Dinosaurs Without Bones (40 page)

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 behind 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.

Along those lines, I had published a paper in 2009 about physical sedimentary structures—such as ripple marks, mudcracks, and so on—and traces—such as invertebrate burrows and vertebrate tracks—next to the Colville River on the North Slope of Alaska. Hence, the rocks in front of us, when combined with what I had learned from that Alaskan riverbank a few years beforehand, almost acted like a time machine. The Cretaceous rocks of Australia and the modern sediments of Alaska could be compared as polar ecosystems, despite being separated by more than 100 million years and thousands of kilometers.

Yet another justification for my growing elation was that these burrows closely resembled trace fossils I had seen in rocks at another place: Knowledge Creek, just a few kilometers east of us. Knowledge Creek is the place where the only well-documented dinosaur track from the Eumeralla Formation was discovered. In 1980, a little more than thirty years before Tom, Greg, and I stepped foot on Milanesia Beach together, Tom and Pat Vickers-Rich found this track. It was probably made by a small ornithopod dinosaur, which was not surprising to them, seeing that nearly all of the skeletal remains of dinosaurs in Victoria belonged to such dinosaurs.

I had visited Knowledge Creek three times in recent years—2006, 2007, and 2009—but did not find any other definite dinosaur tracks there, only a few vague outlines. Still, I was lucky enough to have discovered possible dinosaur burrows and invertebrate trace fossils there, the latter nearly identical to the ones we were seeing that morning at Milanesia Beach. Similar sedimentary rocks and trace fossils at Knowledge Creek and Milanesia Beach implied that similar environments had produced these rocks. So perhaps the conditions at both places were conducive to dinosaurs walking across floodplain surfaces, allowing their tracks to get preserved well enough that they would be identified some day.

After photographing and noting the locations of these trace fossils, Greg and I continued down the eastern extent of the beach. Tom, on the other hand, had already gone well ahead of us, looking for bones. I think he was glad that Greg was helping with my ichnological investigations, which allowed him to concentrate more on finding dinosaur bones or those of other vertebrates. In his previous scouting of Milanesia Beach with others of his body-fossil-hunting ilk more than twenty years before, they had not found any bones or teeth. As a result, they had since written it off as a place to look for such fossils. But he also knew that during that elapsed time, rock falls and weathered surfaces might have revealed previously hidden fossil bones, new to human eyes.

Happy about the invertebrate burrows and their host rocks, I turned to Greg as we sauntered down the beach and said, casually,
“Now all we have to do is find some of those other things we’ve been looking for. But I won’t say what, because I don’t want to jinx it!” Greg grinned and said “Righto, say no more!” Somehow he knew I was talking about dinosaur tracks. In our previous forays, I had mentioned these trace fossils as something we should be looking for at every turn.

Tracing through Recent Time

What was the reason for such vigilance about dinosaur tracks? Well, up until that day, only four dinosaur tracks had been found in all of the Cretaceous rocks of Victoria, a part of Australia just a bit smaller than California. The first track—the one discovered by Tom and Pat at Knowledge Creek—was only about 10 cm (4 in) wide and long. They were doubly fortunate that day: once for finding the track, and twice for having the right tools to collect it. Using a hammer and chisel, they carved it out of the rock, put it in a backpack, and hiked out of the site. Upon their return to Melbourne, they immediately placed the specimen in the Museum Victoria fossil collection. This track later became the template for thousands of reproductions used for science education in Victoria, and photographs of it appeared in many popular articles and books. In other words, it became iconic: For many people, and for more than twenty-five years, this was
the
dinosaur track from the Cretaceous of Victoria.

In 1989, another isolated dinosaur track was found during a fossil hunt by a group led by Tom Rich. They discovered it at a place called Skenes Creek, which was more than 30 km (19 mi) east of Knowledge Creek, but with rocks the same age as those at Knowledge Creek, Dinosaur Cove, and Milanesia Beach. This specimen was also a small ornithopod track and nearly the same as the one from Knowledge Creek. Fortunately, the field crew had a rock saw with them, so they neatly cut the surrounding rock into an easily transportable square slab, and likewise took it to Museum Victoria. Once deposited, it received a catalog number and sat in the museum for the next 21 years. I saw it in its museum drawer in May 2010—only three weeks before our sojourn to Milanesia
Beach—and verified it as the second dinosaur track reported from the Eumeralla Formation. However, it still has not been formally described in the scientific literature, so it remains known to only a few paleontologists and does not get any of the public notice showered on its almost-identical twin from Knowledge Creek.

The other two dinosaur tracks from Victoria were both made by large theropods and are east of Melbourne, at the Dinosaur Dreaming dig site in the geologically older rocks (115–120 million years old) of the Strzelecki Group. I found one in 2006 during my first visit to Dinosaur Dreaming. Sadly, Tom Rich—who was with me at the time—had not yet accepted my ichnological abilities and disregarded it as a dinosaur track. A year later, in 2007, a dig site volunteer, then-student Tyler Lamb, found a nicely defined track of about the same size only about 5 m (16 ft) from the main dig site. Yes, I felt vindicated, but also very happy for Tyler that he’d discovered such an important specimen. Later in 2007, I reported on both tracks in a poster presented at the Society of Vertebrate Paleontology meeting in Austin, Texas. This poster got a fair amount of media attention because it was touted as evidence of a “polar predator” from the Cretaceous, a mixing of theropods and polar dinosaurs that somehow inspired the public imagination.