Assertion 5.3: There are no transitional forms between reptiles and mammals.
Analysis:
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).
References
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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