Systematics, evolution, and biogeography of the ankylosaurid dinosaurs

Abstract

The Ankylosauria is a group of herbivorous, quadrupedal, armoured dinosaurs subdivided into at least two major clades, the Ankylosauridae and the Nodosauridae. The most derived members of the Ankylosauridae had a unique tail club formed from modified, tightly interlocking distal caudal vertebrae and enlarged osteoderms that envelop the terminus of the tail. A review of all known ankylosaurid species, as well as ankylosaurs of uncertain affinities, was undertaken in order to conduct a revised phylogenetic analysis of the clade. Sources of morphological variability were investigated using the relatively large number of specimens referred to Euoplocephalus tutus. Taphonomic distortion can influence the morphology of certain features which were thought to be taxonomically significant. However, the cranial ornamentation of ankylosaurs can be useful for distinguishing species and genera and should not be discounted as being too intraspecifically variable. The overall shape, size, and pattern of the frontonasal caputegulae, the number and shapes of the caputegulae that rim the skull in dorsal view (the nuchal, supraorbital, lacrimal, loreal, and supranarial caputegulae), and the general shapes of the squamosal and quadratojugal horns are all taxonomically important features. Information from the review of Euoplocephalus allows for the recognition of new ankylosaurid species, synonymization of other species, and resurrection of some previously synonymized species. The revised phylogenetic analysis resulted in a monophyletic Ankylosauridae consisting of Aletopelta, Gastonia, Gobisaurus, Liaoningosaurus, Shamosaurus, and a suite of derived ankylosaurids (Ankylosaurinae). There is convincing evidence for the presence of nodosaurids in Asia during the Early Cretaceous. In the mid Cretaceous, Asian nodosaurids were replaced by ankylosaurine ankylosaurids. Modifications to the tail of ankylosaurines occurred at this time, with distinct handle vertebrae appearing potentially as early as the Albian, with Liaoningosaurus. The large osteodermal knob did not appear until the Late Cretaceous. Ankylosaurines migrated into North America from Asia between the Albian and Turonian, where they diversified into a clade of ankylosaurines characterized by arched snouts and numerous flat caputegulae. There is no evidence for any ankylosaurids in Gondwana; the Ankylosauridae appears to be completely restricted to Asia and North America.