A presentation of the current data on the exceptionally diverse non-tyrannosaurid eutyrannosaur and tyrannosaurini genera and species of western North America during the end cretaceous North American interchange

Differences in proportions, anatomy, and species display features between specimens of all sizes from the latest Maastrichtian formations of the North American upper plains, greatly exceed those observed in other eutyrannosaur species, genera and subfamilies. The question is not whether more than one taxon was extant, but how many and at what ranks. Among the large Tyrannosaurus, early T. imperator retains the robust proportions, two lower functional incisors and postorbital boss features derived from earlier tyrannosaurids—the boss that includes highly atypical, large, long spindles not present in stratigraphically higher species. Among the latter, about half are robust T. rex specimens with single lower incisors, and are adorned by unique, very prominent “Mickey Mouse” postorbital display discs not previously observed in Tyrannosaurus. Contemporary T. regina skulls and skeletons are markedly more gracile, and lack either the postorbital spindles or discs. About half a dozen small remains are from juvenile specimens of Tyrannosaurus with the same tooth counts and lack of a prominent lateral dentary groove as the adults. Twice as many cannot be juveniles because their preserved hands are absolutely as large or larger than those of large adults—appendages do not atrophy with maturity in amniotes, and the feature excludes them from being tyrannosaurids. Furthermore, they have more teeth than adult specimens of Tyrannosaurus—tooth count reduction being abnormal or absent among growing reptiles. Growth arcs preserved in bone sections show these non-tyrannosaurids were growth slowing or ceasing subadults or adults, not rapidly increasing mass expansion expected in early ontogeny giants. The large-handed basal eutyrannosaurs are diverse, some but not necessarily all being Nanotyrannus or Stygivenator. Probably having evolved in Appalachia, the basal eutyrannosaurs were invading western North America (over the recently emerged land bridge), where they continued to evolve as they successfully competed with juvenile Tyrannosaurus. The result was an exceptionally diverse assemblage of tyrannosaurs at that end Cretaceous locality—caused by the rare ancient geographical event.