Collectors, sellers on eBay and some dealers periodically ask me to help them in the identification of tyrannosaur type teeth. So I thought I would put something together to share my understanding of these teeth. I am focusing on teeth from the formations that are collected most frequently and available for sale. These include Hell Creek, Lance, Judith River, Two Medicine and the Canadian Formations of the western Provinces. Starting with the Campanian (72-83 mya) deposits there are three types of Tyrannosaurids present: Gorgosaurus, Albertosaurus, and Daspletosaurus being the largest. Teeth of these dinosaurs can exceed 4" (10cm) but most being sold or found are less than 3". Paleontologists views on identification of these teeth has changed over the years and is still evolving. A paper that came out in 2005 which was authored by Phil Currie et al. studied teeth from this period looking at tooth and serration morphology. Their conclusion was that "it is difficult to quantifiably distinguish these teeth reliably by taxon" . Bottom line, in my opinion, all these type of teeth should currently be identified as Tyrannosaurid indet. until further notice Having said that if you happen to be the proud owner of tooth around 4" its fair to say its probably from a Daspletosaurus. I'm not aware of any Gorgo or Alberto skull with teeth that large but have not seen them all. In the Maastrichtian (66-72 mya) deposits there are two Tyrannosaurid present: T-rex and the controversial Nanotyrannus Serrations need to be approximately the same size on both anterior and posterior sides Everyone is focused on the belief that T-rex serrations need to be under 2/mm for it to be a Rex. The facts do not support that belief. First the Campanian Tyranno study showed that tooth serrations do not aid in distinguishing between taxon and that study included juvenile T-rex teeth. Second, since I did not have a similar Maastrichtian research paper to fall back on I did an unscientific study with teeth from my collection. I sampled a dozen maxillary and dentary teeth from both Nano's and Rex's. Nano teeth ranged in size from 7/16" to 2" and Rex teeth were 1/4" to 4 1/2". I used both juvenile and adult teeth. (all counts were done over a 5mm spread on the distal side mid tooth) My findings were interesting and surprised me. In both species the number of serrations decreased in quantity as the tooth became larger. The serration count results:   Nano's : Range from 4.5/mm to 2.7/mm Size: 7/16 to 1" : 4.5 to 3.4/mm 1" to 2" : 3.1 to 2.7/mm Rex's: Range 4.3/mm to 1.6/mm. Size: 1/4 to 1" : 4.3 to 3/mm 1" to 2" : 3.4 to 2.5/mm 2.5 to 3.5" : 2 to 1.8/mm 4.5" : 1.6/mm So how do you tell the difference between Nano and Rex. Well if its 2 1/2" or larger and has bulk its clearly Rex regardless of what the serrations say. It cannot be anything else. The problem arises with smaller teeth, you cannot go by serration count since they are similar. Clearly some small maxillary teeth will mimic each other and those have to be identified as Tyrannosaurid indet. However there are clear morphological difference in smaller teeth. The best way to distinguish these teeth is first in the cross section at the base and second compression Rex dentary teeth are oval at the base and you can see that in most maxillary teeth. Nano teeth are unique as tyrannosaurids go they are compressed and their cross section at the base is rectangular. A good technical way to see Rex teeth is that they are fat. Here are examples of the cross sections of couple small Rex teeth under 1 1/2 inch and Adult Nano's Rex (teeth are oval)   Example of the shape of these teeth at the base.  Although these are tyrannosaurids from the Judith it's applicable to T-rex     Morphometry of the teeth of western North American tyrannosaurids and its applicability to quantitative classification Article (PDF Available) in Acta Palaeontologica Polonica 50(4):757–776 · April 2005 with 762 Reads     Nano (teeth are rectangular) Compression is other attribute. Rex - (teeth are fat) Nano (teeth are flatted) . So if you are comfortable distinguishing between the two you can own a Rex tooth at a price that is much more affordable than the big one. It will not be the statement piece but its still Rex. I have not said anything about the Tyrannosaur Aublysodon and still see premaxillary teeth, those without serrations sold under that name. This dinosaur is considered nomen dubium and teeth ascribed to it belong to other Tyrannosaurs. I also have not said anything about premaxillary teeth. These should all be identified as Tyrannosaurid indet. with one exception T- rex. If you have one where the length is greater than 1 1/2" it Rex and enjoy. These of course are my opinions and I understand that there is a different views by some on Nanotyrannus and that's okay. We will convert you someday