The taxonomics of the diverse, lithe basal eutyrannosaur genera and species of late Maastrichtian western North America

Abstract

In 1946 Gilmore observed that the Hell Creek Formation portion of the late Maastrichtian TT-zone contained more tyrannosaur taxa than just titanic Tyrannosaurus rex, based on a small, ontogenetically mature skull with a higher tooth count than the tyrant lizard. Over four decades later the skull was assigned to its own genus, Nanotyrannus lancensis. Also tagged shortly after was the genus Stygivenator molnari. At the turn of the century it was contended that all TT-zone tyrannosaurs are adult or juvenile T. rex, a view that became accepted by many. Because of the unusual morphological alterations that this hypothesis requires, it was consequently proposed that during ontogeny T. rex experienced a peculiar fish like metamorphosis. Detailing normal amniote growth more broadly than prior work on the subject, this analysis rejects radical ontogenetic speculations, in favor of Tyrannosaurus having shared much the same average amniote/diapsid conventional growth observed in its fellow tyrannosaurids and especially tyrannosaurins. Actual juvenile Tyrannosaurus specimens show the same tooth count and other osteological features observed in the adults, negating the need for the metamorphosis not observed in any amniotes. Most lesser sized TT-zone tyrannosaurs possess a number of features that indicate they are not only not Tyrannosaurus, but represent an array of basal eutyrannosaur taxa that are neither Nanotyrannus nor Stygivenator, with high tooth counts, and at least in some examples elongated hands, prominent dentary grooves and other features. Two new taxa are correspondingly named, Gilmorelarsontyrannus lethaeus and Elegansvenator zannoae. These are postulated to represent at least in part the migration of basal eutyrannosaurs from former Appalachia into the Laramidia region via the new Laralachia land bridge. The total number of tyrannosaur taxa now known to have inhabited the TT-zone for over 1 MA is seven species in five genera from multiple subfamilies, with a smaller portion of this collection extant at any given stratigraphic level.