Tyrannosaurus rex

[DinoFan83]

As of late, I have been of the opinion that Sue, Scotty, MOR 1125, and Bucky exhibit sufficient morphological differences from the holotype T. rex, AMNH 973, to be a different species (and unpublished specimens like Trix and Victoria may be included here as well when they are published). The reasons for this and the implications for T. rex proper are as listed below.

Reasons for this:

-They have proportionally larger and shallower skulls than AMNH 973. This will be elaborated on in a bit.

-They are more robustly built. This is in regards to femur robusticity and ribcage width.

Femur robusticity: As an example. AMNH 973's femur is 127 cm (most recent measurement, Scott Hartman) and 54.5 cm in circumference (Peter Larson & Kenneth Carpenter, 2008). Meanwhile, the femur of MOR 1125 is 107 cm and 51 cm in circumference (Mary Schweitzer, 2016).
This means MOR 1125 is 11.5% more robust than AMNH 973.

Ribcage width: As an example, further back in this thread, there are some images showing dorsal views of AMNH 973 and Sue (156-158). Even though the dorsal view for AMNH 973 shown there is too wide (which I'll elaborate on in a bit), Sue is obviously markedly more robust.

-They have proportionally larger pectoral girdles (this is probably directly correlated to their bigger skulls. These need larger and stronger neck muscles to support, which need more attachment area which in turn necessitates a larger pectoral girdle since that is where neck muscles like the levator scapulae attach to).

As an example, SpinoInWonderland estimates AMNH 973 (which has a 95 cm scapula according to the Theropod Database) at 11.7 meters. Meanwhile, Mickey Mortimer estimates Bucky at 10 meters with a 94 cm scapula (both of those are also from the Theropod Database).
To put that into perspective, AMNH 973 is estimated at 17% longer than Bucky yet only has a scapula 1% longer, meaning Bucky clearly has a proportionally much larger pectoral girdle.

Except for the robusticity difference (which was already addressed with a hyperlink and will be elaborated even more on further down), the outlined differences of a larger and shallower skull together with a larger pectoral girdle are clearly visible between one of the outlined specimens (Sue) and AMNH 973 in these skeletals by GetAwayTrike.

*This image is copyright of its original author

*This image is copyright of its original author

All told, this is more difference than separates lions and tigers (going by that link, they do not appear to exhibit the anatomical disparities the outlined specimens do to AMNH 973, and yet are still different species). Another argument in favor of making the species level split of the outlined specimens.

Implications for T. rex:

Naturally, the exclusion of specimens previously thought to belong to it would have implications for what is currently known about T. rex. They are as follows.

-Size: Sue and Scotty are almost always brought up in discussions about the size of T. rex concerning the maximum size of known specimens, as well as how large it was compared to other large theropods. This is because, when counted as T. rex, they are the largest known specimens of it.
However, since they appear to be a different species for the reasons above, they therefore wouldn't be countable among the largest T. rex anymore. This means 3 things: the largest known specimens of T. rex are not as large as often thought, the sizes of Sue/Scotty have no impact on the size of T. rex, and the sizes of Sue/Scotty also have no impact on how large T. rex was compared to other large theropods.

It also may have implications for the average size of known T. rex specimens (which is ultimately a more useful size parameter than maximum size is).
Because my 6000 kg estimate from earlier used Sue as an overall base and included the other 3 outlined specimens in the sample, it will have to be redone while excluding those 4. The exclusion of those specimens could either increase or decrease the 6000 kg estimate, although that estimate could remain the same even with the exclusions. Ultimately, the only way to know for sure is to redo the size calculations and share them as soon as they're redone.

-Robusticity: Similar to their size, many dinosaur enthusiasts enjoy discussing how robust T. rex is (specifically Sue and Scotty), usually compared to other theropods. 
For the same reasons as listed in the size-related paragraph, however, this is affected by the exclusion of Sue and Scotty (after all, one of the very traits that supports their status as a different species is that exact robust build!). Ultimately, how robust Sue and Scotty are would likewise have no impact on how robust T. rex was, as well as how robust it was compared to other large theropods.

And there's more. Below will be the elaboration on how robust actual T. rex AMNH 973 seems to have been, that was alluded to above.

*This image is copyright of its original author

https://www.deviantart.com/spinoinwonder...-648811067

This model of AMNH 973, as also alluded to above, is far less robustly built than Sue even while being too wide. The latter is stated to be the case by Hutchinson et al. (2011), which (as SpinoInWonderland says in the link) is where the dorsal view is from.

Quote:The largest specimen (Sue) has larger absolute bone volumes than all but the Carnegie specimen's, whose dimensions are partly inflated (especially for the lower limb) by the presence of a supportive metal mounting framework that was too tightly integrated with the bones to easily be separated.

Given that even this too-robust dorsal view is not especially robust or wide, I think it's safe to say that with the exclusion of the robustly built specimens outlined above, T. rex turns out to not be as robust as commonly envisioned. This also has implications for how robust it was compared to other theropods, although specific data concerning that will be saved for future posts.

-No female T. rex: Last, but most certainly not least, is the fact that one of the only dinosaurs and the only tyrannosaurid which we know the gender of (MOR 1125) is excluded from T. rex for the reasons above, and so we don't yet have any T. rex we know the gender of.
This is worth mentioning as MOR 1125 has long been famous as the only T. rex for which gender was known, although it should be noted that neither its erroneous inclusion nor current exclusion from T. rex provide any evidence for or against the popular concept of female tyrannosaurids being larger than males. 

Ultimately I would say all the above merits serious reconsideration of the T. rex mental image most dinosaur enthusiasts have.

@tigerluver, if you're interested, this is the information concerning some specimens of T. rex appearing to represent a different species that I mentioned in our private messages. Reply