The article addresses the question of how to distinguish between a young dinosaur (Tyrannosaurus) and a separate species of Nanotyrannus in the paleontological record.[1] Scientists Griffin et al. and Zanno and Napoli using multiple approaches demonstrated that Nanotyrannus is a separate species, not just a juvenile Tyrannosaurus.[1] Analysis of the skeleton of a small tyrannosaurus found in Montana showed signs of epiphyseal closure (EFS) in the bony tissues and a number of anatomical features that defied the variability expected during Tyrannosaurus growth, including the number of teeth, position of sockets, and hand size.[1] These findings highlight the importance of a comprehensive approach in determining species in extinct animals.[1] The article points out that a deeper understanding of evolution and its limitations is an important direction for paleontology.[1] The case of Nanotyrannus demonstrates how much remains unknown about ontogeny (development of an individual) in both modern and extinct animals.[1]