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).

On Frozen Ground Down Under

The Cretaceous rocks we saw yesterday here in Victoria, Australia gave us a reason to ponder temperature and climate, but in a much grander way than just saying the words, “global climate change.” I cogitated on this matter while enduring the harsh, ravaging effects of the Victorian winter here.


Thinking cold thoughts, post-field work version. Sorry for the omission of a beer, if at least for scale.

Yet despite what we observe here today, this part of southern Australia was connected to northern Antarctica 120 million years ago – during the Early Cretaceous Period – and the rocks here hold clues that reflect environments close to 75° S. (Just in case one of my students is reading this and furiously converting those degrees into Fahrenheit on his/her iPhone, those degrees are not for temperature, but latitude, and the South Pole is 90° S of the equator.)


Paleogeographic map from about 130 million years ago, during the Early Cretaceous Period. So if you think Australia is “Down Under” now, go back to the Cretaceous and see what you think, mate! Map from Ron Blakley’s fantastic paleogeographic map site, hosted by Northern Arizona University.

The Cretaceous outcrops we have walked across for the past eight days are now at 38° S. This means Australia has moved steadily north in the past 120 million years, and at this rate will reach the equator in another 100 million years or so. This is Reason #1,243 why plate tectonics, as a process, rules.

Granted, mean annual air temperature (MAAT) was much higher during the Cretaceous Period than today, at about 20°C, versus 14.6°C today. (OK, now you can do those conversions to Fahrenheit: that’s 58°F versus 68°F.) Still, it got very cold here during the winters, accompanied by months of darkness each winter, cold enough to freeze the ground, and even form permafrost. (Of course, it’s not frozen now, so it’s not really fulfilling the “permanent” part of permafrost. Unless you count rocks as “frozen,” which I don’t.)

What evidence do they have of frozen conditions here? Well, we could use stable oxygen isotope ratios, which are routinely applied to figure out paleotemperatures. Alas, we are mere underfunded field geologists, not the immortals on Olympus who use geochemistry and other means of witchcraft to pry thermally related secrets from the rocks.

We could also use paleobotany, as fossil plants have growth rings, leaf margins, and the assemblage of plants themselves that reveal much about the environments. Indeed, that has been done, and they match plant assemblages that are typical of MAAT of 8-10° C. But we also are not paleobotanists, however much we wish we were (because paleobotanists so cool, they inspire songs).

So how about good old geological evidence, observable in the field using our unaided eyes and not requiring any fancy equipment, or paleobotanical know-how? Hey, that’s a great idea! Sure enough, three lines of geological evidence support that these environments – inhabited by the likes of dinosaurs, mammals, and the “monster newt” Koolasuchus – were indeed frozen at times. These are:

• “Soil drops.”
• Hummocky ground.
• Ice wedges.

All of these collectively could be termed as cryoturbation structures, which may sound a little naughty, and for that reason alone they are probably illegal in much of the southeastern U.S., regardless of their identity. Nonetheless, the name literally means, “cold mixing,” which implies that cold and frozen conditions, along with thawing of ice, can mix sediments, like sand and mud.

Here are the “soil drops” viewed in cross section in an outcrop, in which darker sediments (mud) settled into the lighter colored sand. This was caused by ice melting above, which made the mud heavier, then it plopped into the less-dense sand. These structures are at Flat Rocks, the site of the Dinosaur Dreaming dig.


“Soil drops” at Flat Rocks, near Inverloch, Victoria. Photo scale is a wee one, only 10 cm (4 in) long.

Note that they all have flat bottoms, and they flatten out on some unseen surface about 20 centimeters (8 inches) below the top of the “drops.” This imaginary plane is where the permafrost below them halted their progress. Think of the permafrost acting like the plastic liner of a kiddie pool, thus preventing chocolate pudding from going below it while two consenting adults wrestle above. (OK, I have to work on that metaphor.) Anyway, I have looked carefully at these structures at Flat Rocks during several previous visits there, and lacking any other reasonable explanation, I am convinced that they are what other geologists have interpreted.

Hummocky ground forms the same way as the “soil drops,” only they are more symmetrical, with the troughs of the “hummocks” bottoming out above a permafrost layer. Yesterday, we saw a beautifully exposed example of such a horizon in an outcrop near Kilcunda, Victoria, outlined by a thin, black layer of coal.


A hummocky-ground horizon in vertical section, with its bottom marked by the black line; scale is about 20 cm (8 in) long.

Very close to these at the same outcrop, less than 100 meters (330 feet) away, were the ice wedges. These, of course, are not to be confused with “ice wedgies,” which I suffered at the hands of bullies all through elementary school. (Yeah, but who’s laughing now?). Ice wedges are formed by water freezing in sediments that, as the ice expands, wedges it apart and forms fractures. Here are some of the structures, filled with lighter-colored sandstone, which makes them stand out in the finer-grained and darker shale.


Ice wedge, filled with sandstone in shale, vertical section. (It’s that snaky looking thing in the middle.) Scale is the same as before.


Ice wedge, filled with sandstone in shale, but this time seen from above (what we geologists suggestively call the “bedding plane.") Yup, same scale.

So at the end of this little summary of what may be the most obscure topic you’ve ever had the gumption to read, you might justifiably ask, “So what?” The short answer is, these are the only examples of cryoturbation structures known from the Mesozoic Era in the world. Good enough for you? Also, the fact that they are in the same rocks that contain the remains of dinosaurs, mammals, amphibians, and other critters backs up the interpretations that these animals were not only living in polar environments, but environments that were at least occasionally frozen.


Dinosaurs out foraging on a snowy landscape during the Cretaceous winter in southern Australia, while a small mammal watches, remembering all of those “ice wedgies” the dinosaurs gave him. More gorgeous paleo-art by Peter Trusler; originally in Smithsonian Magazine.

And that’s significant, too: no other polar dinosaur site in the world shows such geological evidence, so clearly indicated and exposed such that geologists can just walk up to an outcrop and interpret them. Not only that, we can later that same day go back to the motel and take in some solar therapy, all while imagining those colder times in the Cretaceous of Australia, helped considerably by holding a cold Tasmanian beer.

(Much of the original research done on these cryoturbation structures is described in this peer-reviewed paper: Constantine, A., Chinsamy, A., Vickers-Rich, P., Rich, T., 1998, Periglacial environments and polar dinosaurs: South African Journal of Science, 94 (3).)

Where Have All the Fossils Gone?

Today (Monday, May 31) was the first where the sun made more than a furtive appearance, with blue skies and brightness illuminating the outcrops. Our goal was to scout a locality called Harmers Haven, which Tom had not seen before this trip. I had visited it three years ago, but remembered little from it, and was reminded why today.


Harmers Haven at low tide.

We went at low tide in the morning, which exposed the rocks along the mostly flat marine platform for kilometers to the east. It was a daunting sight, but we walked to the easternmost end of outcrop and worked our way back, looking for trace fossils and bones. The amount of rock to examine was enormous, and the tide was rising, so not much detailed study could be done of the geology. Within just 2½ hours of arriving, we were driven off the outcrops by the encroaching sea, and had found nothing worth noting. No definite trace fossils or bones were seen, although the rocks did contain a huge amount of carbonized fossil wood, including large chunks of tree trunks.


Fossil plant debris, all shredded and carbonized.



Parts of a fossil tree trunk, with my size 8½ (men's) boot for scale.

Why? How could two paleontologists, one trained in discerning trace fossils and the other in identifying fossil bones, spend hours on a great expanses of Cretaceous-age rocks, yet find nothing more than fossil plant debris? (Not that there’s anything wrong with fossil plants – as my paleobotanist friends will attest.)

The answer is probably related to the science of taphonomy, which is the study of fossil preservation. Sometimes I summarize taphonomy as “everything that happened to an organism after it dies,” but it also can be applied to trace fossils, such as dinosaur tracks or crayfish burrows, which require special conditions to get preserved in the fossil record.

Body fossils and trace fossils are already exceptions compared to all of the life forms and their traces that have lived in the past 4 billion years that did not get preserved. But fossilization could have been made even more rare under given circumstances in the geologic past, even if a place was teeming with life.

First of all, this part of Victoria, Australia was near the South Pole during the Cretaceous Period. The rocks we looked at today, which are 115-120 million years old, were formed near about 75° S latitude. Australia was even connected to Antarctica at the time.



Position of Australia relative to the South Pole during the Cretaceous and today. From a sign at the Flat Rocks (Dinosaur Dreaming) site.

So you would think not much was living here then, but you would be wrong (sorry about that). Global temperature was quite a bit warmer than today, and forests actually extended to the poles. Which is why this area has lots of fossil plant material, as there were many trees living upstream of where these.

And that is a key statement: living upstream of here. All of the trees and other fossil woody materials were transported, perhaps many kilometers from where they lived originally. None of them lived here, as not a single tree stump has been found in place, and neither have any root trace fossils been seen.

The same is true for nearly every fish, amphibian, reptile, dinosaur, and mammal bone found in the Cretaceous rocks here in Victoria. And transport means incompleteness, as traveling downstream can be a little tough on body parts. So nearly all that people find are bits and pieces of these animals. They were very likely not living in the same environments where their remains were buried.

For example, the very first dinosaur fossil found in Australia, a single dinosaur claw, discovered by geologist William Ferguson in 1903.



The “Cape Paterson” claw, found by William Ferguson on the Victoria coast. Source of photo: http://museumvictoria.com.au/prehistoric/fossils/patclaw.html


Here I am at the original site where this dinosaur claw was found.


Oops, sorry - wrong photo! (And I have no idea how it got on my computer.) Here’s me at Eagle’s Nest, at the approximate site of Ferguson’s discovery:


More than 70 years passed by before anyone found any other dinosaur fossils in this area. And despite their re-discovery since the late 1970s and intensive recovery efforts near where we were today and west of Melbourne at Dinosaur Cove, all of the dinosaur bones that have been found from Victoria could easily fit in a single room. Even more depressing, all of the mammal bones could fit in a matchbox. And this is related to how these bones were not buried in the place their former owners once lived. Only a very few lucky parts made it into the fossil record.

Most trace fossils, on the other hand, have a great advantage in that most are not transported. When you see a dinosaur track, it is exactly where that dinosaur stepped on the ground, and when you see a fossil crayfish or insect burrow, it is likewise right where those crayfish or insects were living. The same goes for root trace fossils. If any of these were preserved here, I could say that plants were definitely living in the same places we are walking across.

But now think about these polar environments and how fossil preservation would be affected. This area had a series of rift valleys, which meant high, hilly areas with lots of rocks and steep slopes, and frozen landscapes in the winter. What happened in the spring, when everything thawed out?

You guessed it: torrential run-off of melted ice and snow. And that’s what these rocks show, which are very high rates of flow. Entire trees and lots of other plant parts were transported downstream, along with much more rare bones. I imagine raging rivers here 115-120 million years ago, choked with mud, sand, pebbles, cobbles, and boulders, that scoured deeply into the landscape each spring.


Boulder embedded in sandstone at The Oaks, representing the strength of the Cretaceous river that carried it during a polar spring. Tom Rich for scale.

Later in the polar summers, as these flows subsided, their sediments were dumped into the bottoms of river channels and onto floodplains. Only then could dinosaurs or other animals walk in through here, leaving tracks or donating their bodies to the fossil record for future research. Then came winter, with little to no flow of water and sedimentation, followed in the spring by great eroding floods, erasing body and trace fossils that might have been preserved. And for many of the tracemaking animals, they would not even be living here in the first place to make their traces. So there would be nothing to preserve, anyway.

So for upcoming days of The Great Cretaceous Walk, we will pray to the Goddess of Taphonomy to grant our wishes for more bountiful examples of body and trace fossils. With this hope in mind, we will begin looking at the westernmost outcrops of the Strzelecki Group.