Though Tyrannosaurus rex is considered the “king” of the dinosaurs, the “tyrant lizard” only roamed Earth for about the last two million years of the 150-million-year “Age of Dinosaurs”. Now, researchers believe they have found a missing piece of the tyrannosaur lineage.
Daspletosaurus wilsoni roamed approximately 76.5 million years ago – 10 million years before the cataclysm that caused the extinction of the large dinosaurs, including its T. rex heirs.
The ancient animal’s fossils, described in a paper published in the PeerJ journal, were found at the Judith River Formation in Montana, US.
D. wilsoni is considered a “transitional” species linking the ancestral Daspletosaurus torosus with Daspletosaurus horneri in this particular branch of the tyrannosaurid evolutionary lineage. The new species holotype is nicknamed “Sisyphus” after the arduous task of removing eight metres of rock to reveal the fossilised skeleton. It is identifiable by a unique arrangement of spiked hornlets surrounding its eye.
Sisyphus is known from a skull and partial skeleton designated BDM 107. The specimen has one of the largest skulls of any known Daspletosaurus individual, at 105 centimetres.
Though it is difficult to gauge the size of the full animal, for comparison, the largest known T. rex skulls are about 152 centimetres in length. So, D. wilsoni would have been dwarfed by its 7-tonne, 12-metre-long successors.
Though a lot is known about T. rex, the evolution of tyrannosaurids remains 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,” say first author Elías Warshaw, a Montana State University researcher, and co-author Dr Denver Fowler, Badlands Dinosaur Museum curator in a Sci.News article.
“Reconstructing the evolutionary relationships of Daspletosaurus has been hampered by the rarity of good specimens, and many palaeontologists 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 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,” the authors add.
“In this way, the new species is a ‘half-way point’ or ‘missing link’ between older and younger tyrannosaur species.”
The team’s research supports the hypothesis that Daspletosaurus species are a single evolving lineage, leading to the evolution of T. rex from this group. This suggests that different species once thought to represent diversity of animals coexisting may actually represent evolution of animals across fine segments of time.
“In the late Cretaceous of North America, many dinosaur families are represented by multiple closely-related species,” the palaeontologists explain. “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.”
So, not only does D. wilsoni help us better understand the evolution of T. rex, it may give an insight into the evolution of dinosaurs more broadly.
Read more: How many T. rexes ever lived? Billions
The upshot is that dinosaurs may have tended to evolve in this linear trajectory, rather than branching out into evolutionary “cousins”.
“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,” say the authors.
This is called anagenesis: as opposed to cladogenesis, where many species are “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,” say the authors.
“This seems to suggest that linear evolution is more widespread in dinosaurs, with branching evolution being less frequent than previously thought.”