Saichania vs Tarbosaurus

Saichania


Arbour et al. have an interesting new paper. In it, they name a new armored dinosaur, Zaraapelta, from the red beds of Khermeen Tsav. Of greater interest, however, is new information on Saichania.
Among known Asian ankylosaurs, Saichania is the most derived. It is the only taxon, besides Ankylosaurus, in which the premaxillae are fully covered by armor. In addition, Saichania had a  strongly ossified palate and a large tail club. One specimen, ZPAL MgD I/43, a caudal from Altan IV, includes the biggest club yet found, of any ankylosaur.  Saichania was a veritable tank c 71 Ma.
For many years, though, Saichania appeared paradoxical. It seemed to occur only in the essentially tyrannosaur-free Barungoyotian environment. The beast was not thought to inhabit Nemegtian areas, where the big Tarbosaurus hunted. Tarchia gigantea was supposedly the only Nemegtian ankylosaur yet it was less derived or escalated than Saichania. Why was the latter so well armored and armed if all it faced were dromies amidst the barungoyotian sand dunes?
That view has now changed. PIN 3142/250, the well-known "Tarchia" skull from Khermeen Tsav, has been referred to Saichania. Although Tarchia remains a valid name, the only species is T. kielanae, from the Barungoyot at Khulsan. Saichania is now the only valid ankylosaur in the Nemegt. This finally solves the problem. Saichania did live alongside large tyrannosaurs, and coevolved with them.
In fact, evidence of Tarbosaurus-Saichania combat, albeit unpublished, was noted by researchers long ago. A 1998 abstract discussed a pathological hole in PIN 3142/250. The hole appears to have been a wound, inflicted by one of the anterior teeth of a Tarbosaurus. Even the premaxillary teeth of a subadult T. bataar could penetrate cranial armor. (The premaxillae are not preserved in the big or adult Tarbosaurus specimen, PIN 551/1.) But the Saichania survived, as the wound healed. No doubt, its big bludgeon played a role in the action.
Saichania may not have been the only ankylosaur fighting in the Nemegt. The authors note two club "handle" morphologies, suggesting two taxa. Of four "handle" specimens described so far, three, including ZPAL MgD I/43, have V-shaped neural spines (typical of ankylosaurids). Only one known club handle, ZPAL MgD I/113, has a handle with spines forming a shape intermediate between the typical V and the U-shape of the latter Ankylosaurus. It may be from a more derived taxon, or just due to individual variation. In any case it didn't supplant the typical form--probably Saichania--because it's from Altan III, stratigraphically a bit below Altan IV.
Referral of PIN 3142/250 to Saichania was a bit problematic. Some features suggest it is a different taxon. Among other things, the paroccipital and quadrate (at the rear of the skull) are not fused in PIN 3142/250, as they are in another Saichania skull. Almost a decade ago, I suggested lack of fusion may be due to possible immaturity of the specimen. K. Carpenter disagreed ("no"). But Arbour et al. now concur. It seems I was ahead of my time. The Nemegt preserves many juveniles of other taxa, notably Tarbosaurus, so an immature Saichania wouldn't be shocking. With its big bludgeon, ZPAL MgD I/43 surely represents a full grown Nemegtian "tank."

Arbour et al. The ankylosaurids of the Upper Cretaceous Barungoyot and Nemegt formations of Mongolia.


Addendum: Arbour's research was challenged by Penkalsky and Tumanova, who continue to refer Nemegtian ankylosaur material to Tarchia.