. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. CRANIAL MORPHOLOGY OF THE DIGYNODONT GENUS LYSTROSAURUS 225 ECT. (a) ^l ^ (b) Fig. 56. Comparisons of palates of {a) Emydops sp., No. 2642; {b) LystrosauTus declivis Nat. Mus. No. C. 403. forward, terminating between two posteriorly diverging plates of the vomer, which lie medially to the anterior flanges of the pterygoids. This condition is similar to that found in Dicynodon grimbeeki (Agnew, 1959). Dorsally to the interpterygoidal vacuity runs the parasphenoidal rostrum, visible in ventral v
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. CRANIAL MORPHOLOGY OF THE DIGYNODONT GENUS LYSTROSAURUS 225 ECT. (a) ^l ^ (b) Fig. 56. Comparisons of palates of {a) Emydops sp., No. 2642; {b) LystrosauTus declivis Nat. Mus. No. C. 403. forward, terminating between two posteriorly diverging plates of the vomer, which lie medially to the anterior flanges of the pterygoids. This condition is similar to that found in Dicynodon grimbeeki (Agnew, 1959). Dorsally to the interpterygoidal vacuity runs the parasphenoidal rostrum, visible in ventral view as a broad bar forming an incomplete roof for the interpterygoidal vacuity. In Lystrosaurus the entire palate has migrated ventrally to accompany to some extent the downgrowth of the snout. This ventral migration is more pronounced anteriorly than posteriorly, so that the plane of the choana (see above) lies at an angle to the premaxillary palatal surface. The vomer, accom- panying the pterygoids and palatines, has become a thin vertical sheet of bone, lying approximately in the same plane as the lateral borders of the internal nares. Furthermore, with reduction in the length of the basicranial axis, the interpterygoidal vacuity has become a shortened and lanceolate fossa, clearly demarcated by the vomers in front and the interpterygoidal plate behind. In ventral view the interpterygoidal fossa has therefore become distinct and well separated from the internal nares. According to Cruickshank (1967) the 'interpterygoidal space' of Lystro- saurus is retained as a typical feature in later Triassic dicynodonts such as Tetragonias and Kannemeyeria, this being one of the factors which led him to believe that a species of Lystrosaurus might have given rise to these later dicyno- dont groups. In a further (1968) publication dealing with this region in dicynodonts, he maintained that Triassic dicynodonts, with the Lystrosaurus type of interpterygoidal vacuity, can be separated from
Size: 2038px × 1226px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
