Transition from reptiles to mammals: Pelycosaur synapsids classic reptilian skeleton, inte

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Transition from reptiles to mammals: Pelycosaur synapsids -- classic reptilian skeleton, intermediate between the cotylosaurs (the earliest reptiles) and the therapsids (see next) Therapsids (e.g. _Dimetrodon_) -- the numerous therapsid fossils show gradual transitions from reptilian features to mammalian features. For example: the hard palate forms, the teeth differentiate, the occipital condyle on the base of the skull doubles, the ribs become restricted to the chest instead of extending down the whole body, the legs become "pulled in" instead of sprawled out, the ilium (major bone of the hip) expands forward. Cynodont theriodonts (e.g. _Cynognathus_) -- very mammal-like reptiles. Or is that reptile-like mammals? Highly differentiated teeth (a classic mammalian feature), with accessory cusps on cheek teeth; strongly differentiated vertebral column (with distinct types of vertebrae for the neck, chest, abdomen, pelvis, and tail -- very mammalian), mammalian scapula, mammalian limbs, mammalian digits (e.g. reduction of number of bones in the first digit). But, still has unmistakably *reptilian* jaw joint. Tritilodont theriodonts (e.g. _Tritylodon_, _Bienotherium_) -- skull even more mammalian (e.g. advanced zygomatic arches). Still has reptilian jaw joint. Ictidosaur theriodonts (e.g. _Diarthrognathus_) -- has all the mammalian features of the tritilodonts, and has a *double* jaw joint; both the reptilian jaw joint and the mammalian jaw joint were present, side-by-side, in _Diarthrognathus_'s skull. A really stunning transitional fossil. Morganucodonts (e.g. _Morganucodon_) -- early mammals. Double jaw joint, but now the mammalian joint is dominant (the reptilian joint bones are beginning to move inward; in modern mammals these are the bones of the middle ear). Eupantotheres (e.g. _Amphitherium_) -- these mammals begin to show the complex molar cusp patterns characteristic of modern marsupials and eutherians (placental mammals). Mammalian jaw joint. Proteutherians (e.g. _Zalambdalestes_) -- small, early insectivores with molars intermediate between eupantothere molars and modern eutherian molars.


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