Gorgonopsia: Difference between revisions
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Revision as of 09:00, 4 July 2008
In palaeontology, gorgonopsians are considered as primitive therapsids, despite possessing a number of unique specialisations[1]. The most noticeable characteristic is the exaggerated size of the canines, probably an adaptation to deal with prey of a large size, possibly of a similar size to themselves[2]. Gorgonopsians were the group most likely to have filled the role as the dominant carnivore towards upper Late Permian[1].
The oldest gorgonopsians are known from the sediments of the Tapinocephalus Assemblage Zone of South Africa. These genera are small and have all the characters of later forms, but are considered to be more primitive. By the later Cistecephalus and Dicynodon Assemblage Zones, members of the Gorgonopsia had become very successful, and it was at this stage that they most likely took over the role as dominant carnivore from the more primitive anteosaurs. By the end of the Permian Period, during the Permo-Triassic Extinction event (~250 MYA) the whole group had become extinct. As well as deposits in South Africa, gorgonopsian remains have been found in Malawi, Tanzania and Russia [3] [4] [1] [5].
Early Taxonomy of the Gorgonopsia
The infraorder Gorgonopsia was initially created as a suborder of the Therosuchia [6] in order to isolate the specimen of Gorgonops from other Therosuchia, on the basis that the, ‘temporal vacuities [of the skull] were roofed over,’ (p.1014). Broom[7] recognised that the specimen of Gorgonops was damaged, and noticed its affinities to Titanosuchus. Gorgonops was later placed by Broom[8] into the suborder Dinocephalia, along with several other genera including Delphinognathus, Tapinocephalus and Titanosuchus. Broom [9] reduced the Gorgonopsia to the rank of family, but later[10] he re-established it as a distinct suborder of the Therapsida, after the discovery of some more intact specimens.
Subfamilies of the Gorgonopsia
Between 1913 and 1958 a large number of new taxa were described, often from poorly preserved and fragmentary material. Many of these taxa were also created on the basis of size, or differing numbers of postcanine teeth. In 1970 Sigogneau[3] provided an extensive taxonomic revision of the Gorgonopsia. Many of the genera were synonimised and fragmentary specimens were designated as nomen dubium or incertia sedis. In this revision three subfamilies of were recognised.
The Gorgonopsinae as defined by Sigogneau-Russell[4] contained 18 genera. These genera were placed together on the basis of having a narrow interorbital an intertemporal width relative to the total length of the skull. The postorbital, suborbital and zygomatic arches are all slender, with the latter not possessing a ventral expansion of the squamosal. In contrast the Rubidgeinae, containing 6 genera, were characterised as having a wide interorbital and intertemporal width relative to the total skull length. Some genera have the unusual characteristic of the posterior of the skull being almost as wide as the skull is long. It is not known if this is how the skull would have been in times when the animal was alive or if it is a feature of taphonomic distortion. All the cranial arches are also far more robust than seen in the Gorgonopsinae, with the posterior of the zygomatic arch having a large ventral extension. Finally the Inostranceviinae, which contains only the two Russian genera, has an interorbital to skull length width ratio, and intertemporal to skull length ratio intermediate to that seen in the other two subfamilies. The preorbital length of the skull is much longer than the postorbital length of the skull, whereas in the other subfamilies these lengths are approximately the same.
In 2007, Gebauer[5] performed a phylogenetic analysis of the Gorgonopsia. This was one of the first instances of a comprehensive phylogenetic analysis being performed on the group. Gebauer’s analysis provided support of only one of the three subfamilies, the Rubidgeinae, as proposed by Sigogneau[3] and Sigogneau-Russell[4]. The type of the Inostranceviinae fell as the sister taxon to the Rubidgeinae, while all other taxa are represented as ‘evolutionary stages’ leading up to the Rubidgeinae.
Genera of the Rubidgeinae
Gebauer’s[5] phylogenetic work provided enough statistical support to place the following genera in to the subfamily Rubidgeinae: Aelurognathus (6 species), Sycosaurus (4 species), Clelandina (3 species) and Rubidgea (3 species). Gebauer considers the autapomorphies of this group to include a considerably broadened posterior of the skull, thickening of the supraorbital margin, an intertemporal width that is wider than the interorbital width, thickening of the three skull arches, an anterior slant too the posterior margin of the postorbital bar and a zygomatic arch that is ventrally curved due to an extension of the squamosal.
The diagnosis for each genus, according to Gebauer[5] is as follows:
Aelurognathus
Skull heavy and high, dorsal profile of snout strongly convex, lateral face of snout strongly constricted dorsally, orbit small, temporal opening high, septomaxilla rather large, ridge on maxilla well pronounced, palatal tuberosities well developed and broad, skull arches strong, zygomatic arch curves ventrally to some extent.
Sycosaurus
Snout broad and rather flat, posterior part of skull considerably enlarged, preparietal and no contribution of frontal to supraorbital rim (except for the species S. terror), orbit comparatively large, nasal long and narrow, postfrontal broad, parietal short, interorbital width only slightly narrower than intertemporal width; skull arches massive, antero-dorsal corner temporal opening angular, supraorbital thickening well developed, palatal fossa considerably broad, ectopterygoid elongate, basicranium broad, occiput wide and flat, symphysis somewhat sloping.
Clelandina
Snout extremely broad, wider than high and well rounded anteriorly, posterior part of skull conspicuously broadened and thus at the same time shortened, occiput extremely wide and flat with a broad and distinct median ridge, interparietal wide, frontal excluded from the supraorbital rim, palantines short and broad, palatal tuberosities widely open anteriorly, basisphenoidal tubera elongate, symphysis high but very slightly sloping.
Rubidgea
Heavy and massive skull, snout narrow and high, markedly constricted dorso-laterally and ventrally behind the canine, dorsal surface of the skull with accentuated relief and tuberosities, orbits hardly visible in dorsal view, no preparietal, frontal excluded from supraorbital rim, palatal fossa oval, para-basisphenoidal fossa very broad, basioccipital short, occiput wide and strongly concave, tendency towards pachyostosis.
Genera of the "Gorgonopsinae"
in progress
Genera of the "Inostranceviinae"
in progress
References
- ↑ 1.0 1.1 1.2 KEMP, T. S. (2005) The origin and evolution of mammals. Oxford University Press, Oxford. 331 pp..
- ↑ KEMP (1969a) On the Functional Morphology of the Gorgonopsid Skull. Philosophical Transactions of the Royal Society of London. Series B. 256(801): 1-83.
- ↑ 3.0 3.1 3.2 SIGOGNEAU, D. (1970) Révision systématique des Gorgonopsians Sud-Africains. Cahiers de Paléontologie, Paris. 414 pp.
- ↑ 4.0 4.1 4.2 SIGOGNEAU-RUSSELL, D. (1989) Theriodontia I. In: WELLNHOFER, P. (Ed.) Encyclopedia of Paleoherpetology, Part 17B/ I. Gustav Fischer Verlag, Stuttgart. 127 pp.
- ↑ 5.0 5.1 5.2 5.3 GEBAUER, E.V.I. (2007) Phylogeny and evolution of the Gorgonopsia with a special reference to the skull and skeleton of GPIT/RE/7113 (‘Aelurognathus?’ parringtoni). Unpublished Ph.D. dissertation. Eberhard-Karls Universität Tübingen.
- ↑ SEELEY, H.G. (1894) Researches on the structure, organisation, and classification of the fossil Reptilia. Part IX., Section 1. On the Therosuchia. Philosophical Transactions of the Royal Society of London. Series B. 185: 987-1018.
- ↑ BROOM, R. (1910) Observations on some specimens of South African fossil reptiles preserved in the British Museum. Transactions of the Royal Society of South Africa. 2: 19-25.
- ↑ BROOM, R. (1910) A comparison of the Permian reptiles of North America with those of South Africa. Bulletin of the American Museum of Natural History. 28: 197-234.
- ↑ BROOM, R. (1913) A revision of the reptiles from the Karroo. Annals of the South African Museum. 12: 361-366.
- ↑ BROOM, R. (1913) On the Gorgonopsia, a suborder of the mammal-like reptiles. Proceedings of the Zoological Society of London. 1: 225-230.