Dinosaurs Without Bones

by Anthony J. Martin

  Of course, many small songbirds bathe in what we regard as a conventional way by using water. In such instances, the bodies of water used for bathing might vary from puddles to ponds to lakes to oceans. Anyone who owns a birdbath or has watched songbirds around puddles knows how they partially immerse themselves, and then shake their bodies to ensure that all of their feathers get wet. Although the body of water itself will not preserve traces of this activity, moist mud or sand on their banks and shallow bottoms might record a curious series of tracks, perhaps with splatter marks on emergent areas next to the water.

  So now think of, say, a 1.5-ton Gigantoraptor from the Late Cretaceous, one perhaps rejected by a potential mate. In an attempt to better its health and appearance, it decides to take a dust, sun, or water bath. Now envisage the marks these respective behaviors might have produced. A Gigantoraptor dust bath would be a relatively shallow semicircular structure, but enormous compared to those of all modern birds, probably more than 5 m (16 ft) wide. Linking this wallow to a large theropod could be tough, but doable if it also rolled its neck and head on the ground outside of the main depression, giving more anatomical clues. In contrast, sunbathing would have left body, wing, and tail impressions, with some movement blurring their outlines but not as much as in a dust bath. A water bath would perhaps show a series of walking tracks going from less saturated to more saturated (gooey) sediments, a stopping pattern, then lots of tracks in a small area caused by shuffling of the feet; water droplets on the shoreline and on top of the tracks there would complete the picture. Hopefully this mental exercise serves as yet another example of how modern bird traces can act as predictors for what trace fossils might be out there waiting to be recognized, and the behaviors that might be divined from them.

  Bird Intelligence and Tools as Traces

  At one time, the phrase “bird brain” was an insult hurled at someone perceived to lack common sense. Nowadays, it should be taken as a compliment. Paralleling the extreme makeover for dinosaurs showing they were not slow reptilian-like dullards, research on bird behavior in the past thirty years or so has completely changed our view of birds as simplistic automatons obeying their genetic codes. Instead, we are increasingly seeing birds more as sophisticates with their own complicated individual and social lives, language, tools, and even culture, in which parent birds actually pass down information to their young.

  Nevertheless, if you are skeptical about the term “bird intelligence” and need examples, here are a few. How about recognized gradations of bird language, in which birds can tell one another through alarm calls that a possible threat is coming from above (a hawk) or on the ground (a fox)? This was verified with chickens (Gallus gallus), which are often impugned as the dumbest of all birds and thus deserving of roasting pans. Not complex enough behavior for you? Okay, how about when birds inform one another that not only is a human approaching but a specific individual who harassed them several years before? Or that they then teach this to their children? This sort of learning, recall, and teaching ability has been documented in American crows. Other birds that can learn individual human faces include pigeons, magpies (Pica pica), and northern mockingbirds (Mimus polyglottus); the latter can learn to associate specific faces as threats within about thirty seconds. Still not impressed? How about superb fairy-wrens (Malurus cyaneus) giving their chicks a “password” (a single note) as a cue for feeding, but while the chicks are still snugly inside their eggs? Researchers who verified this found that the fairy-wrens had likely evolved this behavior as a defense against feeding the hatchlings of cuckoo birds.

  Given these samples, it should not be such a stunning revelation that birds are also among the few animals that use and make tools. Nonetheless, when this was first documented starting in the 1960s, it was an eye-opener for behavioral biologists. After all, the conventional wisdom at that time was that only primates used or shaped implements from their environments to accomplish a task. Although bird tools are oftentimes quite understated as traces (how many people can tell whether a wren used a cactus spine to pry out an insect?), it is still good to know about these and add them to their bird-trace checklists.

  Tool-using birds include, at a minimum, New Caledonian crows (Corvus moneduloides), woodpecker finches (Cactospiza pallida) of the Galapagos Islands, Egyptian vultures (Neophron percnopterus) of Africa, bristle-thighed curlews (Numenius tahitiensis) of a few Pacific islands, the palm cockatoo (Probosciger aterrimus) of Australia, brown-headed nuthatches (Sitta pusilla) and burrowing owls of North America, and various herons, egrets, and seagulls. Of these birds, New Caledonian crows are the most impressive of tool users and problem solvers. These crows carefully choose or shape sticks, leaves, or feathers into practical objects, which they use to acquire food. In laboratory settings, they have even used metal hooks provided by researchers, and sometimes bent these into usable shapes. Woodpecker finches latch on to cactus spines or twigs with their beaks and then manipulate these like fine surgical tools to extract insect larvae from tight spots in trees. Like New Caledonian crows, they also may modify these items—such as shortening them—to improve their utility. Egyptian vultures and bristle-thighed curlews share the practice of grabbing a rock with their beaks and throwing these at eggs to break them open. These vultures do this to ostrich eggs, whereas the curlews crack albatross eggs; intriguingly, the vultures seem to pick rocks that are egg-shaped. Male palm cockatoos grasp sticks in their beaks, which they then drum against trees to alert females that they are in the area, echoing the mating habits of beat poets who used bongos in 1950s coffeehouses. Brown-headed nuthatches acquire short pieces of bark or sticks, which they lever against the bark on tree trunks to expose insects. Perhaps my favorite tool-using bird, though, is the burrowing owl. These actually use mammal feces as a tool, by picking up pieces of bison or cattle dung, placing these in front of their burrows, and waiting for dung beetles to arrive, which they then happily devour. Lastly, a few species of herons, egrets, and seagulls go fishing by employing feathers, berries, and even bread as lures. They either dangle these items from their beaks above a water surface or drop them onto the water to attract fish, which they then nab.

  So do birds have memes—culturally acquired behaviors that modify over time—as well as genes? Although acquired knowledge and tool use is certainly shared among contemporary peers and with offspring in some birds, behavioral biologists still have not verified that this knowledge is being passed down or changed over many generations. Nonetheless, our awareness of bird culture and tool use lends to some fun (and not so crazy) speculation about whether or not Mesozoic theropods used tools, had their own way of communicating life-saving information to one another, and whether any of these resulted in trace fossils. Of course, the likelihood of recognizing tools or related traces would be minuscule, unless paleontologists found them in the mouth or hands of a dinosaur, or they noticed an appropriately sized and shaped rock sticking out of the side of a dinosaur egg. Nevertheless, all of these insights on bird behavior make us look at dinosaurs with a little more imagination, allowing us to wonder how many of their behaviors might have been shared with birds’ Mesozoic ancestors and contemporaries, and their traces.

  Thus while keeping in mind bird traces as a bridge to understanding dinosaurs of the past, as well as a few examples of how birds as modern dinosaurs affect the world today, it is time to think on a grander scale with our ichnological perspectives. In the next chapter, we will explore how some dinosaur traces probably changed Mesozoic landscapes, as well as how bird traces are currently changing our landscapes. Indeed, in some instances these changes may have even affected the evolution of modern ecosystems, implying that dinosaurs left a lasting imprint that still surrounds us and likely will persist well into our future as a species.

  CHAPTER 11

  Dinosaurian Landscapes and Evolutionary Traces

  The Living Trace Fossil

  We all live in a dinosaur trace fossil. It’s not a trace fossil in the conventional sense,
like an ankylosaur trackway in Bolivia, a pro-sauropod nest in South Africa, toothmarks on a bone from Canada, or a sauropod coprolite in India. Nor is it a trace made by modern cryptic dinosaurs, which no doubt will be reported breathlessly by actors playing scientists on a made-for-TV “documentary.” It’s not something more human-made, either, such as the worldwide cultural trace of dinosaurs related to their enduring and multigenerational popularity. Instead, the trace fossil is infused in the totality of terrestrial environments, what we sense from those environments, and even some of the earth resources we use. Where we live and what we do today is all somehow related to the former existence of Mesozoic dinosaurs and the continued presence of Cenozoic dinosaurs, birds.

  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 rightfu
l 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.