. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Figure 20. Comparison of the jaw mechanisms in romerilds and primitive ophiacodontids. A. Hylonomus lyelli, RM 12016. X (Carroll, 1964; text-fig. 1); B. Po/eofhyris ocadiono, MCZ 3483. X 1-6 (Carroll, 1969; text-fig. 12); C. Archoeo- thyris llorensis, MCZ 4079. X ; D. Ophiacodon uniformis, MCZ 1366. X (Romer & Price, 1940, plate I). A—fulcrum of lever. B—furthest point from fulcrum on vv'hich the jaw muscle can act. AB mechanical advantage in jav^ mechanism. AD CD—length of tooth row. Angle shown is th
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Figure 20. Comparison of the jaw mechanisms in romerilds and primitive ophiacodontids. A. Hylonomus lyelli, RM 12016. X (Carroll, 1964; text-fig. 1); B. Po/eofhyris ocadiono, MCZ 3483. X 1-6 (Carroll, 1969; text-fig. 12); C. Archoeo- thyris llorensis, MCZ 4079. X ; D. Ophiacodon uniformis, MCZ 1366. X (Romer & Price, 1940, plate I). A—fulcrum of lever. B—furthest point from fulcrum on vv'hich the jaw muscle can act. AB mechanical advantage in jav^ mechanism. AD CD—length of tooth row. Angle shown is the angle of opening of the jaw when jaw muscles extend by 50 percent of their original length. Stippled area—location of adductor jaw musculature. Althougli the Limnoscelidae and the Cap- torhinidae do not de\el()p temporal open- intis, the\' .show an increase in size similar to that seen in pelycosaurs. Limnoscelids are \er\' primiti\e in nature and have little to do with the ancestry of other more ad- \anced reptiles. The\' seem to ha\e sohed the problems inx'ohed with increase in size by developing great lateral expansion of the temporal region to accommodate a greater mass of jaw musculature. The cap- torhinids represent another sterile lineage that has solved this problem in a similar fashion. On the other hand, pelycosaurs retain the narrow configuration of the skull observed in romeriids, but develop a tem- poral fenestra. This temporal opening en- abled the pelycosaurs to reach a position of dominance in the Lower Pennian. The same basic pattern is retained in their de- scendants, the primitive therapsids, which were dominant terrestrial vertebrates for much of the later Permian and the Tri- assic. The entire system of jaw musculature was again reorganized in the later group in relationship to the origin of mammals. Other differences between romeriid and pelycosaurian skulls can also be associated with the changes in the temporal muscula- ture. In romeriids the p
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