Index to Creationist Claims,  edited by Mark Isaak,    Copyright © 2004

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Claim CC215:

Source:

Response:

1. The transition from reptile to mammal has an excellent record. The following fossils are just a sampling. In particular, these fossils document the transition of one type of jaw joint into another. Reptiles have one bone in the middle ear and several bones in the lower jaw. Mammals have three bones in the middle ear and only one bone in the lower jaw. These species show transitional jaw-ear arrangements (Hunt 1997; White 2002b). The sequence shows transitional stages in other features, too, such as skull, vertebrae, ribs, and toes.
   
   
   1. Sphenacodon (late Pennsylvanian to early Permian, about 270 million years ago (Mya)). Lower jaw is made of multiple bones; the jaw hinge is fully reptilian. No eardrum.
   2. Biarmosuchia (late Permian). One of the earliest therapsids. Jaw hinge is more mammalian. Upper jaw is fixed. Hindlimbs are more upright.
   3. Procynosuchus (latest Permian). A primitive cynodont, a group of mammal-like therapsids. Most of the lower jaw bones are grouped in a small complex near the jaw hinge.
   4. Thrinaxodon (early Triassic). A more advanced cynodont. An eardrum has developed in the lower jaw, allowing it to hear airborne sound. Its quadrate and articular jaw bones could vibrate freely, allowing them to function for sound transmission while still functioning as jaw bones. All four legs are fully upright.
   5. Probainognathus (mid-Triassic, about 235 Mya). It has two jaw joints: mammalian and reptilian (White 2002a).
   6. Diarthrognathus (early Jurassic, 209 Mya). An advanced cynodont. It still has a double jaw joint, but the reptilian joint functions almost entirely for hearing.
   7. Morganucodon (early Jurassic, about 220 Mya). It still has a remnant of the reptilian jaw joint (Kermack et al. 1981).
   8. Hadrocodium (early Jurassic). Its middle ear bones have moved from the jaw to the cranium (Luo et al. 2001; White 2002b).

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