I can’t afford cable tv for my home apartment, opting to stick with a cable internet connection instead, so whenever my wife and I petsit we look forward to having access to the National Geographic Channel, the Discovery Channel, and Comedy Central for a few days. This time around, after a special on “Hogzilla” via the NG Channel (I have another entry forthcoming about the pseudoscience documentaries we saw the same night), we were lucky enough to catch most of a film on a juvenile Tyrannosaur named “Jane” (although the sex of the skeleton isn’t known and the species identity wasn’t revealed until the end, in true paleontology documentary fashion). The whole film reminded me of how often paleontologists (and scientists in general) can be “blind men at the elephant,” squabbling amongst each other and how often what a fossil is supposed to be can change.
For my own part, it seemed painfully obvious that the skeleton of Jane was a juvenile Tyrannosaurus rex, the longer legs, larger orbit, higher tooth number, and other similar-yet-different characteristics suggesting a juvenile of a larger species rather than an adult of a new one. Of course, I never examined the skeleton myself nor would I consider myself an expert on Tyrannosaurid cladistics, so I could very well be completely wrong, but I’m not about to let that stop me from discussing the find. Jane was found in the Hell Creek formation (famous for its End-Cretaceous dinosaurs) by a team from the Burpee Museum of Natural History in Illinois and estimated to be 66 million years old. Jane was mostly complete and found articulated in the classic dinosaur “death pose” (the head and tail bend back towards each other during decomposition), her full skeleton being about 21.5 feet in length (about half as long as the largest Tyrannosaurus skeleton, Sue). From what I could see during the documentary and photos on the web, Sue had longer legs, a longer shoulder blade, a larger orbital (eye) opening, and slightly higher tooth number than known adult Tyrannosaurus skeletons, leading some to speculate that the skeleton is really an adult Nanotyrannus, a proposed (but hotly debated) genus of pygmy Tyrannosaur that many (including myself) regard as a juvenile Tyrannosaurus. I actually remember seeing the first proposed Nanotyrannus skull in a documentary as a young boy, Robert Bakker presenting it and its odd features to the viewers, as well as some paintings of the proposed creature, but it always seemed a bit silly to me. This isn’t to say that such a creature doesn’t exist, but it always seemed that the proposed Nanotyrannus remains were too close to what would be expected of a juvenile Tyrannosaurus to justify a new genus.
So, if Jane really is a juvenile Tyrannosaurus, why the differences? A adult Tyrannosaurus has a skull that is shorter and more robust, thicker and stouter legs, a few fewer teeth, and other features that set it apart. Obviously whatever Jane was, it was occupying a different niche, probably more fleet-footed than an adult Tyrannosaurus, which likely was an ambush predator/scavenger whose primary killing device was the head. Indeed, it seems that the whole of the Tyrannosaurus body is constructed to support to massive head, and although there’s no way (as yet) for me to know, I think that as a young T. rex grew up, some teeth and bone got reabsorbed to develop a more robust and heavy head, capable of exploiting carrion and delivering devastating bites. The diet change from quick, small prey to carcasses or larger dinosaurs with lots of bone or armor would have potentially changed the skeleton as well; if you can crush and eat bone, you have access to a lot more calcium and bone-building material to use in your skeleton. Hyenas of today eat plenty of bone, and it is thought that they can nurse their young for longer being their intake of bone is so high. This is all pure speculation, but it seems there is at least potential for these skeletal changes to occur over a lifetime, especially if theropod dinosaurs are closer to birds than to other reptiles.
I’m sure that people will continue to analyze Jane, and perhaps her name may be changed back and forth a few times, but if she is what we think she is (a juvenile Tyrannosaurus) she gives us a unique look into the life history of one of the largest predators ever to live. As I mentioned previously in my cheetah/ratel post, it’s impossible to know all the details of how extinct lineages lived and grew (even those familiar to us today), but finds such as Jane give us wonderful clues as to what life was once like.