Dinosaurs Without Bones (39 page)

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.

Ghosh and his colleagues detected vascular plant remains in the coprolites, as well as fungal spores, algae, and plenty of bacteria. However, their most significant finds came from a chemical analysis of the coprolites, which helped them to narrow down which types of plants the titanosaurs ate. To figure this out, these scientists calculated a couple of stable-isotope ratios in the coprolites—for carbon and nitrogen—and compared these ratios to those in the feces of modern animals, such as deer, camels, buffalo, and big cats (leopards and tigers).

Just to back up with some basic definitions: isotopes are variations of the same element, but with different atomic weights. For example, the isotopes of carbon (C) are
¹²
C,
¹³
C, and
¹⁴
C. Of those isotopes,
¹²
C and
¹³
C are stable, but
¹⁴
C is not, as it undergoes radioactive decay and changes to another element. Nitrogen (N) isotopes are
¹⁴
N and
¹⁵
N, and both of these are stable. Then what are stable-isotope ratios? In this instance, these scientists calculated
¹²
C/
¹³
C and
¹⁴
N/
¹⁵
N. Plants, through different means of photosynthesis, take in carbon and nitrogen isotopes in distinct ways, which is then reflected by their stable-isotope ratios. For instance, C
₃
and C
₄
plants—so called because of the number of certain carbon compounds they form—have dissimilar ratios, because C
₄
plants absorb
¹³
C more easily than C
₃
plants. In short, these ratios are chemical signatures that, under ideal conditions, persist in fossil plants, even if they went through the gut of a dinosaur and were buried for about 70 million years.

As it turned out, the carbon-isotope ratios showed that the titanosaurs ate C
₃
plants, nearly matching a value for birds, and they were much closer to ratios of modern herbivores—goats, camels, and buffalo—than carnivores. Additionally, C
₃
plants make up nearly 90% of all modern vegetation, and include grasses. However, this did not mean these titanosaurs were eating grasses specifically; plant pieces in the coprolites matched those of conifers and other non-flowering plants. But these dinosaurs were certainly
consuming plants with similar modes for photosynthesizing, and the isotopic signatures in their coprolites were close to those of modern plant-eating mammals. Furthermore, the nitrogen stable-isotope ratio matched that of animals that do not use fermentation in their guts (usually aided by bacteria) to help digest their food. So again, this was more like birds and less like mammals that use bacteria in their foreguts (small intestines) or hindguts (large intestines) to ferment their food.

These geochemical clues gleaned from sauropod coprolites certainly gave paleontologists better insights on what these dinosaurs ate and how they digested their food. Yet, as much as ichnologists hate to admit it, they sometimes need a few body fossils in their trace fossils to better interpret the latter. So in 2005, Vandana Prasad and four of his colleagues hit the jackpot, finding grass phytoliths in the same Late Cretaceous coprolites from India studied by Ghosh and others.

Remember phytoliths? These are microscopic bits of silica precipitated by plants and residing in their tissues, which also left microwear on dinosaur teeth when they ate these plants. In many plants, phytoliths can be “fingerprints” for identifying plant clades, and sometimes a specific species. Fortunately for dinosaur ichnologists, phytoliths are also resistant to acids, so these can pass relatively unscathed through an animal’s gastrointestinal tract. This evolutionary innovation by plants thus helped earmark a minimum time for when grasses showed up in Mesozoic ecosystems (65–70
mya
) as well as when dinosaurs started grazing or browsing on them. Not mammals, but dinosaurs.

This discovery of grass phytoliths in sauropod coprolites thus fulfilled the maxim of “You never know until you look,” especially when amended by saying “You never know until you look in dinosaur feces,” or other trace fossils associated with dinosaur digestion. From enterolites, to regurgitalites, to cololites, to coprolites, which can be connected to gut bacteria, plants, snails, insects, mammals, birds, and other dinosaurs, these trace fossils tell us inside stories and intimate details of dinosaur lives.

Because of the sparse and uneven record of dinosaurs in Australia, their fossil footprints are more valuable here than anywhere else on Earth.

—Thomas H. Rich and Patricia Vickers-Rich,
A Century of Australian Dinosaurs
(2003)

Looking for Traces in All the Wrong Places

Dinosaur tracks are hard to find. This humbling realization struck me during the third week of a month-long excursion in May–June 2010, while doing field work along the craggy coast of Victoria, Australia. Just the year before, paleontologist Tom Rich of Museum Victoria invited me to look for trace fossils made by dinosaurs and other Cretaceous animals that might be preserved in the rocks of Victoria. Yet as was often the case with looking for fossils of any kind, there was no guarantee of success. During our time in the field, he and I had already searched more than a hundred kilometers of coastal cliffs and marine platforms east of our present location, with only a few fossil finds in all of its vastness. Now we
were working our way through sites to the west, and so far nothing else had been found worth writing home about.

So a bit of stubbornness underpinned our visit to Milanesia Beach, located in southwestern Victoria, Australia. Milanesia Beach is about a three-hour drive from the big city of Melbourne, but like many places in Australia it feels isolated, far away in space and time from a world where people sip lattes, use mobile devices, or drive cars, sometimes all simultaneously. A testament to its relative inaccessibility is that, despite its having a beautiful beach framed by dramatic sea cliffs, the people who normally see it are not swimmers, surfers, or sunbathers, but hikers. Even then, it is only a brief waypoint for those people as they otherwise enjoy the gorgeous scenery of one of the most famous walking routes in Australia, The Great Ocean Walk. In fact, this trail inspired me to dub, tongue-in-cheek, my excursion along the Victoria coast as “The Great Cretaceous Walk.”

On Monday, June 14, 2010, we were not visiting Milanesia Beach to hike or enjoy the bucolic countryside. Instead, Tom Rich, local guide Greg Denney from the nearby town of Apollo Bay, and I were there to look for fossils from the Early Cretaceous Period, at about 105 million years ago. The landscape was certainly very different back then. It was a time when Australia was close to the South Pole, and dinosaurs presumably walked across broad floodplains of rivers that coursed through its circumpolar valleys. Since then, Australia had drifted north and now was just below the equator. Of course, many animals and plants native to the place had gone extinct, whereas some lineages evolved into the distinctive life of Australia today, such as its rich diversity of marsupials found nowhere else in the world. More than a hundred million years can really change a place.

Milanesia Beach, though, was a new place for me, and it might as well have been new for Tom, as he had not visited it in more than twenty years. His main reason for looking at its rocks was for fossil bones, especially those of dinosaurs or small mammals. As an ichnologist, I was there to look for trace fossils. I knew most
of these vestiges of life would consist of burrows and trails made by invertebrate animals like insects, crustaceans, or worms. But if we were really lucky, these rocks might also reveal trace fossils of vertebrates, such as the burrows or tracks of mammals, dinosaurs, or other backboned animals.

Unfortunately, during the preceding three weeks of field work I had only found a few invertebrate trace fossils (burrows) and no vertebrate trace fossils: no tracks, nests, burrows, toothmarks, gastroliths, coprolites, or anything else that might tell of a former vertebrate presence. Similarly, Tom had not yet found a single scrap of bone. We seemed more than due for a big break.

More Bones than Traces

Throughout this book, I’ve lauded the great advantages of dinosaur trace fossils over bones for all of the insights they give us about dinosaur behavior. Other than telling us about behavior, and especially how dinosaurs interacted with one another and their environments, one of the best benefits of dinosaur trace fossils is their overall great abundance compared to bones. For the many reasons explained before, in Mesozoic rocks of most places in the world you are much more likely to find a dinosaur trace fossil than a dinosaur bone.

Yet there are a few regions that have Mesozoic rocks of the right ages and environments for dinosaurs where dinosaur bones are more common than their trace fossils. One such place is Victoria, Australia, where I accidentally began some research projects in 2006. Up until then and there, I had only dabbled with dinosaur trace fossils in the U.S., mostly through having seen many dinosaur tracks and other trace fossils in the western U.S. As mentioned previously, I was also helping several colleagues with the description of a burrow made by the Cretaceous ornithopod
Oryctodromeus
. In terms of writing about dinosaur trace fossils, I had done a chapter about them in two editions of a dinosaur textbook. Other than this, I was largely ignorant of dinosaur trace fossils. Most of my training, research, and teaching dealt with other traces, both modern and fossil, made
by a wide variety of animals, invertebrate and vertebrate. Dinosaur trace fossils, such as their tracks, nests, gastroliths, toothmarks, and coprolites, were fascinating but did not occupy my every waking thought.

Ironically, then, my interest in dinosaur trace fossils was kindled in an area of the world where they are scarcer than dinosaur teeth, starting with the first day I laid eyes on Cretaceous rocks of Victoria. Although I knew about Lark Quarry, the so-called “dinosaur stampede” (or “dinosaur swim meet”) site far to the north in Queensland, and a few other dinosaur tracksites in the northern and western parts of Australia, I knew next to nothing about any dinosaur tracks in the southern part of the continent. Later I found out this couldn’t just be attributed to laziness or disinterest on the part of other paleontologists; as of 2006, there really were very few dinosaur tracks or other trace fossils known from there.

Just for comparison, let’s take a look at other continents and their dinosaur trace fossils. In North America, the U.S., Canada, and Mexico have tens of thousands of dinosaur tracks and plenty of other trace fossils, such as nests, gastroliths, toothmarks, and coprolites. South America? The same. Africa, Europe, Asia? Ditto. But as of my writing this, no dinosaur nests have been reported from Australia. Not even body fossils associated with dinosaur nests—such as eggshell fragments and embryonic bones—are known from there either, let alone egg clutches. No research has been done on Australian dinosaur gastroliths. In all of Australia, not one study has been done on dinosaur toothmarks. Not a single dinosaur coprolite has been interpreted. It’s almost as if the dinosaurs in Australia didn’t give a crap.