I am writing this guide to help fossil collectors with that often asked question, "How do I know it's really a T.rex tooth?"  Now, I am a collector, not a paleontologist, but do have some information that should be helpful.  Although the question has been frequently asked, I am unaware of any reference that spells out all the unique characteristics of T.rex teeth.  Usually inquiries are met with broad generalities, such as "Rex teeth are fatter than other teeth," or, "Rex teeth are like bananas," or, "Rex teeth are like railroad spikes."

T.rex teeth are highly sought after and command very high prices.  When a kid walks into your fossil room, looks it all over and asks, "Where is T.rex.?" what's a collector to do?  The problem for the buyer is that most of the teeth claimed to be from T.rex on Ebay are not from T.rex, or are, at best "iffy."   Sellers have a strong financial incentive to misrepresent non-T.rex or questionably T.rex teeth as being T.rex teeth.  

Many claim teeth to be from "The African T.rex," which is an animal that never existed.  Such teeth are usually from Carcharodontosaurus, a large therapod dinosaur from Northern Africa that lived 35 million years before T.rex.  It is thought to have been an allosaurid, not a tyrannosaurid, having thin, blade-like teeth.  It was a very cool animal.  But it was not an "African T.rex." 

Another tooth type that is often claimed to be from T.rex, but maybe isn't, is that of Nanotyrannus.  The "Nano-rex" is thought to be a smaller tyrannosaurid that lived at the same time and place as T.rex, but probably weighed under 1000 lb.  Some believe it to have been a juvenile T.rex, and the question is not settled yet. Paleontologist Bob Bakker believes it was a large raptor that hunted in packs.  Others believe it is not a valid genus at all, and should be considered a juvenile T.rex.  There is a trend in thinking among paleontologists that many dinosaurs, including T.rex, were proportionally quite different, behaved quite differently and filled different ecological niches at different times in their growth and development.  Growth series fossils of Albertosaurus (pre-T.rex) do point in this direction, and juvenile Albertosaurus appears to be as lithe and fast as adult ornithomimids, which could probably run 40 mph.  Part of the problem is that definite, agreed-upon juvenile T.rex fossils are lacking, so we really don't know for sure what the teeth of juvenile T.rex looked like.  The teeth attributed to Nanotyrannus, however, have smaller serrations, are smaller, and are thinner than more definite adult T.rex teeth.  However, juvenile T.rex teeth MAY have these characteristics also, as seen in Jane, the "probable" juvenile T.rex at the Rockford, IL Burpee Museum.  Jane has more teeth in her mouth than an adult T.rex does, and they are proportionally smaller.  However, there is disagreement as to whether Jane is a T.rex or some other tyrannosaur.  At this point I would have to say that Nanotyrannus teeth and juvenile T.rex teeth may be indistinguishable.  They are also much more commonly found than the larger, thicker adult T.rex teeth.  I known this sounds confusing, but this is actually a very typical situation in dinosaur paleontology.  Most dinosaurs are known from only bits and pieces and incomplete to non-existant growth series. Much of what the public thinks is known for sure about dinosaurs is really just conjecture.

Two other large tyrannosaurs, Albertosaurus and Daspletosaurus, lived about 8 million years earlier than T.rex and are thought to have been evolutionary forerunners to T.rex.  Their fossils are found in different formations than T.rex.  Their teeth, like those of Nanotyrannus, tend to have smaller serrations and are thinner than T.rex, in general.  The teeth of Daspletasaurus, however, can be just as large and almost as fat as those of T.rex.  Note that Nanotyrannus, Albertosaurus and Daspletosaurus are all Tyrannosaurs, which makes them part of the group of dinosaurs that include Tyrannosaurus rex, and separate from other large meat-eating dinosaurs or therapods.  The similarity of some of the teeth from these dinosaurs to some T.rex teeth shows the importance of knowing the location and the geological formation of origin of a tooth.

There are even more problems you are up against when trying to determine whether or not a tooth is from T.rex.  One is that therapod dinosaur teeth are not like mammal teeth.  You look at a mammal tooth, and if you know your stuff, you know what animal it came from.  Therapod dinosaur teeth are not nearly that specific.  Regardless of what animal it came from, they all are roughly triangular shaped and have serrations.  They are not unique enough to define a species.  The other issue that arises is that the shape of tyrannosaur teeth varies according to their position in the mouth.  In general, the very front teeth will have a somewhat D-shaped cross section, the corner teeth will be more round in cross section, and side teeth will be flatter.  Further, upper (maxillary) side teeth are thinner than lower side teeth.  To make it even tougher, there are individual variations within a species.  I would encourage you to look at museum specimens and casts to see how much variation there really is. 

The bottom line is that to be 100% certain that a tooth is from T.rex, you need to find it in a T.rex jaw!  However, you can greatly IMPROVE THE CERTAINTY that a tooth is from a T.rex by studying various features.  First, if it is over three inches long and looks like a T.rex tooth (fat and relatively straight), and you KNOW it came from a T.rex containing formation, (Hell Creek, Lance, Scollard, etc)  then that is probably what it is.  Rex teeth grew until they broke off or were replaced after about two years in the mouth.  However, most T.rex teeth you will see on the market are 2 inches or less in length.  My personal theory is that most of these smaller rex teeth represent rear side teeth that broke off in bone crushing feeding activity, so didn't have a chance to get larger and had to be more frequently replaced..

Tyrannanosaur teeth curve back less than other therapod teeth, and T.rex teeth tend to be even straighter.  This varies with tooth position in the the mouth, however, and corner teeth have the most curvature.  Rex side teeth tend to have little backwards curvature compared to other therapod dinosaurs.

Next, look at the fatness of the tooth.  Athough T.rex side teeth must have a side to side width at least 60% of the front to back width (a tyrannosaur definition), the likelihood that the tooth is from T.rex will increase as the fatness increases.  All tyrannosaur side teeth  tend to have a block-like oval cross sectional shape, but T.rex is even more extreme in this blocky tendency.  Oval cross sectioned corner teeth and D-shaped cross sectioned front teeth are even fatter.  Most other non-tyrannosaur therapods have a more fusiform or spindle-shaped cross sectional shape., with less variation related to mouth position. 

Finally, look at the serrations.  Tyrannosaurs have broader serrations side to side  than other therapods and the space between them forms the shape of an inverted goblet.  Nanotyrannus teeth will have about 30 serrations per cm.  Albertosaurus and Daspletosurus will have more like 25 serrations per cm.  When you get to 20 serrations per cm, you are into the T.rex range, and even more assuredly when there are 15 serrations per cm or less.  So a rex tooth should have about 12-20 serrations per cm.  Not many other dinosaurs have serrations that large.  One exception is Troodon with its little 1/4" long teeth!  I consider the serration size or number of serrations per cm the single best indicator of whether a tooth is from T.rex or not., but still, there can be some overlap!  

So, what you are looking for in an adult  T.rex tooth is one that is fat, relatively less curved, with a nearly round, blocky-oval, or D-shaped cross section, and has big serrations.  The above guidelines will clarify the issue of T.rex or not T.rex in most cases, but  you may still have to consider some of the teeth out there as "Tyrannosaurid, indeterminate."  Even the experts have to do this, as there is some overlap of features among the tyrannosaurids!  And, there are a LOT of teeth out there that will fall in the cracks.  Are they juvenile T.rex?  Are they Nanotyrannus?  Are these the same animal?  The market usually does reflect this ambiguity and "iffy" T.rex teeth  (juvenile rex vs. nano-rex) will and should be priced considerably less than the much rarer "slam-dunk" adult T.rex teeth.  However, if you are paying the big bucks that adult T.rex teeth command, and you want to be as certain as possible that it really is from a T.rex, choose carefully and figure in the LIKELYHOOD that a particular tooth may not be from a T.rex.  A DEFINITE adult T.rex tooth measuring 2 inches may be worth up to 2K depending on condition, while a 2 inch Nanotyrannus tooth (or juvenile T.rex) may be worth up to 400, depending on condition.  At this time, (May, 2007) there are several teeth on Ebay being sold as definite T.rex teeth.  They are, for the most part attracting more interest and money than their "iffy" status should warrent!

Discouraged?  Confused?  Broke?  Black Hills Institute sells a high quality 11" cast tooth that is hard to tell from a real one for $75, or you can get the entire Stan rex lower jaw cast with teeth (shown in the pics below) for $900.  Also from a tyrannosaur, Albertosaurus teeth cost about a tenth as much as as T.rex teeth.  And, of course, you can get large teeth of Spinosaurus (avoid the composited ones!) and Carcharodontosaurus teeth for a lot less, and both of these were larger than T.rex, with Spinosaurus up to 55 ft! 

T.rex front teeth viewed from the side.

T.rex front teeth viewed from the top.  Note D-shaped cross section.

T.rex corner tooth.  Very large, more curved, very fat.

T.rex corner teeth viewed from the top.  Oval cross section.

T.rex side teeth.  Relatively straight, variably sized.

T.rex side teeth viewed from the top. Blocky oval cross section.

Albertosaurus tooth.  More curved than T.rex.

Albertosaurus tooth compared to T.rex tooth.  T.rex is the wide one!

Addendum:  Kurtis Hooley, an experienced T.rex tooth digger has kindly shared his observation that rex tooth serration size varies with tooth size.  Larger teeth have larger serrations.  I checked the above rex jaw and could see that Kurtis's observation is right on the money, with adjacent but different sized teeth having different sized serrations within the limits I mentioned earlier.

Sue the T.rex at the Field Museum in Chicago.

Now smile!

My source for the information on serration size came from:

Farlow, J.O., Brinkman, D.L., Abler, W.L., and Curie, P.J. 1991.  Size, shape and serration density of therapod dinosaur lateral teeth.  Modern Geology 16: 161-198.