Thursday, 3 November 2011

How Big a Bite is Big?

So Planet Dinosaur has now finished its six episode run and I think it fair to say that the series has been generally well received by both the general public and the palaeoworld as a whole. It was my intention to review the series but I think Marc Vincent has done an excellent job of that over at Love in the Time of Chasmosaurs and presents a reasonable and balanced view. Head over there and take a look if you haven’t already done so.

In one episode, New Giants, there is a sequence depicting a group of Mapusaurus attacking a group of Argentinosaurus and at a one point we witness one of the theropods removing great slices of flesh from the flanks of a distressed, what appears to be, sub-adult sauropod. The theropods withdraw leaving the unfortunate beast with great wounds that bleed profusely and it is intimated in the narration that the sauropod is slowly being reduced to a moving feast as the attackers return again and again to take yet more flesh from the slowly dying animal. But how possible or realistic is this?

Carcharodontosaurid teeth are flat, blade-like and heavily serrated making them ideal cutting tools and are well suited, it would appear, of removing significant amounts of flesh from prey animals. Mapusaurus was a big theropod, comparable with Tyrannosaurus, with a massive skull and powered with significant jaw muscles and it is likely that Mapusaurus was very capable of taking huge bites out of sauropods. Or was it?


From Coria & Currie 2006
Just how thick the skin of sauropods was must remain a matter of conjecture but it is safe to state that it was considerable. The hide of an elephant maybe up to 100 to 125mm thick in some places so it seems reasonable to assume that a sauropods’ skin would have been thicker by an undetermined amount but, for the sake of argument, lets imagine it to be in the region of 150 to 200mm thick which does not appear to me to be unreasonable.

We have some skin impressions of sauropods which display a conventional scale covered surface and, in the case of some titanosauriformes, is supplemented by raised and not insignificant osteoderms (e.g. Bonaparte 1999). Just how durable all this makes the skin is conjecture but this appears to me to represent one hell of a tough mouthful for an attacking Mapusaurus to contend with. The skin alone is probably thicker than the teeth are long and you have to remember that carcharodontosaurid teeth are essentially flat and narrow, even if the crowns are approaching 125mm in length, and do not have the power or durability of, for example, tyrannosaurid teeth. Although theropod teeth were constantly being replaced throughout their lifetime, it seems likely that any theropod would prefer not to lose too many at any one time.

Even if there were no osteoderms, obtaining a mouthful of flesh for the theropod is problematic. The sauropod is certainly not standing around allowing the carnivores to attack at will and will be a moving target, albeit a slow one, and has the added bonus of a significant mass and tonnage which the attacker would do well to avoid.

Looking at other possibilities, maybe there were more vulnerable areas to attack such as the neck but this would be a constantly moving target and it would also be capable of generating significant force that may inflict substantial damage on any attacker. However, as already stated, the teeth of carcharodontosaurids are excellent cutting tools and maybe they were used simply to cut the skin, perhaps supplemented by claws from both the manus and pes, to get the sauropod bleeding as much as possible. Constant hit and run attacks in this way would be an efficient and safer option for any attacking theropod as opposed to trying to tear out great chunks of flesh - slash-and-dash as it is often referred to.

The other tool in the theropods tool box is the fact that they were much faster and agile than any sauropod could possibly be and should be capable of side stepping any flailing neck or tail (remember that we are discussing titanosaurs – not diplodocoids). Having said that, if sauropods moved in herds then it seems possible that they may have defended themselves as a collective group and then the problem is amplified for the attacker and they would need every ounce of agility to avoid injury.


Image from Planet Dinosaur © BBC
All of this assumes that large theropods such as Mapusaurus actively targeted sub-adult and adult animals but this was almost certainly not the case unless one such animal was sick or injured. In such cases as these, the hunters could probably take their time with their assault since the unfortunate beast was almost certainly on its way out anyway. It appears obvious to me that even the largest of theropods would take the easier options of softer targets such as the weak, the sick, the young and, of course, any carrion that they came across.

One thing this particular episode of Planet Dinosaur probably got right were the high mortality rates of young sauropods at the nesting grounds. Using the famous titanosaur nesting ground of Auca Mahuevo as the setting, the episode showed pterosaurs and the abelisaurid Skorpiovenator plundering the nests of the young hatchlings and it seems obvious that the annual? sauropod nesting season would have attracted every predator for miles to join in on the feast and I would include the giant carcharodontosaurs in on this.

Nesting sites such as Auca Mahuevo suggest that sauropods were almost certainly what are known as r-strategists (Chiappe 2003). That is that they laid vast quantities of eggs producing young sauropods that, although tiny in comparison to the adults, were able to survive on their own – if they lived long enough to do so. Mortality rates would have been enormous but those that survived would have grown rapidly and got relatively big very quickly although it would still have taken several years to become a giant. This would have meant that there would have been many different generations of sauropod in circulation at any one time and these animals were likely to be those favoured by theropods – including the giants such as Mapusaurus.

Of course, all of this still has to be considered speculation and it seems likely that very few sauropods would attain their full adult size due to predation but if sauropods were herding animals that grouped together for protection then there may have been an element of sanctuary within the herd for some juveniles at least.

References

Bonaparte, J. F., 1999. An armoured sauropod dinosaur from the Aptian of northern Patagonia, Argentina, pp. 1–12 in Tomida, Y., Rich, T.H., and Vickers-Rich, P. (eds.), Proceedings of the Second Gondwana Dinosaur Symposium.

Coria R. A. & Currie P. J. 2006. — A new carcharodontosaurid (Dinosauria, Theropoda) from the Upper Cretaceous of Argentina. Geodiversitas 28 (1) : 71-118.
 
Chiappe, L.M., Coria, R.A., Jackson, F. & Dingus, L. 2003. The Late Cretaceous Nesting Site of Auca Mahuevo (Patagonia, Argentina): Eggs, Nests & Embryos of Titanosaurian Sauropods. Palaeovertebrata, Montpellier, 32 (2-4): 97-108.

3 comments:

Andrea Cau said...

Footprints usually show sauropods forming ?herds based on individuals of the same size/age thus supporting the idea that juveniles and older individuals did not form ?herds together. It is also plausible that being so different in size, juveniles and fully mature sauropods were ecologically distinct.

Mark Wildman said...

Thanks for the information Andrea. If this was the case then you would think that the amount of sauropods making it to adulthood must have been very small indeed.

Marc Vincent said...

Thanks for the mention and kind remarks Mark. I certainly did try and present a 'balanced view', although I did cock up now and then in my efforts to get a review online as quickly as I could...

This was a very interesting post I might add, so thanks for that too!

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