Tapinocaninus (Greek for "humble"- tapino, and "canine"- caninus) is an extinct genus of therapsids in the family Tapinocephalidae, of which it is the most basal member. Only one species is known, Tapinocaninus pamelae (meaning "Pamela's humble canine"). The species is named in honor of Rubidge's mother, Pamela. Fossils have been found dating from the Middle Permian (Wordian age).
Discovery Tapinocaninus fossils were first found in the Eodicynodon Assemblage Zone of the Karoo deposits, in the Lower Beaufort Beds in Beaufort West. Five specimens are known, four found at Modderdrift farm and one found on Swartgrond farm. A holotype (NMQR 2987) and four paratypes (NMQR 2985, 2986, 3097 and ROZ K95). Two specimens were found by the director of the Bernard Price Institute for Paleontological Research (now the Evolutionary Studies Institute), Professor Bruce Rubidge. Three were found by John Nyaphuli of the National Museum Bloemfontein in the same sandstone. The excavation and preparation of NMQR 2986 the holotype NMQR 2987 was a large undertaking due to the large size of the specimens, and this process took place from 1985 to 2005. Air scribe machinery was used to prepare the specimens, along with manual tools such as a hammer and chisel in areas where matrix was more abundant. Prior to the discovery of the Tapinocaninus, the Anteosaurinae were believed to be the most primitive dinocephalians, and the Tapinocephalinae were believed to be the most derived dinocephalians of South Africa. When comparing features of Tapinocaninus to those discussed of Tapinocephalinae through a cladistic analysis, Rubidge et al. (1991) found that a synapomorphy of the two were the expanded heels on the incisor teeth. Thus, after its discovery, Tapinocaninus is considered the most primitive tapinocephaline.
Description This species is known from several skulls, as most specimens found were lacking of the post cranial skeleton. It was a large animal, measuring 2.5m in length from snout to ilium. With classic regression formulas using the circumference of the humerus and femur bones, researchers approximated that they averaged a body mass of 892 kilograms (1,967 lb) for the taxon. Tapinocaninus were also the largest therapsids from the Guadalupian.
Skull The skull roof and postorbital bar of Tapinocaninus shows pachyostotic thickening, which is consistent with other tapinocephaline dinocephalians. The skull roof is majorly composed of the frontal, which extends between the orbital and temporal fenestre. The external naris is bordered dorsally, anteriorly, and anteroventrally by the premaxilla bone. The maxilla forms a majority of the lateral area of the face, and it is swollen to allow room for the root of the canine tooth. The temporal openings are relatively large, and subsequently, Tapinocaninus has a narrow intertemporal region, which is considered a primitive feature of Tapinocephalinae. The temporal fenestra are bordered ventrally and posteriorly by the squamosal, and dorsally by the postorbital. The squamosal and postorbital touch along the temporal opening, which is a feature commonly found in tapinocephalids. Additionally, these taxon have a thin snout, sloping occipital, relatively small quadratojugal, prominent stapedial foramen, and relatively anterior position of the quadrate.
Palate The palate of Tapinocaninus has narrow vomers with a crest between them that surrounds the internal nares. The premaxilla that overlies the maxilla anteriorly and ventrally, and the maxilla contacts both the palatine and pterygoid medially. Over half the length of the palate is the pterygoid, and it has lateral flanges that extend ventrally and transversally. Additionally, the pterygoid touches the basisphenoid behind the interpterygoid vacuity.
Vertebrae The vertebrae of Tapinocaninus are distinguished by their laterally facing fovea at the base of the neural spine. This feature is primitive for therapsids, another ancestral trait for this clade also includes the presence of intercentra in between the cervical vertebrae. Tapinocaninus have 36 vertebrae, 8 cervicals, 20 dorsals, 5 lumbars, 2 sacrals and 7 caudals. They have a long neural spine that extends all the way to the caudal vertebrae. Their vertebral structure suggests that tapinocaninus had very short tails similar to anomodonts, but in contrast to anteosaurids. The presence of only two sacral vertebrae in Tapinocaninus differs from other therapsids, such as Moschops which has three.
Ribs There are ribs present along the entire vertebral column. In the cervical region, the ribs are shortened and flat. The longest ribs are present in the mid-dorsal region, and dorsal ribs 11-15 are barrel shaped to accommodate the large digestive system of Tapinocaninus. In the caudal region, the ribs are again shortened, in addition to being dorsoventrally flattened, posteriorly directed, and not fused to the caudal vertebrae
Pectoral girdle The pectoral girdle of Tapinocaninus has a prominent scapula, with an enlarged dorsal end. The glenoid is straight, ventral facing, and thickened to comprise the coracoids and scapula. A ridge runs diagonally from the posterior dorsal end of the glenoid to the anterior end of the scapula, and this limits the movement of the clavicle over the scapula. The scapula also flares to connect with the anterior coracoid, which is a round, large bone.
Pelvic girdle The pelvic girdle of the holotype NMQR 2987 is partially preserved, and the other specimens are also lacking of a complete structure. However, parallel grooves are found on the surface of the ischium, which suggest that they may have been connected by tissue.
Humerus Three humeri were retained from various specimens of Tapinocaninus, although the right humerus of the holotype is the most complete, and well described in research. The bone has a narrow center and widens at the ends, with the proximal end larger than the distal. The deltopectoral crest flares and makes up a large portion of the length of the bone. The distal end of the humerus has a small entepicondyle and a larger ectepicondyle with fossa separating the two. Tapinocaninus differs from Ulemosaurus and Moschops in that it has an entepicondylar foramen while the latter two taxon have ectepicondylar and entepicondylar foramen.
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