1 Baby tyrannosaurid bones and teeth from the Late Cretaceous of western North 2 America 3 4 Gregory F. Funston1,2*, Mark J. Powers2, S. Amber Whitebone3, Stephen L. Brusatte1, John B. 5 SCannella4, John R. Horner5, and Philip J. Currie2 6 7 1 School of GeoSciences, University of Edinburgh, Edinburgh, UK;
[email protected], 8
[email protected] 9 2 Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada;
[email protected]; 10
[email protected] 11 3 Department of Biological Sciences, University of Calgary, Calgary, AB, Canada; 12
[email protected] 13 4 Museum of the Rockies, Montana State University, Bozeman, Montana, USA;
[email protected] 14 5 Honors Program, Chapman University, Orange, California, 92866;
[email protected] 15 16 *Corresponding author:
[email protected] 17 1 18 ABSTRACT—Tyrannosaurids were the apex predators of Late CretaCeous Laurasia, and their 19 status as dominant carnivores has garnered considerable interest since their disCovery, both in the 20 popular and sCientifiC realms. As a result, they are well studied and much is known of their 21 anatomy, diversity, growth, and evolution. In contrast, little is known of the earliest stages of 22 tyrannosaurid development. Tyrannosaurid eggs and embryos remain elusive, and juvenile 23 speCimens—although known—are rare. Perinatal tyrannosaurid bones and teeth from the 24 Campanian–MaastriChtian of western North AmeriCa provide the first window into this critiCal 25 period of the life of a tyrannosaurid. An embryoniC dentary (cf. Daspletosaurus) from the Two 26 MediCine Formation of Montana, measuring just three centimetres long, already exhibits 27 distinctive tyrannosaurine charaCters like a ‘Chin’ and a deep MeCkelian groove, and reveals the 28 earliest stages of tooth development.