Newly-Discovered Tyrannosaur Species Fills Gap in Lineage Leading to Tyrannosaurus rex

Daspletosaurus wilsoni, a tyrannosaurid dinosaur that lived 76.5 million years ago in what is now Montana, the United States, displays a unique combination of ancestral and derived characteristics, and represents a transitional stage linking the ancestral Daspletosaurus torosus with Daspletosaurus horneri in an evolutionary lineage evolving from 77 to 75 million years ago.

Four tyrannosaurs dispute ownership of the fresh carcass of a Centrosaurus. Image credit: Rudolf Hima / Badlands Dinosaur Museum.

Daspletosaurus wilsoni roamed our planet during the Late Cretaceous epoch, approximately 76.5 million years ago.

The ancient animal’s fossilized remains were found at the Judith River Formation near Glasgow in Montana, the United States.

Daspletosaurus wilsoni belonged to Tyrannosauridae, a family of large-bodied predatory dinosaurs that includes the well-known Tyrannosaurus rex.

Tyrannosaurids are characterized by massive skulls, thick teeth and short front limbs. They have been known from North America and Asia for over a century, yet many details of their evolutionary history remain unclear.

“Since the 1990s, debate has surrounded Daspletosaurus, a large tyrannosaurid species known from Montana and Alberta, which has been proposed to be an ancestor of Tyrannosaurus rex itself,” said Montana State University researcher Elías Warshaw and Badlands Dinosaur Museum curator Denver Fowler in a statement.

“Reconstructing the evolutionary relationships of Daspletosaurus has been hampered by the rarity of good specimens, and many paleontologists disagree as to whether these tyrannosaurids represent a single lineage evolving in place, or several closely related species that do not descend from one another.”

Daspletosaurus wilsoni is recognized by the unique arrangement of small hornlets around the eye. Image credit: Andrey Atuchin / Badlands Dinosaur Museum.

“Daspletosaurus wilsoni displays a mix of features found in more primitive tyrannosaurs from older rocks, like a prominent set of horns around the eye, as well as features otherwise known from later members of this group (including Tyrannosaurus rex), like a tall eye socket and expanded air-pockets in the skull.”

“In this way, the new species is a ‘half way point’ or ‘missing link’ between older and younger tyrannosaur species.”

The team’s findings suggest that previous research was correct in identifying several species of Daspletosaurus as a single evolving lineage, and supports the descent of Tyrannosaurus rex from this group.

But what does this tell us about dinosaur evolution?

“In the Late Cretaceous of North America, many dinosaur families are represented by multiple closely-related species,” the paleontologists said.

“These were previously thought to represent diversity, i.e. that they lived at the same time, which would be evidence of branching evolution.”

“However, a wealth of new specimens and a better understanding of their placement in time has changed what we think.”

Left maxilla (upper jaw) of Daspletosaurus wilsoni, shown in lateral (A) and medial (B) views. Scale bar – 10 cm. Image credit: Warshaw & Fowler, doi: 10.7717/peerj.14461.

“We can now see that many of these species are actually very finely separated in time from each other, forming consecutive ladder-like steps in a single evolutionary lineage where one ancestral species evolves directly into a descendant species.”

“This is called the anagenesis mode of evolution, and is contrasted with cladogenesis, where successive branching events produce many species that are closely related and therefore look similar to each other, but represent evolutionary cousins rather than ancestors and descendants.”

“Our study supports the addition of tyrannosaurs to a growing list of dinosaurs (including horned and duckbilled dinosaurs) for which anagenesis (linear evolution) has been proposed.”

“This seems to suggest that linear evolution is more widespread in dinosaurs, with branching evolution being less frequent than previously thought.”

The team’s paper appears today in the journal PeerJ.

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E.A. Warshaw & D.W. Fowler. 2022. A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana. PeerJ 10: e14461; doi: 10.7717/peerj.14461

This article is based on a press-release provided by the Dickinson Museum Center.