Sphenacodontidae (Greek: "wedge point tooth family") is an extinct family of sphenacodontoid . Small to large, advanced, carnivore, Late Pennsylvanian to Guadalupian "". The most recent one, Dimetrodon angelensis, is from the latest Kungurian or, more likely, early Roadian San Angelo Formation.
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 , which in life must have been covered with skin and blood vessels, and presumably functioned as a thermoregulation 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.
Classification
The family
Sphenacodontidae is actually paraphyletic as originally described, defined by shared
plesiomorphy synapsid characters; these animals constitute an evolutionary gradation from primitive
synapsid to early
therapsid. The
clade Sphenacodontia is used to designate the
monophyletic group that includes sphenacodontids and all their descendants (including mammals), while Sphenacodontidae in the strict sense includes only specialised pelycosaurs, and not earlier more primitive members of the family like
Haptodus,
Palaeohatteria,
Pantelosaurus, and
Cutleria (in pre-
cladistics classifications all included under the genus
Haptodus). The clade
Sphenacodontoidea is used by Laurin and Reisz 1997 to designate the most recent common ancestor of Sphenacodontidae and
Therapsida and all their descendants, and is defined by certain features of the skull.
Sphenacodontidae in a cladogram after Fröbisch et al., 2011:
See also
Further reading
-
Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
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Colbert, E. H., (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
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Michel Laurin and Reisz, R. R., 1997, Autapomorphies of the main clades of synapsids
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Robert Reisz, 1986, Handbuch der Paläoherpetologie – Encyclopedia of Paleoherpetology, Part 17A Pelycosauria Verlag Dr. Friedrich Pfeil,
-
Romer, A. S., (1947, revised ed. 1966) Vertebrate Paleontology, University of Chicago Press, Chicago
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Romer, A. S. and Llewellyn Price, (1940), Review of the Pelycosauria, Geological Society of America Special Papers, No 28
External links
