Skeletal morphology of Kritosaurus navajovius (Dinosauria: Hadrosauridae) from the Late Cretaceous of the North American south-west, with an evaluation of the phylogenetic systematics and biogeography of Kritosaurini

Abstract

The osteology of the hadrosaurid dinosaur Kritosaurus navajovius (late Campanian of southern North America) is documented in detail, and the taxonomy and phylogenetic relationships of the genus are revised. Kritosaurus is rediagnosed based on the extensive length of the dorsolateral margin of the maxilla and a unique combination of characters that includes a jugal with orbital constriction deeper than infratemporal one, infratemporal fenestra greater than orbit and with dorsal margin greatly elevated above dorsal orbital margin in adults, frontal participating in orbital margin, and paired caudal parasagittal processes of nasals resting over frontals. The taxonomy of numerous hadrosaurid specimens previously referred to Kritosaurus is reassessed; the vast majority of these cannot be positively referred to Kritosaurus. One exception is a specimen collected from the Cerro del Pueblo Formation that extends the geographical range of K. navajovius further south in Laramidia, to present-day northern Mexico. Anasazisaurus is regarded a junior synonym of Kritosaurus; their holotypes are indistinguishable from each other when considering the overlapping elements. However, many characters support distinction of Naashoibitosaurus ostromi as a valid taxon. Kritosaurus, consisting of the sister species K. navajovius and K. horneri, is deeply nested within Saurolophinae as a member of Kritosaurini. The latter clade includes also Naashoibitosaurus, Gryposaurus, and the South American Secernosaurus. Kritosaurini is characterized by a rostral nasal dorsal process not reaching the rostral margin of the narial foramen, frontal with triangular rostrolateral projection ending in a narrow apex (convergent in Brachylophosaurini), and a subrectangular dorsal region of infratemporal fenestra, among other characters. Kritosaurin hadrosaurids are hypothesized to have originated in southern Laramidia no later than the early Campanian. Subsequently, members of the clade reached northern Laramidia and South America via dispersal no later than the early and late Campanian, respectively.