"If we want to understand the environment dinosaurs came from and why they succeeded, we have to use all the information available to us," says Kevin Padian, Professor of Integrative Biology at the University of California at Berkeley. Richard Stucky of the Denver Museum of Natural History agrees. "We want to know what dinosaurs were like, how they interacted with one another, and what kinds of communities they lived in. Studying the environment can give us important clues."
But finding and piecing together these clues is tricky. It is the rare living thing that becomes a fossil in death. The vast majority of organisms, whether they be plants, animals or insects, simply decompose. But sometimes, if conditions are just right, an organism can be preserved for millions of years. Find out how some ancient organisms beat the clock—and what role they may have played in the lives of the dinosaurs.
The first step towards fossilization for vertebrates, or animals with backbones, is rapid burial in sediment. One scenario might be a drowned animal that washes downstream and lodges into a riverbank, where it quickly becomes covered in sand or mud. The animal's soft parts, namely its flesh and organs, rot away, while sediments surround and protect the animal's hard parts—its bones and teeth. Over time, mineral-rich water percolates through the bone's tiny pores and, gradually, the bones absorb these minerals and turn to stone. But this is only the beginning. For a fossil to survive through time, the surrounding rock must withstand the forces of erosion and tectonic activity as well.
The fossil record reveals that dinosaurs first appeared in the Triassic Period, between 250 to 213 million years ago. It also indicates that the first turtle, the first salamander, and the first frog appeared in the Triassic, as did the first crocodilian reptile, the phytosaur. On land, mammals also made their debut during this period, in the form of small insect-and-seed-eating critters that scurried around the forest underbrush. Knowing the vertebrate players is important, according to Kevin Padian, because it gives insight into how dinosaurs may have gotten their first toehold in their climb to supremacy. "It's pretty clear that the first dinosaurs weren't directly competing with other animals for food," he says. "Phytosaurs were in the water eating mainly fish, and there were reptilian herbivores on land, like some of the aetosaurs. But early dinosaurs were small bipedal carnivores, meaning they had their hands free and ate meat. So they did very well."
Figuring out the relationship between plants and dinosaurs can be revealing, but it is difficult to do. The problem is not for a lack of specimens. Plant fossils easily outnumber bone fossils, and they come in many more forms. The billions and billions of tons of coal in the world are ancient plant remains, as are compressed leaves, leaf imprints, pollen grains, and pieces of fossilized wood. The difficulty is that bone fossils and plant fossils are rarely found at the same site.
"It has to do with the chemical conditions that favor preservation of bones as opposed to those that favor the preservation of plant matter," says Scott Wing, Curator in the Department of Paleobiology at the Smithsonian's Museum of Natural History. "Plant matter is preserved best in an acidic environment, like in a peat bog. Whereas bones, having a lot of calcium in them, are essentially demineralized by acidic conditions. If you drop a bone in a bog, it gets rubbery, loses its calcium, and it's difficult for it to be preserved. Conversely, if you take a piece of plant and drop it in a nice alkaline soil where a bone would likely be preserved, the plant is going to be degraded, not only by the chemical conditions but also by all organisms that live in the soil."
Insects may have competed directly with herbivorous dinosaurs for food.
Despite this wrinkle, plant groups of the Mesozoic are well known. The major players were conifers, a tree similar to today's Norfolk Island pines; cycads, a palm-like tree with leathery leaves and great big cones that still grow in some tropical areas; bennettitales, an extinct group of plants that had leaves like cycads but were more closely related to flowering plants; and ferns, which were more varied and abundant than they are today. Flowering plants, or angiosperms, first appeared near the end of the Mesozoic, around 144 million years ago, and they quickly took hold, constituting nine-tenths of all known plant species by the end of the period. Grass, so common today, was non-existent.
If plant fossils and dinosaur fossils tend not to turn up together, how do paleontologists know what herbivorous dinosaurs ate? "You use some manner of correlation," says Peter Dodson, Professor of Anatomy and Geology at the University of Pennsylvania. "Some of the large, important fossil deposits of the American and Canadian West are geographically quite widespread. So you find bones and skeletons in one place, and maybe a few miles away you find rich plant deposits. You build up a picture from all the material that's available from different geographical localities." Richard Stucky concurs: "Oftentimes you get sites with specimens above and below the time period you're looking at, and you can extrapolate that a particular critter or plant lived through the period you're interested in."
Another source of information on dinosaur diets, albeit a rare one, is coprolites, or fossilized dung. "Coprolites give information about what one particular individual was eating, but you don't usually know what genus or even what species was producing the coprolite unless you find it inside a dinosaur skeleton," says Scott Wing. "In general, it's a pretty speculative endeavor. You've got your plant fossils, and you've got your dinosaur fossils—and they're often not from the same places or even from the same times. We can track changes in plants through time, and we can track changes in dinosaurs through time, but we don't have any very good way of establishing cause and effect. So the real answer to 'What did dinosaurs eat?' is that, most of the time, we don't know."
Insects are perhaps the least understood organisms of the Mesozoic. Conrad Labandeira, a research scientist specializing in fossil insects at the National Museum of Natural History, explains that this is due, in part, to their sheer number. "Estimates of modern insect species range from about 5 million to 30 million," he says. "For vertebrates, including fish, it's about 80,000 or 90,000. One of the consequences of there being so many more insects living today is that their proportional 'know-ability' in the past is less. The other issue is that there are so few people studying them." Aside from a colleague in Canada and a small cadre of scientists in Russia, Labandeira is alone in his work.
Ideal conditions for preserving insects are different from those that preserve plants or bones. An insect's exoskeleton is made out of a substance called chitin. "You know when you step on a cockroach or beetle? That snap is chitin," says Labandeira. "And chitin is best preserved when it's entombed in very fine-grained sediments." Typically, insects destined for fossilization were ones that got washed into a stagnant body of water like a lake or pond, where they quickly became buried. Mummification in fossil tree resin or amber, popularized in Steven Spielberg's film Jurassic Park, is a less common method of preservation and only appears in the fossil record after 125 million years ago. "I don't know why that is," says Labandeira. "One possibility is that wood boring insects really didn't savage trees until then, but it's really a mystery."
The story of ancient insects really begins in the period preceding dinosaurs—in the Paleozoic Era, between 590 to 248 million years ago. Insects with piercing and sucking mouthparts, primitive grasshopper-like bugs, and giant dragonflies flourished, as did the primordial ancestors of modern insects. But the Permian Extinction at the end of the Paleozoic wiped out most of these insects, not to mention a number of other life-forms.
The insects that survived into the Mesozoic are basically the insects that we know today. "If you were plopped down into the Cretaceous by a time machine, and you had a basic entomology textbook with the keys to the families, you probably wouldn't have much trouble finding your way around," says Labandeira. "There'd be a few things you'd have trouble with—a few extinct groups like the Kalligrafmatidae, which had butterfly-like wings, or the flea-sized Saurophthirius, which had small antennae and a pump on top if its head that inflated like a balloon to create negative suction to pull up, we think, blood." Labandeira is quick to point out that these creatures may still exist today somewhere in the tropics, but because of our poor knowledge of insects, they may have gone undetected.
Despite their relatively small size, insects played a very big role in the lives of dinosaurs, evidence suggests. In fact, insects may have competed directly with herbivorous dinosaurs for food. "That's certainly been an important question in modern ecosystems," says Labandeira. "In grasslands, vertebrates probably outperform insects in terms of consumption of plant material, but in all other ecosystems, it looks like it's the other way around." Insects may have played a role in recycling dinosaur waste. A researcher named Karen Chin recently looked at cross-sections of dinosaur coprolites and found structures within that indicated the presence of scarab beetles that were probably feeding on the dung. Insects also may have been responsible for transporting diseases like malaria and dysentery, but Labandeira admits this possibility is purely speculative.