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 12C, 13C, and 14C. Of those isotopes, 12C and 13C are stable, but 14C is not, as it undergoes radioactive decay and changes to another element. Nitrogen (N) isotopes are 14N and 15N, and both of these are stable. Then what are stable-isotope ratios? In this instance, these scientists calculated 12C/13C and 14N/15N. 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, C3 and C4 plants—so called because of the number of certain carbon compounds they form—have dissimilar ratios, because C4 plants absorb 13C more easily than C3 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 C3 plants, nearly matching a value for birds, and they were much closer to ratios of modern herbivores—goats, camels, and buffalo—than carnivores. Additionally, C3 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.
CHAPTER 9
The Great Cretaceous Walk
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 Me
sozoic 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.
So what caused Australia to become so poor in dinosaur trace fossils, ranking only above Antarctica in quality and quantity? Dinosaurs certainly lived there, as evidenced by theropod and ornithopod bones and teeth recovered from Early Cretaceous strata (120–105 mya) in Victoria, and theropod, ornithopod, and sauropod bones in central Queensland. The latter area is now bursting with Early Cretaceous dinosaur skeletal material, reminding paleontologists of the beginning of the “Great Dinosaur Bone Rush” during the late 19th century in the western U.S. Still, why are their tracks, nests, gastroliths, toothmarks, and coprolites so rare? Or is it just an apparent scarcity, a combination of wrong conditions for preserving these trace fossils in most of the Mesozoic rocks there as well as not knowing what to look for?
The answer is probably complicated. Regardless, the best way to reach for it was to get out, walk around, and look for trace fossils in the rocks there.
Back to the Cretaceous
It was a fine day on the Victoria coast, started by crisp morning temperatures, a mild breeze, and overcast conditions, but with no signs of the antipodal winter thunderstorms—accompanied by rain, gusting winds, and powerful waves—that had kept us off the coastal outcrops for much of the previous week. Earlier that morning, Tom Rich and I drove from where we were staying in Apollo Bay, picked up Greg Denney at his home, and parked our vehicle near a trailhead, about two kilometers (1.2 miles) uphill from the beach. The walk down to the outcrops, punctuated by muddy, slippery patches, promised a vertically challenging slog later in the day, just when we would be most spent from our explorations below.
Greg, who joined us to scout rocks of the Eumeralla Formation composing the dramatic cliffs near Apollo Bay, had a longstanding relationship with Tom as a field assistant and friend. He also had the good fortune of growing up next to one of the most famous dinosaur sites in Australia: Dinosaur Cove. In the 1980s, Greg and his father, David Denney, assisted Tom, Pat Vickers-Rich, and a crew of volunteers with some of the most technically difficult conditions any dinosaur dig site should ever have to endure, detailed by Tom and Pat in their book Dinosaurs of Darkness, published in 2000. In our more recent ventures, Greg had quickly proved a valuable asset in our field endeavors, suggesting roads and parking spots for our field vehicle and advising on safe access points to outcrops.
Greg had also become my ichnological apprentice during our previous week together in the field and quickly became quite good at spotting small fossil invertebrate burrows in Cretaceous outcrops. I would have liked to credit his rapid success to my extraordinary teaching abilities, but instead chalked it up to his spending much of his life outdoors. After all, he had already trained his eyes to pick up small details in his natural surroundings, such as wallaby tracks, echidna dig marks, and kangaroo feces. These skills had not been sullied by the constant distractions of “big-city life,” a challenge I face every day when not in the field and living at home in the metropolitan area of Atlanta, Georgia. I envied him these opportunities, available to him every day, and in such gorgeous places.
The thirty-minute walk to Milanesia Beach promised by the trail-head sign next to where we parked was surprisingly accurate, considering how carefully we placed our feet while walking down the steep, winding track. Toward the bottom of the trail, we also had to cross a small stream teeming with freshwater leeches, just like those I had encountered at Knowledge Creek a few years before. At the end of the trail, we were greeted by an upside-down sign bearing the usual admonitions about all of the potential forms of mayhem that awaited us if we proceeded. These signs seem to be everywhere in Australia and are meant not just for tourists, but also for anyone who might perform acts of foolishness while celebrating nature.
Once we were on the more level ground of Milanesia Beach, two choices faced us in our fossil explorations: either go to the outcrop on our immediate right—with only beach sand in front of it—or to a more modest outcrop on our left, with our path complicated by numerous blocks of rock that had fallen from cliff faces above. We looked briefly at the closest part of the outcrop to the right, but these rocks seemed too coarse-grained to have many discernible trace fossils. Fine-grained sandstones or siltstones are much better for preserving identifiable invertebrate burrows, vertebrate tracks, and other trace fossils, as opposed to conglomerates. So we chose to go left, a decision also encouraged by high waves already lapping against the outcrop on our right.
While walking parallel to the shoreline, we soon went from a sandy beach to a rocky shore. Some of the blocks of rock we passed were much smaller and more rounded than others, providing indirect clues of their relative time on the beach, in which the surf shaped the smaller and more rounded rocks far longer than bigger ones. In contrast, the larger blocks retained angular corners from their more recent breakage off nearby cliff faces. Normally in a talus field like this, I would just stroll along and not spend much energy looking at each stone for its paleontological value. Nonetheless, I did glance at them, albeit more out of a sense of self-preservation. I wanted to make sure I stepped in all the right places and didn’t slip on any slimy algal films and thereby become too physically intimate with these rocks.
While ambling, we stopped occasionally to scan the rocks in the vertical outcrops, as well as larger angular blocks scattered across the upper part of the shore. The several-meter-high vertical exposures of layered shales, sandstones, and conglomerates were at the top of the beach, marking where the sea had eroded these strata. The surf crashed behi
nd us, giving us no choice but to shout at one another as we pointed out anything of geological interest. We also warily watched the sea for any rogue waves that might catch us by surprise. Field work along the Victoria coast is treacherous enough to encourage a healthy caution in its practitioners—supported by the inverted sign at the end of the trail down—and that day was no different.
In retrospect, we were fortunate to have the winter solstice approaching, which meant the sun would begin to set close to
5:00 p.m., a constraint that urged us to use our time judiciously. Sure enough, within less than ten minutes of our arriving, Greg and I started finding trace fossils—invertebrate burrows—in fine-grained sandstones and siltstones exposed in the outcrop. One type of burrow was a stubby vertical cylinder, some of which were U-shaped. Another was a thin, reddish J- or U-shaped structure, also oriented vertically. Each burrow form was abundant in the thin strata.
These little trace fossils invoked an unprecedented excitement within me, as they provided clues to the ancient ecosystems of the area. Both types of burrows were formerly open tubes, filled with sand very soon after they were made. Furthermore, invertebrate burrows often act as sensitive indicators about the former ecology of an area, and these were typical of what you might see in a modern river floodplain. For instance, some aquatic insect larvae dig burrows in sediments under very shallow water or on the surfaces of emergent sand bars, whereas other insects—such as ants, bees, and wasps—can only make nests above water. These trace fossils looked more like aquatic insect burrows to me, probably used for combined feeding and dwelling.
The presence of these burrows alone was scientifically important, and when put in the context of having been formed in a polar environment, they were doubly significant. Insects and other invertebrates in polar environments cannot burrow into frozen sediment. Rather, they wait until late spring or summer to make their domiciles or brooding burrows, after the uppermost layers of sediments have thawed out. Or they wait until new, soft surfaces have been formed by sediment deposited by spring run-off of melt waters. Moreover, the physical sedimentary structures associated with the trace fossils—ripple marks and cross-bedding—also indicated a healthy flow of water. These structures would have more likely formed during a polar spring or summer following snow melts.
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