Moros intrepidus was a small tyrannosaur that lived 96 million years ago

An article published in the journal “Communications Biology” reports the identification of a new species of tyrannosauroid, the superfamily of carnivorous dinosaurs the famous T.rex belongs to. A team of researchers examined a very incomplete skeleton dating back to about 96 million years ago and named the species, which lived in today’s Utah, Moros intrepidus. It’s the oldest tyrannosauroid discovered so far in North America and this can help to understand the evolution of these dinosaurs in the larger forms that lived later.

Tyrannosauroids (Tyrannosauroidea) probably appeared in the Mid Jurassic period and became extinct together with the other dinosaurs at the end of the Cretaceous period. Some fossils belonging to this superfamily show more or less evident traces of plumage and in recent years one of the major controversies among paleontologists concerns precisely the possibility that these and other dinosaurs, including Tyrannosaurus rex (T.rex), had feathers or not. Another question far from clear is how these animals became dominant during the Cretaceous period starting from species weighing less than 100 kilograms (about 220 lbs) ousting other groups such as allosaurs, which at the beginning of that period, almost 150 million years ago, were much bigger.

In 2013, paleontologist Lindsay Zanno of North Carolina State University discovered a bone in the Cedar Mountain Formation and later other bones that formed the leg and a few teeth of a dinosaur as the top image shows (Lindsay E. Zanno, Ryan T. Tucker, Aurore Canoville, Haviv M. Avrahami, Terry A. Gates & Peter J. Makovicky. CC 4.0). Even a few bones can provide a lot of information, such as that it was a specimen close to adulthood at 6-7 years. Judging from its leg its height at the hips was estimated at about 120 centimeters (3’9″) and the estimates of its weight made based on the characteristics of similar species are of about 80 kg (a little more than 170 lbs).

An interesting discovery is that Moros intrepidus had characteristics more similar to the tyrannosauroids that lived in today’s Asia at the beginning of the Cretaceous than to the North Americans. This suggests that this species’ ancestors were also Asian and at some point migrated to today’s North America by exploiting the mainland bridge that existed at the time. Migrations between continents have been noted for other dinosaur groups as well so this case is not exceptional.

Moros intrepidus was small by the standards of tyrannosauroids but it was also very fast. Along with adaptations such as the advanced sensory abilities that its superfamily already showed in such ancient species, this suggests that it was a predator. Such a dinosaur could hunt small animals and keep away from predators that were bigger but not as fast such as allosaurs.

Probably these are the characteristics that allowed tyrannosauroids such as Moros intrepidus to evolve into other species that became bigger. The lack of fossils makes it difficult to reconstruct the rise of this group of dinosaurs and for this reason even the few bones found of this species are valuable to get an idea of their evolution’s timescale and of their migrations. This discovery suggests that within about 15 million years at least some species doubled in size and then culminate in the T.rex.

Fossils from other organisms, both animals and plants, have also been found in the area where the bones of Moros intrepidus were discovered. Lindsay Zanno and her collaborators are working to study that ancient ecosystem to reconstruct it and better understand which species lived in that period and possibly which ones lived in the previous and subsequent periods to better understand their changes. As often happens in the field of paleontology, each new discovery can add important information, in this case also on the events that led to the birth of the iconic T.rex.

Moros intrepidus reconstruction (Image courtesy Jorge Gonzalez)
Moros intrepidus reconstruction (Image courtesy Jorge Gonzalez)

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