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The first vertebrates to evolve a humerus were the rhipidistian sarcopterygians which preceded the amphibians. They possessed a joint in the region of the
elbow, although the rest of the fin was fairly stiff (Carroll,
p. 161). In amphibians, the humerus was modified so that it could be held out from the
body, allowing a greater range of movement including rotation (Carroll). In amniotes, the humerus
became more narrow (Carroll) and developed a supinator
process (Carroll). Sphenacodonts
evolved more gracile limbs which were longer
and more slender (Carroll, p. 366; Kemp, 1982, p. 33).
The limbs of therapsids were longer still
and allowed a dual gait capable of both sprawling and erect locomotion
(Kemp, 1982, p. 108). In the most
advanced cynodonts, the humerus had become
more slender and was held upright (Kemp, 1982). The Jurassic mammal Fruitafossor represents a basal mammalian lineage which evolved shortly after the monotreme lineage diverged from that of the therian mammals. Its humerus possessed an intertubercular groove but lacked a round head and its radial and ulnar condyles were spherical in shape (Luo, 2005). After the earliest mammals, separate centers
of ossification existed in the epiphyses of the humerus
and the ectepicondylar foramen was lost (Carroll,
p. 409). In Proconsul the elbow was similar to that of higher apes (Carroll). |
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