Dinosaurs Without Bones (33 page)

Weems and his colleagues built their case about the Bull Run gastroliths by noting a number of traits that matched those of known dinosaur gastroliths:

• These rocks stuck out as unusual concentrations of pebbles, gravel, and cobbles weathering out of the Bull Run Formation, most of which is composed of finer-grained strata such as siltstones and sandstones.
  
• More than two hundred of them were recovered from a small area of about 30 m
  ²
  (323 ft
  ²
  ), just slightly more than the size of a baseball diamond.
  
• All of the rocks were made of quartz. Most were quartzite (metamorphosed sandstone), and the rest originated from quartz veins that formed in either igneous or metamorphic rocks. No limestones, sandstones, or other sedimentary rocks are represented.
  
• Nearly all were 2 to 5 cm (<1–2 in) wide, and the biggest was about 10.3 cm (4 in) wide.
  
• Their proportions, such as ratios of lengths and widths, were almost identical to those of gastroliths described from the Early Cretaceous sauropod
  Cedarosaurus
  .
  
• Nearly all were rounded, with few angular corners.
  
• All had some degree of polish to them, and over their entire surfaces.
  
• Their colors ranged from dark yellow to yellow-orange to brown to dark red.
  
• Their surfaces had plenty of pits and gashes in between polished surfaces, details revealed through a closer look with an SEM.
  

Do any of these traits sound familiar? Yes, they should. The paleontologists further concluded that the gastroliths were likely used as gastric mills, helping to mix and wear down food in gizzard-like organs. So these stones were good candidates as dinosaur gastroliths. Still, the paleontologists had a few more questions: Which dinosaurs carried these rocks in their bellies, where did they get them, and how far did they travel from their source area?

The answer to the first question was simple: prosauropods. This was based on the size of the gastroliths—the largest of which could only have been in relatively big animals—and their resemblance to gastroliths in the sauropod
Cedarosaurus
. However, sauropods were rare in the Late Triassic and did not become more widespread until the Middle Jurassic Period. So the closest ecological analog to these in the Late Triassic would have been the largest herbivores at that time, which were prosauropods. Furthermore, the size range of the gastroliths (2–5 cm wide) compares well to that of gastroliths in the Early Jurassic prosauropod
Massospondylus
. Unfortunately, the body fossil record for Late Triassic prosauropods in North America is quite scanty, with no bones or teeth known from the eastern U.S., and only a few prosauropod tracks. These trackmakers might have been the same gastrolith-bearing dinosaurs that lived in the area of the present-day Bull Run Formation, or at least been related to them. But with so little other evidence for prosauropods, to say
anything more would require some great speculative leaps. Nevertheless, these gastroliths represent a great start in better understanding where the first large-bodied dinosaurs lived and how they lived during the Late Triassic.

The second set of interrelated questions—where did these dinosaurs find the original rocks, and how far did they travel with their rocky payloads—was a tougher one to figure out, but answerable by looking for similar stones in other Late Triassic formations in the same area of Virginia–Maryland. The best match came from the Manassas Sandstone, which has conglomerates; these are sandstones that also have chunky bits, such as pebbles, gravel, and cobbles. It turned out these coarser-grained sediments—most of which were also composed of quartz and quartzite—were a good fit for the Bull Run Formation gastroliths. Assuming that the gastroliths are gravestones, marking the final resting places of prosauropods, then these rocks moved a minimum of about 20 km (12 mi) under dinosaur control, and probably much further. If true, this would be a great example of how dinosaurs had already begun changing their environments, transporting large sediments to places where rivers did not reach.

This sort of study demonstrates the great potential of gastroliths as trace fossils, filling in the gaps in lieu of dinosaur bones or tracks and other traces. At first glance, gastroliths might seem like the most boring and misunderstood of dinosaur trace fossils, holding little of the aesthetic appeal of footprints or raw ferocity of toothmarks. But when treated with respect, and through the awesome healing power of science, these humble rocks greatly extend our knowledge of dinosaur presence and behavior. Despite all we know now about gastroliths, though, I still think back to that summer of 1983 when I held that strange rock from the Morrison Formation in Wyoming and later wondered whether it spent time inside a dinosaur before resting in my hand. One thing is for sure, though: if other paleontologists or I were to go back to that same spot today and find that rock, I am confident we could better answer that question.

Dinosaur’s Digest

Dinosaurs had to eat. What we would like to know, though, is just what dinosaurs ate, how they digested this food, how often they ate, and how their eating patterns affected everything else around them. These questions can be answered partly by looking at dinosaur jaws, teeth, arms, legs, and rare soft parts. After all, a theropod with a mouth full of sharp, pointed, and serrated teeth, set in jaws with attachment sites for powerful muscles, accompanied by stout recurved claws on hands and feet, would have become a mite peckish at a vegetarian restaurant. Yet there is also a point where paleontologists admit a dinosaur’s anatomy cannot tell us everything about what went into or out of its body. For example, who would have known that Late Cretaceous hadrosaurs crunched wood? That Early Cretaceous ankylosaurs swallowed fruit? That
Late Cretaceous sauropods grazed on grasses? That Early Cretaceous feathered dinosaurs gulped their avian cousins?

So dinosaur trace fossils come to our rescue again, enlightening us about what passed into or exited the dark passages of dinosaur entrails. We already know how toothmarks help with interpreting what some dinosaurs ate. Microwear on teeth informs us about chewing and whether dinosaurs were low-level grazers or not. Gastroliths tell us how these stony implements assisted dinosaur digestion. But those trace fossils were just the appetizers for expanding our knowledge about dinosaur diets and the ecological consequences of their feeding. More trace fossils—with exotic-sounding names like enterolites, cololites, urolites, and coprolites—await our consumption, absorption, and appreciation, explaining more deeply what dinosaurs ate and how these traces affected other lives in dinosaur ecosystems.

A Brief Tour of Modern Excretions

Based on the malodorous, steaming pile of metaphors related to vomit, urine, and feces crossing all cultures and used throughout history, humans have had a long fascination with bodily wastes and the functions that produce them. For example, here are a few such expressions, best uttered with impassioned aplomb: “You make me want to puke!” “You’re pissing me off!” “Don’t give me any crap!” “You’re full of shit!” “What kind of crap is this?” “I don’t give a shit!” Or, at a more juvenile level, “You poopy head!” All such phrases and the rich heritage behind them hint at how all of us, regardless of age, gender, ethnicity, or socioeconomic status, must puke, pee, and poop. It’s part of what makes us animals.

This intrinsic connection to all things urinal and fecal is probably related to our mammalian heritage, in which these products became not just a way to get rid of bodily wastes but also a form of communication. For example, take your dog out for a walk and you’ll expect it to stop often to sniff at virtually everything. During this walk, your dog is using its nose like you use your eyes, investigating an olfactory landscape holding thousands of invisible clues,
many of which are a result of secretions by dogs and other mammals in the area, such as urinations, defecations, mucus, hair, or body scents imparted by rubbing. Whenever I track canines of any species, whether these are domestic dogs, coyotes, foxes, or wolves, I sometimes refer to these messages, seen and unseen, as “doggy e-mail.” In such exchanges, canines post to one another, many of which simply say “I was here,” or more emphatically “This is my territory, stay out!” This is why the phrase “marking your territory” is so commonly associated with urination, while also serving as an apt allegory in business and academic practices.

Wastes as signposts can get even more extreme, such as when carnivores compete over territory and use feces to intimidate or terrorize. For instance, while I was taking a wolf-tracking course in central Idaho, the instructors told us about what happened when wolves (
Canis lupus
) entered what was formerly coyote (
Canis latrans
) territory. The coyotes picked up the scent of a wolf pack on a trampled trail, so they left scat on the trail, announcing to the wolves “Oh, yoo-hoo! I beg your pardon, but this just so happens to be our neighborhood. We would be most appreciative if you departed from it. Ta!” The wolves responded by hunting down, killing, and eating one of the coyotes. Soon afterwards, they left scat in the same spot, but this time with coyote fur in it. (I could not help but envisage the coyotes’ reactions when they came back to this place, got a whiff of the scat, and realized it contained one of their own.) The tracking instructors said they had seen trace evidence of this Mafia-like behavior not once but twice, suggesting it might have been a standard wolf response to coyote defiance. I have no idea if predatory dinosaurs had similar practices in establishing territory, but this modern example certainly lends credence to such scenarios.

Since then, whenever tracking, I have paid careful attention to the identity, placement, and contents of mammal feces with relation to one another, and have often confirmed for myself other such “scat wars.” These conflicts are normally manifested by one mammal dumping in a prominent spot—such as the middle of a
trail—and then on top of it will be another, fresher pile of feces, left by another mammal. For some mammals, they also make sure their feces cover as much real estate as possible. For example, hippopotamuses do this by rapidly flicking their tails back and forth as they excrete, flinging poo with wild abandon. This windshield-wiper action instantly expands their territory, too, as other animals flee from this fecal assault.

Birds are different. Appropriately enough for a clade represented by about 10,000 species, these modern dinosaurs eat a wide variety of foods, and this sustenance is digested and excreted in myriad ways. As far as I know, though, their wastes are not used for staking claims over territory. This is understandable because most birds are more influenced by sight and sound rather than smells. Their digestion is also more complete than that of most mammals, and instead of the mostly solid feces or purely liquid urine of mammals, each emitted through different orifices, birds excrete uric acid out of a single orifice, the cloaca. This waste is liquid in some birds but more solid in others. Bird droppings that are more solid typically wear a little white cap of liquid uric acid on one end. Prideful car owners everywhere have experienced such calling cards from songbirds; when flocking, these birds leave an impressive amount of waste. Nonetheless, raptors and other carnivorous birds are famous for their white-liquid sprays, which sometimes squirt several meters behind them from their roosts.

However, probably the most charismatic of avian sprayers are penguins. In a paper published in 2003, two researchers, Victor Meyer-Rochow and Jozsef Gal, became intrigued with the radial patterns of feces created by two penguin species, chinstrap (
Pygoscelis antarctica
) and Adélie (
Pygoscelis adeliae
), which they formed by jet-propelled defecating around their ground nests (the penguins, that is, not the researchers). As a result, the researchers calculated the physics behind these intriguing traces, taking into account a variety of factors such as: horizontal distances covered by the spray, poo density, as well as cloacal diameter, shape, and height above the ground. After making a series of measurements, including penguin
cloacal diameters (that would have been fun to watch), they figured these fecal blasts exited at 2.0 m (6.6 ft)/sec, although each volley only took about 0.4 seconds. Once they figured values for density and viscosity of the liquid, which they unappetizingly compared to olive oil, all numbers were plugged into equations, yielding estimated pressures of 10 to 60 kilopascals. For comparison, a kilopascal is 1% of normal atmospheric pressure, so 60 kilopascals is more than half an atmosphere, and much more forceful than what any known human can do. So if someone placed a defecating penguin’s cloaca in front of you, the force generated by its stream would be only about one-fifth that of a garden hose, albeit with uric acid instead of water.

Before wading any more deeply into urine or feces, though, let’s talk about the other end of the voiding spectrum: puking. As many individuals and human societies have learned, purging the upper part of an alimentary canal, however discomforting it might feel at the time, can be therapeutic. In our species, vomiting mainly evicts toxic substances before these are absorbed any further into our bodies, but it can also be a reaction to severe physical or psychological trauma, or induced artificially by manually triggering a “gag reflex” in the back of the throat. In other mammals, eating rotten food or drinking bad water can cause vomiting, but it also can be part of a healthy lifestyle. For instance, cats cough up hair-balls, which are a result of daily grooming, or grasses they chewed earlier. Nonetheless, as many pet owners and trackers know from experience (and shoe cleaning), these excretions are often placed deliberately, such as at doorway entrances, on sidewalks, or trails used by other animals. Hence, regurgitated food can serve a dual purpose, similar to that of urine or feces: keeping digestive tracts healthy, but also sending territorial communiqués to anyone who might be “listening.”