A new species of tyrannosaur, Daspletosaurus wilsoni, could bridge the gap between the early and late dominant predators of Cretaceous Laurasia, revealing a straightforward family tree.
Tyrannosaurus rex may be the most famous dinosaur, but there’s a lot we don’t know about its ancestry. None of its predecessors were nearly as widespread or left as many fossils as T. rex, leaving paleontologists confused about where it came from. The newly published description of D. wilsoni could change that, as it combines features of early tyrannosaurs with those of their more famous descendants.
Although D. wilsoni is new, the Daspletosaurus genus has been known from Montanan and Albertan rocks since 1970, with two species previously identified as having inhabited the area between 80 and 74 million years ago. Previous daspletosauruses, however, raised as many questions as they answered. Some paleontologists saw them as direct ancestors of each other like the biblical begats, whereas others considered them analogous to uncles and aunts on a family tree. The incompleteness of the skeletons available didn’t help matters.
D. wilsoni, however, is very useful. At around 76 million years old, its age is intermediary between the previously known daspletosauruses D. torosus and the sensitive D. horneri. Moreover, it combines features seen in earlier tyrannosaurs with those associated with T. rex and its immediate ancestors, suggesting it evolved from one and was ancestral to the other. For example, the horns around its eyes are reminiscent of early tyrannosaurs, but the eye socket itself and the large air pockets in its skull look like T. rex. There were still a few million years between D. horneri and T. rex, during which the family roughly tripled in weight.
The findings make it likely previous daspletosaurus discoveries represent a single direct lineage, into which D. wilsoni fits. Only one feature of D. wilsoni, a distinctively shaped aspect to its neck, has been found that is not also present in either its predecessor or successor.
The process of one species evolving to another through a series of direct steps is known as anagenesis (linear evolution). It’s what most people think of when they hear humans are descended from apes. However, we are now learning many species, such as ourselves and elephants, have family trees that look more like bramble bushes, known as cladogenesis. This may have pushed some paleontologists too far in the other direction, seeing cladogenesis everywhere. It now seems many dinosaur species evolved through anagenesis.
We still only have one D. wilsoni specimen, but from a single nostril poking out of a hill in 2017 excavators have managed to find the majority of the individual’s skull and much of its skeleton. They had to dig through so much rock to get all this, however, that the dinosaur in question has been nicknamed Sisyphus.
Authors Elías Warshaw, of Montana State University, and Dr Denver Fowler of the Dickinson Museum Center named the species after John "Jack" P. Wilson, who noticed the protruding nose in the course of a bumper season when Wilson and Fowler found four tyrannosaur skeletons in northeast Montana.
The description of the species is published in PeerJ.