New diminutive dinosaur extends the North American fossil record

A new species of tyrannosauroid dinosaur is reported in a paper published this week in Communications Biology. The dinosaur is among the smallest of its kind from the Cretaceous period, and its discovery helps fill a previous 70 million-year gap in the North American fossil record.

One of the most famous dinosaurs, Tyrannosaurus rex, is recognized as a giant apex predator. However, the rest of the known tyrannosauroid species were small, subordinate hunters for most of their 100 million-year-long evolutionary history. Our understanding of the major evolutionary events leading up to T. rex and other giant tyrannosauroids has been limited by the lack of informative fossils from North America, despite a more complete fossil record in Asia.

In Emery County, Utah, USA, Lindsay Zanno and colleagues discovered fossils belonging to an unusually small tyrannosauroid. The new species, named Moros intrepidus, extends the fossil record in North America by 15 million years, filling a previous 70 million-year gap that spanned the Jurassic-Cretaceous boundary and most of the Cretaceous period. The researchers estimated that the adult M. intrepdius was only 78 kilograms and had a limb length of 1.2 meters. The authors’ analysis of M. intrepidusand its close relatives from Asia indicates that tyrannosauroids were restricted to small sizes for about 15 million years and evolved into giants like T. rex during a period of no more than 16 million years.