Tyrannosaurus rex, the most iconic dinosaur of all time, and its closest relatives known as tyrannosaurids, comprise the clade Tyrannosauroidea, a relatively derived group of theropod dinosaurs, more closely related to birds than to other large theropods such as allosauroids and spinosaurids. All tyrannosaurs were bipedal predators characterized by premaxillary teeth with a D-shaped cross section, fused nasals, extreme pneumaticity in the skull roof and lower jaws, a pronounced muscle attachment ridge on the ilium, and an elevated femoral head. The clade was a dominant component of the dinosaur faunas of the American West shortly after the emplacement of the Western Interior Seaway (about 99.5 Mya).
Daspletosaurus horneri, a new species of tyrannosaurid from the upper Two Medicine Formation of Montana, is the sister species of Daspletosaurus torosus. The new taxon was named in honor of Jack Horner, and inhabited northern Laramidia (what is now southern Alberta and northern Montana) about 75 million years ago. Paleontologist Vickie R. Clouse discovered the first specimen in 1989 and more individuals were uncovered in the following decades. The so-called Two Medicine tyrannosaurine, made its first appearance in a study co-written by Jack Horner in 1992, about the phyletic evolution in four lineages of dinosaurs, including tyrannosaurs, from the Late Cretaceous of the American West.
The holotype of Daspletosaurus horneri (MOR 590) consists of a complete skull, partial pectoral limb, and nearly complete hindlimb; and is estimated to be ~9.0 m in total length and 2.2 m tall. D. horneri has taller skull than D. torosus. Because of the excellent quality of preservation of these fossils it was possible to study the type of soft tissue that covered the face (premaxilla, maxilla, nasal, lacrimal, jugal, postorbital, squamosal, dentary). The study revealed that many of the tyrannosaur’s skull features are identical to those of crocodilians. Given the skeletal similarities with crocodylians, tyrannosaurids had a highly sensitive facial tactile system that functioned in prey capture, and object identification and manipulation, for detecting the optimal temperature of a nest site, and, in courtship, tyrannosaurids might have rubbed their sensitive faces together as a vital part of pre-copulatory play.
Thomas D. Carr, David J. Varricchio, Jayc C. Sedlmayr, Eric M. Roberts, Jason R. Moore. A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports, 2017; 7: 44942
Horner, J. R., Varricchio, D. J. & Goodwin, M. B. Marine transgressions and the evolution of Cretaceous dinosaurs. Nature 358, 59–61 (1992) doi:10.1038/358059a0