2 sept 2012
Anyway, to dinosaur tails. A couple of years back I happened to start looking at the tails of various pterosaurs (the flying reptiles that lived alongside the dinosaurs) after a new specimen turned up that was obviously a member of a group that had short tails, but happened in this case to have a relatively long tail. This got me thinking about variation in tail lengths and how this might have evolved between various pterosaurs, and that led me onto thinking about the tails of dinosaurs. At the time I was working at the famous IVPP (understandably shortened from its title of Institute of Vertebrate Paleontology & Paleoanthropology) in Beijing which happens to have a quite exceptional collection of dinosaur fossils, and especially those that have been preserved in unusually good conditions and often are associated with things like skin and feathers.
These fossils one might expect would have pretty much every bone preserved, but a good hunt down of the available specimens soon revealed that even those blessed with feathers and the like very rarely had a totally complete tail. Often it was mostly there (or at least presumably so) but the tip would be gone.
I widened my search and began digging into other museum collections, reading descriptions of fossils in scientific papers, checking out photos of dinosaur specimens and asking colleagues and simply trying to track down every dinosaur fossil I could that had a complete tail. Much effort was expended and the net result was almost literally just a handful of specimens where we had every bone in the tail. The very last bone in a vertebrate tail is an odd shape, so one can tell whether or not the tail is complete relatively easily. Now there were a good number which were all but complete, but while a tail might taper to almost a point which would reveal that nearly every bone is there, one can't be sure without this last one. Some tails do taper considerably but then run on for quite a length without the bones getting any smaller, so tapering alone may be deceptive.
In short it turns out that really, despite having thousands of dinosaurs fossils including a good few hundred that could broadly be considered complete, we've got barely two dozen complete tails. The implication for this is that this is an area of dinosaur anatomy that really we don't know that much about. Yes we do indeed have lots of tails preserved and there's a lot of detail indeed in those specimens, but a fairly simple question of "how long in total was this dinosaur" could be really quite tricky to answer for a very good number of species.
This is not a facile question, aside from the obvious public interest (when was the last time you saw a report on a new dinosaur that didn't suggest how long it was, if only in terms of double decker buses?). Total length is a measure that's been used by various researchers (myself included) over the years as a proxy for the mass of dinosaurs. If we've been over- or underestimating these values it could potentially affect our results quite a bit, so knowing whether or not these measures are right is worth checking.
This is linked to the wider question of dinosaur size in general that I've been writing about over the last few posts. The size of an organism is intimately tied up with how much food and water it needs, what kind of territory it can control, how it moves, what its population might have been like and all kinds of other biological factors. Having a good handle on how large an animal is can tell you a lot about it and it's something worth finding out.
So, are our previous estimates of total length any good? The immediate answer is apparently not, or at least I can't tell. My quick survey of the data showed that actually there was really quite a lot of variation there in the length of the tail. As discussed a few posts back, the length of the femur (thigh bone) is a pretty good indicator of size so I used this as a way of scaling both the lengths of the tails of these species, and then also the length of the head and body of the same animal or a related species.
These scaled results show that not only could tail length really vary a lot between species, but was also markedly more varied than that of the body when compared to the femur. In some ways this shouldn't be too much of a surprise: tail lengths vary a lot in the natural world (think bobcats vs leopards, and some lizards vs skinks) and there had been records for some time of dinosaurs with either very short or very long tails. Even so, such profound differences do seem to have been overlooked and moreover, some differences are startling.
Take the bizarre scansoriopterigids. There are currently only two species known in this group and they are therefore close relatives of one another. One has one of the proportionally shortest tails known for any dinosaur that I recorded, but the other had one of the longest and that is incomplete and would be even longer in life! In short, it may not even be too safe to reconstruct a missing tail from a close relative when things like this happen. I was also lucky enough to get data from a couple of animals of the same genus a couple of times and here the intraspecific variation was also pretty high, again suggesting that length could be pretty variable.
Beyond this we move into the hypothetical, but while the data collected is limited, I don't think it much of a stretch to think that some dinosaurs might have looked rather different to how they are traditionally imagined. We have no good tail material at all for the famous Spinosaurus, for example, and while there's a good few images out there for this animal, there's no real reason it could not have a tail half or double the length of how it's normally portrayed (as shown at the top).
This might not really affect its mass very much – adding or taking off a couple of metres of thin tail and you'd only really make a difference of a few kilos on an animal that was upwards of 7 or 8 tonnes. But talking about an animal that was 12 metres or 16 metres long rather than 14 metres and that both sounds like a major difference and could have a significant effect when scaling things. For a researcher scaling that animal, 16 metres is a long way from 12 metres and would make a bigger difference to a mass estimate.
A bigger change may come in our perceptions and, I hope, how we as researchers talk to the public about dinosaurs and how the media report on them. After all, a 16 metre Spinosaurus does sound a lot bigger than a 12 metre one, when in reality the difference is pretty slight and moreover we can't tell which of those is more likely without a near complete specimen to work from. I'd contend that a great number of oft repeated length estimates are likely grossly out and while many more are probably perfectly sound, it's hard to tell the difference at the moment.
What would be better therefore, is to start talking about the body sizes of dinosaurs, effectively their length from snout to rump. That we know varies less than the tail and tends to be far more complete and is likely more stable between close relatives too as a result, meaning the value is likely to be rather more accurate.
Right, I think that's probably more than enough on dinosaur tails for now. I'm afraid the paper is behind a paywall, but I imagine a few readers may be able to access it here and PDFs generally turn up online sooner or later. My thanks to Scott Hartman for his superb reconstruction and differing tails on Spinosaurus used above.
Source:
http://www.guardian.co.uk/science/lost-worlds/2012/aug/29/dinosaurs-fossils
