Neck ontogeny in Tyrannosaurus rex, redux

Back into 2019, when Matt and I visited the Carnegie Museum, we were struck by how different the necks of juvenile and adult Tyrannosaurus rex individuals are. In particular, the juvenile individual known as Jane has a slender and amost fragile-looking neck compared with the monstrously robust neck of its adult counterpart.

A few weeks ago, Matt and I were at the Los Angeles County Museum (LACM), which has a superb tyrannosaur exhibit with three mounted skeletons at different growth stages:

Here it is from the opposite angle, taken from the balcony above the exhibit:

But what’s this we see in the tiny tyrannosaur that is the baby of the bunch?

A robust neck indeed — very much like that of the adult, and seemingly out of proportion with the slender skull. Let’s compare that once more with a similar shot of Jane the Carnegie juvenile:

(This is the same photo I used in the original post, but cropped a bit differently to better match the photo of the LACM baby.)

And remember: the LACM baby is smaller, and therefore we would assume younger, than Jane. Yet its neck is much, much more robust.

Once you have the two photos side by side, other differences are apparent — notably, that Jane’s forelimbs, scapulae and especially hands are proportionally much longer than in the LACM baby. In comparison, the LACM baby’s postcranium looks like that of a scaled-down adult.

What’s going on here?

If both of these skeletons are legitimate (i.e. mostly based on real fossil material of single individuals) then we have to be dealing with two different species here. There’s just no rational ontogenetic trajectory that goes from the LACM baby via Jane to the adult form.

On the other hand, maybe one or both of the juvenile skeletons is not really legitimate. I don’t know enough about tyrannosaurs to be familiar with individual specimens, but no doubt plenty of people out there are. (If any of them would like to comment here, that would be very welcome.)

I leave you with one more photo of that glorious three-tyrannosaur exhibit.