How large is UCMP 137538, really?

Many of you who are interested in dinosaurs, may have seen people in forums and youtube who fuzz over the legend of the "14-15 meter Tyrannosaurus", and refer to the specimen "UCMP 137538".

UCMP 137538 is known from a single 13-centimeter long pedal phalanx, which has been assumed to be a left pedal phalanx from comparisons with FMNH PR2081. That is a weak assumption, as the bone looks quite different from that of FMNH PR2081.

For starters, it may not even be a tyrannosaurid. It could have been a gigantic therizinosaur, since a herbivorous lifestyle isn't limited by the constraints of a carnivorous one, such as the need to run down prey. If you read the paper about UCMP 137538, you will see that the only real diagnosis done is the one assigning it as a theropod.

It's assignment to Tyrannosaurus is based on nothing but size and location, both of which are weak arguments for assigning isolated fossils to specific genera/species. It's also partly based on the assumption that Tyrannosaurus is the only large theropod living at North America at the Maastrichtian age, which is quite an almost-baseless assumption, considering that the vast majority of the dinosaurs are very likely undiscovered.

It's placement within the foot is also uncertain, the paper bases it's placement on nothing but superficial appearance.


Then how large would it be?




Answer: Completely unknown.


The problem is, it's just an isolated toe bone. Even the enigmatic Amphicoelias fragillimus is known from better remains(A D9/D10 vertebra).

The giant sizes come from scaling it up from FMNH PR2081, and the naive fanboys seemingly only scale from that specimen. FMNH PR2081 isn't the only tyrannosaur specimen however. And Tyrannosaurus isn't the only tyrannosaur. For all we know, UCMP 137538 may actually be a non-Tyrannosaurus tyrannosauroid.

Even IF it was a Tyrannosaurus or a very similar genus, you should still stay away from those 14+ meter calculations. Tyrannosaurus specimens can show quite a lot of variation.

BHI 3033(~10.9 meters, probably around 6 tonnes?), has toe bones that come close in size to that of FMNH PR2081. Not to mention that with pathogeny, digit bones can vary greatly even within individuals.

It is entirely possible that UCMP 137538 may actually be smaller than FMNH PR2081(~12.3 meters, ~8 tonnes).