Multiple lines of evidence support anagenesis in Daspletosaurus and cladogenesis in derived tyrannosaurines

Abstract

Tyrannosaurines were the dominant predators of Laramidian ecosystems during the Late Cretaceous. An anagenetic hypothesis for the evolution of derived tyrannosaurines in Laramidia has received considerable attention in recent years, although studies disagree on the degree of anagenetic and cladogenetic evolution driving tyrannosaurine evolution. These studies have relied on phylogenetic results of equal weights parsimony analysis and do not consider the impact of alternative phylogenetic methodologies. Here, I build on previous work by applying maximum likelihood Bayesian inference and Extended Implied Weights parsimony analyses to competing datasets and show that both of these methods provide additional support for anagenetic evolution in tyrannosaurines. In species-level and specimen-level analyses, species of the Campanian taxon Daspletosaurus form a paraphyletic grade of tyrannosaurines leading to Tyrannosaurini. However, Daspletosaurus is shown as a distinct lineage from Tyrannosaurini by using multiple lines of evidence including the geochronology and biogeography of tyrannosaurine specimens. A fully-resolved specimen-level topology allows for the application of species delimitation techniques and reveals potentially novel metaspecies of Daspletosaurus, although no new taxa are erected here as a conservative practice. Future detailed description of the specimens analysed by this contribution and previous work may provide justification for establishing novel Daspletosaurus metaspecies. Ongoing work in refining the phylogenetics of tyrannosauroids will also aid in elucidating the true mode of evolution in Daspletosaurus. This study underscores the importance of accounting for homoplasy in specimen-level analyses by differentially weighting homoplastic characters.