. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. EVOLUTION OF THE MAMMALIAN QUADRUPEDAL WALKING GAIT 33. Fig. 3. Humeri with the distal end in ventral view, brought to the same length. a. Dimetrodon (after Romer). b. Hipposaurid. c. Pristerognathid. d. Anteo- saurid. e. Titanosuchid. f. Tapinocephalid. g. Dicynodontid. In hipposaurids and dicynodontids, when the distal end of the humerus lies in a horizontal plane, the proximal end would lie in a plane inclined from the vertical. In this position the areas of insertion of the dorsal muscles on the
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. EVOLUTION OF THE MAMMALIAN QUADRUPEDAL WALKING GAIT 33. Fig. 3. Humeri with the distal end in ventral view, brought to the same length. a. Dimetrodon (after Romer). b. Hipposaurid. c. Pristerognathid. d. Anteo- saurid. e. Titanosuchid. f. Tapinocephalid. g. Dicynodontid. In hipposaurids and dicynodontids, when the distal end of the humerus lies in a horizontal plane, the proximal end would lie in a plane inclined from the vertical. In this position the areas of insertion of the dorsal muscles on the upper face of the proximal end of the humerus would have rotated in an anti- clockwise direction (as seen from the left). The pull of the deltoid would then be in an anterior direction. The insertion of the latissimus dorsi is still in the primitive position in hipposaurids and dicynodontids but shifted to a more preaxial position in the other therapsids. On the other hand the untwisting in the other therapsids caused the capitellum to be directed downwards with the proximal end lying horizontally. In these forms the pull of the deltoid would thus be more in a dorsal direction. On the ventral surface of the humerus it is seen that the untwisting has shifted the delto-pectoral crest to a more preaxial position in the pristerogna- thids and the herbivorous dinocephalians. The pull exerted by the pectoralis would thus be in a more backward direction than in the hipposaurids and dicynodontids; where it is inwards. This may explain the presence of an ossified sternum in these forms. In all the early therapsids, except the pristerognathids, the radial condyle (capitellum) is well modelled and faces mostly ventrally. The thrust received is thus from a radius standing vertically with the propodial and epipodial forming a right angle at the elbow joint. To prevent a side slip strong flexors are needed so that we find the entepicondyle well developed and strong antagonistic extensors sim
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
