Palaeos Vertebrates 400.200 Therapsida: Biarmosuchia

*Palæos:*		Unit 400: Therapsida
*The Vertebrates*		200: Biarmosuchia

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Therapsida: Biarmosuchia

Taxa on This Page

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

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 prove that Eotitanosuchus is so close to Biarmosuchus that there is little justification for placing it outside the Biarmosuchia. 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.

Characters: Skull similar to sphenacodonts; generally light construction without thickened bones [I99]; skull with numerous protuberances and horn-like outgrowths [RS01]; septomaxilla broadly exposed on surface; maxilla separates lacrimals and nasals; retains parietal foramen with raised edge; wide tabular; $ orbits greatly enlarged [BS00]; (slightly) enlarged temporal fenestra; broad, concave shelf on the upper margin of the temporal fenestra; supratemporal absent; temporal jaw muscles probably still confined to interior of skull; quadrate reduced; $ squamosal with elongate zygomatic process extending under orbit [RS01]; $ squamosal with long ventral quadrate ramus [RS01]; occiput plate-like, slanted so that top of skull overhangs bottom (opposite of pelycosaurs) [RS01]; occiput strongly attached to braincase; braincase attached firmly to cheek; stapes large, transverse; $ dentary with marked difference in height between canine & postcanine regions [RS01]; reflected lamina of angular with ridges radiating from a common center; epipterygoid excluded from the basicranial articulation; vomers partially fused and extend below palatines; other palatal elements arched; shortened interpterygoid vacuity; pterygoid with postero-median flange; very large canine teeth; $ postcanine teeth with basal swelling & coarsely serrated margins [BS00]; some teeth retained on transverse flange of pterygoid; ectopterygoid teeth absent; glenoid and acetabulum open ventrally (more erect posture and mobile limb); $? cervical vertebrae elongated [RS01]; scapular blade narrow; clavicle & interclavicle retained; $ distal carpals 4 & 5 fused [BS00]; pelvic girdle otherwise plate-like and primitive; humerus primitive, with both ends widely expanded; femur more slender, sigmoid, with inturned femoral head; $ distal tarsals 4&5 fused [RS01]; phalanges small and of more equal length (probably faced forward).

Note: [1] It is often asserted that Biarmosuchia is paraphyletic, i.e. synonymous with Therapsida. [2] See also note and figure under Eotitanosuchia. ATW010817.

Characters: Long dorsal process of premaxilla [S89]; very narrow interorbital roof (?!) [S89]; short lateral postorbital bone, not reaching the level of the ventral border of the orbit [S89]; paroccipital process reaches quadrate [S89]; parasphenoid keeled ventrally [S89]; long palatal dentigerous tuberosities [S89]; mandibular symphysis not sloping [S89]; incisors perhaps without a heel(?) [S89]; short cervical vertebrae, but longer than dorsals, and with a ventral keel [S89]; long neural apophyses [S89]; slightly divergent zygapophyses [S89]; interclavicle very wide anteriorly [S89]; humerus with feeble torsion and with entepicondylar foramen [S89]; humerus wide distally [S89]; ilia widen only slightly anteriorly (means ilia diverge slightly? bones transversely broader? or what?) [S89]; pubis very strongly developed [S89]; slender limbs [S89]; phalangeal formula 23454 [S89].

Ictidorhinidae: (= Hipposauridae) Hipposaurus, Ictidorhinus, Lemurosaurus, Lycaenodon, Pseudhipposaurus, Rubidgina.

Characters: Large (to 6m & 500kg), carnivorous forms; incisors & postcanines small; canines very large; no precanines; vomers incompletely fused; pterygoid narrow posterior to transverse flanges, with quadrate rami almost parallel; interpterygoid vacuities small; dorsal process of premaxilla elongated; maxilla reaches maximum height in posterior; some pachyostosis of upper orbital rim; postorbital bar slightly twisted; $ temporal fenestra larger than biarmosuchids, with expanded (& thickened?) posterodorsal margin for origin of jaw adductors visible in dorsal view (H&B); lack moveable quadrate; paroccipital process contacts quadrate.

Biarmosuchus tener & Eotitanosuchus olsoni

Until recently only two genera -- each of one species, are recorded -- Eotitanosuchus and Ivantosaurus. The latter is known from only two jaw fragments, and seem to be very similar to Eotitanosuchus. Tatarinov (1999) has recently described a new species and genus of eotitanosuchian.

Genus Eotitanosuchus Chudinov 1960
Type species: E. olsoni Chudinov 1960
Diagnosis: Snout long and high; orbit large; interorbital roof narrow; occiput high; step in the alveolar border (?); lacrimal high and long; vomers fusing (= partially fused?).

Eotitanosuchus olsoni Chudinov 1960
Locality: Echovo locality, Ocher Province, Perm Region, eastern European Russia
Age: Upper Kazanian, Late Permian, approx. 267 million years ago (Wordian).
Size: skull: 35 cm, overall length probably about 2 meters or more

The holotype is a crushed and deformed skull. Additional skull and skeletal material is known. A lot of this is juvenile material, such as the famous 35 cm long skull displayed in The Russian Dinosaur Exposition and shown here. An adult skull would likely reach about 1 metre in length, which would give an overall length of some 6 meters. There are 8 or 9 small and flattened postcanines in the jaw. Eotitanosuchus is found preserved in flood deposits (once coastal bogs) containing many skeletons of estemmenosuchids. The legs are quite long, and these animals were probably quite agile in spite of their size

Ivantosaurus ensifer Chudinov 1983 ? = Eotitanosuchus ensifer (Chudinov)
Locality: Echovo locality, Ocher Province, Perm Region, eastern European Russia
Age Upper Kazanian, Late Permian, approx. 267 million years ago (Wordian epoch).
Holotype and only known material, maxilla and quadrate found in association and in their respective natural positions.

Probably a giant individual of Eotitanosuchus (if E. olsoni is a juvenile this would therefore be a large adult), although it may or may not be a different species to E. olsoni. This is a very large animal (length would have been around 6 meters). Maxilla short and high. Two upper canines, long, and with their axes inclined forward. It is not clear if one of the canines is a replacement tooth. Sigogneau-Russell (1989) seems to think this is unlikely, which would make this a quite different animal from Eotitanosuchus. As with the therocephalian "family" Lycosuchidae, these may simply be replacement canines. There are few known animals, living or extinct, with two sets of canines (it would be a very inefficient chewing mechanism)

Kamagorgon ulanovi Tatarinov 1999
Locality: Sokol locality, Udmurtia, Western part of the Middle Urals, Perm region, Russia
Age Upper Kazanian, Late Permian, approx. 267 million years ago (Wordian epoch).
material: Based on an incomplete skull

The snout is relatively short, the canines are massive and long, the parietals are thickened, and the mandibular symphysis is extremely high. The palatal teeth cover the pterygoids and palatines and are not concentrated on special bony tubercles.

Image: Three specimens of Biarmosuchus tener (a-c) compared to Eotitanosuchus olsoni (d) as reconstructed by Ivakhnenko (1999). ATW020727; MAK000808.

Characters: $? zygomatic arch bowed, with laterally expanded temporal fenestra [RS01]; $? ulna without distinct olecranon process [RS01]; $? pes V with only 3 phalanges [RS01].

Therapsida: Biarmosuchia

Abbreviated Cladogram

Contents

Taxa on This Page

Biarmosuchia: Stem Therapsida?

Eotitanosuchia