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