Dinosaurs Without Bones

by Anthony J. Martin

  What more could be learned from these coprolites, the gifts that kept on giving? It turned out that Cretaceous dung beetles were not the only ones taking advantage of bountiful supplies of nutritious dinosaur dung: snails got into the act, too. I remember these gastropods surprising me in 2009 while on a field trip with about thirty other paleontologists, led by Dave Varricchio, Frankie Jackson, and Jack Horner. Most participants would have admitted that the Maiasaura and Troodon nest sites were the main draw of the field trip for them. Yet I was equally excited to know that we would also be strolling through the same area with the hadrosaur coprolites I had seen nine years before when studying fossil insect cocoons and burrows there. Would I notice anything different about the coprolites this time?

  Well, yes. The coprolites, just like before, were easy to spot in the field area, sticking out on the eroded surface of the Two Medicine Formation as dark, baseball- to soccer-ball-sized lumps. Although some of us had seen them before, this did not dim our enthusiasm and we all stopped to marvel at how these formerly squishy vestiges had survived long enough for us to witness them, at that moment, now hard as rock. Sure enough, the finely broken and blackened wood chips of Cretaceous conifers were held together by whitish calcite cement. A few even had circular outlines or lengthwise sections of burrows, the right size to have belonged to dung beetles. We were in awe. Still, as is typical with coprolites, bathroom humor quickly overtook lofty ideals and higher cognitive functions. Within minutes, we regressed to photographing one another squatting above these 75-million-year-old trace fossils and giggling at the resulting images in our digital camera viewscreens.

  Suddenly, a moment of seriousness intruded when I picked up one coprolite—probably to pose in another picture with it—and noticed a snail’s beautifully coiled shell embedded in its matrix. Puzzled, I turned the sample all around and scanned the surface for more such fossils, automatically practicing a basic tenet of science: testing whether or not the initial observations could be repeated. In this instance they were, as I found at least three snails in this coprolite. Excited, I mentioned this to whoever was closest to me, who fortunately shared my interest in seeing something new, too. With an independent observer who had been cued on the same search image, we quickly found more coprolites containing fossil snails sprinkled between the wood and burrows.

  What explained this seemingly odd association with snails and dinosaur poop? My first thought, said jokingly in the field with others, was that the snails were happily grazing on fungi growing on a decaying log. But then a hadrosaur came along, took a big bite of the wood, chewed, and swallowed it, snails and all.

  This idea, however amusing, was wrong. We did not know it at the time, but Karen Chin had already finished studying these snails and surmised how they got into feces in the first place. First of all, none of these gastropods could have survived a trip down a dinosaur’s alimentary canal. If any had been gulped down as inadvertent escargot, their thin calcareous shells, once bathed in the low-pH stomach acids of a hadrosaur proventriculus, would have dissolved instantly. As a result, Chin concluded these mollusks, which were in 40% (6 of 15) coprolites she studied, were either eating feces or whatever might have been growing on the feces. In her paper, published later in 2009, she identified at least seven species of snails, four terrestrial and three aquatic. These ecologically distinct snails painted a complicated post-pooping history for the coprolites before they were buried and fossilized. Some no doubt had been dropped on dry land, where terrestrial snails happily grazed on them. However, some of these land nuggets might have been covered later by nearby floodwaters, or did high dives and made big splashes when exiting their former hosts. Aquatic snails then would have been attracted to them before their final burial and fossilization.

  These Two Medicine Formation coprolites thus demonstrated the potential for such trace fossils as a powerful means for understanding how one species of dinosaur—Maiasaura peeblesorum—could relate to its inner and outer ecosystems. More than most other fossils from their time, they expanded our consciousness of a Late Cretaceous web of life: gut bacteria, conifers, beetles, and snails, all connected through a dinosaur.

  Should I Stay or Should I Go: Dinosaur Gut Parasites

  Speaking of ecosystems, individual dinosaurs harbored their own diverse microflora and microfauna, using their bodies as places to live, eat, and reproduce in what is sometimes called a microbiome. As most people know, everyone, humans and animals alike, carry—on them or inside them—trillions of bacteria, fungi, skin mites, and occasional harmful (pathogenic) microbial or animal parasites. Animals may consist of lice, ticks, fleas, worms, leeches, or whatever else feels like getting a free meal without that person or other animal host noticing it. Not all of this flora and fauna are necessarily harmful to the host, though; for example, some gut bacteria help to produce vitamins K and B12, and about 70% of all bacteria are harmless to humans. Still, parasites seem to get more attention than beneficial members of this microbiome because of the genuine harm they cause, whether through disease transmission, psychological effects, or impurifying our precious bodily fluids.

  Parasites fall into two broad categories: endoparasites (living inside), such as tapeworms, and ectoparasites (living outside), such as ticks. Despite all we know about parasites and how they cycle through multiple animals today, we didn’t know for sure whether dinosaurs had these as parts of their microbiomes. Instead, parasites were just assumed for dinosaurs, especially massive sauropods, which must have represented “The Promised Land” for many parasitic Mesozoic species. Indeed, the main premise of Jurassic Park (the book and movies) was how mosquitoes and other blood-sucking insects (which are ectoparasites) had ingested dinosaur DNA in their meals.

  Although some insects or other Mesozoic arthropods could have parasitized dinosaurs, we had no actual evidence of their having hosted endoparasites. So it was time to look at modern analogs for guidance. Knowing that most modern endoparasites live at least part of their life cycles in animals’ guts, we also know that evidence of these parasites—live or dead bodies, eggs, and dormant stages (cysts)—is often included in host animals’ feces. For that reason, dinosaur coprolites might likewise hold such signs.

  Indeed, some do. In 2006, two paleontologists, George Poinar and Art Boucot, documented three types of endoparasites in a dinosaur coprolite: protozoans, which are one-celled amoeba-like organisms; trematodes, a group of flatworms that includes flukes; and nematodes, which consist of “round worms” and horsehair worms. The coprolites were from an Early Cretaceous deposit in Belgium that also contained many skeletons of the ornithopod dinosaur Iguanodon, which were discovered in the 1870s. This did not mean, however, that the coprolites were dropped by Iguanodon, as some had bone fragments, pointing toward meat-eating theropods as the culprits. The coprolites, first identified in 1903, ranged from 2 to 5 cm (1–2 in) wide and 11 to 13 cm (4.3–5 in) long, or human-sized. Poinar and Boucot took one of them, broke it up, subjected it to strong acids, and centrifuged it: the most action its contents had received in about 125 million years. What survived this preparatory process were protozoan cysts, a trematode egg, and nematode eggs. Incredibly, one of the nematode eggs even held the coiled body of a baby nematode, which had just missed hatching after exiting a dinosaur’s body in its feces.

  Although all of these fossils were new to science, they closely resembled modern pathogenic species seen in amphibians, reptiles, and birds. So thanks to this dinosaur coprolite and its 125-million-year-old secrets, biologists interested in the evolution of endoparasites have a minimum time for when such perfidious behaviors began: at least in the Early Cretaceous, but more likely extending back into the Jurassic.

  Carnivorous Movements

  Everything we know about nearly all large theropods points toward their carnivory: teeth, jaws, claws, toothmarks in bones, bones in abdominal cavities, and with some of those bones etched by stomach acids, to name just a few items. Still, nothing tells us about how much meat could go
through a theropod’s system on a given day like a gigantic tyrannosaurid turd. Fortunately, paleontologists have not just one but two such massive coprolites. Both are credited to tyrannosaurs because of the following: co-occurrence with known tyrannosaur bones; age (Late Cretaceous); environment (river floodplains, which tyrannosaurs may have preferred for hunting); size (extra-long baguettes, anyone?); broken pieces of bone; and, most unexpected of all, fossilized muscle tissue in one coprolite. Both were found in Canada, one in Saskatchewan and the other in Alberta, each of which have body fossils of tyrannosaurids, such as Albertosaurus and Tyrannosaurus.

  The first known coprolite attributed to a tyrannosaurid came out of fluvial (river) deposits in the Frenchman Formation of Saskatchewan. This formation dates from the latest part of the Cretaceous Period, about 66 mya, just before an extraterrestrial object delivered some bad news to all non-avian dinosaurs. The best way to describe this coprolite requires applying an overused word, but justified in this instance: awesome. When found in the field, it was partly eroded, but after having been recovered and put back together, it measured 44 cm (17 in) long, 13 to 16 cm (5–6 in) wide, and had a minimum volume of 2.4 liters (0.6 gallons). Almost half of it was composed of finely ground bones which were from a young ornithischian dinosaur; this bone meal was held together by apatite.

  The paleontologists who were lucky enough to study it—Karen Chin and three others—had little doubt it was a dinosaur trace fossil and related to carnivory. They considered the possibility that it might be a regurgitalite, an enormous cough pellet that became fossilized. However, it was held together so well by apatite and included such tiny bone fragments (some of which were sand-sized), a fecal origin made much more sense. (A cough pellet, in contrast, would have held more complete bones.) Considering its great size and how both its geologic age and location overlapped with that of Tyrannosaurus rex, this coprolite was most likely made by that massive theropod. In their 1998 paper reporting their find, the paleontologists crowned it as a “king-sized coprolite.”

  Once this coprolite was linked with T. rex, it revealed much about the carnivorous behavior of the world’s most famous dinosaur—or at least one individual of that species—and some of it was surprising. For one, the bone was broken into tiny pieces, which implied that this tyrannosaur either chewed its food thoroughly or it nipped bone while feeding. Yet tyrannosaur teeth and jaws were more suited for slicing and crushing, not grinding for hours or nibbling delicately. Furthermore, toothmarks in Triceratops and other dinosaur bones show that T. rex punctured bone. Thus one explanation for so many small bone fragments is that it did come from bone chipped with each puncture, which went along for the ride with any consumed flesh. Another possibility is that these bone bits came from this tyrannosaur scraping meat and sinew off long bones with a sideways motion of its head. In this speculative scenario, numerous denticles on its serrated teeth would have taken off minute pieces of bone.

  Another unexpected point raised by all this bone was how it was still there. Remember how big modern reptilian predators—such as crocodilians—digest bones so well that almost none of it shows up in their scat? They do this by retaining food in their guts for a long time. In contrast, undigested bone suggests that the meal raced through the dinosaur and was not given enough time to be completely absorbed. Did this tyrannosaur have the runs, caused by endoparasites? Probably not, but a short residence time for food also could have made room more quickly for seconds, thirds, fourths, and dessert.

  Despite the impressive size of this T. rex coprolite, it was surpassed by one from the Late Cretaceous Dinosaur Park Formation of Alberta, reported in 2003. Studied again by Karen Chin and five other paleontologists, it was 64 cm (25 in) long and as much as 17 cm (7 in) wide. Like the coprolite from Saskatchewan, it contained many pieces of bone and was cemented with apatite. Nevertheless, it differed in two important ways. First, Tyrannosaurus rex could not have made it because this dinosaur was not alive until about 10 million years after this fecal mass peeked out of a cloaca. The Dinosaur Park Formation did, however, have the tyrannosaurids Aublysodon, Daspletosaurus, and Gorgosaurus living there, so one of these theropods could have dropped it.

  The second way it differed was even more extraordinary. In this coprolite was recognizable muscle tissue, some of it preserved in three dimensions. In their analysis, the paleontologists were astonished to see: striated cell-like structures revealed in thin sections; bundles closely resembling muscle cells and connective tissues in SEM photographs; and high concentrations of carbon in and around these structures. All of these observations meant the paleontologists were looking at fossilized meat, in which minerals had faithfully mimicked the original muscle cells.

  Never before had a tyrannosaurid meal been preserved in such intimate detail. This 3-D snapshot of its former excrement required very unusual conditions, such as brief gut-residence time (food rushing through, resulting in incomplete digestion), dumping it as a cohesive mass on a river floodplain just before it flooded, rapid burial, and anaerobic bacteria in the feces that helped to precipitate minerals in and around the muscle cells. Based on the distinctive broken bits of bone throughout the coprolite, these paleontologists figured these all came from the same animal, such as a pachycephalosaur.

  Just for some biological trivia, these two tyrannosaurid coprolites also tell us something about the soft-part anatomy of their makers, namely their cloacas. Based on widths of the coprolites, their minimum cloacal diameters must have stretched to 13 to 17 cm (5–7 in) to allow these fecal masses to bid adieu to their gastrointestinal tracts. However, the actual diameter was probably greater, considering that the original feces lost volume with dehydration before burial. Too much information? No, it’s more like the more you know, the more you wonder.

  Just how long has such carnivory been a part of dinosaurs? According to coprolites, as long as there were dinosaurs. At the opposite end of the geologic-age spectrum from tyrannosaurid coprolites are the oldest probable dinosaur coprolites. These are about 228 mya, coming from Late Triassic rocks of Argentina and the same strata bearing bones of two of the oldest dinosaurs in the fossil record, Eoraptor and Herrerasaurus. Reported in 2005 by paleontologist Kurt Hollocher and three colleagues, these coprolites, like the tyrannosaurid ones, had bone fragments and were cemented with apatite. Out of the ten found, two were studied in great detail. Both of these were similarly sized, measuring about 2 cm (0.7 in) wide and 3.1 to 3.7 cm (1.2–1.4 in) long, about the average size of what might be encountered in a dog park.

  Like the Maiasaura coprolites, these had invertebrate burrows in them, although only a few millimeters wide. Hence they were probably not from dung beetles but perhaps fly larvae or worms. Because Herrerasaurus was a carnivorous theropod, and its body parts were the most common of any carnivore in those strata, plus it was about the right size for these coprolites, Hollocher and his cohorts proposed that they came from that dinosaur. Still, other archosaurs may have been responsible for them, too.

  These theropod coprolites, from the Late Triassic through the Late Cretaceous, demonstrated how carnivory was a dietary mainstay in this lineage for at least 160 million years. Furthermore, this evolutionary tradition continues today with predatory birds, although fortunately for us and many other animals, none of these theropods are producing feces on the scale of a tyrannosaur.

  Passing Grass: The First Grasses and the Dinosaurs That Ate Them

  Today we take grasses for granted. They occupy most of the soils in our lawns, parks, pastures, athletic fields, and university quadrangles, but are also ubiquitous in our daily diets. Grasses we eat include corn, wheat, or rice, and others we drink in a fermented form, like barley, oats, or hops. With regard to the latter, paleontology, geology, and many other sciences could not be done without these grasses and their by-products, or at least these sciences would be a lot less fun.

  The conventional wisdom about grasses is that they first evolved from non-grassy flowering plants during the Cenozoic
Era—in just the past 65 million years—and especially took off in just the past 30 million years or so, first in South America, then in North America. Grazing mammals, including the earliest horses, were always thought to have facilitated the spread of amber waves of grain. Indeed, changes in horse dentition and limbs were likely linked to changes in grassland habitats over time. Dinosaurs, in contrast, had absolutely no place whatsoever in this comforting story of co-evolution between grasses and mammals. Having all vanished soon after a big rock arrived 65 mya, non-avian dinosaurs only contributed their recycled elements to the soils feeding grasses, with their bones ground to dust under the hooves of grass-grazing ungulates.

  Fortunately, thanks to some Late Cretaceous sauropod coprolites from India, we now know this story needs updating. Two studies, done in 2003 and 2005, showed that sauropods—specifically, titanosaurs—were eating different plants than expected, and the latter study revealed that these plants included grasses. These results surprised nearly everyone for two reasons: few people suspected that grasses had evolved during the Mesozoic, and almost no one expected to find fossils of them in dinosaur coprolites.

  The first detailed report on these coprolites was by Prosenjit Ghosh and five of his colleagues in 2003. How did they know these coprolites came from sauropods? First of all, these grayish masses in the Lameta Formation of central India were actually first identified as dinosaur coprolites in 1939. Their makers were then deduced on their occurring in the same strata as titanosaur bones. Moreover, although they were not very large by titanosaur standards—the biggest were only about 10 cm (4 in) wide—they were big enough not to have come from any other herbivorous animal in those strata. The researchers also noted how the coprolites had dried out and cracked slightly before burial and fossilization, meaning they were originally deposited on dry land.