Stenaulorhynchus

Microscopic analysis of thin-sections of bone have shown that Stenaulorhynchus had a determinant growth pattern, reaching 2/3 of their adult size within one year.[2] The rate of bone deposition in the tibia and femur were 7-10 times higher within that year than in years 2-5. After 5 years of age, less than 1 micron of bone a day was deposited.[2] It would have grown slower than archosauriformes, the South American rhynchosaur species, saurischian dinosaurs, and birds.[2] Stenaulorhynchus continued to grow teeth throughout its life. Juveniles started out with three rows of upper teeth, with only one lateral to the main groove running along the upper surface of the maxilla. By the time they were adults, Stenaulorhynchus had several rows of maxillary teeth, one or more of which were lateral to the main maxillary groove.[4][3] Older teeth would wear down with use and be resorbed where they contacted other teeth.[3]

Stenaulorhynchus may have been adapted for scratch-digging.[8][5]

The fossils were collected in 1930 by the British geologist G. M. Stockley during a survey commissioned by the Tanganyika Geological Survey Department. The goal was to better understand the "economic possibilities" of the Ruhaha Basin within Tanganyika Territory.[1][9] He primarily collected fossils that had weathered out of the rock on the surface.[1] The fossil collection was then described by Sidney Henry Haughton.[1] While Stenaulorhynchus is the dominant vertebrate in the Manda beds, they also included a Dicynodont and vertebrae from an unidentified Theropod.[10]

The specimens weren't all correctly labeled since bones from different localities fit together. More surprisingly, a coprolite was mislabeled as a maxillary fragment.[1][5] Haughton also described a species which he named Stenaulorhynchus major, which he said differed primarily in size, based on a distal left and right humerus fragments.[1][10] Those specimens were later identified as Stagonosuchus nyassicus, a suchian archosaur from the same locality and horizon.[10]

Stenaulorhynchus shares characteristics with other early rhynchosaurs, including their ankylothecodont dentition (teeth within deep sockets and fused to the bone) and precision-shear bite.[3] They also have double-bladed dentaries and grooves on the upper surface of the maxilla.[11] One of the differences between them is that Stenaulorhynchus had smaller maxillary teeth with a larger gap between the tooth rows.[5] Stenaulorhynchus is differentiated from the later Hyperadapedon by its more lateral eyes, more forward braincase, and longer lower jaw with the teeth located more anteriorly.[3][5] Hyperadapedons also lack the double-bladed dentary that the early genera have. The only absolute synapomorphy between all the rhynchosaurs is that the dentary is at least half the total length of the jaw.[5]

Stenaulorhynchinae, a subfamily of rhynchosaurs including Stenaulorhhynchus, has been proposed. Based on an analysis of morphological characteristics, one study defines it as including all taxa that are more closely related to Stenaulorhynchus stockleyi than Hyperodapedon gordoni. The defining traits include having many teeth on the tongue side of the maxilla and a reduced size of the maxillary occlusal teeth allowing more teeth per row.[7]

Stenaulorhynchus has also been labeled as a sister-taxa to the Brazilian genus Brasinorhynchus which also has three or more tooth rows medial to the main maxillary groove but lacks the centrum contribution to the parapophyses and diaphophyses.[1][7]

Stenaulorhynchus is found in the Upper Bone Bed in the Njalili locality, part of the Lifua Member of the Manda Formation.[1][10][8][3] It is composed of proximal floodplain paleosol deposits. Articulated specimens of Stenaulorhynchus have been found, suggesting that they may have mummified before burial.[9] The surface of the bed has been exposed by tributaries eroding away the rock.[9] During the Triassic, the floodplain would have been covered by springs and ponds. Rises in precipitation and mean annual temperature may have led to an increase in perennial vegetation, as well.[9]