Tyrannosaurid premaxillary tooth

Taxonomy

Tyrannosaur

Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Avetheropoda
Family: Tyrannosauridae
Genus: Tyrannosaurus
Species: Tyrannosaurus rex
Author Citation Osborn, 1905

Geological Time Scale

Eon: Phanerozoic
Era: Mesozoic
Period: Cretaceous
Sub Period: None
Epoch: Late
International Age: Maastrichtian

Stratigraphy

Hell Creek Formation

Provenance

Collector: Black Hills Institute
Acquired by: Purchase/Trade

Dimensions

Width: 6 mm
Height: 13 mm
Thickness: 4 mm

Location

Garfield County
Montana
United States

• tyrannosaurus rex
• tyrannosaurus
• hell creek formation
• hell creek
• premax
• premaxillary
• tyrannosaur
• tyrannosauridae
• tyrannosaurid
• juvenile tyrannosaur

Comments

"That some of these teeth are mammalian incisors there can be but little doubt..." - O. C. Marsh¹

This kind of incisor-like ("incisiform") tooth was originally thought to have belonged to a large, Cretaceous mammal.  Later discoveries revealed that these teeth were actually the front teeth ("premaxillary teeth") of Tyrannosaurs - and are now known as a hallmark of their clade, Tyrannosauroidea (along with fused nasals). Closely-spaced, parallel grooves on bones suggest that Tyrannosaurs used these teeth to selectively scrape meat from bone².

Identification

Tyrannosaurid premaxillary teeth have a "D"-shaped cross section, with the lingual face flattened, and often have an apicobasal ridge on the midline of the lingual face.

In more technical language, "...premaxillary teeth bear lingually rotated mesial and distal carinae forming a salinon cross-section at mid-crown height, and a highly convex labial aspect as in tyrannosauroids generally. In mesial/distal views carinae are sinuous, transitioning from lingually convex near the base to lingually concave approaching the occlusal surface. Carinae terminate prior to reaching the root/crown juncture. Mesial and distal aspects of the crown are depressed, yielding a weakly hourglass-shaped cross-section at the crown base... The carinae lack serrations [likely ontogenetically variable]... As in other tyrannosauroids, teeth exhibit a pronounced lingual ridge"³.

Most of the current literature supports only one Tyrannosaurid species in the Hell Creek formation, Tyrannosaurus rex, a hypothesis subject to change in light of new evidence. 

Comments

This tooth exhibits some antemortem wear at the apex (pictured), on the carinae, and near the base of the lingual apicobasal ridge. Given the size, this is from a juvenile animal (smaller than "Jane", BMRP 2002.4.1).

References

1. Marsh, O.C., 1892, "Notes on Mesozoic vertebrate fossils", American Journal of Science, 44: 170-176

2. David W.E. Hone and Mahito Watabe, "New information on scavenging and selective feeding behaviour of tyrannosaurs", Acta Palaeontologica Polonica 55 (4), 2010: 627-634 doi: http://dx.doi.org/10.4202/app.2009.0133

3. Zanno, L., Tucker, R.T., Canoville, A. et al. Diminutive fleet-footed tyrannosauroid narrows the 70-million-year gap in the North American fossil record. Commun Biol 2, 64 (2019). https://doi.org/10.1038/s42003-019-0308-7