Dinosaurs Without Bones (27 page)

Theropod teeth also formed distinctive patterns in how they lined up on both the upper and lower jaws. This row of teeth is appropriately known as a tooth row. Certain theropods had an exact number of teeth per tooth row, but that number could also be divided into two parts, a front (anterior) tooth row, and a rear (posterior) tooth row, which are divided by a gap called its inter-tooth spacing. Regardless, you should mind the gap, as the front teeth and rear teeth served different functions in a theropod mouth. Obviously, the front teeth were what mattered most when it was love at first bite, whether performed on a living or dead animal.

If a theropod’s potential food item, such as a small ornithopod, was still alive and having issues with a proposal that it should devote its life to feeding a theropod, then the front teeth were the most persuasive tools used by that theropod. Depending on tooth forms, these would have been used initially to slash, grip, or crush live prey, then later nip and rake flesh from the body. Conversely, the rear teeth were more likely to have been used well after any disagreement had ceased between the diner and its meal, as these were better suited for chewing meat and crunching bone. These variations in forces within a theropod mouth were a function of physics. For example, think of how a nutcracker is used more effectively when a nut is placed closer to its hinge rather than at its ends.

All in all, theropod toothmarks in bones can be connected quite reasonably to recognized theropod teeth and their intended functions. These trace fossils are also interpretable by applying just a little imagination and keeping in mind how meat is separated from bone. For instance, toothmarks could have been made by a dinosaur scraping its teeth along the length of a bone, trying to get every last little tasty bit of skin, fat, sinew, and muscle off it. (Think of people eating ribs or chicken wings, using just their incisors.) These marks would have been made by a pulling, lateral movement, so they would be shallow, long grooves on a bone surface, maybe with parallel lines made by denticles.

A theropod also could have punctured a bone while accessing deeper sources of meat in a carcass. Because this action would have been more perpendicular to the bone surface, these puncture tooth-marks will be deeper and limited to small areas that more or less reflect tooth cross-sections. A combination of these two actions—puncture and pull—would have inflicted deep toothmarks followed by long, shallower grooves, made when a dinosaur bit into its meal, hit bone, and then pulled meat off those bones with its teeth, like a carnivorous version of raking leaves.

So through a combination of knowledge about tooth anatomy, toothmarks in bones, and experiments on bite forces—including those of living animals—paleontologists can indeed figure out which dinosaurs ate other dinosaurs, what parts they ate, how they ate, and other such gory details. Although dinosaur toothmarks were first interpreted as trace fossils near the start of the 20th century, the first studies to pull together all of these lines of evidence were not done until the mid-1990s. This was when paleontologist Greg Erickson and a few of his colleagues decided it was time to start describing and interpreting what were likely
T. rex
toothmarks in
Triceratops
bones.

What made them think these toothmarks belonged specifically to
T. rex
stemmed from a combination of size, shape, place, and time. Very simply, these toothmarks were big, they matched the shapes of
T. rex
teeth, and this massive theropod lived in the same
places and times as
Triceratops
. Because some of the toothmarks were deep punctures in
Triceratops
hipbones, the paleontologists were able to identify the biter by stuffing putty into the holes, extracting the putty, and presto: they had beautiful molds identical to the shapes and sizes of
T. rex
teeth.

Naturally, just identifying the toothmark maker was not enough to satisfy these curious paleontologists. They also wanted to answer a basic question: How much force was generated by a
Tyrannosaurus
bite? This potentially could have been answered through complicated computer models that took into account muscle attachments, jaw mechanics, pivot points, and other factors. Yet the toothmarks were an actual record of a tyrannosaur biting into bone, so all the paleontologists had to do was try to replicate the toothmarks.

Disappointingly, they did not reanimate a long-dead
T. rex
, but instead set up a “biting” mechanism with proxy tyrannosaur teeth, which were made of aluminum and bronze. These artificial teeth were then used to imitate a tyrannosaur bite, but one in which the force exerted by each “bite” could be measured. Force is measured in newtons (named after Sir Isaac, not the confection), which is equal to a kilogram (2.2 lbs) moving a meter (3.3 ft) per second squared. The formula for force is:

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 predators 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.