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Sphenacodontidae
Sphenacodontidae (Greek: "wedge point tooth family") is an extinct family of small to large, advanced, carnivorous, Late Pennsylvanian to middle Permian pelycosaurs. Primitive forms were generally small in size (60 cm to 1 meter), but during the later part of the early Permian these animals grew progressively larger (up to 3 meters or more), to become the top predators of their environments. Sphenacodontid fossils are so far known only from North America and Europe.
==Characteristics==
The skull is long, deep and narrow, an adaptation for strong jaw muscles. The front teeth are large and dagger-like, whereas the teeth in the sides and rear of the jaw are much smaller (hence the name of the well-known genus ''Dimetrodon'' – "two-measure tooth", although all members of the family have this attribute).
Several large (~3 meters) and advanced members of this group (''Ctenospondylus'', ''Sphenacodon'', ''Secodontosaurus'' and ''Dimetrodon'') are distinguished by a tall sail along the back, made up of elongated vertebral neural spines, which in life must have been covered with skin and blood vessels, and presumably functioned as a thermoregulatory device. However, possession of a sail does not appear to have been essential for these animals. For example, there is the case in which one genus (''Sphenacodon'' – fossils known from New Mexico) lacks a sail, while a very similar and closely related genus (''Dimetrodon'' – fossils known from Texas) has one. During the Permian, these two regions were separated by a narrow sea-way, but it is not clear why one geographically isolated group should evolve a sail, but the other group not.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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