Dinosaurs Without Bones (28 page)

So let’s apply this idea to the geologic past. Did Late Cretaceous ecosystems of Madagascar undergo any sort of stresses, such as droughts? Apparently so, as its rocks show the region was semi-arid while dinosaurs lived there, but with pronounced wet-dry cycles.
Such environments were more susceptible to droughts than, say, a tropical rainforest. One of the more compelling pieces of evidence for droughts in this area during the Cretaceous comes from other trace fossils, namely lungfish burrows.

Modern lungfish, to keep from drying out during times of low rainfall, make burrows. They then reside in these while also enveloping themselves in slimy cocoons, thus doubling their protection. In an ichnologically wonderful paper by paleontologists Madeline Marshall and Ray Rogers published in 2012, they interpreted more than a hundred lungfish burrows in a Late Cretaceous river deposit in Madagascar. These burrows, which were nearly identical to modern ones, pointed toward this as a behavioral response to a long, dry time that required hunkering down. So just like insect cocoons next to
Troodon
nests in Montana, these trace fossils tell us something about how animals around dinosaurs were adapting to their climate. Did such dry periods also affect food supplies of Late Cretaceous dinosaurs in Madagascar? Perhaps, although paleontologists need more details before they can say for sure.

Regardless of reasons why, finds of dinosaur-cannibal trace fossils from so long ago are remarkable enough to prompt paleontologists to reexamine toothmarks on theropod bones, checking to see whether or not these came from the same species. From there, they could then test whether these behaviors were anomalous or more normal than originally surmised.

Along those lines, trace fossils tell us that another large theropod, our old friend
Tyrannosaurus rex
, decided to count on its relatives for some of its meals. In a 2010 study conducted by Nicholas Longrich and three other paleontologists, they examined
T. rex
bones in museum collections at the University of California (Berkeley) and the Museum of the Rockies (Montana) and realized that not only did some of these bones have toothmarks, but big ones. Even before measuring these marks, the paleontologists knew that their depths, lengths, and spacing between teeth limited them to the largest land carnivore that lived at the same time as
T. rex
, which would have been
T. rex
. Unexpectedly, bones from four separate specimens
held these toothmarks. Considering the rarity of
T. rex
specimens in general, and to have four with toothmarks also coming from
T. rex
, cannibalism in that species might have been more common than originally thought. Still, such acts might have been done in desperation, as three of the affected bones were from feet, and one was a humerus. Let’s just say that if you were the world’s largest land carnivore and you were resorting to not only eating your own species but also the least meaty parts, you were much too hungry. Also, no reasonable person can imagine a live tyrannosaur quiescently accepting another tyrannosaur stripping flesh from its toes or arms. In other words, a prone, dead tyrannosaur would have made for a much easier snack for one of its kin.

Trace fossils also tell us that big theropods chewed on one another’s faces, but while their faces were still alive. This idea, first proposed by paleontologists Darren Tanke and Phil Currie in 1998, was based on healed bite marks they noted in skulls of
Sinraptor
(a Late Jurassic theropod from China),
Albertosaurus
,
Daspletosaurus
, and
Gorgosaurus
(all Late Cretaceous tyrannosaurs from Canada). Interestingly, some of this unruly face-biting happened while these theropods were still relatively young. For example, subadults of
Albertosaurus
and
Gorgosaurus
have face bite marks, as did another unidentified tyrannosaur from the Late Cretaceous Hell Creek Formation of Montana that got its nose out of joint (literally) caused by a bite to its nasal and maxilla that bent its face to the left. Because these wounds healed, these were probably from competition or territorial aggression and do not really qualify as cannibalism. Although, considering that these were adolescent dinosaurs, pre-mating “love bites” cannot be discounted, either. If so, these were hickeys from hell.

Dinosaur Dentistry: Looking Closely at Wear on Teeth

Pretend you are a dentist, or, if you already are a dentist, just be yourself. Your patient, who just walked into the waiting room, is a 7-ton hadrosaur, and he is complaining of a toothache. You’ve had hadrosaurs stop in before for checkups, and unlike most theropods,
they’re usually great patients. Nevertheless, you dread taking a look inside any of their mouths because of their dental batteries. Dental batteries are tightly packed arrangements of small teeth, and in a typical hadrosaur, they could have hundreds. Consequently, finding a problem tooth among many healthy ones becomes a time-consuming task. It also doesn’t feel all that necessary, considering that another, newer tooth directly underneath the bad one will replace it anyway.

Using a mirror, directed light, and a magnifying lens, you see thin, shallow scratches on its teeth. These scratches tell you that the hadrosaur moved its upper jaws out and to the sides while the lower jaw stayed put. This motion differs greatly from that of nearly all humans, who can move their lower jaws forward and laterally while their upper jaws are fixed. For this and other hadrosaurs, though, this action is normal because of how its jaw is hinged, and such an arrangement provides the grinding action needed to chew its food, which are various near-the-ground plants. Seeing this evidence, you admonish your patient for eating too many of those succulent low-lying plants along a nearby river floodplain—especially after the river has flooded—and for chewing too much. Your patient is both surprised and impressed that you somehow figured out what, where, and how he had been eating, and he promises to do better. You then schedule him for a cleaning six months from then, and plan your vacation to coincide with that appointment.

Again, thanks to trace fossils on teeth—these tiny scratch marks—we can tell that certain species of hadrosaurs and other herbivorous dinosaurs were grazers, eating plants that grew close to the ground, rather than being browsers, which meant going for greens hanging high in trees. Whole, healthy teeth certainly can have shiny polished surfaces, but if you look at them with a magnifying glass or microscope, you will also see tiny grooves and scratches. You have such wear on your teeth, too, especially if you are in the habit of eating mineral-laden plants.

These marks, called
microwear
, were scored on dinosaur teeth when they chewed plants containing silica or plants with grit on
them. The grit also would have consisted of silica-rich minerals such as quartz. As any geology major will gleefully tell you, quartz is harder than the mineral apatite, and the latter is what composes vertebrate teeth and bones. Hence, silica in plant tissues or silica-rich grit on plants, combined with dinosaurs chewing those plants, would have caused more scratches than if the dinosaur had either swallowed those plants whole without chewing or eaten plants with less silica. On the other hand, browsing on plants growing well above the ground should have resulted in fewer scratches.

To better understand how silica was included in some plant tissues, we look to evolution. Plant-eating dinosaurs may have left few bite marks on fossil plants, but plants certainly “bit back.” Thorns or spikes, toxins, or indigestible parts became common in some plant lineages as a consequence of dinosaurs treating them like indiscriminate items on a salad bar. Among these defenses were
phytoliths
, tiny grains of silica precipitated in plant tissues. Phytoliths represent a war of attrition, which, through sheer numbers and high abrasiveness, slowly wear down herbivore teeth. Hard seeds and nuts are more overtly offensive, inflicting breakage in teeth and thus forming pits.

Phytoliths are common in many plants today, especially monocotyledons, which include all grasses, orchids, bamboo, palm trees, and many others. Monocotyledons also got their start in the middle of the Mesozoic Era. Coincidence? Maybe, but the evolution of the largest land herbivores of all time surely resulted in land plants responding ferociously; after all, plants have no moral compulsion to fight fair, and they want to survive and reproduce, too. Thus, it comes as no surprise that sauropods, hadrosaurs, and other herbivorous dinosaurs also evolved fast-replacing teeth (in sauropods) and dental batteries (in hadrosaurs) to more easily replace teeth worn down by insidiously vicious plants.

Plant-eating dinosaurs also might have had a double threat to their dental health posed by plants with phytoliths: silica-rich grit on their surfaces. How did this grit get on plants? Take a close look at any vegetation alongside a stream that experiences
frequent flooding in a place with silica-rich rocks and you will likely see clay, silt, and fine sand adhered to the leaves, branches, and stems of plants there. As any flood subsides, suspended sediment carried by a stream during a flood settles, and some of it sticks to the plants. I look for such residue whenever tracking animals along stream banks, and wherever noted, it informs me of former flood heights in that stream valley. Of course, wind can also put some grit on plants, but it adheres more easily if already wet. Nonetheless, also think of how dust clouds were likely kicked up by herds of dinosaurs, suspending plenty of fine-grained sediment near the ground and likewise adding these grains to any low-lying flora.

Paleontologists who studied microwear in
Edmontosaurus
found out that this dinosaur—when it was not breaking off tyrannosaur teeth—chewed its food through a definite series of movements, and that it was grazing. Microwear on its teeth consists of four sets of scratch marks, with each set showing different orientations on tooth surfaces. For the set with the deepest scratches that also cut across the others, the paleontologists defined these as having formed during the “power stroke” phase of chewing. This was when the hadrosaurs put the most effort into grinding down their food, moving their jaws vertically and together.

Microwear is also visible on the teeth of other herbivorous dinosaurs, such as ceratopsians, ankylosaurs, and sauropods. Most of these trace fossils, while telling us how these dinosaurs chewed their food, also suggest a grazing habit, instead of their cropping tall tree tops. For sauropods, this may seem to contradict interpretations of how their long necks were used to do just that, reaching higher to sample some tasty canopies. But anatomical studies done on some sauropods now imply that some of these lengthy necks were maybe better suited for sweeping large areas—back and forth—across fields of low-lying vegetation.

Trace Fossils in Bones and Teeth: Greater than the Sum of Dinosaur Parts

Hopefully by now it should be absolutely clear that taking a gander at dinosaur body fossils for their trace fossils was a good kind of
ichnological cheating. For paleontologists, these skin impressions, bones, and teeth with their trace fossils are two-for-one specials that not only are the bodily remains of dinosaurs but also give us valuable insights on dinosaur behavior. Again, just looking at these body fossils without noting their trace fossils is not enough. When viewed through an ichnological lens, these parts unfold into much more than just a piece of a dinosaur, including complex relationships between their former owners with other dinosaurs and many other facets of their Mesozoic ecosystems.

Your Inner Gastrolith

The dark rock stuck out in the pinkish mudstone, looking like an errant raisin tossed into a strawberry mousse. My curiosity properly piqued, I walked over to pick it up and held it in my left hand, admiring its fine qualities. It was a black, polished chert, which is a hard, flint-like mineral. It was beautifully rounded and about half the size of a baseball, but slightly longer than wide. I bounced it up and down to better feel its heft. A closer look revealed a few irregularities on its otherwise flawless surface, little chatter marks where it must have impacted against some other solid object, such as another rock.

Somehow I identified with this stone, feeling like a small oddity pasted onto a homogenous background, with an apparently polished exterior but also bearing blemishes for all to see. It was the summer of 1983 and I was well into the second week of geology field camp in northwestern Wyoming. About a hundred other geology students were there too, all of us being directed by three geology professors. Yet I was an earth-science
neophyte, having just finished my first year of an M.S.-degree program in geology, but entering it with a background in biology and art. I was also a Midwesterner who had been confined to flat, cornfield-dominated landscapes east of the Mississippi River for all of my life. So this was my first trip to the western U.S., and my first time walking on Mesozoic rocks.

Underneath my feet, the weathered mudstone—streaked with white, gray, or maroon, and occasionally interrupted by beige sandstone lenses—was the Late Jurassic Morrison Formation, renowned worldwide for its massive fossil bones. This was dinosaur country, and being in it was absolutely thrilling. I had read about these giant extinct animals all of my life, but at the time my home state of Indiana had no museums with mounted skeletons of dinosaurs, nor did it have any rocks of the right age holding either dinosaur bones or trace fossils. I was a transgendered Alice in Wonderland, albeit with more amiable and slightly saner companions. After studying the rock for a few more minutes, I set it down and walked away, but held on to its memory.

Later that day, the professor in charge of our geology field camp, Dr. Wayne Martin (no relation), casually remarked on these anomalous rocks in the fine-grained mudstone. “Gastroliths,” he informed us with his distinctive slow West Virginian drawl. Geologists regarded these blemishes in the otherwise smooth, fine-grained Morrison Formation as evidence that dinosaurs were there. Sure, you could also find dinosaur bones if you looked hard enough, and the Morrison Formation was rightly celebrated elsewhere in the western U.S. for yielding some of the best-loved of all dinosaurs:
Apatosaurus
,
Diplodocus
, and
Allosaurus
, to name a few. But where we were, these large stones were far more common than bones. Despite nearly two hundred eyes looking at the ground that day, not one person spotted a dinosaur bone, but we found plenty of these strange rocks.