Reassessment of a juvenile Daspletosaurus from the Late Cretaceous of Alberta, Canada with implications for the identification of immature tyrannosaurids

Jared T Voris,François Therrien,Darla K Zelenitsky,Philip J Currie

doi:10.1038/s41598-019-53591-7

Jared T Voris, François Therrien + Show 2 more

Open Access

https://doi.org/10.1038/s41598-019-53591-7

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Abstract

Daspletosaurus is a large tyrannosaurine found in upper Campanian deposits of Alberta and Montana. Although several large subadult and adult individuals of this taxon are known, only one juvenile individual, TMP 1994.143.1, has been identified. This specimen has played a key role in the idea that juvenile tyrannosaurid individuals are difficult to differentiate among species. Here the taxonomic affinity of TMP 1994.143.1 is reassessed in light of a juvenile tyrannosaurine postorbital recently discovered in the Dinosaur Park Formation of Alberta. Anatomical comparisons and phylogenetic analyses reveal that TMP 1994.143.1 is referable to the albertosaurine Gorgosaurus libratus, whereas the new postorbital belongs to a small juvenile Daspletosaurus. This taxonomic reassignment of TMP 1994.143.1 results in the juvenile ontogenetic stage of Daspletosaurus being known only from two isolated cranial elements. The new postorbital provides insights into early Daspletosaurus ontogeny, revealing that the cornual process developed earlier or faster than in other tyrannosaurids. Although some ontogenetic changes in the postorbital are found to be unique to Daspletosaurus, overall changes are most consistent with those of other large tyrannosaurines. Our results also show that diagnostic features develop early in ontogeny, such that juveniles of different tyrannosaurid species are easier to differentiate than previously thought.

Highlights

The discovery of juvenile specimens of tyrannosaurids in the past 30 years has greatly improved our understanding of the life history of these non-avian theropods[1,2,3,4,5,6,7,8,9]
Detailed anatomical examination and phylogenetic analyses of these and other tyrannosaurid specimens reveal that TMP 1994.143.1 is a juvenile-subadult Gorgosaurus libratus, whereas the small isolated postorbital TMP 2013.18.11 belongs to a juvenile Daspletosaurus
The specimen can be unambiguously identified as Daspletosaurus based on the presence of one autapomorphy of the genus: the dorsal articular surface for the squamosal extends to a point posterior to the anterior margin of the laterotemporal fenestra[9]

Summary

Introduction

The discovery of juvenile specimens of tyrannosaurids in the past 30 years has greatly improved our understanding of the life history of these non-avian theropods[1,2,3,4,5,6,7,8,9]. Studies have shown that TMP 1994.143.1 appears to share few features with Daspletosaurus (and other tyrannosaurines) but surprisingly many with albertosaurines[4,13] These similarities may have played a large part in the idea that juveniles of albertosaurines and tyrannosaurines (and most tyrannosaurids) possess few diagnostic characteristics, rendering species difficult to differentiate among juveniles. Detailed anatomical examination and phylogenetic analyses of these and other tyrannosaurid specimens reveal that TMP 1994.143.1 is a juvenile-subadult Gorgosaurus libratus, whereas the small isolated postorbital TMP 2013.18.11 belongs to a juvenile Daspletosaurus. These results alter our understanding of Daspletosaurus ontogeny, they demonstrate that juvenile albertosaurines and tyrannosaurines, and probably juvenile tyrannosaurids in general, can be more readily differentiated than previously thought

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