An evolutionary "missing link" species was just discovered, and it was a very big and scary link in a very big and famous chain.

The newfound species, Daspletosaurus wilsoni, is thought to be the direct ancestor of Tyrannosaurus Rex, and had a unique set of facial features like a set of ridges called "hornlets" above its eye socket.

Everyone knows T. rex, the giant almost 10,000 pound dinosaur that stood atop the pinnacle of the Cretaceous Era food chain, but scientists don't know how it came to be, or what it evolved from.

The family Tyrannosauridae contains 9 known animals, and it's believed that the genus Daspletosaurus were the progenitors of the genus Tyrannosaur. However a lack of substantial fossil remains has limited the possibility of drawing that connection until now.

Daspletosaurus wilsoni was identified from parts of a fossilized skull and skeletal fragments, including a rib and toe bone, that date to about 76.5 million years ago during the Cretaceous period (145 million to 66 million years ago).

Paleontologists from the Badlands Dinosaur Museum in North Dakota uncovered the fossils at the Judith River Formation in northeastern Montana, between 2017 and 2021, according to a new study published November 25th in the journal Paleontology and Evolutionary Science.

The team initially stumbled across the fossils after crewmember Jack Wilson noticed a small, flat piece of bone projecting out from the bottom of a cliff, which later turned out to be part of the dinosaur's nostril. Excavating the bones, however, proved to be immensely challenging because they were buried beneath 26 feet (8 meters) of solid rock.

The researchers had to painstakingly chisel away large parts of the cliff with jackhammers before they could even start excavating the individual bones. Rather than something like "Sue," the team named the specimen "Sisyphus" after the Greek character of legend who was forced by Hades to push a boulder up a mountain for eternity.

The new species 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 T. rex), like a tall eye socket and expanded air-pockets in the skull. In this way, D. wilsoni is a "half way point" or  "missing link" between older and younger tyrannosaur species.

In the Late Cretaceous of North America, many dinosaur families are represented by multiple closely-related species. These were previously thought to represent diversity, ie. 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.

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.

The new 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.

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