. Bulletin. Natural history. 32 PEABODY MUSEUM BULLETIN 36 concavity. In cynodonts a basic alteration of the structure of the notochordal remnant would account for the loss of the notochordal fossa and the development of an incipi- ent dens. An "apical ligament" probably extended to the basioccipital between the condyles, thus strengthening the atlanto-occipital articulation. At a later and as yet unknown stage the caudal part of the ligament may have been strengthened by ossi- fication in situ or, with the same result, replacement by an increasingly longer protu- berance of bone fro
. Bulletin. Natural history. 32 PEABODY MUSEUM BULLETIN 36 concavity. In cynodonts a basic alteration of the structure of the notochordal remnant would account for the loss of the notochordal fossa and the development of an incipi- ent dens. An "apical ligament" probably extended to the basioccipital between the condyles, thus strengthening the atlanto-occipital articulation. At a later and as yet unknown stage the caudal part of the ligament may have been strengthened by ossi- fication in situ or, with the same result, replacement by an increasingly longer protu- berance of bone from the body of the atlas centrum. Whatever were the caudal proc- esses in the development of a dens, selection pressures for its development must have been high once the possibility of rotation at the atlanto-axial joint was realized. A tonguelike or toothlike dens is one of the most invariable features of the mammalian postcranial skeleton. Among modern mammals, only some cetaceans lack a dens. The dens is unquestionably one of the earliest definitive structures to appear in mammalian phylogeny: Kermack (1963) reported a "fully formed odontoid" in the Late Triassic mammal Morganucodon. Kiihne (1956) described and figured a dens of mammalian proportions in the Liassic tritylodontid Oligokyphus. Although Oligo- kyphus is not a mammalian ancestor, but a specialized and late-surviving therapsid, the morphology of the axis is exactly what would be expected in a mammalian ances- tor: the atlas centrum is fused to the axis centrum, as in some cynodonts and in all mammals. But in place of the slight protuberance of cynodonts is a typically mam- malian dens. This may be termed the "prototherian" stage in the evolution of the dens and will be now considered with reference to Ornithorhynchus. The final stage in the evolution of the dens is represented by the Ornithorhynchus dens, which in morphology and size is entirely mammalian (d. Fig. 7). However, the relationship
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