Synapsids, often called "mammal-like reptiles," were a group defined by their skull structure and their persistent quadrupedalism.
Pelycosaurs were early synapsids, predominantly Pennsylvanian and Permian, with a sprawling stance.
Many pelycosaurs had a sail-like membrane along their back; sails were well vascularized and probably served as heat-regulating devices,
showing a metabolism already on its way to mammalian homeothermy. Derived from the pelycosaurs were the later synapsids (Therapsida),
most of which had further evidence of homeothermy, such as a higher stance and the evolution of a bony palate. Different groups of therapsids
are characterized by different food-slicing adaptations, including the movable quadrate bone in the Theriodonta. The transition to mammals
involved the evolution of more complex molar teeth, the stabilization of tooth replacement to fewer than two complete waves, and the
replacement of the quadrate/articular jaw hinge by a new hinge between the squamosal and dentary bones.
The first two-thirds of mammalian evolution took place during the Mesozoic. Several early mammals, grouped together as
Prototheria, share an extension of the petrosal bone into the wall of the eye socket; these include the morganucodonts, triconodonts, docodonts,
multuberculates, and the living egg-laying mammals or monotremes. The remaining mammals, grouped together as Theria, share a type of molar tooth
that originally had three cusps arranged in a triangle. Therian mammals include the symmetrodonts, eupantotheres, and the marsupials and placentals
described in later chapters.
- Synapsids:
- Pelycosauria:
- Fish-eating Ophiacodonts
- Herbivorous Edaphosaurs
- Carnivorous Sphenacodonts
- Problem of the sail
- Therapsida:
- Therapsid origins
- Eotitanosuchians
- Food-slicing adaptations
- Dinocephalians
- Dicynodonts
- Theriodonts: Cynodonts and their relatives
Illustrations: Synapsids
Article: spine changes in early mammals
- Mammalian characteristics
- Transition to mammals
- Mesozoic mammals
- Prototheria:
- Morganucodontida
- Triconodonta
- Docodonta
- Monotremata
- Multituberculata
- Theria:
- Kuehneotherium
- Symmetrodonta
- Eupantotheria
- Ancestry of Marsupials and Placentals
Illustrations: Early mammals
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