Wednesday, 8 December 2010

Complex Dinosaurs

Sauropods were a group of large plant eating saurischian dinosaurs found all over the world. For some time now I’ve always thought that when you could solve the biomechanics of sauropods, you could more or less solve the secrets of the dinosauria as a whole. Sauropods were amazing animals.

The functional morphology of sauropods has long come under intense scientific scrutiny. For example, how did sauropods eat enough food to maintain a constant metabolism and support their enormous body sizes? The size of a sauropods’ mouth appears, on the face of it, not large enough to eat sufficient plant matter. This is particularly fascinating when you consider the small size of the head in relation to the body and the relatively poor chewing mechanisms of some (but not all) sauropods (Christiansen 2000; Upchurch & Barrett 2000).

The long necks of sauropods are under continual scrutiny and have been since they were first discovered. There have been all sorts of theories regarding their posture, range, and movement and we are still unsure (Martin et al 1998; Stevens & Parrish 1999; Taylor et al 2009). Were their necks curved? Did they raise them vertically, hold them horizontally, were they sloping down?


Then how on earth did they pump enough blood to the brain and back again? The distances are enormous – in the case of Barosaurus, the neck is approximately 25 feet long, in Seismosaurus, even longer (Choy & Altman 1992; McIntosh 2005). Just how did they manage this? Obviously they did since the giraffe-like Brachiosaurus and Giraffatitan held their necks aloft and must have fed in the tree tops. The engineering required to achieve such blood flow defies belief. Theories are legion but none are universally agreed on.

The reason I mention the issues above is that I think we sometimes lose sight of what nature is capable of. We are all very astute in being able to say what is not physically or physiologically possible and yet we find it difficult to provide any suitable hypotheses or alternatives – at least any that can be agreed on. Totally understandable of course and yet this somehow misses the point.

These animals diversified and radiated for millions of years and were extremely successful. Sauropods did exist in vast numbers; they did eat enough to, not only survive, but proliferate. They did have a cardiovascular system more than capable of pumping blood around their bodies. These animals were truly natural wonders – these dinosaurs were complex creatures.

It was while studying the premaxillae of tyrannosaurs that it really occurred to me how complex they were – indeed how complex all theropods are, both avian and non-avian. If we concentrate on tyrannosaurs, however, we can see what has happened over the years. Certainly, despite vast amounts of data to the contrary, these animals are sometimes portrayed as slow scavengers who were not even capable of staying on their own two feet unless they stood still. This is just plain wrong.

Naturally enough, mention tyrannosaurs and it is Tyrannosaurus that comes to the fore and rightly so. For me this has to be the single most impressive land predator ever and it is understandable that huge amounts of study are devoted to this species. After all, the amount of material recovered in the last twenty years is astounding – over forty known specimens and the numbers are rising (Larson 2008).

Back to tyrannosaurid premaxillae then. Tyrannosaurus is continually portrayed as a huge bone shattering, crunching set of jaws that was able to consume vast amounts of flesh and bone in huge gulps. Perfectly reasonable – there are even coprolites full of pulverised bone (Chin et al 1998). Except that tyrannosaurs also had an arcade of eight premaxillary teeth (Holtz 2004). What were they for?

A typical D-shaped tyrannosaurid premaxillary tooth.

Maybe they were used for grooming, intraspecific communication or, as seems to be the general consensus, these teeth were used for selective feeding (Hone & Watabe 2010), stripping flesh from the bone and nibbling bits of flesh off, maybe, to even feed their young . Either way, this immediately does away with the common depiction of Tyrannosaurus as a simplistic bone-mashing-eat-everything carnivore. If Tyrannosaurus just fed like this then why have a premaxillary? The premaxillary suggests complex behavioural patterns – otherwise why bother with it?

And then when you start to look at tyrannosaurs in more detail, their general biomechanics, just like sauropods, are astounding. The overall combination of good eyesight, superb sense of smell and hearing is generally accepted and the brain was relatively large to process all the details (Witmer & Ridgely 2009). These are adaption’s, not only for hunting and scavenging, but for intraspecific communication and environmental awareness.

Another adaption to consider is bipedalism – the general body plan for all theropods is more or less the same, pot-bellied therizinosaurs not withstanding. High running speed in tyrannosaurs has come under intense scrutiny over recent years and whilst it is generally accepted that a top speed of 40mph is somewhat exaggerated, a 40 foot long, 7 ton tyrannosaur running at 15 mph is not exactly hanging about and was more than adequate to run down its contemporaries (Farlow et al 1995b; Paul 1998).

However, even a tyrannosaur briskly walking has come in for some tough scrutiny. It has been suggested that a tyrannosaur would hardly dare break into a trot just in case it tripped or fell. I mean what would a theropod do if it broke a leg or, as has been suggested, couldn’t even get back to its feet and remained grounded?

Whilst this may have an element of mechanical soundness about it, the truth actually stares us in the face. Let’s take a couple of steps back and think about it. An entire clade of dinosaurs, the theropoda, evolved into bipedal animals that walked on their toes and endured for around 150 million years – considerably more if you include aves. These animals ranged in size from critters the size of a chicken to huge multi-tonne mega-carnivores. If these animals were continually falling over, breaking bones, and not able to get up then evolution would have demanded a change in their way of moving around the terrain or theropods would have probably become extinct within a very short time span. After all, ostriches don’t worry too much about whether they will trip and fall when it is time to outrun a predator – albeit an ostrich doesn’t weigh a few tons.

I’m being a little simplistic here but the point is essentially correct. Theropods did not worry about falling over and did not hesitate to put their foot on the gas when food was required. And again the ability to combine agility, judgement and hunting skills is indicative of complex animals.

Finally, a look at the tail of a tyrannosaur. The tail was a massive part of the animal, held proud from the ground, heavily muscled and not only acted as a counter balance for the large head of the animal, but was also the driving force that propelled the animal forward at speed and would have been vital in maintaining equilibrium when the animal moved. And, indeed, while writing this post, a newly published paper (Persons & Currie 2010) makes a point of this very issue and puts some substance behind the theory.

This combination of characters, from the massive head through to the tail, and all of the specialised traits in between suggests a level of complexity comparable with any extant mammal of today. Behavioural inference can only be that – inferred but that still leaves an enormous amount of physical evidence that is totally indicative of complexity within species.

Dinosaurs were complex animals of the highest order. Of that there is and should be no doubt.


Referances

Chin, K., Tokaryk, T.T., Erickson, G.M., Calk, L.C., 1998. A king-sized theropod coprolite. Nature 393, 680–682.

Choy, D. S. J. & Altman, P. 1992 The cardiovascular system of Barosaurus: an educated guess. Lancet 340, 534-536.

Christiansen, P. 2000. Feeding mechanisms of the sauropod dinosaurs Brachiosaurus,Camarasaurus, Diplodocus and Dicraeosaurus. Historical Biology 14, 137–152.

Farlow, J. O, M. B. Smith, and J. M. Robinson. 1995b. Body mass, bone “strength indicator,” and cursorial potential of Tyrannosaurus rex. Journal of Vertebrate Paleontology, 15:713–725.

Holtz, T. R., Jr. 2004. Tyrannosauroidea; pp. 111–136 in D. B. Weishampel, P. Dodson, and H. Osmolska (eds.), The Dinosauria, second edition. University of California Press, Berkeley, California.

Hone, D.W.E., and Watabe, M. 2010. New information on scavenging and selective feeding behaviour of tyrannosaurs. Acta Palaeontologica Polonica 55 (4) 627-634.

Larson, N 2008. One Hundred Years of Tyrannosaurus rex: The skeletons: pp. 1-55 in P. Larson and K. Carpenter (eds.), Tyrannosaurus rex, the Tyrant King, Indiana University Press, Bloomington, Indiana

Martin, J., Martin−Rolland, V., and Frey, E. 1998. Not cranes or masts, but beams: the biomechanics of sauropod necks. Oryctos 1: 113–120.

McIntosh, J. S. 2005. The genus Barosaurus Marsh (Sauropoda, Diplodocidae). In Tidwell, V. & Carpenter, K. (eds) Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press (Bloomington & Indianapolis), pp. 38-77.

Paul, G.S.1998. Limb design, function and running performance in ostrich-mimics and tyrannosaurs. Gaia 15:257–270.

W. Scott Persons, Philip J. Currie 2010. The Tail of Tyrannosaurus: Reassessing the Size and Locomotive Importance of the M. caudofemoralis in Non-Avian Theropods. The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology, 2010.

Stevens, K. A. & Parrish, J. M. 1999 Neck posture and feeding habits of two Jurassic sauropod dinosaurs. Science 284, 798-800.

Taylor, M. P., Wedel, M. J. & Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54, 213-220

Upchurch, P. and Barrett, P. M. 2000. The evolution of sauropod feeding mechanisms. pp. 79–122 in Sues, H.D. (ed.): The evolution of herbivory in terrestrial vertebrates. Perspectives from the fossil record. Cambridge University Press.

Witmer, L.M. and R.C. Ridgely. 2009. New insights into the brain, braincase, and ear region of tyrannosaurs, with implications for sensory organization and behavior. Anatomical Record 292:1266–1296.

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