Probactrosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Ankylopollexia > Styracosterna > Hadrosauriformes > Hadrosauroidea

Overview: Probactrosaurus was a medium-sized iguanodont belonging to the Hadrosauroidea. This made it a relative of the true “duck-billed” hadrosaurids, including Hadrosaurus itself, but it seems to have been too “primitive” to belong to the Hadrosauridae proper. It may’ve been the closest of the hadrosauroids to the clade Hadrosauromorpha, a group containing the hadrosaurids and all of the hadrosauroids most closely related to them. One such hadrosauroid was Bactrosaurus, a genus Probactrosaurus was once thought to be directly ancestral to. They were clearly relatives, but this is no longer accepted as likely. Their overall anatomy was similar, however. Probactrosaurus was a sturdily built, low-grazing herbivore capable of walking on all fours or as a biped. Its arms appear to have been more slender than those of its later relatives, so it was less well adapted for the former.

Sources of food for this animal included conifers, ferns, cycads, horsetails, and tubers. The front of its jaws were equipped with a keratinous beak, used to snip branches and stems, quickly processed by the many rows of grinding “cheek teeth” behind the beak. Probactrosaurus had narrower jaws, so it may’ve been more selective about what it ate. Like other hadrosauroids, it probably would’ve lived in groups or even large herds for added protection against predators. It had no real means of physical defense like armor or spines. Probactrosaurus was first described in 1966 by the renowned Russian paleontologist Anatoly K. Rozhdestvensky, based on fossils unearthed from the rock layers of the Miaogou Formation, in China’s Inner Mongolian region. It lived in a fairly warm environment alongside the ankylosaur Gobisaurus and the carcharodontosaur Shaochilong.