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).
