Abstract
A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic (ancestor-descendant) lineage of Late Cretaceous tyrannosaurids. In concert with other anagenetic lineages of dinosaurs from the same time and place, this suggests that anagenesis could have been a widespread mechanism generating species diversity amongst dinosaurs, and perhaps beyond. We studied the excellent fossil record of the tyrannosaurid to test that hypothesis. Phylogenetic analysis places this new taxon as the sister species to Daspletosaurus torosus. However, given their close phylogenetic relationship, geographic proximity, and temporal succession, where D. torosus (~76.7–75.2 Ma) precedes the younger new species (~75.1–74.4 Ma), we argue that the two forms most likely represent a single anagenetic lineage. Daspletosaurus was an important apex predator in the late Campanian dinosaur faunas of Laramidia; its absence from later units indicates it was extinct before Tyrannosaurus rex dispersed into Laramidia from Asia. In addition to its evolutionary implications, the texture of the facial bones of the new taxon, and other derived tyrannosauroids, indicates a scaly integument with high tactile sensitivity. Most significantly, the lower jaw shows evidence for neurovasculature that is also seen in birds.
Highlights
A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic lineage of Late Cretaceous tyrannosaurids
A hypothesis of anagenesis amongst tyrannosaurids (D. torosus ->D. horneri ->T. rex) is defensible if: (1) the taxa are sister species or a phylogenetically successive series of species, (2) the species are stratigraphically sequential, (3) the phylogenetic relationships do not conflict with their stratigraphic sequence, and the taxa (4) are from the same landmass or adjacent landmasses that had connections that do not conflict with their chronological sequence
Our new phylogeny (Fig. 2A) shows that D. torosus and D. horneri are sister species, whereas T. rex is nested in a separate clade
Summary
A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic (ancestor-descendant) lineage of Late Cretaceous tyrannosaurids. We studied the excellent fossil record of the tyrannosaurid to test that hypothesis Phylogenetic analysis places this new taxon as the sister species to Daspletosaurus torosus. We report here the discovery of a new large tyrannosaurine (Figs 1–4) from the Upper Cretaceous of Montana Phylogenetic analysis places this taxon as the sister species of Daspletosaurus torosus (Fig. 2). The new species adds to the number of late Campanian tyrannosaurines and shows that this clade was a dominant component of the dinosaur faunas of the American West shortly after the emplacement of the Western Interior Seaway, in contrast to the depauperate, and geographically and temporally restricted Albertosaurus lineage that makes up the sister lineage of Tyrannosaurinae. Correspondence and requests for materials should be addressed to T.D.C.
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