Therapsida: Biarmosuchia

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1. Biarmosuchia X?

Biarmosuchia: Stem Therapsida?

The term "Biarmosuchia" is a more or less artificial term used to unite a number of primitive and 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 Sphenacodontidae and the proto-therapsid Tetraceratops) and more derived therapsids. Biarmosuchia currently contains the Russian forms (Biarmosuchidae) and the South African forms (Ictidorhinidae and Burnetidae), which may or may not form a monophyletic group Accordingly, this is not a clade but an organizational grade (a "horizontal" taxon). Nevertheless, the group has enough similarity -- largely through shared primitive characteristics -- to be a useful paraphyletic group for didactic purposes, if nothing else. This very problematic group is currently being considered for review by South African therapsid specialist Bruce Rubidge. For what relevance it may have, the authors of this site find themselves taking the opposite of their usual views, with ATW suspecting monophyly and MAK asserting paraphyly. The weight of scientific opinion probably favors the view that the Biarmosuchia are probably just the stem line of therapsids, not a clade.

Basically, the Biarmosuchia are moderately sized, lightly built carnivores, appearing somewhat more modern and athletic than sphenacodontids. The skull is very similar to a sphenacodont skull; however, it shows a more derived condition in the large temporal openings, 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 skull formed a single sturdy unit.

The post-cranial skeleton also seems to show changes from the "pelycosaur" condition. Although the vertebrae of an animal like Biarmosuchus resemble those of sphenacodontids (but absent the long neural spines) the structures of the shoulder and pelvic girdles and the limbs indicate a rather more derived 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, including 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 therapsids, the temporal opening is larger, thus enabling a large suite of muscles, and a more powerful and versatile bite. MAK000802.

The best known species is the Russian biarmosuchid, Biarmosuchus tener Chudinov 1960 from the Late Permian, Zone I (Ocher Faunal Assemblage), Upper Kazanian, Ezhovo, Ocher locality, Fore-Urals, Perm Region. Biarmosuchus was about the size of a large dog (skull length 15 cm (immature) to 21 cm, with an overall length of about 2 meters. This genus is abundantly represented, but most of the specimens remain inadequately prepared and have not been used for an anatomical and functional analysis. Such details as the configuration of the palatines are unknown, as well as numerous characteristics of the postcranial skeleton. It may be that several taxa are represented, but in the present state of our knowledge, it is not possible to define them. Biarmosaurus antecessor is based on a larger (206 mm as opposed to 153 mm) skull than the holotype, representing a mature individual. This animal is similar in size to Phthinosuchus, but opinions differ as to how distinct the two forms are. The large size of the orbit (eye socket) constitutes the most notable difference. For a long time Biarmosuchus was placed in the same family as Eotitanosuchus, but it is now believed the two forms appear to be quite distinct (see however Ivakhnenko 1999) for an opposing opinion). Biarmosuchus would seem to represent one of the most primitive of the Biarmosuchia, and could be taken as a good generalized model for the other, mostly later, forms. In 1999, Biarmosuchus tchudinovi, a new species, was described by Ivakhnenko from the Sokol locality, Udmurtia, Russia. MAK000801.

Similar, but perhaps more derived forms are known from the Late Permian of South Africa under the family names of Ictidorhinidae, Burnetidae and Hipposauridae. An attempted life reconstruction of Hipposaurus is shown at right. At the moment, we can draw few conclusions other than that the basal Therapsida were widely dispersed and so similar overall, that their monophyly is unquestionable.

Eotitanosuchia

The Eotitanosuchia were large (skull length 45 cm, overall length perhaps 2.5 meters) to very large (possibly up to 6 meters long, weights of up to 500 kg or more, assuming a very large jaw fragment belongs to this group and is proportional) predatory therapsids of the middle Permian (Wordian epoch). Like the Biarmosuchidae they are rather poorly known, although they were probably more common than their few remains indicate. They seem to be more advanced than the Biarmosuchia in that the temporal opening behind the eye socket, although small, is still somewhat larger than the biarmosuchians, being expanded in the upper rear (posterodorsal) margin, allowing the area of attachment of the adductor (jaw closing) muscles to be visible from the dorsal (top) view looking down. The eotitanosuchian bite was stronger and more efficient than the biarmosuchian bite.

For this reason, some paleontologists see the eotitanosuchids as transitional between the biarmosuchians and higher therapsids. They may indeed be so, but it is just as - if not more - likely that features of a larger temporal opening and hence increased muscle mass and biting power evolved simultaneously, among a number of early therapsid groups, due to the obvious advantages this adaptation conferred. One must be wary in applying cladistic methodology to characteristics that are likely to evolve simultaneously among many competing lineages. It is interesting that in other respects the eotitanosuchians are quite primitive -- they were the least modified in their jaw apparatus from their sphenacodont ancestry.

Eotitanosuchus is often grouped with the Phthinosuchidae and the Biarmosuchidae. In fact, Ivakhnenko 1999) argues that Biarmosuchus tener and Eotitanosuchus olsoni are the same organism, which would eliminate the Eotitanosuchia as a separate taxon  This conclusion does not seem to have been widely accepted. Regardless of the eventual outcome of this debate, Ivakhnenko's paper does seem to show that Eotitanosuchus is very similar to Biarmosuchus. Further, given the rather close similarity between Eotitanosuchus and later therapsids, this observation supports the view that Biarmosuchia is paraphyletic. Others view Eotitanosuchus as quite distinct from other basal therapsids and perhaps closer to the Gorgonopsia. But gorgonopsian specializations are either not present in Eotitanosuchus or, as is more often the case, the state of the characters is unknown. This genus is characterized by many primitive features of the septomaxilla, the postorbital, the parietal, the interparietal, the basioccipital, the quadrate rami of the pterygoid and the vomers of the skull. The length of the dorsal process of the premaxilla (front jawbone) and the postorbital twisting (rear side of the skull) constitute specializations that indicate it is not a direct gorgonopsian ancestor. These features however are shared by the anteosaur and biarmosuchid lineages. MAK000808. ATW010819.

Biarmosuchia: Burnetia, Hipposaurus, Ictidorhinus, Niuksenitia, Proburnetia.

Comments:

It is often asserted that Biarmosuchia is paraphyletic, i.e. synonymous with Therapsida.

See also note and figure under Eotitanosuchia.

Links: stars.

Image: Biarmosuchus tener from Gondwana Studios. ATW010817.