Dinosaurs Without Bones (49 page)

With regard to resources and a cultural presence of dinosaurs, in the 1960s I grew up hearing the statement “Oil is made from dinosaurs.” Sinclair Oil Corporation encouraged this illusion by sponsoring dinosaur exhibits at the Chicago and New York World’s Fairs in 1933–34 and 1964–65 respectively, in which they overtly connected “dinosaurs” and “oil” in the public mind. (As a legacy of the 1964–65 Fair, statues of
Tyrannosaurus
and
Apatosaurus
constructed for it can still be seen in Dinosaur Valley State Park, Texas, less than a kilometer from real Early Cretaceous theropod and sauropod tracks.) Sinclair even adopted a green
Brontosaurus
as a symbol of its company, using this logo on service station signs and in magazine ads, while also selling plastic dinosaurs at their service stations. The plastic, of course, was also partially made of petroleum, which in retrospect seemed as if Sinclair Oil was into recycling long before it was hip.

Naïvely, I accepted the adage “dinosaurs make up oil” as true until a few science classes in college—particularly those in geology—straightened me out. It turns out that nearly all petroleum is from algae, most of which were deposited and buried in marine environments; no dinosaurs contributed their bodies to the original organic matter, and they had no role in helping to bury it, let along mature the organic compounds sufficiently that these later became oil and gas deposits. Indeed, some of the most prolific petroleum reservoirs in the world are filled with oil that post-dates the end-Cretaceous extinction of dinosaurs. Given all of these revelations, I had learned a lesson in not blindly accepting popular assumptions no matter how much we want to believe them, and to beware of the power wielded by smart, pervasive advertising.

Yet it was not until I became a geologist, paleontologist, and ichnologist that my perspective started coming back to this childhood thought and I wondered how, in some small part, it could be justified as true. Sure, dinosaurs did not directly contribute their remains to petroleum reserves. My mind is not going to change on that point. Furthermore, some petroleum deposits definitely formed millions of years after the last of the non-avian dinosaurs
had left their traces. But did dinosaurs somehow change environments globally so that algae—which did contribute their bodily remains to oil—became more prolific in the world’s oceans during the Mesozoic Era? Did they alter their local environments so that rivers changed their courses, which affected the locations of river deltas where many oil reservoirs are located? Did dinosaurs affect the evolution of terrestrial ecosystems and their organic productivity so much that marine ecosystems were impacted by these landscapes, thus affecting what happened in ocean waters, shallow and deep?

Up until now, we’ve learned that dinosaur ichnology applies to dinosaur trace fossils like tracks, nests, burrows, gastroliths, toothmarks, and coprolites, ranging in scale from two-meter-wide sauropod tracks to microscopic scratch marks on dinosaur teeth. Yet dinosaur ichnology also could be expanded to a more global view. Going back to a basic definition—that a trace is any indirect evidence of behavior aside from body parts—this concept can be taken further. For instance, to use a well-documented phenomenon, global climate change today is largely a human-caused trace. Did dinosaurs affect the world in a similar (albeit non-industrial) way? Could it be that the burning of fossil fuels today is really a composite trace, one that would not be happening if it were not for dinosaurs changing the earth to one conducive for making those fuels?

Maybe not. But let’s explore anyway. The worst that will happen is to learn something new, while also expanding our perspectives by considering how dinosaurs may have been the original “ecosystem engineers” of terrestrial environments, altering them in ways that never would have happened without them and their behaviors and resulting traces. We will also take a look at how these alterations constitute dinosaurian traces that still affect us in significant ways today, and how these traces will continue to influence our future.

That One’s Going to Leave a Mark: Dinosaur Trails and Their Effects on Landscapes, Rivers, and Ecology

I’d seen plenty of large sauropod tracks in the western U.S. and parts of Europe, but never ones this big. I tried to informally
measure a few of the larger ones by making a circle with my arms above them. But my hands were always wide apart, making only semi-circles. Had I been doing ballet, I would have failed to complete the first position
bras au repos
, meaning the tracks were well over a meter wide. Once recognized, they were easily visible along the seashore as shallow rounded or oblong pits in the reddish Cretaceous sandstone exposed there. Once my wife Ruth and I picked out a few as search images, hundreds revealed themselves, accentuated by indirect light as the sun began to set over the ocean. It was a dinosaur-trampled mess, and a glorious one.

Although the marine platform was heavily eroded, a few of the flat sandstone bedding surfaces were continuous enough for trackway patterns to emerge. With one, a sauropod had made a “narrow gauge” diagonal-walking trackway, and one where its rear feet stepped directly on top of its front footprints. In other places, though, tracks were paired and closely spaced, either offset or overlapping. These were front- and rear-foot impressions, with the offset ones reflecting an understep (slow walking) pace. We could even see some of the sauropod tracks in vertical sections of the coastal outcrops. The normally near-horizontal layering of the sandstones had been distorted and contorted, showing where massive dinosaur feet had deeply compressed soft sandy layers about 130 million years before we were there. Sprinkled between the sauropod-made pits on the marine platform were three-toed theropod tracks. These seemed minute in comparison to the sauropod footprints, but were still 30 to 40 cm (12–16 in) long, indicating theropods with hip heights of about 1.4 to 1.6 m (4.6–5.2 ft)—big enough to stare us in our faces had they come back to life just then.

It was May 2009, and Ruth and I were on vacation in Broome, Western Australia. We had just finished a week of field work in Victoria, and to celebrate we were fulfilling one of the items on our Australian checklist, which was to visit Broome. Although it’s a long way from anywhere else, friends told us that it was a lovely place to visit, with a gorgeous beach, art galleries, cultural tours, and some quirky, unconventional touristy attractions such as an open-air
theater and camel rides on the aforementioned beach. What about the dinosaur tracks? Well, okay, as a card-carrying ichno-nerd, I have to admit these factored into our decision, especially once I learned the dinosaur tracks at Broome were only a few kilometers outside town and publicly accessible at low tide.

I first heard about these tracks at a scientific meeting, the first International Palaeontological Congress, which was held in Sydney in 2002. At this meeting, Tony Thulborn—introduced previously as one of the original paleontologists to study the Lark Quarry tracksite—gave a talk simply titled “Giant Tracks in the Broome Sandstone (Lower Cretaceous) of Western Australia.” The audience of 25 to 30 paleontologists attending his presentation was in for a treat. Along with some of the preliminary scientific findings—that the Lower Cretaceous Broome Formation held a huge number and variety of dinosaur tracks—Thulborn showed photographs of what were then known as the largest extant footprints made by any land animal in the history of the earth. Some of the sauropod tracks were nearly two meters across; I’d slept in beds smaller than these tracks. At the end of his presentation, he announced with rightful pride, “Mine’s the biggest!” (Just for context, he was talking about the tracks.)

Additional photos shown by Thulborn effectively communicated another point he wanted to make, which was that the dinosaurs—which were mostly sauropods, but also included some large theropods—had literally impacted their environments. Through sheer quantity of footfalls, as well as those footfalls coming from massive animals, the sauropods—and to a lesser degree the theropods—had altered the surfaces of their landscape enough to change the topography of their local environments. In 2012, Thulborn elaborated on that idea in an article titled “Impact of Sauropod Dinosaurs on Lagoonal Substrates in the Broome Sandstone (Lower Cretaceous), Western Australia.” In that paper, he provided evidence that the dinosaurs had stomped soft sediments along a lagoonal shoreline so much that they formed low-lying areas flanked by higher areas, like levees on either side of well-worn trails.

Furthermore, these trails may have been routes used habitually by the dinosaurs. Once established, they became paths of least resistance for moving about, as if they made their own highways. Photographs in Thulborn’s article showed huge sauropod tracks in depressed areas, but no tracks on the elevated areas on either side. The sandstones also lack plant-root trace fossils or other evidence of fossil plants, so it either was an already clear area for the dinosaurs to saunter through there or they denuded it by stomping plants into submission, while also compacting the soils, which prevented further plant colonization.

So imagine an Early Cretaceous shore next to a lagoon, with these deeply impressed trails running parallel to the average high-tide mark along that shore. With no vegetation along the way, the sauropods would have had a clear view, just in case they needed advance warning of the big predatory theropods waiting for them out there. Occasionally one or several of these theropods came down to the shoreline too, hoping to pick off a straggling sauropod for a big score, but for the most part they stayed away; this was sauropod country, and the uneven ground made ambush hunting and quick pursuits problematic. If seen from a pterosaur’s point of view, the coastal trails would have connected to more inland ones, criss-crossing the forested interiors and freshwater wetlands like a great spider web.

Such grand disturbances of pliable mud or sand, in which great numbers of overlapping footprints made by immense dinosaurs made trails or left churned messes in the geologic record, are sometimes called “dinoturbation.” I personally dislike this term, because it literally means “terrible [or awe-inspiring] mixing.” This handiwork, however, is not the exclusive domain of dinosaurs. After all, earthworms and ants also mix tons of sediment every day. This term also distracts from how a few modern vertebrates, such as elephants and hippopotamuses, are capable of doing their own awesome mixing of sediment, which we somehow manage to restrain ourselves from labeling “elephanturbation” or “hippoturbation.” Semantics aside, huge-sized dinosaurs, which sometimes
traveled together in herds like the proverbial ships passing in the night, would have left sedimentary wakes with their passage, massively disturbing and altering terrestrial and freshwater ecosystems wherever their feet landed.

As the largest living land animals, elephants are the first analogs ichnologists reach for when trying to estimate the potentially far-reaching ecological effects of dinosaur trails. Elephants consist of three species: the African bush elephant (
Loxodonta africana
), African savannah elephant (
L. cyclotis
), and Indian elephant (
Elephas maximus
). Of these, the African bush elephant (
L. africana
) is the largest, with males weighing more than 7 tons, but Indian elephant males can also reach 5 tons. Elephants of all three species travel extensively and migrate annually. They normally walk in groups led by an adult female (matriarch), although adult males will go off on their own to make their own tracks. Thanks to fossil trackways recently discovered in the United Arab Emirates which show a series of parallel and overlapping tracks (group behavior) crossed by one trackway (a lone male), we know elephants and their relatives have likely held these same behaviors minimally for the past seven million years.

These behaviors also imply that local vegetation is normally worn down and sediments compacted by groups of elephants, not individuals, and that once a path has been cleared, it will be used repeatedly, perhaps by generations of elephants. Elephants also need plenty of water, so they try to stay near rivers or ponds, which they often enter and exit to drink or bathe. These habits mean they wear down banks, form wide divots on those banks, and muck up water-body bottoms, especially if they start wallowing. Elephant trails can also form depressions deep enough for water to flow along them, creating canals that connect previously isolated rivers or ponds. Trails on riverbanks similarly allow easier passage for floodwaters to cut through levees and pour out onto floodplains, depositing sediment in what are called
crevasse splays
.

Modern hippopotamuses (hippos), despite being smaller than elephants—with adults weighing in at 2.5 to 4 tons—have an even
larger impact on their aquatic environments, which is where they spend most of their time. In a study by geologist Daniel DeoCampo published in 2002, he documented how hippos in Tanzania made a 30 m (100 ft) wide and 2 m (6.6 ft) deep muddy wallow pond, which connected to 1 to 5 m (3.3–16 ft) wide trails that imparted radiating and branching patterns onto the surrounding landscape. Hippos made these trails by frequently moving into and out of the wallow pond to feed on nearby vegetation; their activities, combined with their bulks, compressed and otherwise altered sediments. Most important, hippo trails actually changed the direction for water flow in the area, in which channels followed the trails, a type of channel abandonment called an
avulsion
. This channelization via hippo traces, in which their trails eventually turn into new river channels, is also well documented in the Okavango Delta of Botswana.

So did dinosaurs affect the courses of rivers with their trails, carving out new routes for flowing water through avulsions, or connect previously isolated water bodies? Given the known effects of much smaller modern large animals on rivers, the probable effects of individual dinosaurs that weighed 10 to 20 tons or more, herd sizes of these dinosaurs, and their geological longevity, I would be extremely surprised if they did not. If so, these effects may be detectable by geologists by more closely examining Mesozoic river deposits that also contain plenty of dinosaur bones and trace fossils—such as those of the Late Jurassic Morrison Formation in the western U.S.—for sauropod-width incisions in ancient river levees. They might also reexamine crevasse splays to see whether these connect to such divots, and whether these contain sauropod or other dinosaur tracks. In this respect, in 2006, two geologists—Lawrence Jones and Edmund Gustason—did indeed propose that avulsion features in the Morrison Formation of east-central Utah were likely caused by sauropod trails that created “channels” for the flow of floodwaters. Indeed, some former river-channel sandstones in the Late Jurassic Morrison Formation have dinosaur tracks on their bottoms, which may have been made by theropods or sauropods crossing rivers.