Jin-Young Park et al. have a new paper out on MPC-D 100/1353, a specimen of Tarchia tumanovae from the Nemegt formation. Based on their research and that of others, the authors reach a number of conclusions. Six require comment, below:
1. Citing Arbour, the authors opine that ankylosaurine clubs evolved primarily for agonistic behavior not antipredator defense. Like modern mammals, ankylosaur males engaged in combat to secure mates or territory. They used their clubs during such contests, causing pelvic and rib fractures. The claim that the club evolved primarily for this purpose does appear logical. Had large theropods been the principal challenge, a stegosaur-like thagomizer would've sufficed. As Carpenter's research shows, it could easily impale a theropod. Tail spikes, however, were ill-suited for fighting an armored opponent. Had they struck large osteoderms, spikes would've failed to penetrate sufficiently, or even shattered on impact. In contrast, a club was best for hitting another "tank." That does not mean clubs played no role in repelling theropods. It was an important secondary function. Ankylosaurs lived alongside large theropods and had no other means of active defense.
2. The authors assume pathologies observed in MPC-D 100/1353, including grooves on the major osteoderms of the tail club knob, were incurred during intraspecific combat. Grooves, however, do not suggest impact related trauma, but raking wounds inflicted by a predator. One Triceratops horn was partially bitten off, almost certainly by a T. rex. The same specimen displays raking wounds on its frill, interpreted as toothmarks. Inasmuch as theropods occasionally struck at the defenses of herbivores, it wouldn't be surprising if the club of an ankylosaur was bitten, resulting in the observed damage. Indeed, if a tyrannosaur bit the knob of MPC-D 100/1353 and the ankylosaur yanked it out of his grasp, the knob would likely be scarred by toothmarks appearing as grooves.
3. Park et al. reiterate Arbour's view on the irrelevance of predators in club evolution. In their judgement, there is no correlation between knob size and predator mass. Large knobs are known from paleoenvironments with rather diminutive predators, and modest knobs co occur with tyrannosaur giants such as T. rex. The illustration below appears to make this point. Anodontosaurus had a more impressive knob than that of Ankylosaurus, the contemporary of T. rex, yet faced the much weaker Albertosaurus. Likewise, a Tarchia specimen, ZPAL MgD I/43 had the largest known club of any ankylosaur even though its contemporary, Tarbosaurus, is usually thought to have been smaller than Tyrannosaurus. Lastly, Saichania was well-armed yet may have been threatened by just the small Velociraptor.
Yet the maximum sizes of knobs and predators is uncertain. The best-known Ankylosaurus knob, that of AMNH 5214, is from an immature individual. As CMN 8880 makes clear, an adult Ankylosaurus was larger, and so presumably was its club. And while Tarchia had a big knob Tarbosaurus, known mostly from juvenile material, may have roughly equalled T. rex as an adult. The sample of adult material is too small to rule this out. The supposed ecological separation of Saichania and tyrannosaurs is probably fallacious (see below). As for well-armed Campanian ankylosaurs such as Euoplocephalus and Zuul, the mid Dinosaur Park horizon yielded a large Daspletosaurus (an element of which was reported by P. Currie). The latter genus may have persisted into the early Maastrichtian, co occurring with Anodontosaurus (although the latter's knob morphology appears optimal for shattering armor and bone. Like the replacement of nasal horns by bosses in centrosaurines, it was probably symptomatic of predator-prey deescalation at the time).
The appearance of the largest known knobs in Campanian and Maastrichtian time mirrored the the larger sizes of both ankylosaurs and tyrannosaurs. Prior to this interval, Talarurus had a diminutive knob and Gobisaurus none at all; both were of modest size and contemporaries of relatively small predators-- Alectrosaurus and Shoachilong respectively. Pinacosaurus was late Campanian and had a small knob but preceded Tarbosaurus and was not in the Zhuchengtyrannus environment. (Known from a single, probably allochthonous element, Sinankylosaurus probably wasn't either.) Despite the primacy of intraspecific interaction, clubs became larger in part to perform their secondary function when necessary.
Given the dimensions of some tyrannosaurs, it was almost certainly necessary. Just as the fractured pelvis of a Dyoplosaurus (and a presacral fracture in MPC-D 100/1353) points to agonistic encounters there is evidence for predation by tyrannosaurs.
In 1998 Dodson and Tumanova investigated a pathological hole in a Tarchia skull. The authors determined the hole matches the anterior teeth of a Tarbosaurus. Presumably the attacker was the size of PIN 551-3 since the premaxillae are not preserved in the larger PIN 551-1. As P. Currie noted, there is evidence for gregarious behavior or pack hunting in Tarbosaurus. The defender may have swung its club at a large individual while a smaller one moved in to strike. The attack was not decisive but attests to the threat faced by ankylosaurs as well as others, hence the need for an antipredator defense.
4. Park et al. suggest ankylosaurines evolved wide bodies to protect their internal organs from the trauma of club blows. In view of the extensive armor that would appear superfluous. Wide bodies evolved to accommodate large digestive masses. Since the ankylosaur body plan was squat they could not produce enough room for their stomach and intestines by expanding upward or vertically, like sauropods or stegosaurs. They had to expand horizontally.
5. The authors note evidence that, unlike earlier ankylosaurs, Saichania and Tarchia were selective feeders. Indeed the Nemegtian environment, being wetter than the Djadokhta, supported a far greater abundance and variety of plants. Whereas a desert ankylosaur, with few plants available, could not subsist on just one species a nemegtian one could. (Incidentally if Saichania was a selective feeder it probably did not actually feed in a desert, hence was not ecologically separate from tyrannosaurs and susceptible to attack.)
6. While the authors are undoubtedly right to cite a wetter environment as a cause of the dietary shift, they also, and dubiously, attribute it to the advent of Saurolophus in Mongolia. Unlike ankylosaurs, certatopsids, sauropods etc, large ornithopods, including hadrosaurs, preferred riparian habitat. Saurolophus was no exception. Abundant tracks, as well as skeletons, in the fluvial beds testify to this. Since Saurolophus presumably foraged in its riparian habitat, it is unlikely to have competed with Tarchia.
Reference
Jin-Young Park, Yuong-Nam Lee et al. A New Ankylosaurid from the Upper Cretaceous Nemegt Formation of Mongolia and implications for paleoecology of armoured dinosaurs. Scientific Reports Vol. 11, article #22928 November 2021
