Valuable lessons from gore-kings and thieves

Oh, yes. This pic again. Lythronax by Andrey Atuchin.

The Cretaceous was undoubtedly one of the most tumultuous times in earth’s history. Life was rapidly changing: over the course of eighty million years, the world saw a wave of never-before-seen groups of life, including flowering plants (and their accompanying pollinators), snakes, and “modern” birds. By the late Cretaceous, the world would have been alien to any animal living in the Jurassic. This was a world of giants, oddballs, tyrants, and tanks. The Cretaceous, despite being the last period of the Mesozoic, was the golden era for dinosaurs worldwide.

As the Cretaceous progressed from its vaguely Jurassic beginnings to its cataclysmic end, faunal groups the world over were surprisingly homogeneous. In North America and Asia, there was a relatively consistent fauna. Generally, the families that were present in such faunal group can be linked to Asian origins. It would make sense, then, that such species, over the course of tens of millions of years, migrated from their Asian motherlands to the brave new world of western North America. With each successive generations moving farther and farther across the land bridge connecting the two continents. However, two newly-described species contest this unidirectional migration. They seem to show that our understanding of late Cretaceous faunal shifts and the evolution of the families composing these faunal groups did not take straightforward paths to reach their eventual burial sites; rather, the migration between the two continents was much more complex.

A "tyrant map" from Wiki. Click to embiggen. The abundance of
tyrannosaurids in North America, and their lack in Asia, were thought
to represent the Asian origin of Tyrannosauridae.

The first recently-described species to raise intriguing questions about dinosaur biogeography and evolution is one that has been making the media rounds lately: Lythronax agrestes, the nowfamous “goreking of the southwest.” Apart from winning Most Badass Scientific Name of the Year, Lythronax reveals interesting aspects of tyrannosaurid evolution. It is the earliest known tyrannosaurid, dating back to about 80 million years, to a time when the North American dinosaur fauna was starting to take the form it would keep until the last day of the Mesozoic. Alongside Lythronax lived some of the first centrosaurine ceratopsians, which themselves would become major ecological players in a few more million years (more on that in an upcoming post), as well as hadrosaurine hadrosaurs. What makes Lythronax special is that it dispels the idea that tyrannosaurids first evolved in Asia. The earliest tyrannosauroids, as well as several species of advanced tyrannosaurids, have been found in China and Mongolia, leading to this logical conclusion. However, it appears that tyrannosaurids may have emerged in North America, evolving their characteristic juggernaut build and binocular vision there before migrating back to Asia. And, although only one Asian ceratopsid has been described thus far, it is likely that the centrosaurines with which Lythronax shared its environment travelled the same way, migrating north in giant herds to the floodplains of Canada and Alaska before returning to Asia.

Acheroraptor by Emily Willoughby. 

Another recently-described species has actually been known for quite a while, but has only recently been given a name. Acheroraptor temertyorum is a dromaeosaurid from Hell Creek, a formation bearing rocks from the end of the Cretaceous. The fauna of this formation is unmistakable, consisting of some of the most wellknown dinosaurs of all time. Tyrannosaurus was the apex predator, stalking lowland plains and forests populated with Triceratops, Ankylosaurus, and Edmontosaurus. For many years, it was labelled as a close relative of the Canadian Dromaeosaurus, a dromaeosaurine dromaeosaurid. Dromaeosaurines were North Americanan through and through, with no other species thus far discovered outside of the continent. It would make sense to assume that Acheroraptor shares a similar story, evolving from endemic early Cretaceous dromaeosaurines. However, Acheroraptor appears to have its roots not in North America, but yet another continent: it was not a dromaeosaurine, but a velociraptorine, an almost entirely Asian group. Even at the very end of the Cretaceous, species were still migrating between the continents of the northern hemisphere.

The relationship between where a species is found and where it comes from are not as straightforward as they may seem. Evolution is an enormous tale of unexpected outcomes and unlikely beginnings, as well as the forces which dictate such results. The wealth of fossil information we have found on ancient biogeography reveals a great deal of surprising new insight into the outward reasons for their long, successful time on this earth.