| Sphenacodontidae Fossil range: Late Carboniferous - Middle Permian | |
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 Palaeohatteria | |
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The Sphenacodontidae is a family of small to large, advanced, carnivorous, Late Pennsylvanian to middle Permian pelycosaurs. Primitive forms (Haptodus, etc) 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.
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.
The family Sphenacodontidae is paraphyletic since it does not include its evolved descendants therapsids and ultimately mammals. It is also monophyletic in that it is descended from a single common ancestor. Note: Monophyly and polyphyly have to do with ancestry. Paraphyly and holophyly have to do with descendancy. In the cladistic perspective the Sphenacodontidae and their evolved descendants are smeared together.
See also[]
References[]
- Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
- Colbert, E. H., (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
- Laurin, M. and Reisz, R. R., 1997, Autapomorphies of the main clades of synapsids
- Reisz, R. R., 1986, Handbuch der Paläoherpetologie – Encyclopedia of Paleoherpetology, Part 17A Pelycosauria Verlag Dr. Friedrich Pfeil, ISBN 3-89937-032-5
- Romer, A. S., (1947), revised ed. 1966) Vertebrate Paleontology, University of Chicago Press, Chicago
- Romer, A. S. and Price, L. I., (1940), Review of the Pelycosauria, Geological Society of American Special Papers, No 28