New Evidence Suggests T. Rex Has Deep American Roots
A recent discovery highlights changing thought on tyrannosaur evolution.
If you had to name one dinosaur, odds are you would pick Tyrannosaurus rex. Most people are keenly aware of the tyrant lizard king, its literal bone-crushing jaws, and the terror it reigned over Mesozoic fauna. Most people are unaware that paleontologists do not know its full history.
A new study led by paleontologist Sebastian G. Dalman, published in the journal Nature, provides fresh insight into the origins of T. rex and its closely-related kin. This research depicts a significantly more American-centered genesis than previously believed.
T. rex belongs to a group of dinosaurs called tyrannosaurids. These dinosaurs had huge skulls, banana-shaped teeth, absurdly tiny forelimbs, and finely developed senses that kept them at the top of the food chain. They also had jaws strong enough to crush bone. To quote from another paper:
Tyrannosaurids characteristically bit deeply into carcasses, often through bones… Some T. rex bite marks… indicate that bone was fractured, ingested, and used for sustenance… The bite forces needed to crunch through bone would have been enormous.
Tyrannosaurids ate bone so regularly it wore down their teeth. They also ate giants with large bones.
Early in the Cretaceous (145 to 66 million years ago), tyrannosaurids had yet to evolve. They were preceded by smaller species — called tyrannosauroids — that were seldom larger than horses. It wasn’t until the final 20 million years of the Cretaceous that they emerged as massive hypercarnivores like the T. rex.
T. rex’s size in particular was incredible. It could grow up to 42 feet long (13 m) and weigh 20,000 lbs (10 tons). The Toyota Rav4 is 15 feet long and 3,500 lbs. You would need six Rav4s stacked two cars high and three cars wide to match the size and weight of T. rex.
As Dalman notes, T. rex was “the last and largest tyrannosaurid.” It was possibly the largest apex land predator to ever exist. Unfortunately, it appeared just 1 to 2 million years before the asteroid that ended the Age of Dinosaurs. It had a short run.
Until recently, tyrannosaurid evolution has been somewhat mysterious. There is a 20-million-year gap in the fossil record where the transition from small precursor species (with normal forelimbs) to large megapredators takes place. Because of these missing fossils, paleontologists struggle to understand tyrannosaurid geography: T. rex is found exclusively in the southwestern United States, while its most closely-related sister species are found in Mongolia.
Fossil evidence to this point indicated an Asian origin for tyrannosaurids. Tarbosaurus bataar, a tyrannosaurid from Mongolia (and a species actor Nicolas Cage once bought at auction), and Zhuchengtyrannus magnus, an even older tyrannosaurid from China, predate the evolution of T. rex by millions of years. This age difference suggests the tyrannosaurid ancestor spread through Asia before heading east into North America.
Another paper phrased it thusly, clarifying that T. rex would later evolve from this migrant ancestor:
The abundance of definite, large [tyrannosaurid and other closely-related fossils from Asia], which are older than Tyrannosaurus rex, favors the hypothesis of Tyrannosaurus rex as the descendant of immigrant species from Asia.
Darlan’s findings challenge this hypothesis. His team unearthed remains of a new tyrannosaurid, Tyrannosaurus mcraeensis, from the Hall Lake Formation in New Mexico. These remains are older than T. rex and shift the hypothesis toward a North American genesis.
T. macraeensis, affectionately named “NMMNH P-3698,” was part of an ecosystem of large dinosaurs local to the southwestern United States (in places like Utah, Texas, and New Mexico). This group included other giants like Sierraceratops, Hadrosaurid, and the amusingly named Alamosaurus. The North American tyrannosaurids likely evolved their massive size to prey on these gigantic megafauna.
Initially, T. macraeensis was mistaken for another T. rex. It is comparably large and shares many anatomical features. Darlan clarifies how his team changed their classification:
the characters that diagnose T. mcraeensis and differentiate it from T. rex are relatively subtle characters relating to the shape and articulation of the skull bones, but because T. rex is known from multiple individuals, it is possible to show that T. mcraeensis lies outside of the range of individual variation seen in T. rex.
Darlan continues, stating that
NMMNH P-3698 is unlike any other specimen referred to T. rex… each bone has at least one diagnostic character, it is therefore an outlier from all other specimens referred to T. rex, in every element.
Radiometric dating suggests the remains, including teeth, a mandible, and parts of the skull, predate T. rex by 7 million years. Morphological and phylogenetic analyses corroborate these findings and establish that T. mcraeensis is more closely related to T. rex than any of its Asian counterparts. They also clarify that T. macraeensis is a sister species to T. rex, not a direct ancestor.
Darlan posits two distinct tyrannosaurid lineages to explain this discovery: one that “dispersed to Asia giving rise to Tarbosaurus and Zhuchengtyrannus,” and another that stayed in the southwestern United States to yield T. macraeensis and T. rex. To phrase it differently: the direct ancestor of T. rex was no migrant.
Future findings will certainly clarify these migration patterns. We live in the Golden Age of tyrannosaur research, where new discoveries are frequent. It is possible future research might point us in another direction. For now, the United States can celebrate the T. rex-sized privilege.
