EVOLUTION AND DEVELOPMENT OF DENS OF MAMMALIAN AXIS

Certain members of the extinct reptilian group from which mammals evolved possessed both a dens and an atlas body. Available paleontologic evidence supports the conclusion that the dens evolved as an addition to the atlas body. There‐fore, the dens is not homologous with the atlas body as is generally claimed on the basis of supposed developmental evidence. The atlas body is large in the most primitive of living mammals, the monotremes, which also possess a dens of typical mammalian proportions. In metatherian and most eutherian mammals, both a dens and an atlas body remnant of variable size are present. The development of the dens in the Virginia opossum, Didelphis marsupialis, confirms the fact that the dens arises from, but does not replace, the atlas body anlage. The dens evolved as a functional replacement of the atlanto‐axial articular processes which were lost when the mammalian atlanto‐axial joint became specialized for rotational movement. Copyright © 1969 Wiley‐Liss, Inc.