A new albertosaurine from the Oldman Formation, Alberta, Canada, illuminates ambiguity between albertosaurine and tyrannosaurine characteristics

Tyrannosaurid species have often been considered to have high longevity and low diversity relative to contemporaneous megaherbivores. Recent chronospecies assertions around tyrannosaurid lineage evolution have reinforced this idea. An albertosaurine, Tromerosaurus solumons, from the Oldman Formation of Alberta, Canada, shares features with Daspletosaurus and Gorgosaurus, and prompts reassessment of tyrannosaurid characteristics and the Late Cretaceous radiation of Tyrannosauridae. Maxillary morphometrics reveal clear intermediate morphologies in Tromerosaurus between Daspletosaurus and Gorgosaurus, including an anteroposteriorly longer maxillary fenestra and narrower pila promaxillaris than in Gorgosaurus libratus. An anatomical diagnosis of Tromerosaurus solumons allowed referral of specimens previously attributed to Gorgosaurus. Restricted stratigraphic overlap between Tromerosaurus and Gorgosaurus offers support to this distinction. A modified recent character-taxon dataset recovers Tromerosaurus as the earliest diverging member of Albertosaurinae under parsimony or Tyrannosaurinae under Bayesian inference. The difference in topologies along with reduced node support values for clades such as Lythronax + Teratophoneus, or Bistahieversor and other derived tyrannosaurines, suggests uncertainty as to their recovery within Albertosaurinae or Tyrannosaurinae. Together, these results support a younger divergence of the two main tyrannosaurid lineages with rapid character acquisition and transition during the Late Cretaceous (Santonian–Maastrichtian).