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.
CHAPTER 7
Why Would a Dinosaur Eat a Rock?
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.
I remember being intrigued by this vague, indirect evidence of a dinosaurian presence. Filled with naivety and impressionable enthusiasm, I continued to wonder about that black rock from my field camp and gastroliths in general. Was it, along with similar rocks there in the Morrison Formation, really connected to dinosaurs? And if so, how? I wanted to believe they were. Yet when I read more about gastroliths much later in my graduate studies, doubt began to erode my faith. Indeed, in a class the year after this field camp, Dr. Martin seemingly contradicted himself, saying that such rocks should not be called “gastroliths” unless found in the ribcage of a dinosaur. It later seemed that many geologists and paleontologists told me, to paraphrase Freud, “Sometimes a rock is just a rock,” saying that these rounded stones could be readily explained by non-dinosaurian means, such as rivers, wind, or ocean waves. This creeping uncertainty was fueled by later trips to the western U.S., in which I would find similar rocks in Mesozoic strata and ask if these were gastroliths. In response, other geologists and paleontologists would scoff at the suggestion, as if I had asked about Bigfoot, aliens, or Bigfoot aliens.
Were these skeptics completely or partially right, or should they have been laughing at themselves for being gastrolith deniers? But maybe they had a point. After all, what exactly were gastroliths? How could a mere rock tell us whether a dinosaur had been on or near a given spot in the western U.S.? Moreover, if gastroliths were real, how could these curios actually be trace fossils, supplying deep insights on dinosaur behavior and evolution?
Between a Rock and a Soft Place
Starting nearly 200 million years ago, well before our ancestors were attracted to certain rocks, chose them with care, and began using them as tools, dinosaurs had their own rock collections. Some of these rocks even had fossils in them, which meant that dinosaurs might have been unwittingly the first fossil collectors, too. The biggest difference between these dinosaur- and human-made assortments of stones is that dinosaurs carried them on their insides. “Gastrolith” literally means “stomach stone” (in Greek, gastros = stomach, lithos = stone), and they thus refer to rocks that somehow made it into the digestive tract of an animal. Because many modern animals—from invertebrates to vertebrates, marine to terrestrial—have rocks in their innards, no one questions the physical reality of gastroliths. Nevertheless, with dinosaurs of the long-lost past, disagreement stems from exactly how those rocks got there in the first place, whether they served any purpose, and if so, their intended functions. Thus it becomes easy to see why these are among the most controversial of dinosaur trace fossils.
If one can stomach it, a little bit of jargon goes a long way in better understanding these enigmatic trace fossils. For one, they can be divided into two categories: bio-gastroliths and geo-gastroliths. As these hyphenated prefixes imply, biological processes make one of them, whereas the other is formed by geological means. However, the latter type certainly has to involve an animal at some point in a gastrolith’s lifecycle by making its way into a gut. So these could be called geo-bio-gastroliths, or bio-geo-gastroliths. But either of those terms would be needlessly pedantic and cause science editors everywhere to flinch and twitch uncontrollably, especially if these bio-geo-gastroliths are touted as “missing links” in a “living fossil.” And we wouldn’t want to do that.
With bio-gastroliths, the animals make these themselves, secreting minerals internally and forming concretions in their body cavities. Sound strange? Not really. If you’ve ever heard of gallstones or kidney stones, or been unfortunate enough to have experienced them directly, then you know about bio-gastroliths. I once learned more about bio-gastroliths and their effects on human health while waiting in a hospital emergency room with a fractured right radius, a consequence of a bicycle accident. Meanwhile, the patient lying next to me passed a
kidney stone, which took him about an hour or so. (Let’s just say that after that, I stopped feeling so sorry for myself.) Later, once the memory of his screams faded enough for me to think more objectively, I read about kidney stones. These bio-gastroliths are deposits of calcium oxalate, which form in people’s kidneys as a result of calcium imbalances and insufficient fluid intake. (Fortunately for many of us, myself included, regular beer drinking is excellent preventative medicine.) Gallstones, on the other hand, are more organic than mineral and are normally composed of cholesterol, although these can sometimes have calcium mixed in as well.
Remarkably, some crustaceans, such as marine crabs and freshwater crayfish, secrete their own bio-gastroliths of calcium carbonate. The purpose of these secretions is to absorb calcium from their exoskeletons just before they molt, then put that calcium back into new exoskeletons. Hence, these bio-gastroliths are not pathological but act more like tiny biochemical ATMs, from which these crustaceans can deposit or withdraw calcium as needed. These concretions are not only in modern crustaceans but also are preserved in the geologic record, trace fossils that let us know about a former crab or crayfish presence.
In contrast, geo-gastroliths are rocks made outside of animals’ bodies by normal geological processes—whether igneous, metamorphic, or sedimentary—but that later somehow make their way inside while that animal is still alive. Most of these rocks are composed of silica-rich minerals, such as quartz and feldspars, and originally were parts of much larger rocks that were broken down and perhaps rounded by streams over many years before residing in a gut or two.
Sizes of these rocks can vary considerably from sand-grain-sized to as big as baseballs, and even in the same animal. Size is roughly correlated with the body size of the host animal, meaning small gastroliths are in smaller animals and big gastroliths are in bigger critters. However, the width of a gastrolith seemingly never exceeds 3% the length of the animal. In terms of numbers, an animal with gastroliths could have as few as two or three, or more than a hundred. Many gastroliths are rounded and have smooth, polished surfaces, but some are duller and irregularly shaped, with angular outlines. In short, gastroliths have a few traits in common, but each can have its own personality, and a “population” of these in any given animal can be quite diverse.
How these rocks get in the belly of an animal is one of the most important questions surrounding them, but answers to that inquiry can be reduced to just two: accidental and purposeful. In the first instance, an animal could be going about its daily business and accidentally swallow a few rocks. For example, an alligator might use its snout to help dig a den, and a few rocks then get in its mouth and go down its throat while it is shoveling. Similarly, nest-building birds that are picking up ground debris, especially soil, also might inadvertently take in small stones. Alternatively, an alligator or bird might see some rocks in its environment, and this triggers a behavioral response along the lines of “must eat rocks.” They then accordingly scoop these up in the mouth and swallow.
Birds certainly do this, intentionally selecting sand-, pebble-, or gravel-sized particles and ingesting them. Anyone who has owned or otherwise watched chickens knows about this behavior, in which these backyard or barnyard fowl grab grains of sediment with their beaks. I have watched videos of crows doing the same thing, deliberately walking along and selecting pea-sized gravel to eat. Most telling with respect to dinosaurs, ostriches and other large flightless birds regularly include rocks on their lists of items to get into their bellies.
However, where this seemingly simple behavioral division of “Oops!” or “I meant to do that” gets a bit more blurred is when, say, our same alligator purposefully eats another animal for a meal, but in its eagerness swallows a few rocks sticking to or just underneath the prey. Even more complicated would be if the alligator ate another animal with gastroliths inside of it. In either case, this means the gastroliths were ingested accidentally, regardless of whether the original gastroliths were inside or outside of another animal, or biologically or geologically formed. Powerful digestive juices inside that alligator then may reduce the body of its prey item, but the gastroliths might be left behind, a ghostly clue that another animal might have been there. Of course, we also do not know if the prey animal had accidentally or purposely eaten these rocks; all we know is that the rocks were being recycled, spending time in yet another animal’s gut.
Now, it might seem unlikely that an animal would knowingly consume a rock, but think of how many pet owners fool their pets into taking unwanted pills by folding these into their food. Once a feeding frenzy starts, all sorts of items might unintentionally get included and make it into a gullet. Even herbivores could consume rocks that just happen to be part of the surrounding soil sticking to plant roots. Moreover, animals sometimes mistake rocks for food. For example, snails in a pond might look just like some of the stones lying along a pond bottom. So a vertebrate that likes to eat these snails, such as a fish, turtle, or alligator, might just indiscriminately ingest anything that looks like those snails, some of which will be rocks. For those of you who think “I would never make such a stupid mistake,” recall all of the times your eyes were drawn to brightly colored beads that looked like hard candies and how tempted you were to pop them in your mouth (and perhaps did). Visual triggers can be quite powerful, particularly when accentuated by hunger.
All this gastrolith goodness is nice to know, but a nagging question about intentional rock eating should be asked: Why? As in, why would an animal eat a rock on purpose? Perhaps surprisingly, the reasons are many and quite sensible from an evolutionary perspective. Yet the best-understood one is to use these geologic materials as substitutes for teeth: letting the rocks do the “chewing” of food so that its surface area is increased and more easily digestible. This process, also known as trituration, is like having a food processor in the upper part of an animal’s digestive system.
In birds that use gastroliths for just this function, their alimentary canal, from top to bottom, goes like this: mouth, esophagus, crop, proventriculus, ventriculus, small intestine, large intestine, and rectum, the latter also known as the cloaca. (Recall that in females, this is also where eggs exit.) Let’s say a chicken has just eaten some grass seeds mixed in with delicious, nutritious insect larvae. This chicken may briefly chop its food with its beak, but more grabbing and swallowing happens than chewing. Her food, some of it still squirming, travels down her esophagus through a series of muscular contractions, or peristalsis. It may then reside briefly in her crop, which is an enlarged storage area at the end of the esophagus that acts as a holding room. Once ready for digesting, this mixture of grains and insects is passed down to her proventriculus, sometimes nicknamed “soft stomach.” The proventriculus has low pH (acidic) fluids which start dissolving the food and otherwise regretfully informing any insects still holding on to their lives that resistance is futile.
Still, this chemical attack might not be enough for proper digestion, especially if the seeds had tough outer coatings and the insects had some hard parts, such as mandibles. This is where the chicken’s ventriculus (more popularly known as a gizzard) comes to the rescue, aided by its super geo-friends, gastroliths. The food is then moved down to her gizzard, a strong, muscular organ hosting the gastroliths. The gizzard squeezes and releases, squeezes and releases, over and over, repetitively and redundantly. The hard stones inside the gizzard act like a mortar and pestle, mechanically breaking down these seeds and insect parts into tinier particles, and correspondingly increase their surface areas. This food then may be passed back to her proventriculus, which applies more stomach acids that react more efficiently with more food surface area available. Then it’s back to the gizzard again for yet more grinding. Back and forth: a digestive tennis match, but one in which the final volley goes below the gizzard to the small intestine for further absorption.
This winning combination of muscular activity, acids, and geological materials works very well for birds that use gastro
liths as digestive aids. However, this picture is not so simple as saying “grinding good, not grinding bad.” Gastroliths serve other more subtle but important roles in bird digestion. One is in predatory birds, such as hawks, which may swallow a few pea-sized rocks, keep them in their digestive tracts for a while, and then regurgitate them before hunting. This process better prepares that bird’s digestive system for upcoming meals. The gastroliths apparently are used to scour a bird’s crop, proventriculus, and gizzard of any impurities (fat, mucus) left behind by previous meals, which, when coughed up, rid it of those nasty residues. Ornithologists and falconers—people who raise and train birds of prey—refer to these types of gastroliths as rangle.
Yet another use of gastroliths in birds is at the dietary opposite end of the spectrum from carnivores, which is in those birds that get lots of fiber in their diets by eating grasses, leaves, and other plant parts. Large, flightless grass-eating birds—such as emus or ostriches—regularly cut into leaves and grasses, or pull up plants with their beaks. Down their necks these plants go, and once they find their way past the crop, they begin to be digested. However, for plants like grasses, their long, hard-to-process fibers may start to bunch and bind like a tangled ball of string. If you have ever pushed or driven a lawnmower through a patch of tall grass, you may have experienced something similar to this, where the lengthy stems and leaves wrap around the blades, requiring regular stopping of the motor and pulling these out. Now imagine this happening in your stomach, in which the grasses form a big, knotted mass and get stuck there, rendering constipation most fatal. Fortunately for these birds, though, gastroliths not only help to grind these plant fibers but also separate them enough to prevent balling up and turning into impassable objects.
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