Dinosaurs Without Bones (31 page)

The results were surprising. In the land-based chick, its gastroliths stayed put. Although insects, dehydration, and bacterial decomposition successfully caused rapid weight loss, tissues tightened around the gastroliths in its crop, a biological shrink-wrap that kept all of these stones together and in one place. In contrast, the water-immersed chick lost very little flesh, but it opened up and lost all of its gastroliths, which fell to the bottom of the barrel. Although quite simple and limited to only two samples, this little experiment gave Wings and other dinosaur paleontologists a few insights on gastrolith residence times after death, and differences to keep in mind for a dinosaur carcass that stayed on land or ended up in a body of water.

Nevertheless, before such experiments, so many uncertainties about dinosaur gastroliths caused paleontologists to reach for the only weapons that made sense for handling such a desperate situation: lasers. As most people know, but most cats do not, lasers use amplified beams of light that maintain nearly the same diameter
over long distances. Like all light, though, laser beams can be bent (refracted) or bounced back (reflected). These properties of light are then applicable to measuring the degree of polish on a surface. Very simply, less polish causes laser light to scatter more, whereas more polish, such as a mirror might have, causes it to scatter less.

How these properties of laser light could be applied to dinosaur gastroliths was thus related to one of the problems with identifying gastroliths outside of dinosaur bodies. Smooth, lustrous surfaces on rocks are not necessarily the result of being inside a dinosaur gut. Instead, such surface textures could be attributed to all sorts of natural, non-biological processes, such as wind and water erosion. Could laser reflectance detect the differences between rocks polished by wind or water versus dinosaur viscera?

So in the early 1990s, paleontologist Kim Manley took a look at the differences of laser-generated light scattering on genuine gastroliths, which were taken directly from a sauropod skeleton (
Diplodocus
), wave-rounded rocks from beaches, and river-rounded rocks. All three rocks had some degree of polish to them, but paleontologists assumed that gastroliths, having spent time grinding up food in a dinosaur’s crop, proventriculus, or gizzard, and subjected to stomach acids, were the most polished.

So science struck with a blow delivered at the speed of light. It turned out that some beach rocks were just as polished as the gastroliths, although both of these were more polished than river rocks. This technique was refined in a 1994 study by other scientists who looked at gastroliths from much more recently dead avian dinosaurs, moas from New Zealand. These large, extinct flightless birds lived on both the north and south islands of New Zealand up until less than a thousand years ago, which is about when the Maori people arrived and found them far too delicious for their continued existence. The scientists, Roger Johnston and two others, again used laser reflectance to look for differences in polish between moa gastroliths and beach rocks. However, they also employed a video instrument that recorded the reflectance in three dimensions, rather than just a horizontal plane. They concluded that the moa
gastroliths were more polished than beach rocks, stating that it was “fairly” successful. This faint praise came with an admission that the amount of polish on a gastrolith may have depended on the time these gastroliths spent in a moa’s gizzard. This meant former beach rocks, swallowed by a moa but only put to work for a few weeks before that moa’s death, would be nearly indistinguishable from unused beach rocks.

In 2000, scientists Christopher Whittle and Laura Onorato used a scanning electron microscope (SEM) to peer more intensely at gastroliths in general. In their study, they picked out a variety of gastroliths from dinosaurs, plesiosaurs, birds, and alligators, and then calculated percentages of areas that were pitted or gashed. At a magnification fifty times what we would see with unaided eyes, they found these gastroliths were mostly smooth, but also had a good number of pits and gashes on about 20% of gastrolith surfaces. In contrast, rocks that had been polished by waves, river flows, or wind were much less pitted or gashed than the gastroliths. These dinks and nicks on otherwise smooth surfaces are assumed to have come from rock-to-rock contacts more frequent than those in outside environments. These were promising results, and even though Whittle and Onorato used such expensive equipment for this study, they also pointed out that paleontologists could use normal binocular microscopes at the same magnification to look for these diagnostic features.

These outcomes also point to how gastroliths and microwear in dinosaur teeth tend to overlap in a few ways. Recall that minerals harder than apatite tend to scratch teeth, whether as quartz sand on plant surfaces or phytoliths in plant tissues. Similarly, stones of equal or different hardness mashing against one another would have imparted a sort of microwear on gastroliths, evident as pits and gashes. These marks also might have been from quartz sand or other minerals in the stomach, perhaps including those on or in plants. Both types of microwear also involved much muscular activity behind it, whether through gnashing of teeth or squeezing of gizzards. In principle, the only major difference with gastroliths
is their exposure to low-pH stomach acids, which presumably would leave a more chemical overprint.

Still, despite these studies that used lasers, microscopes, and other technologically advanced tools to study suspected gastroliths, healthy skepticism about the recognition of dinosaur exoliths in Mesozoic rocks still lingered well after the 1990s. Regardless of these non-believers, though, the good number of gastroliths found inside dinosaur skeletons in the early part of the 21st century ensures that paleontologists have plenty more of these trace fossils to study, and bodes well for finding more dinosaur gastroliths in the future.

Dinosaur Gastroliths? Get Real!

So how important are these dinosaur gastroliths, considering the continued skepticism and theories that still surround them? First of all, despite previous attempts to diminish these important trace fossils via the almost-clever pun “gastromyths,” they really do exist. Admittedly, our views of gastroliths in dinosaurs have changed considerably since their discovery. Yet a little history lesson about gastroliths helps to understand just how far our concepts about these dinosaur trace fossils have progressed, with novel insights about them emerging in just the past ten years or so.

The earliest description of possible gastroliths in a dinosaur was in 1838, when French paleontologist Jacques Amand Eudes-Deslongchamps noted about ten pebbles underneath the ribs of the Middle Jurassic dinosaur
Poekilopleuron
, which had been discovered in France. He concluded that these rocks were in this dinosaur’s stomach, just like those found in fossil crocodiles from the same area. Much later, in the late 19th century and leading up to 1900, paleontologists began spotting gastroliths in dinosaurs and marine-reptile contemporaries of dinosaurs, such as Late Cretaceous plesiosaurs. Large stones were also associated with sauropod skeletons excavated in the 1870s, although the people digging out these dinosaurs did not call these “gastroliths,” just “stones.”

Most of the credit for the linking of gastroliths with some function in dinosaurs was bestowed upon paleontologist Barnum
Brown, who was doubly famous for originally naming
Tyrannosaurus rex
and wearing full-length fur coats while conducting field research. Sometime around 1900, Brown noticed a collection of rounded cobbles inside a hadrosaur skeleton that he identified as “
Claosaurus
.” He imagined that these stones might be similar to those found in plesiosaur skeletons, which he discussed in a brief paper published in 1904. In 1907, Brown followed up this study with another paper on dinosaur gastroliths, which he very simply titled “Gastroliths.” (To this day, no one knows if Brown’s brevity was a direct affront to the verbose titles employed by the previous generation of Victorian-era scientists.)

Later, it turned out that Brown was wrong on two counts. For one, the “gastroliths” in this particular dinosaur were probably river stones that washed into the body cavity of the hadrosaur soon after it died. Second, the hadrosaur was misidentified and instead was a species of
Edmontosaurus
, mentioned in the first chapter and taken literally by a
T. rex
when taunting it by saying, “You want a piece of this?” Brown was also beaten to press on dinosaur gastroliths by a not-as-famous paleontologist, G.L. Cannon, who in 1906 published a short paper with twice as many words in its title as Brown’s: “Sauropodan Gastroliths.” This was the first publication to mention gastroliths in sauropods, and it was an idea that has stuck around since. Other paleontologists, such as Brown’s protégé Roland T. Bird, later promoted this supposed association between sauropods and gastroliths. Consequently paleontologists began looking for, finding, and interpreting anomalous assemblages of rocks in dinosaurs, and especially sauropods.

So starting in the early 20th century, and continuing for quite a while afterwards, the conventional wisdom about gastroliths has been twofold. First of all, gastroliths were indeed present in some dinosaurs, but mostly in sauropods, and maybe a few other plant-eating dinosaurs such as hadrosaurs, and almost never in theropods, stegosaurs, ankylosaurs, or ceratopsians. Second, these gastroliths were used to help grind up hard-to-digest food in gizzard-like organs because sauropods did not have the right teeth
for chewing their food. Related to these two assumptions was the tacit agreement that theropods lacked gastroliths because they were all carnivorous, with big, sharp, pointy teeth, powerful jaws, and marvelously corrosive stomach acids. In other words, only wimpy herbivorous dinosaurs needed these digestive aids.

We now know that this story about gastroliths and dinosaurs, neatly framed, displayed in a prominent place, and highlighted with artfully angled lighting, is mostly wrong. Given what we’ve learned about the varied uses of gastroliths in modern animals, combined with a broader knowledge about dinosaur diversity, evolution, and behavior, and all topped with healthy distrust about what constitutes a real dinosaur gastrolith, this picture, much like paleontologists’ wardrobes, has changed considerably since the days of Barnum Brown.

First of all, the dinosaurs that are now the most likely to have gastroliths are
not
the plant eaters, but theropods. Yes, you read that right: some of those meat-eating, über-macho, Mesozoic killing machines of yore may have needed a little help from their geological friends. Among the theropods documented thus far with gastroliths are: the Early Jurassic
Megapnosaurus
(previously known as
Syntarsus
); the Late Jurassic
Nqwebasaurus
; the Early Cretaceous
Caudipteryx
,
Shenzhousaurus
,
Sinocalliopteryx
, and
Sinosauropteryx
; and the Late Cretaceous
Sinornithomimus
, among others.

Well, of course these theropods had to use gastroliths, you might say. These are small, effete theropods, some of which, such as
Nqwebasaurus
, had nubby teeth, and
Sinornithomimus
, which completely lacked teeth. But then tell that to more imposing theropods such as the Late Jurassic
Allosaurus
and
Lourinhanosaurus
, the Early Cretaceous
Baryonyx
, or the Late Cretaceous
Tarbosaurus
, all of which have had possible gastroliths directly associated with their skeletons, too. Granted, the supposed gastroliths found with these larger, well-toothed theropods may be evidence of accidental ingestion, whether through gulping down a prey animal with gastroliths or swallowing stones underneath the body of an animal as it was eaten. In fact, the few gastroliths in the abdominal cavity of the
small theropod
Sinocalliopteryx
have been attributed to inadvertent gulping, unknowingly including them with a meal. Still, some large theropods have definite gastroliths in their bodies, thus directly disputing the previously held idea that only sauropods and plant eaters made use of gastroliths.

There is one additional and important facet of this continuing debate that has yet to be mentioned regarding theropods and gastroliths. Remember the very first description of gastroliths by Eudes-Deslongchamps in
Poekilopleuron
, in 1838? Well,
Poekilopleuron
was not only a theropod, but also a big one, estimated to have been about 9 m (30 ft) long. From a dinosaurian perspective, this matched an average-sized
Allosaurus
and was about 3/4 the length of an adult
Tyrannosaurus
. Yes, that’s right, gastroliths in dinosaurs as a concept actually began with them in a very large theropod, not sauropods. Unfortunately, we can’t study the original bones of
Poekilopleuron
, as these were lost in bombing raids during World War II. Nor can we study the original gastroliths as they too have been lost, which from an ichnological standpoint is an even greater tragedy. Still, we are left with this satisfying piece of paleontological history that connected theropods with gastroliths, which we are now revisiting with much vim and vigor through their discovery in many other theropods of varying clades and sizes.

What’s even more exciting about this renewed recognition of gastroliths in theropods is how it coincides with the evolutionarily based revelation that birds are dinosaurs, as many modern birds have gastroliths, too. Of the small theropods found with undoubted gastroliths in their abdominal cavities, some of these—such as
Caudipteryx
,
Sinocalliopteryx
, and
Sinosauropteryx
—also have feathers. Although gastroliths are trace fossils and mostly reflect behavior, which is more fluid and open to interpretation and variety than genetically determined anatomy, natural selection must have favored theropods with the visual acuity to pick out the right rocks (ones that were rich in silica) and the mental ability to think “Must eat rocks,” as those rocks would then help them with digestion and overall good health.