. Annali della South African Museum = Annale van die Suid-Afrikaanse museo. Storia naturale. 2l8 Annali della South African MUSELIM RC.POSTMAJ. R C POST MIN. R.C. LAT Fig. 51. Lystrosaurus declivis. Nat. Mus. No. C.403. Vista occipitale del cranio che mostra le aree della muscolatura occipitale inserimento. alleato sono stati parte di muscoli interspinalis acceso tra successivi spine vertebrali. La fusione di questo atlantal neural della colonna vertebrale con il processo trasversale della atlas sarebbe, secondo Cox, produrre la condizione di mammiferi, dove il obliquus capitis caudalis e muscoli cranialis inserto e orig

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. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 2l8 ANNALS OF THE SOUTH AFRICAN MUSELIM RC.POSTMAJ. R C POST MIN.. R.C LAT Fig. 51. Lystrosaurus declivis. Nat. Mus. No. C.403. Occipital view of skull showing areas of occipital musculature insertion. ally have been part of the interspinalis muscles running between successive vertebral spines. Fusion of this atlantal neural spine with the transverse process of the atlas would, according to Cox, produce the mammalian condition, where the obliquus capitis caudalis and cranialis muscles insert and originate on the enlarged atlantal transverse process. If Cox's theory were correct, signs of this transition would be expected in the atlas-axis complex in cynodonts, since these therapsids are accepted as being closely related to the forms which gave rise to the first mammals. How- ever, the neural arch of the cynodont atlas bears no process equivalent to the spine in Lystrosaurus and Kingoria (Jenkins, 1968) and the transverse wing retains its primitive vertical position, in contrast to the more horizontal orientation of this process in mammals. Jenkins (1968) has for several reasons preferred to regard the cynodont atlas-axis complex as reptilian, rather than as closely foreshadowing the mammalian condition, as Evans (1939) thought was the case. Cox's theory is thus further weakened, since as the cynodonts have a primitive transverse process, the neural spine would be expected to be present to provide attachment for the two obliquus capitis muscles. It seems more likely that in the therapsids the m. obliquus capitis caudalis inserted on the dorsal surface of the atlantal neural arch and 'spine', near the postzygapophysis, and that the m. obliquus capitis cranialis originated on the wing-like transverse process, as reconstructed by Ewer (1961). If this view is correct, Lystrosaurus had retained a basically reptilian atlanto-axial musculature, with, however, the loss of the m. ob

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• bookcentury1800
• bookdecade1890
• booksubjectnaturalhistory
• booky