Introducing Thanatotheristes degrootorum, the Reaper of Death

Thanatotheristes degrootorum. Illustration by Julius Csotonyi

Tyrannosauroidea, the superfamily of carnivorous dinosaurs that includes the iconic Tyrannosaurus rex, originated in the Middle Jurassic, approximately 165 million years ago, and was a dominant component of the dinosaur faunas of the Northern Hemisphere. 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. But for most of their evolutionary history, tyrannosauroids were mostly small-bodied animals and only reached gigantic size during the final 20 million years of the Cretaceous. To explain their geographic dispersal through Laurasia, one phylogenetic hypothesis suggested a scenario where basal tyrannosaurids (i.e., albertosaurines and Daspletosaurus) occurred in northern locations (Alberta and Montana) and derived tyrannosaurids (i.e. Bistahieversor, Lythronax, and Teratophoneus) occurred farther south (New Mexico and Utah), with the Tyrannosaurus evolving from the southern group during the Maastrichtian.

The recently described Thanatotheristes degrootorum, from the middle Campanian of the Foremost Formation of Alberta, Canada, is the oldest tyrannosauroid known from Canada and is approximately 2.5 million years older than Gorgosaurus libratus and Daspletosaurus torosus from the Oldman and Dinosaur Park formations. This new specimen provides a new tool to understand the evolution and paleogeographic distribution of Tyrannosauridae.

Jaw bones of Thanatotheristes degrootorum. Image credit: Jared Voris

The holotype (TMP 2010.5.7) includes the right maxilla, right jugal, right postorbital, right surangular, right quadrate, right laterosphenoid, left frontal, and both dentaries. The presence of a well-developed ornamentation on the maxilla suggests the individual was probably sexually mature. A referred specimen (TMP 2018.016.0001) is based on a partial right maxilla from a subadult individual, and it was found approximately 10 km northeast of Hays, Alberta. The name derived from Thanatos the Greek god of death, and theristes (Greek), harvester or reaper. The specific name honors John and Sandra De Groot, who discovered the holotype specimen.

The new taxon is one of the earliest tyrannosaurid from North America, and is roughly equivalent in age to Dynamoterror dynastes from the Menefee Formation of northern New Mexico, but is slightly younger than Lythronax argestes from the Wahweap Formation of southern Utah.

Thanatotheristes skull. Image credit: Jared Voris

Phylogenetic analysis suggests that Thanatotheristes is most closely related to Daspletosaurus. Together, they form the clade Daspletosaurini, a group of long and deep-snouted tyrannosaurines. Thanatotheristes differs from Daspletosaurus in several characters. For instante, in Thanatotheristes the contact surface for the jugal on the postorbital is braced by a robust ridge that extends to the posterior margin of the postorbital. Furthermore, Thanatotheristes lacks two synapomorphies diagnostic for Daspletosaurus. First, the posterior bifurcation of the antotic crest of the laterosphenoid is an indistinct ridge rather than a prominent shelf-forming crest. Second, the symphyseal surface of the dentary only displays low anteroposterior ridges and lacks large interlocking bony projections.

The addition of more basal tyrannosaurid clades reveals that at least five major lineages evolved within Tyrannosauridae, with different skeletal morphotypes linked to differences in paleoecology, such as prey composition or hunting/feeding strategies.

 

References:

Voris, J.T., Therrien, F., Zelenitsky, D.K., Brown, CM., A new tyrannosaurine (Theropoda:Tyrannosauridae) from the Campanian Foremost Formation of Alberta, Canada, provides insight into the evolution and biogeography of tyrannosaurids, Cretaceous Research, https://doi.org/10.1016/j.cretres.2020.104388.

 

Meet Ferrisaurus sustutensis, the iron lizard from the Sustut River.

 

Preserved elements of the holotype of Ferrisaurus sustutensis. From Arbour and Evans, 2019.

In 1971, during uranium and thorium exploration in the Sustut Basin of northern British Columbia, Canada, Kenny F. Larsen registered above-background levels of radiation from a talus slope near the confluence of Birdflat Creek and the Sustut River. The source of this radiation were the fossil remains of an unknow dinosaur. Larsen, an economic geologist, donated the bones to the Dalhousie University (Halifax, NS) in 2004. Later, the specimen was accessioned into the collection of the Royal British Columbia Museum in Victoria, BC.

Initially described as a pachycephalosaur or a basal ornithopod similar to Thescelosaurus, a new study determined that the remains belongs to a new genus and species: Ferrisaurus sustutensis. The holotype (RBCM P900) includes portions of the pectoral girdles, left forelimb, left hindlimb, and right pes. The name derived from Latin ferrum (=iron) and Greek sauros (=lizard), referencing to the specimen’s discovery along a railway line. The specific name honors its provenance near the Sustut River and within the Sustut Basin.

Pedal elements of Ferrisaurus sustutensis compared to other Laramidian small-bodied ornithischians. From Arbour and Evans, 2019.

Despite the lack of cranial material Ferrisaurus can be placed within leptoceratopsids based on several aspects of the preserved phalanges. Leptoceratopsids were short-frilled, hornless ceratopsians that lived in Campanian–Maastrictian aged dinosaur assemblages from Asia and North America. Ferrisaurus measured about 1.75 metres in length and 150 kilograms in weight and is similar in size to large specimens of Leptoceratops and Cerasinops.

 

References:

Arbour VM, Evans DC. 2019. A new leptoceratopsid dinosaur from Maastrichtian-aged deposits of the Sustut Basin, northern British Columbia, Canada. PeerJ 7:e7926 https://doi.org/10.7717/peerj.7926

Arbour, V. M., & Graves, M. C. (2008). An ornithischian dinosaur from the Sustut Basin, north-central British Columbia, Canada. Canadian Journal of Earth Sciences, 45(4), 457–463. doi:10.1139/e08-009