The term "Bairmosuchia" is a more or less artificial term used to unite a number of primitive and mostly little-known basal (ancestral) therapsids from the middle and late Permian period. These forms are intermediate in both morphological development and geologic age between the sphenacodont pelycosaurs (and the proto-therapsid Tetraceratops) and more advanced therapsids. Biarmosuchia currently contains the Russian forms (true biarmosuchids) and the South African forms (ictidorhinids/hipposaurids], which may or may not form a monophyletic group group. This is not a clade but an organisational grade (a "horizontal" taxon), but have enough similiarity (through shared primitive characteristics) to be definable as a typical paraphyletic group. This very problematic group is currently being cosidered for review by South African therapsid specialist Bruce Rubidge).
Basically, the Bairmosuchia are moderately sized, lightly built carnivores, probably somewhat more active than sphenacodontids. The skull is very similar to a sphenacodont skull; however, it shows advances toward the therapsids in large temporal opening, in the depression of the jaw articulation, and in the presence of single large canine teeth in both upper and lower jaws. The larger, more completely differentiated canines are backed by the increased mass (and hence power) of the jaw-closing muscles, as indicated by the flaring of the rear part of the skull where these muscles were attached. The primitively movable braincase was fused to the palate so that the entire all formed a single sturdy unit.
The skeleton also seems to show advances toward the more advanced therapsid condition. Although the vertebrae of an animal like Biarmosuchus resembles that of primitive sphenacodontids (although without the longe neural spines) the structure of the shoulder and pelvic girdles and the limbs indicate a much more advanced posture. The feet are more symmetrical, indicating that they faced more directly forward throughout the stride, and the number of joints and length of some phalanges (toes) is greatly reduced, indicating a condition more like that of later therapsids and of mammals.
Nevertheless, these were primitive animals, close to the Therapsid ancestral stem. Relative to later therapsida, the temporal (synapsid) opening just behind the eye socket is small. This means there is less room for the attachment of muscle that closes the lower jaw. In more advanced mammal-like reptiles, the temporal opening is larger, thus enabling a large suite of muscles, and a more powerful and versatile bite.
Biarmosuchidae | Ictidorhinidae | Burnetiidae
|some Links and References|
Bairmosuchia - Palaeos
Denise Sigogneau-Russell, "Theriodontia I - Phthinosuchia, Biarmosuchia, Eotitanosuchia, Gorgonopsia" Part 17 B I, Encyclopedia of Paleoherpetology, Gutsav Fischer Verlag, Stuttgart and New York, 1989
Carroll, R. L. Vertebrate paleontology and evolution. -W. H. Freeman and company, New York, 1988
James A. Hopson, "The Origin and Adaptive Radiation of Mammal-Like Reptiles and Non-Therian Mammals, Annals of the New York Academy of Sciences 167:199-216, 1969
James A. Hopson and Herbert R. Barghusen, "An Analysis of Therapsid Relationships", in The Ecology and Biology of Mammal-Like Reptiles ed. by Nocholas Hotton III, Paul D. MacLean, Jan J. Roth and E. Carol Roth, Smithsonian Institute Press, Washington and London, 1986, pp.83-106
Palaeos Page (incorporates some of this material, plus a lot of additional material)