Biarmosuchus

Biarmosuchus tener (Tchudinov 1960) Late Permian ~260 mya is the most primitive therapsid. Derived from a sister to Ophiacodon and Haptodus, Biarmosuchus phylogenetically preceded Aelurognathus and the rest of the Therapsida, including mammals and man.

Distinct from Haptodus garnettensis, the skull of Biarmosuchus was elongated with an elongated premaxilla dorsal process (both as in Ophiacodon). Huge canine teeth dominated the jawline, which otherwise had fewer teeth. The maxilla was much deeper to house the canine root. The lacrimal did not contact the naris, obstructed by the rising maxilla. The quadratojugal did not descend but the quadrate did. Rather than a premaxillary notch, the premaxill rose anteriorly. The pineal opening (third eye) was raised like a little volcano. The anterior dentary developed a strong chin. The reflected lamina became a thin sheet of bone reinforced by a cross of thicker bone.

The neck was slightly longer and included more vertebrae. The posterior dorsal ribs were shorter.

The scapula was longer, less bulky and rode more loosely on the ribcage. The lateral fingers and toes were shorter, brought about by the reductions of m3.2, m4.2 and m4.3 (see below). This permitted the hands and feet to swing more directly beneath each limb, rather than out to the sides (as in Gephyrostegus), during the recovery stroke of each stride. This change helped reduce the amount of torso undulation and thus enabled Biarmosuchus to breathe more easily while walking.

The pelvis was relatively much larger. The femur was longer and elevated the torso higher off the ground while walking.

If you want to read the book, "From the Beginning - The Story of Human Evolution" by David Peters (Little, Brown 1991), which is where the above images were first published, click here for the PDF.

Tetraceratops (Matthew 1908) was considered the most primitive therapsid by Laurin and Reisz (1996), but cladistic analysis indicates it shares more characters with Tseajaia, a limnoscelid, which has never been included in previous cladistic analyses of Tetraceratops.

The right hand of three synapsids, Haptodus, Biarmosuchus and Homo. In Haptodus manual 3.2, 4.2 and 4.3 are all unreduced. In Biarmosuchus they are reduced to discs. In Homo, and all mammals, these phalanges do not ossify.

Proburnetia was another basal therapsid in which various skull bones grew protuberances or "horns". These structures play into the hypothesis that Tetraceratops (see above) was also a basal therapsid, between Dimetrodon and Biarmosuchus, but in recent cladistic anaylsis the two were not related. So in any scenario, the horns were a convergent development.

Adding Ophiacodon to the Origin of the Therapsida
Therapsids were derived from pelycosaurs but the key taxon has not been found yet. The sphenacodonts, Haptodus and Dimetrodon have been traditional sisters, but both have a relatively shorter, taller skull, a short premaxillary ascending process, a kink at the premaxilla/maxilla jawline, a shorter, taller rostrum and a deeply concave posterior jawline.

The resemblance of Eotitanosuchus to Dimetrodon (convex rorstral margin, loss of pre-canine maxillary teeth) are identified here as homologous traits. Eotitanosuchus nested between Biarmosuchus and higher therapsids.

Here Biarmosuchus nests closer to Ophiacodon. Haptodus and Dimetrodon were likewise derived from a sister to Ophiacodon. The base of the Therapsida lies somewhere inbetween.

Several Biarmosuchus traits shared with Ophiacodon are not found in Haptodus, Sphenacodon and Dimetrodon: 1) Premaxilla longer than naris; 2) Rostrum twice as long as tall; 3) Quadratojugal not reduced to anearly invisible nub; 4) Premaxilla rises anteriorly; 5) Transition from premaxilla and maxilla without a kink.

Fewer Biarmosuchus traits shared with Haptodus are not found in Ophiacodon: 1) Reflected lamina present. 2) Anterior dentary deep and ventral margin sharply angled. These traits would be expected to appear in the last common ancestor of the Therapsida originating between Ophiacodon and Haptodus.

In Therapsids the nasal is relatively narrow, but in sphenacodonts it is broader. The purported septomaxilla in therapsids appears to be the anterior lacrimal beneath the ascending process of the maxilla, perhaps laminated over it.

Basal Therapsid Skulls. Starting with Biarmosuchus, these basal therapsids branch off to form various recognized clades. Only a few of these taxa have pages on this site yet, but look for them in the near future.

Amson E and Laurin M 2011. On the affinities of Tetraceratops insignis, an Early Permian synapsid. Acta Palaeontologica Polonica 56(2):301-312. online pdf

Laurin M & RR Reisz 1996. The osteology and relationships of Tetraceratops insignis, the oldest known therapsid. Journal of Vertebrate Paleontology 16: 95-102.

Rubidge BS and Sidor CA 2002. On the cranial morphology of the basal therapsids Burnetia and Proburnetia (Therapsida: Burnetiidae). Journal of Vertebrate Paleontology 22(2):257-267.

Tchudinov PK 1960. Diagnosen der Therapsida des oberen Perm von Ezhovo: Paleontologischeskii Zhural, 1960, n. 4, p. 81-94.

Synapsida