Dinosaurs Without Bones (26 page)

Their conclusions were that a few modern head-butting mammals and
Stegoceras
shared the same types of skull tissue and other cranial traits for absorbing head-to-head blows. Having a more rounded, dome-like head was an advantage, as well as having more compacted (
cortical
) bone supported by more porous (
cancellous
) bone, like a motorcycle helmet with cushioning just below its hard exterior. Somewhat surprisingly, duiker skulls were the most similar to those of the pachycephalosaurs, despite these animals being much smaller than pachycephalosaurs. This led these paleontologists to suggest that we should look at these hoofed mammals in particular as models for pachycephalosaur behavior.

But wait! Don’t give up on trace fossils just yet. If pachycephalosaurs were knocking noggins with one another, surely this left marks on their skulls where they impacted. Alternatively, if pachycephalosaurs went for flank butting, this behavior would have broken ribs in the one receiving the blow, and in a definite, localized pattern. Just to put their bone-breaking potential in perspective, the largest of pachycephalosaurs,
Pachycephalosaurus
, may have weighed as much as 400 kg (880 lbs), or about three times that of the largest linebackers in the NFL. Imagine, then, this bipedal dinosaur running at full speed, head down and pointed forward, and the consequences of its head meeting any other solid object, such as another head, and with the rest of another
Pachycephalosaurus
behind it. Adaptations or no adaptations, traces of this behavior should have been imparted on the bodies of the “butter” and “buttee” as trace fossils.

In 2012, in a bit of good timing as a follow-up to Snively and Theodor’s 2011 study, two other paleontologists—Joseph Peterson and Christopher Vittore—scrutinized a
Pachycephalosaurus
skull and found exactly the sort of damage one would expect from a head-on collision. Two big closely spaced depressions, each about 5 cm (2 in) wide and 1.6 cm (0.6 in) deep, are on the top surface of this skull, together making a figure-8 pattern. Closely associated with these are about twenty pits, ranging from 1 to 10 mm (less than 0.4 in) wide and deep. The paleontologists discounted post-death erosion of its skull, such as breaking or dissolving. Likewise, they did not find any reason to think that bone-boring insects made any of these marks. Instead, they concluded these were lesions caused by some sort of skull trauma that did not result in
the death of the pachycephalosaur but left it with a dented dome and a terrific headache.

What could have caused these marks? Quite reasonably, these researchers concluded that it was a result of the pachycephalosaur smashing its head into a solid object: perhaps another pachycephalosaur head, but certainly something that did not yield easily, like a tree trunk. The only problem with this evidence was that it came from a single specimen, and in science, we always like to see results repeated before confidently stating an idea might actually have some truth behind it. In a follow-up study published in 2013, Peterson and two other paleontologists found many more examples, verifying the previous study. As a result of these studies, dinosaur paleontologists may now look more carefully at dinosaur skulls for similar trace fossils, testing whether this head banging happened in other species.

Tell Me Where It Hurts: Distinguishing Dinosaur Trace Fossils from Other Injuries

Do trace fossils show that some species of dinosaurs hurt other species of dinosaurs? Yes. But this is also where ichnologists and paleopathologists alike urge caution when looking at formerly broken dinosaur bones that healed. For example, it is awfully tempting for paleontologists to look at the smashed (but mended) leg bones of a Late Jurassic
Allosaurus
and say, “Look, it’s the trace fossil of an ankylosaur or stegosaur!” Such leaps of logic are connecting the formidable tail clubs of an ankylosaur or tail spikes of a stegosaur contemporary of
Allosaurus
, and one that successfully whacked its tail against this particular theropod marauder’s leg. Alternatively, the
Allosaurus
might have tripped over a log and broken its leg from the fall. Again, just like the
Edmontosaurus
skin injury discussed earlier, paleontologists are at their best as scientists when they consider many possibilities for the origins of dinosaur wounds. Then, in a Sherlock Holmes sort of way, each possibility is contemplated, tested, and eliminated until only one, however improbable, remains.

Fortunately, two types of trace fossils recorded in the bones of both
Stegosaurus
and
Allosaurus
tell tales of their tails. One is of injured but
healed bony tissue in a few tail spikes of
Stegosaurus
. These wounds are consistent with damage it would have suffered from striking solid objects. Another is of a hole in a tail vertebra of an
Allosaurus
, but one that probably healed around a piece of a
Stegosaurus
tail spike, an unwanted souvenir.
Stegosaurus
had four pointy spikes on the end of its tail, held horizontally and paired on each side, which surely became lethal weapons when whipped by powerful muscles attached to the tail. Paleontologists since the first half of the 20th century had figured that stegosaur tail spikes were used for self-defense, but lacked further confirmation until these trace fossils were described in 2001 (broken spikes) and 2005 (a healed hole in the bone of a predator).

This was not the only example of
Allosaurus
having had a bad day or two. One
Allosaurus
in particular had its rough-and-tumble life recorded all over its body. Nicknamed “Big Al” and studied by paleontologist Rebecca Hanna in the 1990s, this
Allosaurus
was unusual in two ways: Its skeleton was about 95% complete, and it had 19 instances of bone injuries, including breaks that healed. Among the bone breaks suffered by “Big Al” were two of its ribs, and its right foot was badly hurt, meaning that its tracks would have reflected a pronounced limp. These breaks may be trace fossils from an interaction with another dinosaur, whereas other breaks may have been self-inflicted, which would also qualify as trace fossils. However, where bone ailments would not count as trace fossils is if diseases caused them, because these would
not
be related to dinosaur behavior.

Unlike
Tyrannosaurus
,
Allosaurus
is one of the best represented of all dinosaurs in the fossil record, with thousands of its bones identified. This means that paleontologists are seeing a more complete range of its bones, which accordingly more closely reflect the overall health of an
Allosaurus
population at a given time. In that respect,
Allosaurus
was more likely to get hurt than another copiously represented theropod dinosaur, the Late Triassic
Coelophysis
of the southwestern U.S. Hundreds of specimens of
Coelophysis
have been studied, and very few of these show signs of healed bone injuries. One of the
reasons for this may be an easy one:
Coelophysis
was a much smaller dinosaur than
Allosaurus
, measuring about 3 m (10 ft) long and weighing about 45 to 50 kg (100–110 lbs). In contrast, an adult
Allsaurus
could have been as much as 10 m (33 ft) long and weighed about 2.5 tons (more than 5,000 lbs). Yet another factor to keep in mind is that
Allosaurus
(on average) probably lived longer than
Coelophysis
, meaning it had more opportunities to rack up injuries.

Bigger was not better for a bipedal dinosaur, owing to the effects of gravity. As elaborated in a previous chapter, if a large two-legged dinosaur tripped, both its greater weight and height would conspire against it once it hit the ground, fulfilling the old saying “the bigger they are, the harder they fall.” Big theropods also may have gone for accordingly larger prey items, which would have objected to a theropod’s invitation to dinner and fought back. Last but not least, big theropods, just like ceratopsians or pachycephalosaurs, may have competed with other large members of their own species, whether over real estate, sustenance, or desirable mates. All of these possibilities add up to a greater likelihood of a hefty theropod suffering physical damage during its lifetime, especially if compounded by aggressive behaviors.

Inconveniently, other than the few examples here, most healed bone breaks in dinosaurs have not been interpreted more narrowly as trace fossils made by other dinosaurs or by the dinosaur itself. In most instances, paleopathologists are rather conservative in interpreting these abnormalities, offering a basic diagnosis—such as “tibia fractured, later became infected”—and leaving it at that. Perhaps more precise attributions to bone injuries—such as “tibia fractured by
Ankylosaurus
tail club, later became infected”—will be made in future studies. But in the meantime, one can always turn to one of the most unambiguous of dinosaur trace fossils known: toothmarks.

When Tooth Met Bone: Dinosaur Toothmarks as Trace Fossils

Dinosaur toothmarks are lovely trace fossils, simply because most leave little doubt about the dinosaur’s motivation, which was eating. Other than the few exceptions of toothmarks
interpreted in skin impressions mentioned earlier, almost every example of a dinosaur toothmark described thus far was registered in bone. However entertaining it might be to imagine dinosaurs gnawing random, non-living items in their surroundings, such as sticks, mud, or rocks, no trace fossils of this behavior are known. Similarly, dinosaur toothmarks have not yet been interpreted in fossil plants, despite our surety that sauropods, ornithopods, ankylosaurs, stegosaurs, ceratopsians, and some theropods enthusiastically consumed vegetation. One of the most obvious problems with such a discovery is that plant material, such as leaves, stems, roots, and so on, may not have readily preserved such evidence.

When a dinosaur bit into another dinosaur and its teeth reached bone, the resulting punctures or scrapes are trace fossils of the dinosaur that did the biting. Just like mentioned before, though, a good question to always ask about toothmarks as trace fossils is whether that bone belonged to a living or already-departed dinosaur. For instance, if a bitten dinosaur happened to be alive when chomped by another dinosaur, it very likely would have reacted to this assault and may have added a trace of its own behavior to the wound. Think about how the first reaction of a person bitten by their pet dog, cat, parrot, snake, or Komodo dragon would be to pull away from the source of pain. Unless the biter has really clamped down on its victim, this defensive motion should cause the wound to elongate, making the trace a composite one that is a result of both the pet and pet owner’s behaviors. The same principle should then be applied to dinosaur toothmarks. When paleontologists look at a bone with dinosaur toothmarks in it, they most often assume that the bone belonged to an already-dead animal. Nevertheless, they could also look closely for signs that an animal may have still been breathing, and possibly adding its traces to that of its attacker.

More good questions to ask include “Who did it?” which relates to the tooth anatomy of the dinosaur that made the toothmark, and “How did this dinosaur bite?” For example, did the dinosaur rake its teeth across the surface of a bone as it was tugging meat from it? Or did it punch past skin, muscle,
and other soft tissues and go directly into the bone? Did it mostly use teeth from its upper jaw (maxilla), or did it also use its lower jaw (mandible)? Which teeth were used in the biting: ones toward the front of the mouth, ones more toward the back, or a combination of the two? Did the dinosaur bite multiple times in the same general area, inflicting an overlapping array of toothmarks? And in a related question, did more than one dinosaur snack on this delectable treat, adding its toothmarks to a varied collection?

Before going on any further with toothmarks, though, it is probably a good idea to learn about dinosaur teeth, and with a focus on theropod teeth, because theropods included a large number of dinosaurs that ate other dinosaurs. Like many other groups of toothed vertebrates, theropods had a wide variety of tooth shapes and sizes appropriate for their varied uses. Although paleontologists now know that not all theropods were meat eaters, and some were even toothless, we are also aware that “meat eating” is too broad a category for toothed dinosaurs to have evolved “one-shape-fits-all” tooth forms, too.

For example, a few smaller theropods, such as the Late Cretaceous theropods
Velociraptor
and
Saurornitholestes
, had thin, curved, and beautifully serrated teeth. In contrast, larger Late Cretaceous theropods, such as
Daspletosaurus
and
Tyrannosaurus
, had thick, stout teeth with only minor serrations. Serrations on theropod teeth were formed by numerous raised and bladed bumps called
denticles
, which were evenly spaced along the narrowed edge of a tooth. These functioned just like the serrations on a typical steak knife, helping the teeth to saw through flesh. How this helps ichnologists is that toothmarks can also act as a “fingerprint” for identifying a dinosaur species when examining any bones bearing long, parallel series of grooves. In such marked bones, each groove corresponds to a denticle. Hence the spacing and numbers of these traces should match those of known theropod teeth.

Tooth forms even varied within the same dinosaur, a condition called
heterodonty
(“different teeth”). Humans, by having incisors, canines, bicuspids, and molars all in the same mouth, are well acquainted with heterodonty, which is a typical condition for
mammals with varied diets. However, heterodonty can be subtler in other vertebrates, such as theropods. For instance, if you were to glance at a live tyrannosaur’s open mouth, all you would see at first are a large number of its big pointy teeth, which understandably might all look alike as you quake in fear. But if you looked deeper into its mouth, say, while being eaten, you would see that the teeth in the front curve more toward the rear of the mouth, are more incisor-like on their ends, and have D-shaped cross sections, with the rounded part of the “D” facing the front of the mouth. Then, just before you pass down its gullet and become a bolus, you would note that its rear teeth are rounder and blunter than those in the front. This will be explained further in just a minute, but in the meantime, just digest the preceding.