specimen as NMMNH P-70796. While the specimen was being excavated, the skull was accidentally sawn through, leaving a vertical cut 6.25 mm across through the jaws and orbit.
A description of the specimen was published in 2018 by palaeontologists Spencer G. Lucas, Larry F. Rinehart and Matthew D. Celeskey, where it was named as the holotype of a new genus and species, Gordodon kraineri. The generic name is from the Spanish "gordo", meaning "fat", and the Ancient Greek suffix "-odon" to mean "fat tooth", referring to its characteristically large front teeth. The name is also a play on "Alamogordo", the name of the city close to where the fossil was discovered. The specific name kraineri was chosen in honour of Karl Krainer, a geologist of the University of Innsbruck in recognition of his extensive work on palaeontology and geology in New Mexico.
The only known specimen of Gordodon is an incomplete but articulated skeleton exposed mostly on its right side that consists of the front portion of the animal, including the skull and lower jaws, five cervical vertebrae in the neck, four complete dorsal vertebrae from the back and parts of the tall neural spines of 12 other vertebrae not visible on the slab, ribs, parts of both right and left scapulae and clavicles in the shoulder, and two partial digits likely from the hand. Although relatively small in size, suturing of both the vertebrae and the shoulder suggests that the specimen was not a young juvenile.
The type specimen of Gordodon was discovered stratigraphically low in the Bursum Formation, at a site Lucas and colleagues referred to as the "edaphosaur locality". This locality is only above the base of the formation, which has been approximately dated to the earliest North American Wolfcampian stage and typically considered to be of the Early Permian, equivalent to the globally defined Asselian stage. Gordodon straddles the boundary between the Pennsylvanian subperiod of the latest Carboniferous and the very earliest Permian at approximately 299 million years old, and may alternatively be considered Pennsylvanian in age based on alternative definitions for the base of Permian (e.g. conodont biostratigraphy). The fossil was found in fluvial facies of olive-grey sandstones deposited by river channels.