Escalation in late Campanian Time

Tarbosaurus is found in the Nemegt formation of early Maastrichtian age, but it probably originated in the late Campanian


Pinacosaurus was a rather small and weak ankylosaurid. Known from the late Campanian of Asia (the Alag Teeg, Djadokhta and other units) it attained a wide distribution. Specimens have been found from the Ulaan Nuur basin in Outer Mongolia to Laiyang in China. At the time, roughly 75 million years ago, tyrannosaurid prowess must have been at a low ebb. Or perhaps tyrannosaurid ranges were severely limited in the arid Djadokhtan interval. Pinacosaurus was not only small for an ankylosaurid, it had a diminutive tail club, and limited armor protection (e.g. no premaxillary armor).

Not surprisingly, Pinacosaurus was soon supplanted. It is absent in the Barun Goyot horizon, directly atop the Djadokhta (the two probably should be considered the same formation, albeit with different levels). By barungoyotian time, larger and better armored taxa appeared. Saichania had more complete armor, greater overall size and a bigger tail club than Pinacosaurus. So did Tarchia. By about 72 million years ago, Mongolian ankylosaurs were more escalated, hence better able to withstand attack.

The evolutionary stimulus for this change is clear.  Known from the base of the Nemegt formation (as well as higher levels) Tarbosaurus almost certainly evolved in the earlier, arid interval. Its appearance then is suggested not only by ankylosaur turnover and escalation, but by an interesting anatomical feature. As P. Currie noted, Tarbosaurus had the most rigid lower jaw of any tyrannosaur. This conferred an ability to flatten heads, especially the small ones of Asian ankylosaurs. The diminutive, weak skull of Pinacosaurus was most vulnerable. Given the abundance of Pinacosaurus in alagteegian and Djadokhtan exposures (bonebeds are known) it's not surprising that the means to hunt ankylosaurs (besides other taxa) were selected for in Tarbosaurus.

In contrast to known specimens of Tarbosaurus, Zhuchengtyrannus was an actual contemporary of Pinacosaurus. (The Hongtuya, overlying the Xingezhuang, which yielded Z. magnus, has basalt dated at 73.5 million years, roughly the same age as the Alagteegian.) Zhuchengtyrannus is not, however, from Mongolia but China, where environmental conditions were much different. Unlike Tarbosarus, and like T. rex, Zhuchengtyrannus inhabited lowlands. The dominant forms were edmontosaurus-like hadrosaurs (Shantungosaurus) and ceratopsids (Sinoceratops). Ankylosaur material is known from the Xingezhuang but ankylosaurids were mostly excluded from lowlands. Nodosaurs were present but appear rare in Asia. Zhuchengtyrannus probably wasn't particularly well-adapted to hunt armored quarry. It would be interesting, though, to see what kind of ankylosaur lived alongside it.

In any event, Zhuchengtyrannus is important in that it shows tyrannosaurs comparable to Tarbosaurus  existed before the Nemegt deposition, or early Maastrichtian time. Evidently, they compelled defensive escalation in the Campanian.

Reference

Currie, P. J., et al. The Crushing Bite of Tyrannosaurids   Journal of Vertebrate Paleontology 2002