Tuesday, August 9, 2011

The Dinosaur Tracks of Milanesia Beach: Part I

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).

The Preamble

Dinosaur tracks are hard to find. This humbling realization struck me during the third week of a month-long field excursion in May-June 2010, while doing field work along the craggy coast of Victoria, Australia. Why was I there, engaging in such an apparently fruitless quest? Paleontologist Tom Rich of Museum Victoria had 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 were no guarantees of success. He and I had already searched more than a hundred kilometers of coastal cliffs and platforms east of Melbourne, and were then working our way through sites west of there.

Here are four three-toed dinosaur track,s preserved on a block of sandstone at Milanesia Beach, Victoria (Australia). They’re faint, but there – look closely for all four. These tracks were probably made by small theropods on a river floodplain during a polar summer about 105 million years ago, when Australia was close to the South Pole. On June 14, 2010, I discovered the block that contained these tracks, and a few hours later, Greg Denney found another block with more tracks. This is a big deal, as they represent the greatest number of polar dinosaur tracks found in any one place in the Southern Hemisphere. It’s enough to make you want to do a happy dance. Scale bar in photo (lower left) = 10 cm (4 in).

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 is a relatively isolated place. A testament to its relative inaccessibility is that, despite its possessing a beautiful beach, framed by dramatic seacliffs, the people who normally see it are not swimmers, surfers, or sunbathers, but hikers. Moreover, it is only a brief waypoint for these recreationalists as they otherwise enjoy the gorgeous scenery of one of the most famous walking routes in Australia, The Great Ocean Walk.

On Monday, June 14, 2010, we were not visiting Milanesia Beach to hike and enjoy the bucolic countryside. Instead, Tom Rich, local guide Greg Denney (of nearby Apollo Bay) and I were there to look for fossils in that area that area dating back to 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 walked across broad floodplains of rivers that coursed through its circumpolar valleys.
Milanesia Beach 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 20 years. His main reason for looking at its rocks was for fossil bones, especially those of dinosaurs or mammals. My purpose, however, was different. I am an ichnologist, and was there to look for trace fossils, vestiges of life that would most often consist of burrows and trails made by invertebrate animals, like insects, crustaceans, or worms. Tom had invited me to come to Victoria to fulfill this purpose, which was to find as many trace fossils as possible. If we were really lucky that day, though, these rocks might also reveal trace fossils of vertebrates, such as the burrows or tracks of mammals, dinosaurs, or other back-boned animals. Unfortunately, during the preceding three weeks of field work, I had only found a few invertebrate trace fossils and no vertebrate trace fossils. We seemed more than due for a big break.

It was a fine day, denoted by crisp morning temperatures 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 and I drove from where we were staying in Apollo Bay, picked up Greg at his home along the way, and parked our field vehicle near a trailhead, about 2 kilometers upslope 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.

The sign at the trailhead (top) along the Great Ocean Walk telling us the distance to Milanesia Beach. Annoyingly enough, it said absolutely nothing about uphills and downhills along the way. But we were gratified that none of us had brought along bicycles, dogs, cats, tents, fires, or any combination of those items.

Within about 15 minutes down or up this trail, you get some great views, including this first glimpse of Milanesia Beach the morning of the dinosaur-track discovery. It’s a beautiful place with beautiful trace fossils. Now if only it had a pub at the trailhead, it’d be perfect.

Greg, who had joined us for our scouting rocks of the Lower Cretaceous Eumeralla Formation composing the dramatic cliffs near Apollo Bay, had a long-standing relationship with Tom as a field assistant and friend. He had grown up in the area and had the good fortune of living 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 the book Dinosaurs of Darkness (2000, Indiana University Press). In our more recent venture in June 2010, 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 for outcrops via trails leading from wherever we parked.

Here’s a brief interview I conducted with Greg Denney about his and his father’s (David Denney's) role in the recovery of dinosaurs at Dinosaur Cove. Fittingly, this interview was done at Dinosaur Cove, and the day before we discovered the dinosaur tracks at Milanesia Beach, only 9 km (5.5 mi) west of there.

Greg had also become my ichnological apprentice during our previous week together in the field, and was now quite good at spotting small fossil invertebrate burrows in Cretaceous outcrops. I would have liked to take credit for his rapid ichnological success because of my extraordinary teaching abilities, but (less delusionally) I 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 poo. These skills also 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 (USA).

The 30-minute trip promised by the trailhead sign next to the parking area 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. Other than these blood-sucking parasites, it was no big deal, and all three of us had been through far worse. At the bottom of the trail, we were greeted by an upside-down sign bearing the usual warnings about all of the potential forms of mayhem and death that awaited us if we proceeded.

An upside-down trail sign at Milanesia Beach, which was either auspicious or cheeky, depending on perspective. (Naturally, I choose “cheeky.”) Note all of the graphic admonitions of impending doom if you bother to go on from that point, none of which we heeded.

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 or vertebrate tracks. Thus we chose to go left, a decision that was helped by high waves already lapping against the outcrop on our right.

A view over our shoulders after exiting the trail onto Milanesia Beach, showing a coarse-grained sandstone without any trace fossils. In other words, boring. What was exciting, though, was getting a glimpse of the Milanesia Beach fishing hut (arrow), which apparently has its own Facebook fan page. (You didn’t believe me, did you?)

While walking parallel to the shoreline, we quickly went from a sandy beach to rocky shore. Some of the blocks of rock we passed were much smaller and rounded than others, providing indirect clues of their relative time on the beach. For example, the surf had shaped the smaller and more rounded rocks for a longer time than the bigger ones, and 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 across it 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 while making sure I stepped in all the right places and didn’t slip on any slimy algal films.

Meanwhile, we stopped occasionally to scan the rocks in the vertical outcrops, as well as the larger angular blocks scattered across the upper part of the shore. The several-meter-high vertical exposures of layered shale, sandstone, and conglomerate were at the top of the beach, marking where the sea had eroded these sedimentary strata. The surf crashed behind us, causing us to shout to one another as we pointed out anything of geological interest. We also warily watched the sea for any rogue waves or elevation of the tide that might catch us by surprise. Field work along the Victoria coast is treacherous enough to encourage a healthy paranoia in its practitioners, and that day was no different.

Cretaceous outcrop to the left, ocean waves to the east on Milanesia Beach. You might say we were stuck between rock and a wet place. Tom is walking in front, Greg is just behind him, and I was lollygagging behind, which later turned out to be a good thing. You can even see how my footprints (left) show where I looked at the outcrop, then backtracked to get a better overall view.

In retrospect, we were fortunate to have the winter solstice approaching that time of the year, which meant the sun would begin to set close to 5:00 p.m., thus urging us to use our time judiciously. Sure enough, within less than 10 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. Another was a thin, reddish J- or U-shaped structure; both were formerly open tubes that had been filled with sand very soon after they were made, more than 100 million years ago. Each burrow form was abundant in the thin strata.

Wow, look – invertebrate trace fossils! Two types were at Milanesia Beach, identifiable by size, color, orientation, and other features. The first (top) are sand-filled burrows that are mostly horizontal, but with some short vertical shafts. These sometimes have an internal structure that show their tracemakers – probably insect larvae – packed their burrows with sand behind it as they moved forward. The second (middle) are thin J- or U-shaped burrows, also filled with sand, but colored red by hematite (iron oxide). Sometimes the “U” form manifests as a “J” if only part of the burrow is exposed, but the tops will show a pairing of tubes (bottom) that reveal their true “U’s”. Such burrows may have been made by larval insects, such as dipterans.

These little trace fossils invoked an excitement in me that cannot be described with words, but I’ll try anyway. Such burrows are sensitive indicators about the ecology of this area 105 million years ago, and typical of what you might see in a modern river floodplain. For instance, some insect larvae will dig burrows in the sediment under very shallow water or on the surfaces of emergent sand bars. 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.

You see, insects and other invertebrates in polar environments can’t 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 once new, soft surfaces have been formed by sediment deposited by spring run-off. Moreover, the physical sedimentary structures associated with the trace fossils, consisting of ripple marks and cross-bedding, also indicated a healthy flow of water, which 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, invertebrate burrows, and vertebrate tracks associated with a point bar in 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 point bar 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, even though they were separated by more than 100 million years and thousands of kilometers.

Poverty Bar, a point bar of the Colville River on the North Slope of Alaska, which during the summer holds heaps of insect, nematode, and vertebrate traces. This makes for a nice modern analogue when looking at sedimentary rocks formed in Cretaceous polar environments of Australia. Why yes, those are grizzly bear tracks just to the left of mine, which is why you won’t hear me whinging about freshwater leeches in Victoria. By the way, those are Cretaceous rocks (partially covered by vegetation) in the background, which contain dinosaur bones and tracks from other Cretaceous polar environments. But that's another story.

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- which was just a few kilometers east of us. Knowledge Creek is the place where the only well-documented dinosaur track was discovered from the Eumeralla Formation. In 1980 – a little more than 30 years before he, Greg, and I stepped foot on Milanesia Beach together – Tom and Pat found this track, which was probably made by a small ornithopod dinosaur.

The only previously documented dinosaur track from the Lower Cretaceous Otway Group, probably made by a small ornithopod dinosaur, and discovered in 1980 by Tom Rich and Pat Vickers-Rich at Knowledge Creek, only 2 km (1.2 mi) east of Milanesia Beach. It’s in Museum Victoria in Melbourne, and a rare example of a dinosaur track from Victoria. Or, should I say, was.

Although I had visited Knowledge Creek three times in recent years (2006, 2007, and 2009), I had not found any other definite dinosaur tracks there. Nonetheless, I was lucky enough to have discovered possible dinosaur burrows, and invertebrate trace fossils 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 similar environments 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 to look for bones. I think he was glad that Greg was helping with my ichnological investigations, which allowed him to concentrate more on finding the bones of dinosaurs or other vertebrates. In his previous scouting of Milanesia Beach with others of his body-fossil-hunting ilk more than 20 years before, they had not found any skeletal material. Thus they had since written it off as a place to look for such fossils. Nonetheless, he also knew that during that elapsed time, rockfalls and weathered surfaces might have revealed previously hidden body parts, new to human eyes.

Excited 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.

What was the reason for such vigilance? Well, up until that day, only four well-defined dinosaur tracks had been found in all of the Cretaceous rocks of Victoria, a part of Australia only slightly less in area than California. The first track, found by Tom and Pat at Knowledge Creek, was only about 10 cm (4 in) wide and long. Tom and Pat were doubly lucky 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 and hiked out of the site. The latter task was so difficult, they vowed never to return.

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-outreach articles and books. Hence it became iconic. For many people, and for more than 25 years, this was the dinosaur track from the Cretaceous of Victoria.

Attentive readers who are also well versed in arithmetic are now waiting for the other three shoes to drop, so to speak, and I’m glad to oblige. In 1989, another isolated dinosaur track was found during a fossil-hunting group led by Tom Rich. Discovered at a place called Skenes Creek, more than 30 km (19 mi) east of Knowledge Creek, but bearing rocks of the same age as those at Knowledge Creek, Dinosaur Cove, and Milanesia Beach. This specimen was also a small ornithopod track, nearly identical to the one from Knowledge Creek. 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 there, it received a catalog number and sat in a drawer 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. (Note to all: I got dibs.)

This space left intentionally blank, but some day will be filled with a photo of the second ornithopod track from the Eumeralla Formation, found in 1989 at Skenes Creek, Victoria, and relegated to a museum drawer for more than 20 years. Ignored no longer, I should be describing it in the upcoming year and will compare it directly to the track 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 (not pictured here, because it’s not very pretty), then a dig-site volunteer, Tyler Lamb, found a nicely defined one in 2007, only about 5 meters from the main dig site.

A single dinosaur track from the Strzelecki Group, made by a large theropod dinosaur, and only the fourth definite dinosaur track from Victoria. The track is about 35 cm (14 in) long, which when multiplied by 4.0 gives the approximate hip height of the dinosaur: 1.4 m (4 ft. 7 in). That’s eating size, as in, it would have been big enough to have eaten us, had it been interested in consuming slightly fatty bipedal mammals.

So there you go: four definite dinosaur tracks from all of the Cretaceous of Victoria, after more than 100 years of paleontological research in those rocks. The two tracks from 105-million-year-old rocks west of Melbourne – Knowledge Creek and Skenes Creek – were attributable to small ornithopod dinosaurs. Large-sized theropods likely made the two 115-million-year old tracks in rocks east of Melbourne.

An abundance of dinosaur bones found during the past 30+ years in this part of Australia, though, indicate a more diverse fauna should be there, and these dinosaurs should have been leaving tracks during the springs and summers in between harsh polar winters. Moreover, in most places where dinosaurs used to live, dinosaur tracks tend to be more abundant than their bones. Frustratingly enough, the situation in the Cretaceous rocks of Victoria was just the opposite. Paleontologists deal with gaps in the fossil record all of the time, but this was a big one for the fossil record of polar dinosaurs in Australia.

Fortunately, that gap was about to get filled. How? That's the subject of my next entry, coming soon.

Further reading:

Polar Dinosaur Tracks Open New Trail to Past, by Carol Clark: Emory University news release, Aug. 9, 2011

Martin, A.J., Rich, T.H., Hall, M., Vickers-Rich, P., and Vazquez-Prokopec, G., 2011, A polar dinosaur-track assemblage from the Eumeralla Formation (Albian), Victoria, Australia. Alcheringa: An Australasian Journal of Palaeontology. DOI:10.1080/03115518.2011.597564

1 comment:

  1. Finally! A blog I actually want to bookmark and keep up witb. A thousand thanks!