Nobody cares about Ophiacodon, but we should.
Ophiacodon is an overlooked key taxon in the evolution of synapsids, therapsids and by all accounts, mammals and humans.
Figure 1. Ophiacodon, large, squat and amphibious - not the perfect therapsid precursor... or is it?
Overlooked for Good Reason
Ophiacodon was large, low-slung, pretty darn ugly and apparently nothing like the lithe little mammals it would give rise to. (As an aside, let’s not forget that — way back — pterosaurs also arose from bulky diadectids and birds had their origins with equally bulky and amphibious erythrosuchids.) Various Ophiacodon species grew larger and more specialized throughout the Early Permian, so therapsids and sphenacodonts would have arisen from less specialized, smaller, earlier members.
Figure 2. Biarmosuchus, the most basal therapsid.
The Basal Therapsid
Most studies (other than those including Tetraceratops) place Biarmosuchus at the base of the Therapsida. Now all we have to do is find the pelycosaur that most parsimoniously matches Biarmosuchus.
Biarmosuchus vs. the Sphenacodonts
Traditional studies have always placed sphenacodonts like Haptodus, Sphenacodon and Dimetrodon (Figure 3) as predecessors to Biarmosuchus largely due to the presence of the reflected lamina as a shared trait. A reflected lamina is that thin, circular bony leaf peeling off the back of the mandible. In reptiles that mandible bone is called the angular. In mammals the angular and reflected lamina shrinks to frame the eardrum.
The reflected lamina is important, but overall Ophiacodon looks more like Biarmosuchus (Figure 3). However, it’s not good practice to rely on just one character, but a whole suite to make a most parsimonious nesting.
No doubt therapsids were derived from pelycosaurs, but the key sister taxon has not been found yet.
Figure 3. Ophiacodon and its phylogenetic successors, the pelycosaurs and the therapsids.
The Problem(s) with Sphenacodonts as Therapsid Ancestors
Traditionally the sphenacodonts, Haptodus and Dimetrodon have been considered the closest sisters to the Therapsida, but sphenacodonts 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. Biarmosuchus has none of these traits. But Eotitanosuchus does.
Eotitanosuchus
Eotitanosuchus (Figure 3) has often been compared to Dimetrodon. Both share a convex rostral margin and both lose or greatly reduce the pre-canine maxillary teeth. However, taken as a whole we find that Eotitanosuchus nests between Biarmosuchus and various higher therapsids, especially gorgonopsids in the lineage of mammals. So the characters Eotitanosuchus seemed to share with Dimetrodon were convergent.
The Reptile Family Tree
Here Biarmosuchus nests closer to Ophiacodon. Haptodus and Dimetrodon branch off as sisters to this node. However, if we consider all the clues together, the base of the Therapsida actually lies somewhere between Ophiacodon and Haptodus, with a lean toward Ophiacodon.
Biarmosuchus vs. Ophiacodon
Several Biarmosuchus traits shared with Ophiacodon are not found in Haptodus, Sphenacodonand 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.
Biarmosuchus vs. Haptodus
Fewer Biarmosuchus traits shared with Haptodus are not found in Ophiacodon: 1) Reflected lamina. 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. Check all these out on Figure 3. Finally, let’s take a look at the right hand of our candidates. Biarmosuchus had a robust manus, not as robust as Ophiacodon, but not nearly as gracile as Haptodus.
Figure 4. Comparing the right manus of Haptodus, Biarmosuchus and Ophiacodon. Biarmosuchus is right in the middle, literally and morphologically. The reduction of those three disc-like phalanges in Biarmosuchus signals a more erect stride.
We’ll Keep Looking
Someday we’ll find a small, early ophiacodont with longer legs, a pretty big canine, a shorter postorbital region and a reflected lamina. Essentially I’ve just described Biarmosuchus, haven’t I?
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
References
Marsh OC 1878. Notice of new fossil reptiles: American Journal of Science, 3rd series, v. 15, p. 409-411.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.
Tchudinov PK 1960. Diagnosen der Therapsida des oberen Perm von Ezhovo: Paleontologischeskii Zhural, 1960, n. 4, p. 81-94.
The Pterosaur Heresies 