Mark I. Vuletic
Last updated 21 March 2008
Two genera (Probelesodon and Massetognathus) of the cynodonts, a small subdivision of the synapsids, display characteristics of both reptiles and mammals, as well as qualities that are ambiguous between reptiles and mammals. The reptilian features are:
- lower jaw comprises several bones
- jaw joint formed between articular and quadrate bones
- small cranium
- ribs in neck region
- number of bones in fingers and toes exceeds 2,3,3,3
The mammalian features are:
- teeth specialized for different functions
- lower jaw with prominent coronoid process
- double condyle at back of skull for neck articulation
- axis with odontoid process
- ilium slopes forward
The ambiguous characteristics are:
- cheek teeth have simple cusps
- jaw joint formed between hollow in the lower jaw and flat surface in the skull
- prominent ribs confined to chest region, but there are short ribs in front of the pelvis
- legs not splayed, but not vertically beneath body either (list from McGowan 1984:138)
Probainognathus, another genus of the cynodonts, has both reptilian and mammalian jaw joints, and shows the first step in the change of the reptilian jaw joints into the mammalian ear ossicles. This does away with the creationist charge that a reptile-mammal transitional form could not have chewed its food while its jaw was being unhinged and repositioned for hearing (McGowan 1984:139). Also, Probainognatus would have had no trouble hearing during rearticulation, as reptiles exploit the transmission of sound from the ground through their jawbones (Kitcher 1982:111).
Kitcher P. 1982. Abusing Science: The Case Against Creationism. Cambridge, Mass: MIT Press.
McGowan C. 1984. In the Beginning...: A Scientist Shows Why the Creationists Are Wrong. Buffalo: Prometheus.
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