Daspletosaurus & Gorgosaurus

Judith River Formation

Tyrannosaurid fossils from the Judith River Formation are frequently available to collectors, and occasionally, similarly aged deposits like the Two Medicine Formation. However, one common mistake with this material, particularly the teeth, is that they are often labeled as Daspletosaurus or Gorgosaurus. Occasionally they are also labeled as Albertosaurus, a genus found only in younger deposits.

Daspletosaurus and Gorgosaurus have almost indistinguishable teeth with some rare exceptions. Daspletosaurus being the bigger of the two, it is common for crowns larger than 3 inches to be tentatively assigned to Daspletosaurus.

If a specimen is found in a mass mortality site of Daspletosaurus or Gorgosaurus, then a bone or tooth could be assigned to either of those two genera. More so if a piece is directly associated with a diagnostic specimen.

Tyrannosaurid material from the Campanian deposits in North America are almost always, at best, assigned as Tyrannosauridae indet. (Indeterminate tyrannosaurid).

Campanian Albertosaurus

It is not rare to see indeterminate tyrannosaurid specimens from the Campanian Judith River Formation and similar aged deposits to be identified as Daspletosaurus or Gorgosaurus. However, once in a while some of them are labeled as Albertosaurus or cf. Albertosaurus.

The confusion here may derive from the occasional lumping of Gorgosaurus and Albertosaurus, with Gorgosaurus being a synonym of Albertosaurus.

Albertosaurus sarcophagus is currently only known from the younger Maastrichtian aged Horseshoe Canyon Formation in Drumheller, Alberta, Canada. This makes true Albertosaurus fossils quite rare as they are exclusively from old collections, most often with a disposition issued by the Royal Tyrrell Museum for commercial sale. This location does also contain the even younger Scollard Formation that contains Tyrannosaurus, but the vast majority of material available from Drumheller appears to be from Horseshoe Canyon Formation.

While Daspletosaurus or a similar species was once suspected to be found in the Horseshoe Canyon Formation, all known tyrannosaurid specimens are now assigned to Albertosaurus. This makes Albertosaurus the only tyrannosaurid in the deposit, at least for now. It is common for multiple tyrannosaurids to be found in the same deposit, thus, it would not be surprising if a second species of tyrannosaur was eventually found alongside Albertosaurus.