Dinosaurs Without Bones
Page 24
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 clad
es 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.
The gastrolith connection between non-avian feathered theropods with gastroliths in early birds was bolstered by the discovery of gastroliths in the Early Cretaceous bird Yanornis martini of China. These gastroliths, mostly composed of sand- and gravel-sized quartz particles, were in exactly the same place where the bird’s gizzard should have been. Another insight provided by this avian rock collection was how an absence of gastroliths in other specimens of Y. martini, but the presence of fish remains in one, implied that this species might have been switching its diet seasonally. Some modern shorebirds do the same, eating seeds and insects in the spring through fall, but chowing down on seafood during the winter. So this Early Cretaceous bird may have died before the winter while its gizzard was still full of gastroliths, which it would have needed to fully digest and process fibrous seeds and insects.
Other theropods with gastroliths include a few species of ornithomimids. In 1890, nearly a hundred years before the theropod–bird connection was firmly established, O. C. Marsh named one dinosaur Ornithomimus velox; bones of another species, O. edmontonicus, are the most common Late Cretaceous theropods in North America, and Asian ornithomimids are not exactly rare either. Because “ornithomimid” means “ostrich mimic,” it seems only appropriate now that some of these dinosaurs, like modern ostriches, also had gastroliths. Ornithomimids with gastroliths include the Early Cretaceous Sinornithomimus, the Late Cretaceous Shenzhousaurus, and a dozen specimens of an unnamed species, all from China. The most recent, and one of the most exciting examples of gastroliths in an ornithomimid, was reported in 2013. More than a thousand rocks, all relatively small, were in a skeleton of the Late Cretaceous Deinocheirus mirificus of Mongolia. The hundreds of gastroliths from the dozen unidentified ornithomimids ranged from sand- to gravel-sized and were angular to rounded, affirming how gastroliths are the most diverse and unpredictable of dinosaur trace fossils. All of this points toward how these so-called “ostrich mimics” were 70 million years ahead of ostriches in using gastroliths, meaning these modern birds are actually the “mimics,” not their non-avian predecessors. These gastroliths, combined with their toothless condition, also imply that ornithomimids were herbivorous, lending to the still-radical concept of vegetarian theropods.
Given this knowledge both old and new about gastroliths in theropods, what about sauropods and gastroliths? The long-held assumption is that huge sauropods, many of which only had puny, pencil-like teeth, used gastroliths to grind their food. However, this idea is now seriously doubted. The biggest problem with the previous explanation is that these gastroliths, like those used for “buoyancy control” in marine reptiles, are too few to have made any real difference in digestion. For sauropods to have actual functional “gastric mills,” they would have needed many more gastroliths than the ones found in sauropod skeletons so far.
For instance, in modern ostriches and other birds that employ gastroliths to help with their food, these rocks make up about 1% of their total body mass. The Early Cretaceous theropod Caudipteryx matches this ratio, implying that these rocks served a similar purpose in its lifestyle. However, for sauropods, the proportion between gastroliths and estimated body mass was about 10% that of birds. So if an ostrich were scaled up to 50 tons (scary thought), then it would need about 500 kg (1,100 lbs) of gastroliths to digest its food, which is about the weight of the largest Harley-Davidson motorcycle. Yet the greatest mass of gastroliths described thus far from a sauropod (Diplodocus) was only 15 kg (33 lbs), which is about the weight of a Huffy bicycle. Also, gastroliths are relatively rare in sauropod skeletons, and if used for something as essential as food processing, these trace fossils should be much more common. This huge disparity between extant and extinct gastrolith-using animals led paleontologists to conclude that these stones surely were not used for the same purposes, effectively pulverizing the “gastric mill” hypothesis. Alternatives may not be so exciting, but include: accidental ingestion, especially if rocks were adhered to plant roots; mineral supplements, such as for calcium or trace elements; or used as separators for keeping fibrous food from bunching in their stomachs.
Nonetheless, the disproving of one explanation for gastroliths in sauropods doesn’t mean they suddenly vanished. For example, one specimen of the Late Jurassic sauropod Diplodocus hallorum (formerly named “Seismosaurus hallorum”) of northern New Mexico had more than 200 gastroliths, which were carefully mapped inside and around its former body cavity. These gastroliths were all igneous and metamorphic rocks with varying degrees of polish to them, and most were about 2 to 8 cm (<1–3 in) wide. An assemblage of 115 gastroliths, totaling about 7 kg (15.4 lbs), was also packed into a small area within a skeleton of the Early Cretaceous sauropod Cedarosaurus from Utah. Other sauropods with gastroliths directly associated with their bones include: Apatosaurus, Barosaurus, and Camarasaurus of the western U.S., as well as Dinheirosaurus of Portugal (all Late Jurassic sauropods), and the Early Cretaceous Rebbachisaurus from Morocco. In one instance, 14 gastroliths were directly associated with the remains of a juvenile Camarasaurus scavenged by Allosaurus, the latter indicated by tracks, toothmarks, and dislodged teeth. Several specimens of a Late Triassic prosauropod from South Africa, Massospondylus—the same dinosaur affiliated with nests, eggs, and babies mentioned in a previous chapter—had gastroliths too. Moreover, the skeleton of another prosauropod, the Early Jurassic Ammosaurus of North America, had gastroliths. In short, although gastroliths are relatively rare, having been found in less than 4% of all sauropod skeletons, these trace fossils are abundant enough in them that they should not be ignored, either.
So imagine you are a 20-ton sauropod and walking along a stream bank during the Late Jurassic. At some point during your stroll, you get a serious hankering for some geo-gastroliths, and you don’t know why, because your brain is smaller than that of a 0.075-ton (150 lbs) human. How do you pick the right rocks? The cerebral processing needed to discriminate between “good” rocks (solid silica-rich ones) and “bad” rocks (crumbly ones or limestone) can’t be too complicated. One thing is for sure, though: it can’t be based on smell, because most rocks lack good scents. It also can’t be from sound, although the distinctive crunching of silica-rich metamorphic or igneous rocks underfoot might have given off tones different from those of, say, limestone or shale. How about touch? Your feet might have somehow felt the right rocks based on their size, shape, and surface texture, but that would have required an unexpected sort of sauropod sensitivity akin to the princess and the pea.
Hence you are probably left with sight and taste. Depending on your visual spectrum, brightly or darkly hued rocks of a certain size might catch your eye. If too small, though, these rocks might not be worth the effort. If too big, you risk choking to death, which would be very bad for transmitting your genes. So you needed proper search images for potential gastroliths before lowering your head to the ground, grasping a rock or two (or three) with your teeth, and gulping it down. Perhaps during the short time a rock was in your mouth, taste might have had some influence on your choice, too. Do taste buds in your mouth help say “Yes, swallow this!” and you comply? Or does the opposite happen, a spit-take of rejected rocks, striking and killing nearby small theropods like stray missiles?
Imaginative scenarios aside, sauropods, theropods, and other dinosaurs that ate rocks on purpose must have applied some sort of pattern recognition to select the ones best for them, for whatever reason. Based on the presence of gastroliths in the Early Jurassic prosauropod Massospondylus, this ability and its outwardly expressed behavior probably started in dinosaurs early on, such as in the Late Triassic Period. From an evolutionary view, one of the more interesting aspects of gastroliths is how prosauropods, sauropods, and theropods (birds, too) all share a common ancestor as saurischians.
In contrast, only a few ornithischians have gastroliths. These include the Late Cretaceous ankylosaur Panoplosaurus of Canada, the Early Cretaceous ceratopsian Psittacosaurus of China, the Late Cretaceous ornithopod Gasparinisaura of Argentina, and nearly no others. So although gastroliths are rare in the vast majority of dinosaurs, they are much more likely to be in saurischians, showing up in those dinosaurs from the Late Triassic through the Late Cretaceous, and then continuing in birds. That means this viewing, recognition, grabbing, and eating of stones has been going on in dinosaurs for nearly 200 million years, and will keep on happening as long as birds are around and also need gastroliths.
Speaking of birds, another type of dinosaur behavior that was possible, but difficult to test scientifically, is that parent dinosaurs taught their young by example, showing them how to select rocks for their own internal collections. Mammals are of course well known for passing on skills to their offspring, such as how mother grizzlies teach their cubs to fish for salmon. But in the past few years, behavioral scientists are gaining a greater appreciation for how some species of birds, such as crows and ravens, learn through experimentation and watching one another, while also imparting newly acquired knowledge to their chicks. If any dinosaurs performed similar behaviors, such as teaching their offspring how to become rudimentary geologists, then this would be a glimpse of dinosaur learning.