By now I’m sure most readers will have picked up on the new paper describing the skull of a tiny Tarbosaurus. It’s estimated to have been just 2 years old when it died and was around 2 m in length – that’s rather smaller than the 10-12m long adults! A few images of this hit the news when the specimen was first announced around three years ago and can be hunted down if you look. There is one above of me with a cast of the specimen as it was when discovered (it’s now been fully prepared out) which gives you a pretty good idea of just how small it was.
While above is just the cast, I was lucky enough to see some of the material while it was undergoing preparation. Even better I was allowed to take some pictures for my own research and have been told I can show off a couple of them here now that the paper is finally out. My thanks for this to Mahito Watabe and his team for this generosity. Though perhaps inevitably with Larry Witmer on the team, there are even better pictures and 3D animated scans of the skull out there too!
This is post is (a little) more than just a couple of photos though as the paper itself has some interesting things to say about tyrannosaur ontogeny. This is the smallest / youngest tyrannosaur we have and coming from an especially big genus makes the size discrepancy even greater. This is pretty handy as there are a variety of problematic tyrannosaurs specimens out there that may or may not represent distinct taxa but being known only from juveniles make this hard to work out. However, if you have a really good handle on how some characters do (or do not) change as individuals get older and bigger then you know what you can and cannot rely on when looking for unique characters in other juveniles.
For me the most interesting characteristic was that of the number of teeth. While there is a little intraspecific variation in tooth numbers, there are also some discreet differences between taxa too – the big tyrannosaurines like Tarbosaurs and Tyrannosaurus have fewer teeth than do smaller ones like Daspletosaurs or Alioramus. In the past it’s been suggested that this number actually changed during ontogeny with the number starting relatively high and the number of teeth reducing as the animal got bigger. However this little guy has exactly the same number of teeth as an adult Tarbosaurus. While this doesn’t exactly disprove the hypothesis, it does at least show that at best it’s not always true, and so a smaller tyrannosaurine skull with lots of teeth could grow into a big adult with lots too.
All in all a very revealing paper (and superbly illustrated I should add) and with more to come on the postcranium, this is going to be an important specimen for many a long year. Sadly, it’s parent institution may not be able to say the same as I will talk about tomorrow.