Thursday, May 6, 2010

T. rex Down!

It can be worthwhile revisiting what perhaps have become rather over-familiar scenes portraying dinosaur life. One of these is the much-pictured confrontation between a Tyrannosaurus rex and the armoured ankylosaur Euoplocephalus (my painting below). Just how likely would such an encounter have been? And if so, then what would have been the probable outcome?

If we place the two dinosaurs alongside each other (my painting below, with the skeletons giving a human scale), then one thing is clear straight away: Euoplocephalus, the largest of all the ankylosaurs, had at least as much body mass as the formidable predator. And it certainly was armoured - it even had a small bony plate covering its eyelid. In addition to the various spikes and plates, its most obvious weapon of defence was the bony 'club' at the end of its tail. This club (actually modified tail vertebrae) was not solid, but was honeycombed with spongy air pockets, making it in life both comparatively light and extremely strong and resilient, with the honeycombed bone acting as a shock absorber. That the fossilized tail club is so massively heavy is simply due to this honeycomb of air cells becoming filled with solid mineral deposits.

So how would a T. rex have tackled such an animal? *Studies have shown that the jaws of a T. rex could close with a staggering 2,900 pounds of bite force per side of the jaw: the most powerful bite of any animal ever known. This is certainly powerful enough to crunch straight through solid bone - and if a few teeth were broken in the process, then they were simply replaced, as was normal throughout a theropod's life. To put a Euoplocephalus on the menu, an experienced T. rex probably would go for the vulnerable neck or legs of the animal, or even *gulp* bite straight through the damage-dealing tail. But just how vulnerable was the T. rex itself?

The old-style reconstructions of tyrannosaurs portrayed them with legs as sturdy as tree trunks (by Charles R. Knight, above, painted almost a century ago), but we now know that T. rex would have had a 'chicken drumstick' leg, with hefty calf muscles, but with the ankle being little more than skin and sinew over bone. A well-timed blow from a tail club to a T. rex ankle surely would have done serious damage, injuring or even crippling the animal if wrong-footed. And a T. rex that went down after such a blow, even when its injury was not immediately fatal, might not get back up again.

My preparatory sketch for the scene (above) includes three T. rex, with one down and two still on the attack. Even such a top-of-the-food-chain predator might have scored better with such odds. My more finished drawing (below, prior to scanning and digital painting) narrows the odds down to two to one. Pathologies on the fossil of 'Sue', the largest T. rex known, suggest several partially-healed injuries, although at least some of these could be due to *parasitic infections.

So in the end, what would have been the outcome of such an encounter? As with animals today, it probably would have been down to the individual dinosaurs involved. A mature ankylosaur certainly might have downed a younger T. rex lacking the experience to know how well-enough to avoid a crippling blow. And a mature T. rex would have been on the lookout for easy-to-take-down young or sick prey. And as with anything else, that chance factor - a randomly-struck but lucky blow, a sudden well-aimed bite - could have drawn the line between living and dying.

*Breathing Life into Tyrannosaurus rex, by Gregory M. Erickson. Scientific American, Sept. 1999.
*Paleontologists Assess T. rex Sue's Pathologies, by Kate Wong. Scientific American, Oct. 2001.