Dinosaurs Without Bones Read Free Page B

ago) of Utah in 2010. These are interpreted as claw marks made by predatory theropods. The close association of these marks with underlying fossil burrows, inferred as those of mammals, adds another previously unconsidered dimension to dinosaur behavior, which was their preying on small subterranean mammals. Sediment-rimmed nests made by theropods like Troodon and some sauropods also imply that these dinosaurs dug up and mounded soil to make these protective structures.
    Related to this, a renaissance in our understanding of dinosaur eggs, babies, and the rearing of young has revolved around their trace fossils, too. Troodon , a Cretaceous dinosaur from 70 to 75 million years ago and found in parts of western North America, was the first known North American example of a theropod that made rimmed ground nests. These nests also contained clutches of paired eggs, which were arranged vertically in the nests by one or both of the parents after egg-laying. All three trace fossils of Troodon behavior—the making of rimmed ground nests, pairing of the eggs, and their post-laying arrangement—provide insights we never would have figured out from their skeletons.
    Similarly, a spectacular find of Late Cretaceous nests in Argentina from 70 to 80 million years ago and attributed to gigantic sauropods called titanosaurs shows that dinosaurs other than Troodon made ring-like enclosures for their eggs. The sauropod nest structures, however, only superficially resemble those of Troodon and are bigger, more abundant, and stacked on top of one another, representingmany episodes of sauropod breeding in the same general area. In this sense, then, did these enormous dinosaurs act like modern migratory birds, returning to the same nesting grounds for hundreds of thousands of years? Once again, this and other questions are ones that trace fossils can help to answer.
    The seemingly odd depiction of the gangly theropod Struthiomimus consuming rocks along a riverbank and using these as gastroliths is not too far off from the truth, either. Paleontologists have long suspected that some herbivorous dinosaurs, similar to modern birds or crocodilians, swallowed rocks and used them in their digestive tracts to grind food. This especially made sense for dinosaurs with teeth poorly adapted for chewing yet somehow needing to eat difficult-to-digest plants. What has surprised paleontologists in recent years, though, is the realization that a few theropods, a group of dinosaurs once assumed to have been exclusively carnivorous, also have these “stomach stones.” Paleontologists just assumed that strong stomach acids were sufficient for digesting anything consumed by a theropod. Although no one has yet found gastroliths directly associated with Struthiomimus , some of its relatives, collectively called ornithomimids (“ostrich mimics”), do have them. This fact has prompted paleontologists to start thinking about what these theropods might have eaten other than meat: insects, plants, or a blend of both? Or did these gastroliths have some other uses we still don’t quite understand? And why would some herbivorous dinosaurs with teeth unsuited for chewing, such as most sauropods or stegosaurs, not have gastroliths?
    Speaking of food, yet another dimension of dinosaur behavior that is much better comprehended through their trace fossils regards what they ate. Traces woven into the opening narrative, such as healed bite marks, toothmarks on bones, wear on teeth caused by plants, and coprolites (fossil feces), tell us much more about dinosaur dietary choices than any other means of fossil evidence. For instance, we can now surmise that Edmontosaurus and Triceratops must have been quite tasty for some tyrannosaurs. This is backed by healed toothmarks caused by a large predatory theropod preserved in a few bones of Edmontosaurus , including at least one with a smoking gun (or tooth, as it were) linking it to Tyrannosaurus or its close relatives.
    Triceratops