The discovery of an embryonic tyrannosaur of an as-yet-unknown genus suggests that tyrannosaurids developed their distinctive skeletal features while developing in the egg. Furthermore, the size of the specimen, a dentary from the lower jaw found in the Two Medicine Formation of Montana in 1983 and a foot claw found in the Horseshoe Canyon Formation in 2018 and described in 2020, suggests that neonate tyrannosaurids were born with skulls the size of a mouse or similarly-sized rodents and may have been roughly the size of a small dog at birth. The jaw specimen is believed to have come from an animal roughly while the claw is believed to belong to a specimen measuring around . While eggshells have not been found in association with either specimen, the location where these neonate tyrannosaurids were uncovered suggests these animals were using the same nest sites as other species they lived with and preyed upon. The lack of eggshells associated with these specimens has also opened up speculation to the possibility that tyrannosaurids laid soft-shelled eggs as the genera "Mussaurus" and "Protoceratops" are believed to have done.