Daspletosaurus

Did You Know?

• One large specimen of Daspletosaurus sp. (CMC VP 15826) suggests the genus could reach lengths of over 9 m, potentially up to 11 m and 4–5 MT in weight.
• Daspletosaurus had a massive skull that could reach more than 1 m in length, with bones heavily constructed and some fused for strength.
• Unique skull features included the rough outer surface of the maxilla and pronounced crests around the eyes on the lacrimal, postorbital, and jugal bones.

Discovery and naming

The first known Daspletosaurus torosus fossil, cataloged as CMN 8506, is a partial skeleton unearthed in 1921 near Steveville, Alberta, by Charles Mortram Sternberg. Sternberg initially believed it belonged to a new species of Gorgosaurus. It was not until 1970 that Dale Russell fully described the remains and designated them as the type specimen for a new genus. He coined the name Daspletosaurus from Greek words meaning "frightful lizard," with the species name torosus being Latin for "muscular" or "brawny." The skeleton includes the skull, shoulder, a forelimb, pelvis, femur, all vertebrae from the neck, torso, and hip, plus the first eleven tail vertebrae. Only one other well-known specimen exists, RTMP 2001.36.1, a fairly complete skeleton found in 2001. Both fossils come from the Oldman Formation in Alberta’s Judith River Group, which dates to the middle Campanian stage of the Late Cretaceous, roughly 79.5 to 77 million years ago. Russell later suggested that a juvenile Albertosaurus specimen (CMN 11315) from the younger Horseshoe Canyon Formation might actually be a third Daspletosaurus, which he called D. cf. torosus, extending the genus’s time range by about 3.5 million years into the Maastrichtian. This idea was not widely accepted, and further study returned the specimen to Albertosaurus sarcophagus. Additionally, a maxilla and teeth from an Edmontosaurus-dominated bonebed in the same formation were mistakenly assigned to Daspletosaurus, but all tyrannosaurid material from that site has since been confirmed as Albertosaurus.

Classification and systematics

Daspletosaurus is a member of the tyrannosaurine subfamily, a group that also includes Tarbosaurus, Tyrannosaurus, and the Alioramini. These dinosaurs are more closely tied to Tyrannosaurus than to Albertosaurus, and aside from Alioramus, they tend to have sturdy bodies with bigger skulls and longer thigh bones compared to the albertosaurines. Within this subfamily, Daspletosaurus sits in the tribe Daspletosaurini alongside Thanatotheristes. Its relationship to Tyrannosaurus is often seen as very close, with some scientists suggesting it might be a direct ancestor through a gradual evolutionary line. Gregory Paul once proposed moving D. torosus into Tyrannosaurus as Tyrannosaurus torosus, but this idea hasn't caught on widely. Many experts view Tarbosaurus and Tyrannosaurus as sister groups or even the same genus, placing Daspletosaurus as a more primitive relative. However, Phil Currie and his team argue that Daspletosaurus is actually more closely linked to Asian tyrannosaurids like Tarbosaurus and Alioramus than to the North American Tyrannosaurus.

Paleobiology

The snout of Daspletosaurus horneri may have been covered in sensory scales that helped with touch, temperature, and detecting prey, while large flat scales could have shielded its face during fights or while hunting. A fully grown Daspletosaurus from the Dinosaur Park Formation has tyrannosaur bite marks on its skull. These could have come from another species, but face-biting is common among predators like Gorgosaurus and Tyrannosaurus. Researchers Darren Tanke and Phil Currie think the bites resulted from fights over territory, food, or social rank. A younger tyrannosaurid specimen, first thought to be a Daspletosaurus but later identified as Gorgosaurus libratus, also shows healed bite marks on its face, meaning it survived the attack. Evidence for social behavior comes from a Montana bonebed containing three Daspletosaurus—a large adult, a small juvenile, and one of medium size—along with at least five hadrosaurs. Geological signs point to all being buried together at the same time, and the hadrosaur bones have many tyrannosaur tooth marks, suggesting the Daspletosaurus were eating them when they died. Currie proposes they may have hunted as a pack, though this is uncertain. Other scientists, like Brian Roach and Daniel Brinkman, argue that Daspletosaurus interactions were more like Komodo dragons, where individuals gather to feed on carcasses without cooperating, sometimes attacking and even eating each other.

Paleoecology

Daspletosaurus fossils are known only from rock layers laid down between 78 and 74.4 million years ago, during the middle to late Campanian stage of the Late Cretaceous. At that time, the Western Interior Seaway split North America in two, submerging much of modern Montana and Alberta. The Laramide Orogeny, the mountain-building event that created the Rockies, began during Daspletosaurus’s era and pushed the seaway back to the east and south. Rivers carrying sediment from those rising mountains formed the Two Medicine Formation, the Judith River Group, and other deposits. Around 73 million years ago, the seaway reversed course, advancing west and north to become the Bearpaw Sea, which left behind the extensive Bearpaw Shale. Daspletosaurus inhabited a broad floodplain along the seaway’s western edge. Large rivers watered the land, occasionally flooding and depositing fresh sediment. When water was abundant, the region teemed with life, but droughts caused mass die-offs, visible in numerous bonebeds—including one containing Daspletosaurus. Volcanic eruptions to the west periodically covered the area with ash, killing many animals while enriching the soil for future growth; these ash layers also enable precise radiometric dating. Shifting sea levels created diverse environments within the Judith River Group: offshore and nearshore marine zones, coastal wetlands, deltas, lagoons, and inland floodplains. The Two Medicine Formation, deposited at higher elevations farther inland, contrasts with the other two formations.