June 3, 2014 11:10:04 AM
Think back to your last visit to a natural history museum, and remember the dinosaur halls. What do you see? Why, bones and bones, some on shelves, and some in framework that allows the strange creatures to stand up in skeletal form. There they stand, inert. The bones are wonderful for giving us an idea of what the dinosaurs looked like, but not so good at telling us what dinosaurs did. For that, ask an ichnologist. I didn't even know what ichnology was before I read Dinosaurs Without Bones: Dinosaur Lives Revealed by Their Trace Fossils (Pegasus Books) by Anthony J. Martin. The word dates only from 1851, and means literally the study of footprints, and indeed ichnologists are wild about footprints. "I like to argue," says Martin, "that dinosaur tracks constitute the 'real' fossil record of dinosaurs rather than their bones, which are nice but, well, just a little too dead." There's more to it, though; dinosaurs left footprints, but also nests, and burrows, and feces. And, surprise, evidence of urination. For those of us who don't know anything about ichnology, but appreciate stories of how real science is done, Martin's book is a great introduction.
When an ichnologist comes across footprints, identifying the dinosaur which made them is important, but it is only a small part of what sort of information might be drawn out of the prints. One of the good aspects of Martin's book is that he tells us what sort of questions ought to come up in the head of anyone investigating such prints, and the list of questions is far longer than, "What made this?" How big was the creature who made the print? (Even if you know exactly what species it is, it might not be a full-grown specimen.) What was the gait? (Like most of us, even marathoners, dinosaurs almost always walked, but they might run or trot.) Did the animal take a break and just stand, or just sit? Was it with others of its own kind? Is there evidence of any injury? (There are plenty of bone fossils that show injury, but whether the injury affected the behavior of the dinosaur is harder to say. An asymmetric gait with unequal strides can tell the story.) Was it male or female? Patiently, Martin shows that careful measurements and a few calculations can help give answers to such questions, however tentative. There's an equation, for instance, for the hip height of a dinosaur, which turns out to be the footprint length quadrupled. A much more complicated formula gives the speed, but the tracks themselves hint at speed: in both bipedal and quadrupedal dinosaurs, the narrower the track, the more likely it is that the animal was going fast. Martin says this is as true now as it was eighty million years ago, and invites us to take our dogs to the beach, run and walk them around, and see.
Martin often draws comparisons to current animals, especially birds, which as even kids know nowadays, evolved from dinosaurs and so are the dinosaurs' representatives in our time. He also is a fan of Jurassic Park ("the first one, that is, not its awful sequels"). He reminds us, however, that the dinosaurs depicted in the movie came from a different age, and the movie should have been called Cretaceous Park. You remember the scene where the tyrannosaurus runs at 45 mph after the jeep. Paleontologists and ichnologists have been having a field day ever since debunking this scene. Calculations show that a rex that could run that fast would need a body mass of 85% muscle all in the leg area; and also, a six ton beast which happened to trip at such speeds would die instantaneously. 25 mph is the calculated expected maximum T. rex speed.
"Before talking about dinosaur eggs, babies, and nests," says Martin, "a discussion of the potentially weighty subject of dinosaur sex is warranted, if for no other reason than that these behaviors were a necessary prelude to fertilization, but mostly because such banter is great fun." There are, as yet, no fossils of a dinosaurs making the two-backed beast (although there has been discovered Protoceratops engaged in fighting a Velociraptor). But dinosaurs in coitus would have left prints; again, no one has found them yet, "a postponing of evidence that tries the already-stretched patience of intellectually lusty paleontologists who desperately seek the warm afterglow of scientific gratification that comes from a sudden, ecstatic release of suspense." Science has predictive power, however, and Martin tells us what we should be looking for in such footprints; good luck to us all. As for the eggs that came from such couplings, we have known about those since the mid-19th century. Definitive examples of nests, however, were not recognized until around twenty years ago. Questions to be asked upon finding a depression that might be a dinosaur's nest: Does it hold eggs? Does it hold baby skeletons of the same species? Does it have a rim around it? Does it have different layers of soil? He picks apart the possible answers to these questions; it is clear that he spends plenty of time teaching students (and entertaining them), and that he truly enjoys his duties of imparting scientific information, in the classroom and in print.
He also enjoys taking academic looks at the grosser side of dinosaur activity. There really is a diagram here of a long-necked brachiosaurus projectile vomiting from a height of 14 meters, with resultant traces (and below, little theropods fleeing in distress). It's speculative; no one has found dinosaur vomit yet, but I bet they call for Martin's help when they do. They have discovered urolites, depressions made in the sand when a dinosaur urinated. Dinosaurs were big animals, and Martin discusses large liquid volumes dropped from suitable heights. "An idealized urination structure made under such conditions should have a central impact crater, closely associated splash marks, and, if a slope is present, linear rill marks caused by excess fluid running down that slope." Onward, science.
Among the most enjoyable pages here are the ones Martin devotes to his own discoveries in the field. In 2005 a friend called him to come look at diggings in Montana, where bones were found in what could have only been a burrow a dinosaur had made. That conclusion came afterward; he documents the painstaking process of evaluating the area but of also speculating on other things they were uncovering. His companion asked, "Are you sure we're not just making this up?" and Martin emphasizes that this is just the sort of question that needs to be asked repeatedly, and evidence sought to ensure the answer is yes. It turns out that their excitement was justified; they had found a new species of dinosaur, adapted for burrowing, within its burrow, with juveniles in the den. "Clearly it was time to do the next step in the scientific method, which was to hold a press conference." He's joking, but he then goes on to describe the excruciating difficulties of getting their discovery published and having others pick holes in their findings. Digging in the ground sounds much more fun.
Martin has lots to tell, and he is an entertaining writer with a jocular tone and an eagerness to descend into groan-worthy puns, especially when explaining the investigation of dinosaur excreta. ( To understand why there are so few specimens of fossilized dinosaur urination, for example, "doesn't take a whiz".) Those like me entirely unacquainted with his field of expertise will find him a genial guide. Readers will also have increased wonder at the activities of the dinosaurs, and at the cleverness of ichnologists to explain what the beasts were doing all those millions of years ago.