Thanatotheristes (meaning "reaper of death" according to the original authors;[1] literally, "death harvester" in Greek: Greek, Modern (1453-);: Θάνατος,, "Death"[2] and Greek, Modern (1453-);: θεριστής,, "harvester"[3]) is a genus of tyrannosaurid dinosaur from the Late Cretaceous of Laramidia, approximately 80.1-79.5 Ma. Thanatotheristes contains only one species, T. degrootorum. Fossils of this taxon are found in the Foremost Formation of Alberta, Canada, coexisting with medium-sized ceratopsids like Xenoceratops foremostensis and small pachycephalosaurids like Colepiocephale lambei.
The holotype specimen of Thanatotheristes degrootorum (TMP 2010.5.7) is based on a right maxilla, right jugal, right postorbital, right surangular, right quadrate, right laterosphenoid, left frontal, and both dentaries. The length of the skull has been approximated to be . It was smaller than the closely related Daspletosaurus, but the holotype individual was not osteologically mature at the time of death. A referred specimen, TMP 2018.016.0001, is based solely on a partial right maxilla from another subadult individual. It was found at the Twelve Mile Coulee in the upper Herronton Sandstone of the Foremost Formation. The specific name honours John and Sandra De Groot, who discovered the type specimen.
T. degrootorum is characterised by:
The type specimen of Thanatotheristes has been estimated at 8m (26feet), based on the skull material and comparisons with related genera.[4]
The formal description of T. degrootorum erected a new clade within Tyrannosauridae, Daspletosaurini, a sister taxon to a clade comprising Zhuchengtyrannus, Tarbosaurus, and Tyrannosaurus. It includes T. degrootorum, Daspletosaurus torosus, D. horneri, and a tyrannosaurid from the Dinosaur Park Formation (FMNH PR308). The existence of Daspletosaurini shows that there is geographic segregation of clades within Tyrannosauridae, with the clade of Bistahieversor and the clade formed by Lythronax and Teratophoneus living in the south of Laramidia, and Albertosaurinae and Daspletosaurini occupying Canada and northern USA.
The cladogram below is the result of the phylogenetic analysis performed by Voris et al. (2020):
An analysis of a subadult Daspletosaurus torosus frontal by Yun, 2020 suggests that features of the bone would effectively demonstrate that autapomorphies of some tyrannosaurid taxa would be inadequate, due to the broad range in morphology of frontals within particular clades. This may suggest that any autapomorphies or synapomorphies pertaining to the frontal of T. degrootorum are inadequate, and may therefore render it a species of Daspletosaurus.[5] This identification was based on the 2003 study by Philip J. Currie.[6] A subsequent analysis of Gorgosaurus ontogeny by Voris et al, 2021 found that Philip Currie's initial identification of this frontal as Daspletosaurus was in error and that the frontal actually belonging to a Gorgosaurus. This study further reinforced the frontal autapomorphies of Thanatotheristes initially proposed by Voris et al (2020) as valid noting that Yun (2020)'s consideration of the characters as invalid was due to his misidentification of the specimen as Daspletosaurus and a misinterpretation of the frontal morphology of tyrannosaurids overall.[7]