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.
Hawkwood


*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.

Wednesday, March 10, 2010

The Stones on Tour

It must have been one of the most dinosaur-intense environments in the world. During the last several million years of the Cretaceous, a shallow sea divided the eastern and western sides of the North American continent, with the western side stretching from the ice-free North Pole - then centered on land that is now northwest Alaska - all the way down to present-day Central America.

On the eastern side, Quebec's Manicouagan impact crater and the Appalachian mountain chain were already-ancient features of the Cretaceous landscape. On the western side lived all the dinosaurs that have come to embody what the term 'dinosaur' represents to us: predatory tyrannosaurs, ceratopsians such as Triceratops, long-necked titanosaurs, and tank-like armoured ankylosaurs. In the wide skies above the flat Texas floodplain cruised the giant pterosaur Quetzalcoatlus (above), as large as a light aircraft. And in the shadows of volcanoes south of the present Canada-United States border, huge herds of hadrosaurs were on the move as they migrated to fresh grazing. With the discovery of the remains of a herd of the hadrosaur Maiasaurus estimated at a staggering 135 thousand individuals - apparently overcome by volcanic ash - we know that such migrations must have taken place: such herds would have been too large for a single location to have sustained them.

The waters of the central sea, known as the Western Interior Seaway or the Cretaceous Interior Seaway (my map, above), were home to marine reptiles as well-known to us now as the dinosaurs in the land to the west. Predatory mosasaurs, the giant turtle Archelon, with a shell as large as a Humvee, long-necked plesiosaurs - and the even-longer-necked Elasmosaurus. The teeth of Elasmosaurus (the skull, below) were well-adapted to grasping fish, and its meal certainly had a way to travel along the length of the neck before it reached the animal's gut. Now, there is more to tell about the gut of the elasmosaur, but first this:

An animal's fossil can teach us much, but it might not necessarily have a lot to offer when it comes to learning about that animal's social behavior. For this, analogs can be used. That is: the behavior of an extinct species can be inferred by comparing it with the behavior of a living animal with a similar morphology, or body shape. It was while watching a wildlife documentary of two snakes neck-wrestling that I thought of the snake-like necks of elasmosaurs, and later made a sketch (below) to illustrate the idea. We know from its neck vertebrae that an elasmosaur had more flexibility laterally than dorsally. That is: it could wave its neck from side to side more readily that it could up and down. So the two animals in my picture would have had to have been almost 'standing' in the water - an extra factor in such a trial of strength.

So would rival elasmosaurs, perhaps struggling for the right to mate with a female, really have fought in this way? As with any analog behavior, it can neither be proven nor disproven. But to me it does seem both reasonable and likely. The writhing serpentine necks weaving around each other among the white crests of the waves: this was the image in my mind as I began to draw. Strange creatures of another time and another place, like Moby Dick escorted by a cloud of white sea birds. Except in this case, the 'sea birds' are circling pterosaurs (Pterandon) excited by the struggle, as other animals often are in such situations.

Intriguingly, I came across in a *book the information that elasmosaurs apparently swam through present Kansas, Nebraska and the Dakotas to reach estuaries on the northeastern and northwestern coasts (the map detail, above). There they would swim upriver as much as 100 km (over 62 miles) away from the Seaway. But what drove them to do so? The question gnawed at me for a while. Then I remembered that elasmosaurs are associated with finds of gastroliths: stones which an animal swallows to aid digestion, using the stones like millstones to grind up hard-to-digest material before it passed to the animal's gut. Perhaps in the case of aquatic elasmosaurs these gastroliths served the important dual function of ballast to stabilise the animal's bouyancy.

All gastroliths wear down with use and need to be replaced. Were the reaches of these Cretaceous rivers the ideal source for these stones, pre-smoothed in the tumbling river waters, and lying on the riverbed for the taking? An elasmosaur *excavated in Kansas revealed gastroliths (above) whose source appears to have been these northern rivers. The river journeys of these animals clearly had a purpose, and that purpose would seem to have been to collect stones suitable for use as gastroliths. Stones from northern rivers found in the stomach region of an aquatic fossil reptile in Kansas can tell us much, both about the environment in which that animal lived, and about how that animal used the resources of that environment for its needs.
Hawkwood


*An Odyssey in Time: The Dinosaurs of North America, by Dale A. Russell. The University of Toronto Press, 1989.

*Cicimurri, D. J. and M. J. Everhart, 2001. An elasmosaur with stomach contents and gastroliths from the Pierre Shale (late Cretaceous) of Kansas. Kansas Acad. Sci. Trans 104(3-4):129-143.

Gastroliths photograph from the Oceans of Kansas website (link on my sidebar). All other images © Hawkwood. The map of Cretaceous North America has been compiled from material drawn by Ron Blakey and Christopher R. Scotese, and from data by Dale A. Russell. Any misinterpretations of the data from these authors are entirely my own! The superimposed outline of contemporary North America is from D-Maps.

Friday, February 5, 2010

Hoots, Honks and Bellows

Some eight years ago I remember writing that, although the colors given to dinosaurs in artists' reconstructions are conjectural, it could only be a matter of time before some new imaging technology might provide us with real evidence of actual colors in their fossils. Well, last month's issue of *Nature contained news of exactly that. The discovery of cells known as melanosomes in the preserved fossil feathers of the dinosaur Sinosauropteryx indicated that the tail of this animal was a lemur-like striped russet brown. Although the colors themselves were not preserved in the fossil, the distinctive different shapes of the cells acted as a code that allowed each color to be determined. This is the first time ever that colors have been described in a dinosaur fossil, and of course it created much stir. But.. this particular fossil was in an exceptional state of preservation. We are still a long way from knowing - if indeed we ever will - just how colorful (or not) T. rex actually was. So the reality is that I find that my own stance as a reconstructional artist has not greatly changed.

Take a group of dinosaurs - the hadrosaurs - about which, thanks to partially mummified fossil specimens, we know a remarkable amount. Herbivorous hadrosaurs (sometimes informally known as 'duck-billed' dinosaurs) were extremely successful, ranging in the last few million years of the Cretaceous over several continents (the Parasaurolophus herd, above). Their crests contained a complex system of air passages that would have led between the nostrils and the lungs (the Lambeosaurus skull section, below), and they would have been capable of producing a variety of hoots, honks and bellows to fill the mists of a Mesozoic morning. Think of the passage through which a player must blow from lungs to mouthpiece through to the bell of an instrument such as a trombone or a french horn, and you have an analogy for a hadrosaur orchestra.

Hadrosaurs could also be large animals (my comparative skeletons of human and Lambeosaurus, below), and when we consider that fossil finds suggest evidence in some species of herd behaviour, plus the vocalizing abilities of these dinosaurs, then the picture emerges of animals with a reasonable degree of social interaction - and pehaps even complex social behaviour. But did color also play a role in this? We do not know, but it seems reasonable to assume that it did so. Snakes and birds have color vision. Mammals do not. Dinosaurs were certainly closer to reptiles than to mammals, and birds are the living equivalents of raptorial dinosaurs. Perhaps, in addition to vocalizing, the crests of hadrosaurs were used to send recognition signals of distinctive patterns and colors to others of their kind.

Such were my thoughts when I came to create my own 'portraits' of six hadrosaurs. With two of these (Anatotitan and Kritosaurus), it was an inflatable sac of skin rather than a crest which formed the vocalizing function, and the crest of one (Saurolophus) seems to have had limited vocalizing range. But the remaining three (Lambeosaurus, Parasaurolophus and Corythosaurus) all had distinctively individual head crests. My usual technique is first to make a detailed drawing in pencil (below), and to include in the drawing enough detail which commits me to establishing any skin patterns, which - however accurately I can portray the rest of the anatomy - clearly are speculative.

This detailed pencil drawing I then scan in and paint with a variety of digital brushes (below). And although this is the stage of creating the artwork that makes these animals seem the most alive, it also is the stage during which the most conjecture is used. From a purely palaeontological perspective, the reconstruction has by now become too conjectural to be of real value. Now, I can keep the thing within reasonable limits of zoological credibility by having a look through, and taking my lead from, my studio reference library of reptile and bird photographs and applying various analogous patterns, colors and textures.

Poring over such reference material - and studying the real thing in zoos and natural history museums - is a way of understanding generally the form and appearance of such skin markings in nature. Now, to build up the life appearance of a Lambeosaurus using the fossil skull as a basis on which to construct muscle and skin tissue: that's science. But to make you believe that a Lambeosaurus really did look the way in which I have portrayed it here, with patterns, colors, and all; that's where the art comes in!
Hawkwood


*Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Zhang, F., Kearns, S.L., Orr, P.J., Benton, M.J., Zhou, Z., Johnson, D., Xu, X. and Wang, X. Nature, advanced online publication, 27 January 2010.