Dinosaurs Without Bones (30 page)

Additional health-related roles of gastroliths include their possible use as mineral supplements, especially if these are calcium-rich rocks, like limestone. Read the label on the bottle of nearly all over-the-counter antacids, and under “Active Ingredients” you will see calcium carbonate listed. This means the antacids are made of either calcite or aragonite, which also happen to be the primary minerals in most limestones. These minerals, once they encounter and react with low-pH acids in your stomach, increase the pH, making it less acidic and more basic. In this reaction, the calcium carbonate dissolves, liberating calcium from its bonds with carbon dioxide, which is released as a gas. (This is where former geology students fondly remember identifying limestone in lab exercises, in which they repeatedly dropped diluted acid on these rocks and satisfyingly watched them fizz.) How would we know that a bird or other animal ate limestone? We would not, unless we caught it
in the act. The limestone, once inside an animal’s stomach, would quickly disappear, leaving no sign it was ever there, except for maybe sudden releases of gas.

No firm evidence supports the notions that gastroliths are used for other self-medication, such as for re-introducing gut bacteria, or destroying parasites—the latter the internal equivalent of crushing a tapeworm between two rocks. There is also no reason to suppose that stones are used to either alleviate or increase appetite, thus fulfilling a recipe for “stone soup.”

Nonetheless, a few birds, mammals, and even humans sometimes ingest clay or soil as a digestive aid, a practice called geophagy. This is not such a crazy practice, as some clay minerals have adsorptive properties, in which toxins latch on to the minerals and are later passed out of the body. For example, kaolinite—a white clay common in Eocene and Cretaceous sedimentary deposits of Georgia—has been long used for medicinal purposes, dating back from its traditional use in West African societies, and then continued in slave communities in North America. This remedy was used more broadly in Kaopectate™, a commercial liquid used to treat stomach ailments, in which kaolinite was an active ingredient. However, ornithologists who have studied geophagy in South American birds—such as macaws and parrots—found that these birds were also attracted to soils with high sodium content. Hence, consumption of these soils may provide trace elements and minerals in these birds’ diets, but while also incorporating some gastroliths along the way.

Everything mentioned so far applies to geo-gastroliths in land vertebrates. What about aquatic ones? It turns out that gastroliths are indeed used in a variety of swimming vertebrates, such as pin-nipeds—marine mammals such as sea lions, seals, and walruses—penguins, and crocodilians. For pinnipeds and penguins, rock swallowing may be therapeutic in the same way it is for raptors, in which these stones are used to clean stomachs, then regurgitated. Yet, considering how all of these animals swim, these gastroliths also might help with buoyancy control in all three groups of
animals. This is not necessarily like a scuba diver wearing a weight belt, though, and the “buoyancy control”
vis-á-vis
gastroliths in aquatic vertebrates was questioned in a 2003 study on alligators, which demonstrated that gastroliths only added 1 to 2% to their body weights. In other words, these rocks had a negligible effect on alligator buoyancy. As a result, the explanation was adjusted to state that these gastroliths were used more for stabilizing bodies while swimming. Instead of weight belts, think of the tiny weights placed on car tires when these are balanced and rotated, helping the tires to wear more evenly.

Oddly enough, gastroliths are completely absent in whales and sea turtles, which in their evolutionary histories managed to find other ways to move from deep to shallow water and back again in a balanced way without the use of gastroliths. Yet these trace fossils show up as concentrated masses of rocks in the skeletons of big, non-dinosaurian Mesozoic marine reptiles, such as plesiosaurs. Paleontologists who found these gastroliths were at first perplexed by them, as digestion was presumed to have been an unlikely reason for their presence. After all, these vertebrates had seafood-only diets, much of which should not have required as much grinding as, say, plants from terrestrial environments. This supposition, combined with the negligible amount of weight these rocks added to a multi-ton marine reptile as ballast, added up to a big mystery. Why even have these rocks in their guts in the first place?

One possibility for these gastroliths would have been accidental ingestion of rocks from the seafloor. This might seem like a bizarre idea, but is actually backed up by even more ichnological evidence, which means it must be taken seriously. These spectacular trace fossils, evident as long, wide grooves (“gutters”) preserved in Jurassic strata of Switzerland and Spain, are interpreted as gouge marks made by plesiosaurs as they swam along a sea bottom and scooped up seafood. Paleontologists concluded these were trace fossils based on the sizes of the grooves: some were as much as 60 cm (24 in) wide and 9 m (about 30 ft) long. Trace fossils this big could only have been made by marine vertebrates with large
noses, and plesiosaurs were the only vertebrates living then with such massive snouts.

Based on this information, paleontologists interpreted them as a result of plesiosaurs diving to the bottom, inserting their muzzles into the bottom sediment, and snatching up hapless invertebrates hiding in that sediment that had no idea death would come from above. Any rocks on the sea bottom would have been accidentally consumed with the seafloor as a sort of by-catch. Nowadays, some whales and walruses make similar gutters while feeding, so this behavior is not completely unknown in modern animals, either.

Of course, the alternative to the “accidental” hypothesis is the opposite one, which is that these marine reptiles ate rocks on purpose. But if so, why? How about a synthesis of two previously competing explanations, in which gastroliths were used for both buoyancy control—but for balance more than ballast—
and
helped to digest their food? This idea got more support when two Early Cretaceous plesiosaurs described in 2005 from Queensland, Australia not only had gastroliths but also stomach contents. These contents included bits of clams, snails, crustaceans, and other invertebrate animals that lived on the ocean bottoms during the Cretaceous. This evidence came as a big surprise, because paleontologists had always assumed that these plesiosaurs only ate fish, and they never thought of them as bottom feeders.

Gastroliths in some plesiosaurs would have helped to grind down those hard-shelled critters, increasing the surface area of any calcium-carbonate shells for easier dissolution in stomach acids. So this evidence syncs well with the “scooping” hypothesis, while also explaining how feeding behaviors might have gotten those rocks into marine-reptile bellies in the first place. Other plesiosaurs, though, might have just accidentally consumed rocks as part of their wholesale ingestion of seafloor sediment, and gut contents of some specimens suggest just this.

This wide range of past and present animals that use gastroliths is by no means completely known, and more surprising examples surely await our detection. For instance, the recent discovery of
gastroliths in pterosaurs—dinosaurs’ flying cousins—was an unexpected one, but welcomed by paleontologists. In 2013, Laura Codorniú and several other paleontologists reported on an assortment of coarse sand to fine gravel in the body cavities of two specimens of the petite Early Cretaceous pterosaur
Pterodaustro guinazui
from Argentina. These pterosaurs were always assumed to have used their specialized beaks to strain their food from water, much like modern flamingos. Also similar to flamingos, they probably ate small crustaceans as part of their diet. The gastroliths, then, would have helped the pterosaurs to mash the harder-shelled crustaceans. This gastrolith presence also prompted the paleontologists to reconsider the front teeth of
Pterodaustro
, which they noticed were more robust than the teeth toward the back of the mouth. Accordingly, they proposed that the front teeth were adapted for grabbing sand and rocks. In other words, paleontologists gained new insights on the feeding habits and evolutionary history of this pterosaur because of the trace fossils they contained. Gastroliths: is there nothing they can’t do?

How to Recognize a Dinosaur Gastrolith

Armed with this comprehensive knowledge of gastroliths in modern animals and a few ancient ones, it now should be a breeze to identify dinosaur-related ones, right? The answers to that rhetorical question are yes, no, and maybe, and not necessarily in that order. The likelihood of correctly diagnosing these enigmatic trace fossils depends on slavishly asking and addressing a few important questions:

• Are the suspected gastroliths inside a dinosaur skeleton?
  
• Are there just a few of these stones, dozens, or more than a hundred?
  
• Are these stones clustered in a compact mass, taking up a small volume compared to most of the dinosaur’s body?
  
• Is this mass of stones in the right location in the dinosaur’s skeleton, namely its abdominal cavity?
  
• Are the stones appropriately sized for this dinosaur?
  

Ideally, if all of these criteria are fulfilled, gastroliths will also outline the approximate size and shape of whatever organ was holding it in a dinosaur, such as a gizzard. As trace fossils, then, these can actually help fill in that part of a dinosaur’s soft-part anatomy as a proxy.

However, if a dinosaur body is not associated with your gastrolith suspects, then confirming that these rocks formerly resided in a dinosaurian alimentary canal gets much tougher, albeit not impossible. When faced with an absence of bones but with some possible gastroliths, you must ask a second set of questions:

• Are these rocks in strata of the right age for gastrolith-bearing dinosaurs?
  
• Are these rocks in strata of the right environments for dinosaurs?
  
• Are these rocks the right composition to have survived time in an acidic environment: not limestone, but more like chert, quartz sandstone, or quartzite?
  
• Do they have the same size, shape, and surface texture as those of known gastroliths found inside dinosaurs?
  

A few paleontologists have suggested that if a gastrolith is found outside of the body cavity of a dinosaur or other animal that used gastroliths, with nary a bone in sight, then you should stop calling it a gastrolith and instead refer to it as an
exolith
, as in “exotic.” Although this word has not yet caught on with dinosaur paleontologists—let alone ten-year-old dinosaur enthusiasts—it does help paleontologists and geologists make a mental shift when thinking about these as trace fossils. Unlike most other dinosaur trace fossils, such as tracks and nests, gastroliths might have been transported far away from where a dinosaur originally lived. In other words, whenever suspected gastroliths show up outside of a dinosaur skeleton, serious doubt tends to pummel any sunny optimism.

Yet another factor to consider with dinosaur gastroliths is that dinosaurs might have been recycling gastroliths before they died. Recall how certain birds of prey use rangle: they swallow a few rocks, use these to clean out the upper part of their digestive tracts, then cough them out. If some dinosaurs similarly ate and regurgitated gastroliths, then these rocks would have been left behind and perhaps later picked up by another dinosaur that thought “Hey, look—free gastrolith!” For accidentally ingested gastroliths, these might have passed through dinosaur bodies and come out the other end, deposited along with their feces. However grotesque it might seem to those of us who do not regularly gulp rocks, nor pick through feces for little undigested treasures, these stones could have been removed or eliminated and then ingested by other dinosaurs if no others were readily available.

What was the fate of gastroliths after the death of their host? Given that many dinosaurs and dinosaur parts were deposited after floating in rivers, lakes, or oceans, paleontologists have to keep in mind that gastroliths still in dinosaur body cavities went along for the ride too. Even worse, after floating for a few days, these dinosaur bodies could have burst open from decay. Given their greater density, gastroliths would have been among the first objects to leave a dinosaur body and sink to the bottom of a lake, stream, or ocean. Millions of years later, geologists might find them, scratch their heads, and wonder how these larger rocks got into such fine-grained sediments.

On land, gastroliths might have had a chance staying in or close to a dinosaur’s body. However, once that body was opened, whether by predators, internal decay, scavengers, or just the ravages of time, these gastroliths could have scattered around those remains or been accidentally consumed by whatever was eating the dinosaur. The same would have been true for a floating carcass scavenged by sharks or marine reptiles. These animals could have either hastened gastrolith departures from dinosaur bodies while tearing open a dinosaur’s body cavity, or inadvertently eaten some while enthusiastically noshing on dinosaur entrails. All of these considerations
mean that gastroliths may have been deposited far away from a dinosaur body and gone through multiple “lives” as trace fossils.

In 2003, paleontologist Oliver Wings decided to find out under what conditions gastroliths might stay or go after the death of a dinosaur. In setting up this research, he asked himself what happened to gastroliths in modern dinosaurs—ostriches (
Struthio camelus
)—after they died and their gastrolith-holding bodies were placed in different environments. Following the death of two ostrich chicks from bacterial infections on a South African farm, he donated their bodies to science by watching their corpses decay for six days. One chick, the smaller one of the two—weighing only 2.1 kg (4.5 lbs), or about the size of a roaster hen—was left out in the open and on land, where both South African sunshine and cadaver-loving insects quickly got to work on it. Meanwhile, he placed the other, much larger chick, which weighed 11.5 kg (25 lbs), in an uncovered water-filled barrel. After three days, he turned over each body so that both sides had been exposed to air and water, respectively.