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.