Tetragonias is an extinct genus of dicynodont from the Anisian Manda Beds of Tanzania. With tetra meaning "four," and goni meaning "angle," the name references the square shape of the Tetragonias skull when viewed dorsally. Not to be confused with the plant Tetragonia, Tetragonias were dicynodont anomodonts discovered in the late 1960s by paleontologist A. R. I. Cruickshank in the Manda Formation. Only the type species, T. njalilus, has been recognized. Originally thought to have existed during the lower Middle Triassic age, also called the Anisian age, Gay and Cruickshank (1999) later postulated that the Manda Formation may have actually been of the Ladinian age. This terrestrial herbivore was determined to have relation to the genera Lystrosaurus and Kannemeyeria.
Discovery and historical information Initially described by F. von Huene in 1942 as Dicynodon njalilus, Cruickshank renamed this species Tetragonias njalilus to correctly point out how different the palate and intertemporal bar structures of this Triassic creature were from the other mostly Permian dicynodonts. He mainly aimed to revise the earlier von Huene descriptions of the skull and postcranial skeleton. Cruickshank's contribution to the scientific classification for Tetragonias is what is used today, although, there have been questions as of late concerning the "taxonomic integrity" of Tetragonias due to Cruickshank's misattribution of certain size differences in features to sexual dimorphism as opposed to feeding differences or other taxa.
Description and paleobiology As for Cruickshank's specific diagnosis, T. njalilus was described as a "medium sized dicynodont, tusked in both sexes" with an overall rectangular skull shape in dorsal view. He included details about the maxillary processes surrounding the tusks differently in males and females, the fact that the postorbital bones do not meet the squamosals at the back of the temporal fenestrae, information about the size of the interpterygoid space in relation to the length of the internal nares, and most significantly, that the intertemporal bar is "drawn out into low parietal crest."
Skull T. njalilus skull bones are quite large. One notable feature is the "sharply downturned snout," which inspired the hypothesis of possible relation to the genus Lystrosaurus. However, T. njalilus has much larger temporal fenestrae than Lystrosaurus murrayi, as well as a shallower skull, no postfrontals, a parietal crest, and differences in size of some parts of the palate. The postorbitals don't cover the parietals to the same degree as von Huene had described, and as mentioned before, do not meet the squamosals. The palatal openings to the internal nares of Triassic dicynodonts are significantly larger than the palatal openings of Permian dicynodonts, which is a consistent pattern that have prompted some have tried to explain using the idea that concentrations of atmospheric oxygen have been gradually declining and contributed to end-Permian extinction.
Postcranial skeleton
Pelvic girdle and locomotion Source:
To further expound on the posture of the Tetragonias pelvic girdle, the iliac blade forms a ventromedial angle with the ischium and the pubis. All the pelvic bones in the acetabulum are fused. There is a curvature in the pelvis of Tetragonias that causes the pubis and ischium to extend far medially and ventrally (see Fig. 4 of Fröbisch). This causes the iliac blade to form the major vertical component of the pelvic bone, which contrasts with what Cruickshank's 1967 paper suggests. It appears there was a misinterpretation about the entire pelvis being vertical. In Tetragonias the straight medial margins of the puboischiadic suggest an articulation which is supported by ligaments or cartilage, although there's no evidence for a suture or fusion along the midline. All pelvic bones in the holotype of T. njalilus at both the left and right sides of the pelvic girdle of are fused. This indicates that movement b/n the pelvic bones of one body side was impossible (in the mature animal). The pelvis is separated into a vertically & a more horizontally oriented part. It possesses an upward-arched backbone, though less pronounced than Cruickshank had postulated. As mentioned earlier concerning the biomechanical strain on the hind limb, most of Tetragonias's body weight would have been carried by the adducted hind limb rather than pelvis: "In this orientation of the pelvic girdle, the supraacetabular crest is strongly overhanging the acetabulum, forming a well-developed dorsal roof to the laterally pointing articulation surface. This feature emphasizes a direct transmission of the body load from the pelvis to the femur, resulting in a more adducted position of the hind limb." This inhibits Tetragonias from sprawling motion, but rather, a parasaggital gait.
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