BY RICHARD C. FOX University of Kansas Lawrence 1964 University of Kansas Publications, Museum of Natural History Editors: E. Raymond Hall, Chairman, Henry S. Fitch, Theodore H. Eaton, Jr. Volume 12, No. 15, pp. 657-680, 11 figs. Published May 18, 1964 University of Kansas Lawrence, Kansas PRINTED BY HARRY (BUD) TIMBERLAKE, STATE PRINTER TOPEKA, KANSAS 1964 30-1522 BY RICHARD C. FOX Information about osteological changes in the groups of reptiles that gave rise to mammals is preserved in the fos

The outlines of the skulls of Captorhinus differ considerably from those of the skulls of the primitive captorhinomorph Protorothyris . Watson (1954:335, Fig. 9) has shown that in the morphological sequence, Protorothyris—Romeria—Captorhinus , there has been flattening and rounding of the skull-roof and loss of the primitive "square-cut" appearance in transverse section. The quadrates in Captorhinus are farther from the midline than in Protorothyris , and the adductor chambers in Captorhinus are

The morphology of the skull of Dimetrodon closely resembles that of the primitive Haptodus (Haptodontinae, Sphenacodontidae), and "hence may be rather confidently described as that of the family as a whole" (Romer and Price, 1940:285). The major differences between the two genera are in the increased specialization of the dentition, the shortening of the lacrimal, and the development of long vertebral spines in Dimetrodon . The absence of gross differences in the areas of the skull associated wi

A comparison of the general pattern of the adductor musculature of Captorhinus and Dimetrodon reveals an expected similarity. The evidence indicates that the lateral and medial temporal masses were present in both genera. The anterior pterygoid aided in initiating adduction in Captorhinus , whereas in Dimetrodon this muscle was adductive throughout the swing of the jaw. Evidence for the presence and extent of a pseudotemporal muscle in both Captorhinus and Dimetrodon is lacking. The posterior di

The evidence for the position and extent of the external adductors of the lower jaw in Thrinaxodon was secured in part from dissections of Didelphis marsupialis , the Virginia opossum. Moreover, comparison of the two genera reveals striking similarities in the shape and spatial relationships of the external adductors. These are compared below in some detail. The sagittal crest in Thrinaxodon is present but low. It arises immediately in front of the pineal foramen from the confluence of bilateral

Dissections reveal the following relationships of the external adductors of the jaw in Didelphis marsupialis ( Fig. 8 ). 1. Masseter Origin: ventral surface of zygomatic arch. Insertion: posteroventral and lateroventral surface of mandible. 2. External temporalis Origin: sagittal crest; anteriorly with internal temporalis from frontal bone; posteriorly with internal temporalis from interparietal bone. Insertion: lateral surface of coronoid process of mandible. 3. Internal temporalis Origin: sagi

In discussions of the morphology and functions of the adductor mechanism of the lower jaw, the problem of accounting for the appearance of temporal openings in the skull is often encountered. Two patterns of explanation have evolved. The first has been the attempt to ascribe to the constant action of the same selective force the openings from their inception in primitive members of a phyletic line to their fullest expression in terminal members. According to this theory, for example, the synapsi

Adams, L. A. 1919. Memoir on the phylogeny of the jaw muscles in recent and fossil vertebrates. Annals N. Y. Acad. Sci., 28:51-166, 8 pls. Estes, R. 1961. Cranial anatomy of the cynodont reptile Thrinaxodon liorhinus . Bull. Mus. Comp. Zool., 125(6):165-180, 4 figs., 2 pls. Hotton, N. 1960. The chorda tympani and middle ear as guides to origin and development of reptiles. Evolution, 14(2):194-211, 4 figs. Olson, E. C. 1961. Jaw mechanisms: rhipidistians, amphibians, reptiles. Am. Zoologist, 1(2)