e bone or part of the bone in question while increasing the metabolic
efficiency of the organism. The bone is no longer essential for
support, the contribution of the mass of bone to calcium metabolism and
the contribution of this part of the skeleton to protection have not
been compromised, and the available energy can be diverted to other
needs.

The study of _Captorhinus_ has indicated that the central area of the
cheek was subjected to less stress than the border areas. A similar
condition in basal reptiles may well have been present. A continued
trend in reducing the thickness of the bone of the cheek in the manner
described above may well have resulted in the appearance of the first
reptiles with temporal fenestrae arising from the basal stock.

Such an explanation adequately accounts for an increased selective
advantage in the step-by-step thinning of the cheek-wall prior to the
time of actual breakthrough. It is difficult to see the advantage
during such stages if explanations of weight reduction or bulging
musculature are accepted.

After the appearance of temporal fenestrae, selection for the classical
factors is quite acceptable to explain the further development of
fenestration. The continued enlargement of the temporal fenestrae in
the pelycosaur-therapsid lineage undoubtedly was correlated with the
advantages accrued from securing greater space to allow increased
lateral expansion of contracting mandibular adductors. Similarly,
weight in absolute terms can reasonably be suggested to explain the
dramatic fenestration in the skeletons of many large dinosaurs.

Literature Cited

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1919. Memoir on the phylogeny of the jaw muscles in recent
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ESTES, R.

1961. Cranial anatomy of the cynodont reptile _Thrinaxodon
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HOTTON, N.

1960. The chorda tympani and middle ear as guides to origin
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OLSON, E. C.

1961. Jaw mechanisms: rhipidistians, amphibians, reptiles.
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