Tiny Tarbosaurus

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.

Notes on the taxonomy and identity of Zhuchengtyrannus

After the quick intro to the new taxon, now it’s time to talk in a bit more detail about the bones of Zhuchengtyrannus. All we have is a maxilla and a dentary but that’s actually quite useful. A few ribs and some caudal vertebrae wouldn’t have told us much as these are rather conservative in tyrannosaurs, but happily maxillae are not and have lots of useful and important taxonomic characters in them. It is also worth remembering that pretty much any bone, or even part of one, that is diagnostically different from everything else out there is reasonable to use as the basis for erecting a new taxon (like Brontomerus and various others).

I should of course add, before I go much further, that this is a blog, not a paper. There is more detail and commentary in the actual publication than here in internet land and if you really want to dive in then go read the paper (though again the paper as it stands online is an uncorrected proof, and that has cut off part of the taxonomy stuff!!!). This here is little more (as ever) than a surface discussion of the issues for general consumption and if anything more general than normal as I hope (expect?) that a few more people than normal will be finding the Musings right now as a result of the media coverage.

For those that don’t know their tyrannosaur taxonomy as well as they’d like it’s worth noting that Zhuchengtyrannus is a tyrannosaurine and that puts it in the group of especially large and derived tyrannosaurs and as part of a Late Cretaceous group that was restricted to eastern Asia and North America. We can tell this at least in part because it is a huge theropods from the end Cretaceous of China, but the relatively straight anterior edge of the maxilla supports this, and the shape of the teeth and dentary put it well within the tyrannosaurs in general.

Zhuchengtyrannus teeth. From Hone et al., in press

At this juncture, it’s worth remembering that there are different ways of identifying species, or more specifically, distinguishing them from others. Obviously with a fossil we’re working on a morphological species concept (that is, identifying a species buy it’s anatomy), but more specifically we can separate out differences in different ways. First off we can look for genuinely unique features – a giant tooth in socket 5, only one finger on the hand, a skull twice as long as tall etc. Things that appear in our new species that don’t appear in any others (or at least any other close relatives – stripes are characteristic of tigers since even though other cats are stripey, you’d never confuse the two because of the obvious size differences etc.). Secondly though, you can look for unique combinations of characters. One species may have a long and wide skull, another a short and narrow skull. The characters of ‘long’, ‘short’, ‘wide’ and ‘narrow’ are all in play here, but you could distinguish a possible new species with a clearly distinct combination of ‘long and narrow’ or ‘short and wide’.

So onto Zhuchengtyrannus. This is diagnosed in our paper by two unique characters – a short of shelf on the anterior part of the maxilla and an odd notch in the maxillary fenestra (see figure below). Neither of these does, to our knowledge (or indeed that of the referees or various other colleagues we consulted), turn up in any other tyrannosaur specimen ever. There is also a unique combination of characters to further separate it from other tyrannosaurs in the position of the antorbital fossa and size of the maxillary fenestra.

Drawing of the Zhuchengtyrannus maxilla. The unique shelf is labelled 'S' and the notched fenestra 'mf'. From Hone et al., in press

Although the specimen was not entirely complete when recovered (and sadly the maxilla was later damaged as can be seen from the picture of the maxilla in the previous post) it was initially in very good condition. We have good reason to think therefore that all of these characters are valid ones. The bones were not broken (well they are a bit, but not where the critical characters appear) or distorted or altered and there was no sign of disease or pathologies. This is also not an issue of ontogeny (age-related changes). The animal is easily big enough that it’s hard to credit that it was anything other than an adult, and possibly a large one at that. Moreover, there is a decent literature on both ontogenetic changes in tyrnanosaur skulls and on intraspecific variation. Zhuchengtyrannus exhibits several characters that are normally only seen in adult tyrannosaurs (like the heavy sculpting on the maxilla, more on that later) and the characters we use in our diagnoses are not known to vary either through growth, or within putative populations.

For all of this, lumping taxonomists out there might well not regard this as valid (“To synonymy, and beyond!”). There are those who would still have Tarbosaurus as ‘just’ a species of Tyrannosaurus and I can only suspect they won’t like this much as a result (though I don’t know for sure of course). All I can say is that we are happy with the distinctions, and the referees and other colleagues who have examined the material were too. Taxonomy really does operate at little more than a consensus level and while this can all change, already (from what I have seen and discussed so far) the consensus is that this is a perfectly valid taxon. Of course there’s also a good chance that we will get more material of this species (indeed as noted previously, we may already, even if referral is currently an issue) which will help our cause. Given what bones we currently have, Zhuchengtyrannus seems to be as diagnostic as any other large tyrannosaur and while it could be better (we don’t have that much material), it is sufficient.

Even so, this is only the second tyrannosaurine from China and one that very probably overlaps in time, and space, with Tarbosaurus. As such, it is worth making special note of the differences between these two and again there are some more in the palatal shelf and at the back of the maxilla. In short, it should be very hard to confuse the two if you have a maxilla of either in your hands and there is even better reason to think the two are different and thus again that Zhuchengtyrannus is a genuinely new genus.

I was also reminded in comments in yesterday’s post about ‘Tyrannosaurus zhuchengensis‘ which is detailed in the paper, but initially forgotten here! Whoops. back in the 1970’s several tyrannosaurus-like teeth were recovered from this quarry and, in the manner of the day, named as a new species: Tyrannosaurus zhuchengensis. Later on a single isolated metatarsal (foot bone) was assigned to this species. What of this? Well none of these teeth or the metatarsal show any unique features that would make them diagnostic from any other tyrannosaur – in short, if you got a Tarbosaurus or Tyrannosaurus tooth or metatarsal and compared them to the T. zhuchengensis material, you wouldn’t seen any real difference. As such we cannot consider this to be valid and we therefore call Tyrannosaurus zhuchengensis a ‘nomen dubium’ –  a dubious name that should never have been created and should no longer be used (and hence the use of quote marks around it in it’s initial appearance here). Of course this material might be a much earlier record of Zhuchengtyrannus, but we can’t be sure, it might belong to the second taxon, or who knows, even another tyrannosaur!

That’s rather more than I intended to say so I’ll cut it ‘short’ there. More to come tomorrow where I’ll delve into the ecology of ZT and then we’ll be onto the glorious artwork and its genesis and importance in science communication.

Selective feeding by tyrannosaurs

So having covered the fact that tyrannosaurs were both predators and scavengers in yesterday’s post, we can now talk about how they were feeding. Previous records of feeding by large tyrannosaurs have tended to show a pretty rough approach – bite bites going deep into the bones which are sometimes part-dragged out leaving scores in the bone. This is no great surprise, they have big and robust teeth, and huge skulls with a lot of muscle power behind them and a powerful bite. It makes sense that they can be quite careless as it were in their approach – when you have a bite that can break pretty much anything open then go ahead and bite.

Tyrannosaur scrape marks left in a Saurolophus humerus. From Hone & Watable, in press.

This is however, not what we see here with the Saurolophus specimen. The humerus has suffered a large number of bites, but most of them are shallow, not big deep gouges. What’s more the marks are far from randomly distributed across the specimen. There are some relatively big and deep bites at the ends of the bone, but along the low and flat deltopectoral crest (where the main muscles would attach) the marks are shallow and clearly made by teeth being dragged across the surface of the bone. It’s worth noting that the deep bites at the end were heavy enough that if they have been performed on the DP crest then this would have broken. In short, the animal was being relatively careful here when it was not at the ends.

Deep bites by a tyrannosaur on a Saurolophus humerus. From Hone & Watabe, in press.

What this shows therefore is that the tyrannosaur was making an active choice in how it fed on the bone. It chose to make heavy bites deep into the bone ends, but deliberately did not do this on the deltopectoral crest even though it could have done so. Tyrannosaurs did not just rip apart carcasses and crunch through bones even though they could. Sure on some occasions they did, but not always. They could, and did, chose how to feed and could be relatively delicate.

Differing feeding styles by Tarbosaurus. Image courtesy of Matt van Rooijen.

Why the different styles? Well the deep bites might have been to try and get to the cartilage on the bone to eat it, but might also have been to simply free the bone from the rest of the skeleton to make feeding easier. This is supported by the fact that the marks on the DO crest are on both sides but at different angles to each other. The different angles means that they were not made by the top and bottom teeth together in unison. That also means that the bone must have been turned over at some point for the Tarbosaurus to get to the other side, and thus the fact that freeing the bone from the rest of the body would have assisted feeding.

This specimen does then give us rather greater depth to our understanding of tyrannosaur behaviour both in terms of the practice of scavenging and in how they dealt with carcasses. This shows a surprisingly delicate touch by a 10 m and 6 ton* reptile.

* Note – vague, but probably not inaccurate estimate.
Hone, D.W.E., and Watabe, M. 2010. New information on scavenging and
selective feeding behaviour of tyrannosaurs. Acta Palaeontologica Polonica.

Artwork courtesy of Matt van Rooijen over at the Optimistic Painter. More superb work I have to thank him for.

So tyrannosaurs were scavengers then…

The argument over ‘was Tyrannosaurus (or big tyrannosaurs) a predator or scavenger’ has been going on for far too long and is, I think, not only dull, but largely pointless. It rather assumes that they were one or the other, when pretty much no carnivore is, what they are, is both. Jackals might favour scavenging and lions predation, but jackals hunt and lions eat random carcasses. Where a given carnivore (or even omnivore) lies on the scavenger-predator spectrum is of interest, but still not worthy of the coverage Tyrannosaurus has had on this subject.

Interesting though a great many papers have been on the subject, and clever and intriguing that many analyses have been, getting the ‘answer of predator or scavenger was never likely to be satisfactory *because* the answer was always very likely to be ‘both’. All of this has recently been reviewed and covered by Tom Holtz and thus the book might appear to be closed on the subject, since both are in evidence. But there is always room for more data, especially when it comes from a new fossil find.