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MESOZOIC ERA

TRIASSIC PERIOD

251-201 million years ago

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cynodont

skeleton

The vertebral column, rib cage, hips, legs, and tail of intermediate cynodonts became more mammalian.

In the vertebral column, intermediate cynodonts developed better zygapophyses between vertebrae. The atlas and axis decreased in size and the centrum of the atlas attached to the axis. Intermediate cynodonts stabilized the number of cervical vertebrae at seven, as in mammals. (This means that even though a giraffe has a longer neck than a bat, both mammals have seven homologous vertebrae in their necks.) Intermediate cynodonts, such as Thrinaxodon, lacked a shaft on its lumbar ribs. The lumbar ribs were reduced and attached to the vertebrae without any possible movement between them. Lateral undulation of the vertebral during locomotion was gradually reduced from the therapsids through the cynodonts. The size of the tail was reduced in both therapsids and cynodonts. Cynodonts incorporated a third vertebrae into the sacrum. Intermediate cynodonts developed a shorter rib cage and advanced cynodonts lost the costal plates of ribs.

Early cynodonts possessed a phalangeal count of 2,3,4,4,3 while intermediate cynodonts possessed a modern mammalian count of 2,3,3,3,3. While cynodonts possessed two centralia in the carpus, there is only one in the mammalian carpus. The pelvis of cynodonts included an obturator foramen. In intermediate cynodonts, the ilium was expanded and the pubis reduced to produce a hip which was practically mammalian in its form. The tibia and fibula were held more vertically and joints were modified to accommodate a more erect posture.