Sphenacodon
Sphenacodon | |
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Skeleton of Sphenacodon ferox in the Field Museum of Natural History | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Synapsida |
Family: | †Sphenacodontidae |
Subfamily: | † Sphenacodontinae
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Genus: | †Sphenacodon Marsh, 1878 |
Species | |
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Synonyms | |
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Sphenacodon (meaning "wedge point tooth") is an
Researchers currently recognize two species: Sphenacodon ferox (the type species) and Sphenacodon ferocior. Sphenacodon ferocior can be up to 40% larger in overall size (at about 3 m [9.8 ft] long) compared to Sphenacodon ferox (at about 2 m [6.6 ft]). In addition, the dorsal spines in Sphenacodon ferocior are proportionately 45% taller than in Sphenacodon ferox. The recent discovery[1] of a nearly complete skull of Sphenacodon ferox has helped clarify other distinctions between the two species, including the number of teeth in certain parts of the jaws and the size of the indented notch between the maxillary and premaxillary bones in the upper jaw. The two species occur together in some formations, but Sphenacodon ferox apparently survived later into the Early Permian.
Sphenacodon and Dimetrodon typically have been found in different geographical areas that were separated by the ancient Hueco Seaway that penetrated equatorial
Description
The skull of Sphenacodon is very similar to that of Dimetrodon.[7] It is narrow from side to side and vertically deep, with an indented notch at the front of the maxillary bone in the upper jaw. The upper and lower jaws are equipped with an array of powerful teeth, divided into sharp pointed "incisors" [precaniniforms], large stabbing "canines" [caniniforms], and smaller slicing back teeth [postcaniniforms]. The orbit is set high and far back with a single opening (temporal fenestra) behind and partly below the eye, a characteristic of synapsids.
Body proportions are also similar to Dimetrodon, with a very large head, short neck, robust trunk, relatively short front and hind limbs, and a tapering tail that makes up about half the animal's entire length. However, the tops of the neural spines along the back bone are strikingly different in each genus. In Dimetrodon, the neural spines develop into long, narrow, cylindrical projections that support a tall vertical dorsal sail that ends near the base of the tail. In Sphenacodon, the neural spines are enlarged but retain a flat-tipped, blade-like shape along the back and tail, and form a crest rather than a tall sail. (The sphenacodontid genus Ctenospondylus also has blade-like neural spines, but its dorsal crest is taller than in Sphenacodon, although not as tall as the sail in Dimetrodon.)
There is evidence for strong epaxial muscles along the base of the raised neural spines in both Sphenacodon and Dimetrodon, likely helping to stiffen and strengthen the backbone for walking and for lunging at prey by restricting side-to-side flexing motion. A recent study[8] of the structure of the neural spines on Sphenacodon confirms that the upper parts were not encased in a thick muscular hump and instead protruded above a layer of muscle to form a low dorsal crest. Finds of sphenacodontid specimens in which postmortem distortion of the body caused the dorsal spines to overlap suggests that the spines were not connected by hard or particularly tough tissue. The possible function of a low, skin-covered crest in Sphenacodon is debated. A thermoregulatory role seems unlikely, although the taller crest in Sphenacodon ferocior is allometrically larger than in S. ferox. Recent research has favored a display role for the tall sails in Dimetrodon and Edaphosaurus.[9]
Both Sphenacodon and Dimetrodon have been depicted with their short limbs splayed outward at 90 degrees from the body in a wide pushup position and with the tail (and even belly) dragging on the ground, similar to modern lizards and crocodiles. A sprawling stance is also typical for Sphenacodon and Dimetrodon skeletons as currently mounted in museums. However, trackways called Dimetropus ("Dimetrodon foot") that match the foot configuration of large sphenacodontids show animals walking with their limbs brought under the body for a narrow, semi-erect gait without tail or belly drag marks. Such clear evidence for a more efficient upright posture suggests that important details about the anatomy and locomotion of Sphenacodon and Dimetrodon may not be fully understood.[10] Some well preserved narrow Dimetropus tracks found in parts of the Prehistoric Trackways National Monument in New Mexico match the smaller size of Sphenacodon, a genus known from skeletal fossils in the state, but could also come from a small Dimetrodon.
Discovery and classification
The American paleontologist O. C. Marsh[11] named Sphenacodon (from Greek sphen "wedge" + ake "point" + odous (-odon) "tooth") in 1878, based on part of a lower jaw (dentary) bone found in the redbeds of northern New Mexico by fossil collector David Baldwin. In his very short description of the jaw, Marsh cited the back teeth as characteristic ("crowns are much compressed, and have very sharp cutting edges without crenulations") and assessed the animal as "about six feet in length, and carnivorous in habit," although the rest of the skeleton was not known. He did not provide an illustration of the specimen. Marsh gave the genus the Latin specific name ferox "fierce" and erected the new family Sphenacodontidae, placed under the primitive reptilian order "Rhynchocephala" (=Rhynchocephalia), then including nearly all groups of early reptiles in addition to the living tuatara.
Other paleontologists overlooked Marsh's brief mention of Sphenacodon for almost three decades.
In 1937,
A third species, Sphenacodon britannicus, has sometimes been cited in the literature. In 1908 German paleontologist F. von Huene[15] described Oxyodon britannicus, based on part of a maxilla found in England, (The generic name Oxyodon is preoccupied by a fish (Oxyodon Baur, 1906) and so is invalid.) The specimen had been identified earlier as a possible Triassic dinosaur, but von Huene recognized a "pelycosaur." Paton[16] transferred the species to Sphenacodon in 1974, noting it would have been an animal about the size of Sphenacodon ferox. However, more recent studies[17] have questioned whether such limited fossil material can be used to distinguish between Dimetrodon and Sphenacodon—or its own genus. The species "Oxyodon" britannicus (or as Sphenacodon (?) britannicus) is now generally classified as Sphenacodontidae incertae sedis (of uncertain placement).[1][8]
Sphenacodon in a cladogram after Fröbisch et al., 2011:[18]
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See also
References
- ^ a b c Spielmann, J. A.; Rinehart, Larry F.; Lucas, Spencer G.; Berman, David S.; Henrici, Amy C.; Harris, Susan K. (2010). "Redescription of the cranial anatomy of Sphenacodon ferox Marsh (Eupelycosauria, Sphenacodontidae) from the Late Pennsylvanian-Early Permian of New Mexico". Bulletin of the New Mexico Museum of Natural History and Science. 59: 159–184.
- ^ Lucas, S. G. (2011). Traces of a Permian Seacoast. Prehistoric Trackways National Monument. pp. 1–48.
- ^ Berman, D.S. (1977). "A New Species of Dimetrodon (Reptilia, Pelycosauria) from a Non-Deltaic Facies in the Lower Permian of North-Central New Mexico". Journal of Paleontology. 51 (1): 108–115.
- ^ Lucas, S.G.; Spielman, J. A.; Rinehart, L.F.; Martens, T. (2009). "Dimetrodon (Amniota: Synapsida: Sphenacodontidae) from the Lower Permian Abo Formation, Socorro County, New Mexico". New Mexico Geological Society Guidebook, 60th Field Conference, Geology of the Chupadera Mesa Region: 281–284.
- ^ Synapsids Sphenacodon, Dimetrodon, Edaphosaurus, and Ophiacodon in a Permian Period landscape as depicted by Rudolph Zallinger for The Age of Reptiles Mural at the Yale Peabody Museum in 1947. https://remodernreview.files.wordpress.com/2015/03/zallinger2.jpg
- ISBN 1-84028-152-9.
- ^ doi:10.1130/spe28-p1.
- ^ S2CID 40899700.
- .
- ^ Hunt, A. P.; Lucas, S. G. (1998). "Vertebrate tracks and the myth of the belly-dragging, tail-dragging tetrapods of the Late Paleozoic". Bulletin of the New Mexico Museum of Natural History and Science. 271: 67–69.
- ^ Marsh, O.C. (1878). "Notice of new fossil reptiles". American Journal of Science. 3 (15): 409–411.
- ^ Case, E.C. (1907). Revision of the Pelycosauria of North America. Washington, D.C.: Carnegie Institution of Washington. pp. 1–176.
- ^ Romer, A. S. (1937). "New genera and species of pelycosaurian reptiles". Proceedings of the New England Zoological Club. 16: 89–96.
- ^ https://www.researchgate.net/publication/282291788_REDESCRIPTION_OF_THE_CRANIAL_ANATOMY_OF_SPHENACODON_FEROX_MARSH_EUPELYCOSAURIA_SPHENACODONTIDAE_FROM_THE_LATE_PENNSYLVANIAN-EARLY_PERMIAN_OF_NEW_MEXICO/figures?lo=1 [bare URL]
- ^ Huene, F.v. (1908). "Neue und verkannte Pelycosaurier: Reste aus Europe". Centralblatt für Mineralogie, Geologie und Paläontologie. 14: 431–434.
- ^ Paton, R. L. (1974). "Lower Permian pelycosaurs from the English midlands". Palaeontology. 17: 541–552.
- ^ Eberth, D.A. (1985). "The skull of Sphenacodon ferocior, and comparisons with other sphenacodontines (Reptilia: Pelycosauria)". New Mexico Bureau of Mines and Mineral Resources, Circular. 90: 1–40.
- .