The ancestor of T-rex: Newly-discovered species may solve a long-standing mystery

But the lack of good specimens hindered the reconstruction of the evolutionary relationships of Daspletosaurus, and eventually, researchers couldn't agree if the tyrannosaurids represented "a single lineage evolving in place, or several closely related species that do not descend from one another", as per a release.

Until now.

Ancestors of T-Rex confirmed

In the latest research published in PeerJ on Monday, Elias Warshaw and Denver Fowler, research associates at Badlands Dinosaur Museum, report the discovery of a new species of Daspletosaurus, which contains a combination of features found in older tyrannosaurs - such as a set of horns around the eyes, along with features that were known from later members of the family, like T-Rex. These features included a tall eye socket and expanded air pockets in the skull.

These proved that D. wilsoni is a "halfway point" or “missing link” between older and younger tyrannosaur species, the researchers said in a statement. 

The findings substantiated previous research that identified "several species of Daspletosaurus as a single evolving lineage, and supports the descent of T. rex from this group," they said.

Dinosaurs followed linear evolution

In the Late Cretaceous period of North America, dinosaur families represented by several closely-relate species were thought to have lived at the same time. 

"However, a wealth of new specimens and a better understanding of their placement in time has changed what we think. 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," the researchers said.

The new study explains the addition of tyrannosaurs to an increasing list of dinosaurs for which linear evolution has been proposed. Known as the "anagenesis" mode of evolution, it contrasts with "cladogenesis", where successive branching produces species that are closely related and look alike. 

The research suggests that linear evolution is more prominent in dinosaurs. 

Study Abstract:

Here we describe a new derived tyrannosaurine, Daspletosaurus wilsoni sp. nov., from Judithian strata (~76.5 Ma) intermediate in age between either of the previously described species of this genus. D. wilsoni displays a unique combination of ancestral and derived characteristics, including a cornual process of the lacrimal reduced in height relative to D. torosus and more basal tyrannosaurines, and a prefrontal with a long axis oriented more rostrally than in D. horneri and more derived tyrannosaurines. The description of this taxon provides insight into evolutionary mode in Tyrannosaurinae, lending strength to previous hypotheses of anagenesis within Daspletosaurus and increasing the resolution with which the evolution of this lineage can be reconstructed. Cladistic phylogenetic methods, stratigraphy, and qualitative analysis of the morphology of relevant taxa supports an anagenetic model for the origin of morphological novelty in this genus, highlighting the predominance of anagenetic evolution among contemporary dinosaur lineages.