Estemmenosuchus mirabilis was one of the most baroque, grotesque, bizarre and hellzapoppin’ reptiles of all time. In 1991 I chose it as the cover creature for A Gallery of Dinosaurs & Other Early Reptiles. Those bony horns above the eyes and those bony bosses jutting out from the cheeks must have been fearsomely AWESOME in life.
So, how does a reptile like Estemmenosuchus (Tchudinov 1965) come to be? Cladistic analysis provides the answer. Every paleontologist knows that Estemmenosuchus was a synapsid, a therapsid and a dinocephalian. What we’re going to do today is point the finger at several specimens in its ancestral lineage.
The beauty of having a very large reptile tree is being able to trace the ancestry of any taxon all the way back to Ichthyostega. In this way we can also trace the evolution of any character trait through time as it evolves from one state to another or is retained essentially unchanged while the rest of the reptile evolves.
We’ll Start with Paleothyris, the first reptile with a possible synapsid opening appearing anterior to the squamosal (that’s how I traced it, but that is not the traditional interpretation). One tooth, the canine, was notably larger than the others.
The next Estemmenosuchus ancestor was Archaeothyris in which the canine was larger, the temporal opening was larger and the posterior skull angled down. Ophiacodon was phylogenetically closer, but some sister between these two would have been ancestral to the Therapsida, as blogged earlier. Ophiacodon was a little too derived to have been the direct ancestor.
Biarmosuchus was a basal therapsid with a longer deeper rostrum to house the root of a deeper canine, which was protected with a deeper dentary. The ventral premaxilla was raised. The posterior skull was reduced in length, but the temporal fenestra was larger. The lacrimal no longer contacted the naris, or is the septomaxilla the anterior lacrimal overlapped by the maxilla? The posterior mandible was developed into a thin flange and the quadrate was lower than the squamosal.
Eotitanosuchus had a lower longer skull with a longer canine. The jawline below the orbit was deeply concave as the lower temporal arch expanded laterally until it was a little larger than the orbit.
Sinophoneus was similar, with a smaller orbit and a larger temporal fenestra. By contrast…
Deuterosaurus had an altogether deeper skull with almost no concavity beneath the orbit. The nasal was raised to form a horn. The rim of the orbit was enlarged to form a brow. This is where the horns of Estemmenosuchus would eventually develop. The premaxillary teeth were deeper and the maxillary teeth (other than the canine) were smaller. Deuterosaurus was likely a plant-eater.
Estemmenosuchus uralensis was the first in this lineage to develop distinct horns from the frontals along with laterally expanded jugals forming cheek bosses. The temporal fenestra was much deeper than the orbit. The skull was not so tall overall and a slight indentation remained below the smaller orbit.
Estemmenosuchus mirabilis caps off our journey into the evolution of this Middle Permian weird-oh. Distinct from E. uralensis, the dorsal horns and lateral bosses were larger. The temporal fenestra was further expanded.
While Paleothyris was larger than its contemporaries, it still had an elongated lizard-like body. Biarmosuchus was larger with longer legs, but they moved in a parasagittal motion, as determined by the more symmetrical and reduced toes with reduced mid phalanges on digits 3 and 4. Deuterosaurus had a shorter tail, a bulkier body and shorter, thicker legs. Estemmenosuchus took these traits to further extremes, increasing its bulk and reducing its relative surface area. Such a strategy helps conserve heat, rather than radiate it.
I know of no taxa closer to the lineage of Estemmenosuchus. If you do, please drop a line.
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.
(T)chudinov PK 1965. New Facts about the Fauna of the Upper Permian of the USSR”, Journal of Geology, 73:117-30.