Tiny Tyrannosaur trod lightly

Modern science and ancient fossils are slowly letting us watch the fearsome Tyrannosaurus rex evolve, and a new discovery has filled in another part of the puzzle.

Long before it produced the giant predator everyone knows, the T.rex lineage was marked by much smaller species, known collectively as tyrannosauroids.

In April 2019, palaeontologists led by the North Carolina Museum of Natural Sciences in the US unveiled Moros intrepidus, a previously unknown T.rex ancestor that is small even by tyrannosauroid standards, coming in at about 1.2 metres and 78 kilograms.

 Now another team, led by Sterling Nesbitt from Virginia Tech, also in the US, has reported the discovery of two juvenile skeletons in the Zuni Basin of New Mexico that takes the story further.

Younger than M. intrepidus, dating to around 92 million years ago, they are also a little bigger – but still hardly huge. They are about the size of a Labrador, with a skull measuring between 25 and 32 centimetres in length.

They represent the most complete specimens of a mid-Cretaceous tyrannosauroid ever found, the researchers say, and so provide “our best glimpse yet” at what dinosaurs from this period looked like. 

The species has been named Suskityrannus hazelae: “suski” being the local Zuni word for coyote. 

Of note, the researchers say, is the fact that it had a specially adapted running foot and a strong bite force – a combination not present in either early tyrannosauroids or later, larger ones.

“This new species was phylogenetically intermediate between the grade of early-diverging, small to medium-sized tyrannosauroids that originated in the Middle Jurassic and the enormous, bone-crunching apex predator tyrannosaurids of approximately the final 20 [million years] of the Cretaceous,” they write in a paper in the journal Nature Ecology & Evolution. 

“The new species shows that several integral components of the tyrannosaurid-style body plan, including a cursorially adapted arctometatarsalian foot, evolved at small or medium body size.”

Gigantic tyrannosaurs thrived in the Late Cretaceous, from 80 to 66 million years ago, but their origins are poorly understood because of a global sampling gap associated with high sea levels in what is now North America that occurred earlier in the epoch. 

Bit by bit, that gap is being filled.

S. hazelae is important, Nesbitt and colleagues say, because it is another addition to the Moreno Hill assemblage, which “now boasts several dinosaurs represented by partial skeletons that can be placed in a phylogenetic context”.

As such, this fossil-rich part of New Mexico helps illuminate the transition between Early Cretaceous faunas and the classic, well-studied latest Cretaceous dinosaur ecosystems.

“The discovery of Suskityrannus helps fill major phylogenetic, morphological and temporal gaps in tyrannosauroid evolution and in doing so clarifies how these dinosaurs transitioned from small, gracile species into enormous apex predators that characterized the last-surviving dinosaur faunas of Laurasia,” the researchers write.