Dinosaurs Without Bones

by Anthony J. Martin

  F = ma (Force = mass 3 acceleration)

  However, force for a biting tyrannosaur did it no good if it just bit air. Once applied to an area on whatever it was biting, this force then translated into pressure. To better understand the difference between force and pressure, gently place a heavy book on the upper parts of both of your feet. Not so bad, is it? Now imagine what it would feel like if you dropped this book from waist height, and onto just the big toe of one foot. Dropping the book would certainly increase the force exerted by the book, but pressure would also increase dramatically because of this force striking a smaller area, and even more so if the corner of the book was the first point of contact. Now think about a Tyrannosaurus mouth, the muscles connected to its jaws, those jaws opening and then closing, with the huge force of the bite transmitted into the small areas represented by points of teeth.

  Still, the researchers’ biting mechanism needed something to actually bite, such as real bone. This would effectively test whether depths of the original toothmarks in bone could be produced artificially. Seeing that Triceratops bones were both too valuable and too fossilized to use, the researchers turned to a more easily procured supply of fresh hipbones from domestic cattle. So with this biting machine and “victims” in place, the researchers were ready to start making their own traces.

  The results of this experiment were astounding. Although nearly everyone suspected that T. rex was a terrific chomper, no one had been able to put numbers to this presumption. Bite-force estimates came out to 6,400 to 13,400 N, which at the time were greater than those known for any living animal, and confirmed that T. rex could have easily crunched its food, bones and all, living or dead. In later experiments done on modern alligators and crocodiles, Erickson and other researchers found the largest American alligators (Alligator mississippiensis) had bite forces within the same range as T. rex (9,400 N). But most impressive of all were estuarine crocodiles (Crocodylus porosus)—known warmly by Australians as “salties”—which had maximum bite forces of 16,400 N. These findings further implied that Deinosuchus, a gigantic Late Cretaceous crocodilian, probably had a bite far more powerful than that of T. rex or any other land-lubbing theropods. (It was not too surprising, then, when paleontologists later found toothmarks attributable to Deinosuchus in dinosaur bones.)

  Despite a usurping of tyrannosaurs by crocodilians as champion biters, other T. rex toothmarks invoked awe-inspiring scenarios of this dinosaur occasionally saying “Off with their heads!” This gruesome idea came about when Denver Fowler and several other paleontologists noticed, while looking at Triceratops bones from the Late Cretaceous of Montana, that some of the outer frills of Triceratops skulls had toothmarks on them, which once again matched those of Tyrannosaurus rex. The most perplexing aspects of the toothmarks, though, were their specific locations. Some were along the edges of Triceratops head frills, whereas others on the same skeleton were on the neck vertebrae. The toothmarks on the skull showed no signs of healing and were from teeth on upper and lower parts of the jaws puncturing opposite sides of the frill: no scraping or pulling, just clamping. For feeding, these traces did not make much sense, because there was very little ceratopsian meat to be enjoyed on its skull. In contrast, toothmarks on the neck vertebrae were more puncture and pull, definite traces of where the tyrannosaurs bit into and pulled off scrumptious hunks of ceratopsian flesh.

  To answer this mystery, take another look at a Triceratops skeleton, and you will note that its huge frill covers its neck quite thoroughly and effectively. Hence, it is intuitively obvious to even the most casual observer that this dinosaur not only must have been already dead for a tyrannosaur to feed on its neck muscles, but also must not have had a frill in the way. Accordingly, the easiest way for a tyrannosaur to access that flesh would have been to remove the frill. For T. rex to do this, given their jaw strength and body mass, all it would have needed to do was bite down onto the edge of a Triceratops frill, tug, and yank the head from the body to expose the good stuff below. In such a scenario, it also may have used one or both of its feet to stabilize the Triceratops body while pulling.

  So these trace fossils tell us that Tyrannosaurus ate dead dinosaurs, which is good to know. But some of us want something more to satisfy our bloodlust. In our minds, T. rex was not a big vulture, however noble vultures might seem to be in their own way. Instead, we imagine T. rex or other tyrannosaurs as apex predators of their environments, among the greatest that ever lived. So do toothmarks ever show where a tyrannosaur bit into a living dinosaur?

  Recall again the story at the start of this book, in which a Tyrannosaurus took a bite-sized chunk out of the tail of an Edmontosaurus. It turns out this is a story based on fact, and the trace fossil evidence backing it up can be viewed publicly. In the dinosaur hall of the Denver Museum of Science and Nature is a beautiful mounted skeleton of the Late Cretaceous hadrosaur Edmontosaurus, flawless in nearly every way save for one small blemish. About halfway down its tail vertebrae, on the top surface, its vertebral spines look as if they were clipped, forming a semi-circular pattern, almost as if someone used a cookie-cutter on them. When paleontologist Ken Carpenter took a close look at this oddity, he saw signs of healing around the bone, which meant this pattern was from an injury and not from, say, vertebral spines snapping off after death. The curvature and width of the wound was also intriguing, as it was the right size and shape for the tooth row of a large theropod dinosaur. The only theropod large enough to own such big jaws and that lived in the same area and time as this Edmontosaurus was Albertosaurus, a tyrannosaur closely related to T. rex, or T. rex itself.

  This was among the first firm indicators that tyrannosaurs did indeed go after live prey, refuting naysayers who have put forth the case that the T. rex was just a lowly scavenger versus a fearsome predator. The reality, like most realities, is much more nuanced. Since Carpenter’s study, other healed toothmarks found in Edmontosaurus bones, either attributed to T. rex or Albertosaurus, have both affirmed and restored the original reputation of tyrannosaurs as predators. Both trace fossils tell us that tyrannosaurs ate both dead and live dinosaurs; hence, these theropods probably used a mixture of predation and scavenging to feed, much like many modern top predators today, such as African lions, grizzly bears, and hyenas.

  Fine Young Cannibal Dinosaurs

  Consider being armed with knowing a theropod’s individual teeth sizes and shapes, the number of teeth and spacing within its jaws, and how to interpret toothmarks. All of these bits of knowledge then become handy forensic tools for figuring out who ate whom. So this is exactly how three paleontologists—Ray Rogers, David Krause, and Kristi Rogers—discerned that Majungasaurus, a large theropod from Late Cretaceous rocks of Madagascar, was a cannibal. In one specimen of Majungasaurus, its ribs and vertebrae had toothmarks on them, and rather distinctive ones. The toothmarks were a series of thin, evenly spaced grooves caused by serrations, ones that perfectly matched those on the teeth of Majungasaurus. These grooves were also separated by a gap that corresponded with the intertooth distance of—you guessed it—Majungasaurus.

  Based on similar toothmarks these paleontologists had seen on sauropod bones, they knew that this theropod normally ate those dinosaurs. But at least one decided its dead relative looked too appealing to pass up as a snack. Based on the bodily locations of the toothmarks, the Majungasaurus must have been dead when they were made, so these trace fossils are not only of cannibalism but also of scavenging. Such evidence is a bit perplexing, leading paleontologists to wonder if there was a time when ecological conditions during the Late Cretaceous in Madagascar became bad enough that large theropods resorted to eating their own.

  From an evolutionary standpoint, one might think that cannibalism is uncommon in modern large carnivores. After all, any long-term reliance on consuming your own species could lead to eventual extinction. Yet cannibalism happens. Komodo dragons, crocodilians (including alligators), big cats such as lions and tigers, and bears are among the large predato
rs that will eat their own species. Some cannibalism is a consequence of competition, in which a male lion kills and eats the cubs of a rival male, or bad timing, such as when a baby alligator swims too close to a hungry adult. But cannibalism also can be more opportunistic, such as during hard times; when you’re starving and nothing else is around, you might as well eat your brother. This situation is especially more likely to take place during times of ecological stress, when food supplies tend to shrink. Think of droughts, which cause many plants to wither and die, negatively impacting herbivores, which would in turn affect carnivores. Hence, the biological taboo of cannibalism becomes less of a barrier when food becomes scarce.

  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.