For example, Austrobaileya scandens, a perennial flowering vine that composes part of the shady undergrowth in the rainforest, is the only species representing its clade worldwide. To see any of its relatives, you would have to look at the fossil record; its pollen closely resembles that of the oldest known flowering-plant pollen in Australia, dating from the Early Cretaceous at about 120 million years old. Unlike many flowering plants, flies—not butterflies or bees—pollinate its flowers, which is better understood when you sniff its broad, light-green flower and recoil from the rotten-meat smell. From these flowers, it produces plum-sized fruits with about a dozen large seeds in its middle covered by only a thin layer of fleshy goodness. Nonetheless, this fruit is on the extensive menu of cassowaries, hence this plant probably depends on them to carry its seeds farther away from where they might just fall off the vines. No doubt other local birds—including bowerbirds, fairy-wrens, honeyeaters, and riflebirds—also help with seed dispersal in the forest, but none had the voluminous guts of cassowaries. Even on a dare, I wouldn’t swallow some of these fruits whole, yet cassowaries were doing it every day, transporting the seeds far and wide through the forest, while also helpfully depositing them with warm nitrogen- and phosphorus-rich fertilizer on top.
Earlier in the day, Chris and I explained to the students some of this basic information about the ecological importance of cassowaries, but also how these huge birds are endangered owing to a lethal combination of habitat degradation and premature deaths. With regard to the latter, while driving into the area that morning we saw many large yellow cautionary road signs saying “Cassowary Crossing” that also graphically depicted a car striking a cassowary. Young cassowaries are also vulnerable to predation by dogs, not being big enough to take on an aggressive canine, let alone a ferocious pack.
We were thus becoming filled with this academic knowledge that was being reinforced by reality, a most satisfying way to learn. So after walking another kilometer down the trail past more tracks and scat, everyone had relaxed, including me. Patches of sunlight reaching the forest floor surely contributed to our sense of ease. A cyclone that hit this area just a little more than a year before had ripped open parts of the canopy, illuminating the way. Otherwise, deep shade in the middle of the afternoon might have subdued our collective mood. Confident that our loud, boisterous group—with American accents, no less—had probably frightened off all wildlife for the day, I stopped paying attention to the songbirds in the rainforest canopy above and around us.
This was a mistake. Had I been eavesdropping on these avian conversations, they first would have alerted me to two people walking on the trail about a hundred meters behind us. These songbird scolds would have been followed by a brief moment of quietude, as a second large disturbance—which slipped into the forest, hid, and waited for our group and the couple to pass by—emerged and walked on the trail behind them. Instead, I only saw the couple come out of the woods and walk on a low, narrow footbridge over a stream our group had just crossed. I returned to idle chatter with a few of the students, all of us snapping photos of the gorgeous rainforest around us. We were clueless.
The low-frequency boom was subtle enough that at first I dismissed it. This was a lesson in observational blind spots, in which a lack of training in perceiving something new, but very real, can be quickly unnoticed or rejected. The second boom registered more firmly, though, and I realized it came from something alive, in the forest, and getting closer. A behavioral relic of my Cenozoic primate heritage was handily expressed as the hairs on my neck sprang up in direct response to a shot of adrenaline. Something was wrong, but I didn’t know what.
Nearly every dinosaur documentary that attempts to recreate dinosaur noises uses mammalian or avian voices, or a blend of animal calls that somehow manage to reach some familiar place in our expectations of what dinosaurs should sound like. Even in fiction, such as during the unforgettable scene of the first Tyrannosaurus attack in the movie Jurassic Park, this theropod’s petrifying roar actually was an amplified mix of sounds from an alligator, tiger, and elephant. Shaped by cinematic conditioning, we expect large theropods to roar or snarl, smaller theropods to hiss or growl, and herbivorous dinosaurs to moan or bay. They are almost never depicted emitting low-frequency booms just within the hearing range of humans. Yet that was exactly the voice used by the theropod that emerged from the rainforest that morning onto the trail and only about twenty meters behind me.
She was a beauty. Tall and resplendent with bright red and blue patches around her head and a mass of black feathers covering her torso, she moved with firm intention, exuding confidence. This was her rainforest, or at least her part of it. She let out another boom, which connected the sound to its producer and confirmed my previous feelings of trepidation.
It was then that I realized one of my students—Heather—was standing on the footbridge, blissfully oblivious that a dinosaur was behind her. I had already raised my digital camera to take a photo of the cassowary and the instantly replayed viewscreen included Heather in the foreground, wearing a big grin as she marveled at her surroundings, but with the cassowary in the background. Feeling a sense of responsibility, but while also wondering how many other professors have to worry about a theropod walking into their classrooms and confronting their students, I used an urgent stage-voice whisper to warn her.
“Heather, there’s a cassowary behind you!”
She smiled and replied, oh-so-cheerfully, “Are you joking?” (Her question reflected how, during the previous weeks of the program, I had earned a reputation among the students as a prankster. I have no idea how this happened.)
“No!” I said with what I hoped was enough gravitas to change her mood.
My tone must have worked, because Heather turned to look over her left shoulder and saw that, yes, indeed, there was a large adult cassowary walking toward her. Quite sensibly, she froze in place, and her expression changed according to this newly perceived situation, which I dutifully recorded by taking another photo. You have to admit, however perilous the moment might have felt at the time, “Before Cassowary Awareness” and “After Cassowary Awareness” photos might have some good comic potential later in my teaching, and I was not going to waste this opportunity for the future educational benefit of others.
Fortunately for both of us, the cassowary eschewed taking the footbridge, stepped to its left, and began wading into the stream. Like Heather, I stood still, but continued to take pictures, most of which were blurred because of my shaking hands. Ambling at an even pace through the shallow water, her head turning jerkily to glance at each of us, the cassowary kept parallel to the bridge. Her crossing took only half a minute, although it felt more like five. She strode smoothly out of the water and onto the bank, only about three meters away, and circled behind me. I rotated in place to watch her, because nothing in the world would make me turn my back on an adult cassowary while I was standing on her home turf.
By now, other students farther up the trail had seen her and excitedly spread the word, resulting in their dashing directly toward Heather and me, and with the cassowary in between us. This was not good. Despite my admonishments to approach more quietly, some ran squealing with cameras in front of them, unknowingly risking more than most paparazzi might while approaching Russell Crowe, Lindsay Lohan, or Russell Crowe with Lindsay Lohan. Fortunately, instead of charging, the cassowary continued to walk around me, decided she’d had enough of these noisy humans, and re-entered the stream on the other side of the footbridge. Awestruck, we all stopped to watch her go. She paused briefly in the stream to step around a downed tree on the opposite bank, and then slipped into the rainforest. The whole incident, from first detected boom to her disappearance, had lasted just a little more than three minutes.
Later, on the other side of the world and safe at home in Georgia, I looked at my photo collection from that memorable day. Amidst the out-of-focus photos was one shot that came out absolutely clear. It showed the cassowary in full body an
d color, her legs pulling through the water, swirling the filtered sunlight, with greenish rainforest reflections on the stream surface. I often come back to this photo and stare at it as a reminder that I do not need a time machine in order to track a living dinosaur. These dinosaurs are everywhere around us, here and now, and making traces.
We call them birds.
A Brief Introduction to Modern Dinosaurs, Their Evolution, and Their Traces
No matter where you live in the world, it is nearly impossible to avoid seeing newly made theropod traces. Given about 10,000 species of modern birds, then billions of individual birds, multiplied by the thousands of traces a single bird can make in a normal lifetime, it all adds up to a whole heap of traces. Go to the most inhospitable places on the earth’s surface, look long enough, and you are likely to find some sort of sign that a bird was there. Even urban environments are rich in bird traces, especially those where birds such as pigeons (Columba livia) and American crows (Corvus brachyrhynchos) have adapted and thrived. Bird traces thus add yet another, deeper dimension to our appreciation of their makers.
Bird traces include their tracks, cough pellets, beak marks—probe marks in sand, piercings in fruit, and drill holes in wood—as well as talon marks, nests, burrows, gastroliths, droppings, food caches, dust baths, and broken seashells, to name a few. Very broadly, most bird traces are related in some way to feeding and reproducing, but these and other behaviors are so incredibly varied and nuanced, it is well worth taking a second look at their traces for whatever insights they might supply.
The ubiquitous presence of birds and their traces throughout the history of humanity also led to their infusion into our lives as stories, idioms, and metaphors, even featuring prominently in world religions. For example, a Cherokee story tells how the Appalachian Mountains were made from the beating of a vulture’s wings against the earth; most Native American traditions also feature birds as important parts of their spirituality. The Ngarrindjeri people of Australia have a Dreamtime story about a rivalry between the emu (Dromaius novaehollandiae) and brush-turkey (Alectura lathami) that resulted in the emu becoming flightless and the brush-turkey having smaller egg clutches in its nests.
Ancient Egyptians revered a heron-like god, Bennu, as the creator of the universe, as well as the sun god Ra, who had a human body but the head of a hawk. Incans viewed Andean condors (Vultur gryphus) as representing a higher realm of being in a three-level universe. Both Hinduism and Buddhism mention Garuda (“eagle” in Sanskrit), a bird-like god that was particularly skilled at dispatching snakes. In Judaism, King Solomon understood bird languages and used this knowledge to help his people. The Quran mentions the Ababil, flocks of birds that protected Mecca against an invading army. Christianity features doves in many of its teachings, including its symbolism as the Holy Spirit.
In the secular world, millions of people worship birds every day by looking for, identifying, and marveling at them. Birdwatching is among the most popular of all outdoor activities in the U.S., enjoyed by about 60 million people. In terms of an unconscious homage to birds, nearly every rock concert in the southeastern U.S. since 1973 has witnessed someone at some point yelling the words “Free Bird!” In short, these dinosaurs of the everyday have become powerful and omnipresent symbols for much of humanity, interwoven with the biological and cultural development of our species.
As nearly every precocious five- to ten-year-old can tell us nowadays, birds first evolved from non-avian theropods in the Jurassic Period. Also, for nearly as long as we have formally studied dinosaurs, we have known about fossil birds near the time of that evolutionary transition. First discovered in 1861, Archaeopteryx lithographica, from Late Jurassic (150 mya) rocks of Germany, was long regarded as the oldest known bird. However, some paleontologists now regard Archaeopteryx as more of a non-avian dinosaur, and two Chinese fossils, Anchiornis huxleyi and Aurornis xui from earlier in the Jurassic (circa 160 mya), may be closer to a hypothesized “first bird.”
As far as bird trace fossils are concerned, the oldest suspected bird tracks come from Late Jurassic–Early Cretaceous rocks in various places from about 130 to 140 million years ago. Based on these and geologically younger tracks from the remainder of the Mesozoic Era—as well as the preservation of these tracks in rocks from river floodplains, lakeshores, and seashores—birds spread very quickly throughout the world and adapted to a wide variety of environments. For example, bird tracks are preserved in Cretaceous rocks formed in previously polar environments of Alaska as well as a few I recently co-discovered with colleagues in Victoria, Australia. These trace fossils and a few body fossils show that birds, within only 50 million years of evolving from non-flighted theropods, already had a worldwide distribution.
Explaining the evolutionary history of birds is a massive undertaking, and other people have written excellent, lengthy books on this topic, such as Thor Hanson’s Feathers: The Evolution of a Natural Miracle (2011) and Living Dinosaurs: The Evolutionary History of Modern Birds (2011), a volume with many authors and edited by Gareth Dyke and Gary Kaiser. Accordingly, I will not attempt to duplicate their fine efforts here. Instead, I will point out that soon after the extinction of the non-avian dinosaurs, avian dinosaurs filled ecological niches left behind by their ancestors, while also carving out new ones. Birds of the past also had their own awe-inspiring qualities worth knowing, some of which made traces we know of, whereas others we may discover some day.
For example, if you took a trip back in time to the Eocene of North America, you might meet up with Gastornis (formerly known as Diatryma), a flightless bird as tall as the largest known cassowaries. Mostly because of its size, but also because of its massive hooked beak, this bird was originally interpreted as carnivorous. It also lived when the ancestors of modern horses were the size of domestic dogs. Hence, paleontologists painted lurid scenarios of these birds eating horses, which artists then literally painted. Regrettably for people who enjoy the concept of a bird saying the Eocene equivalent of “I’m so hungry I could eat a horse,” no trace fossils, such as sliced or crunched little horse bones with beak marks, have confirmed this reputation for Gastornis. Owing to this lack of evidence, as well as recent research on this bird’s skull and its shearing capabilities, this rapacious image of Gastornis has softened over the years. Paleontologists now suggest its huge beak was better suited for large fruits and nuts or for scavenging. Still, an encounter with one of these birds surely would have inspired at least as much fear in a small mammal as running into a medium-sized theropod during the Jurassic.
Other birds of the past also echo feelings normally evoked by dinosaurs, or at least a few of their Mesozoic contemporaries such as pterosaurs. For an example of the latter, the largest flying bird known from the fossil record is the giant teratorn (Argentavis magnificens) from the Miocene Epoch (about 15 million years ago) of South America. This bird had an estimated weight of 75 kg (165 lbs) and a wingspan of 7 m (23 ft). If transplanted into the Mesozoic Era, it would have been bigger than most pterosaurs, including Pteranodon. On land, terror birds—known by paleontologists as phorusrhacids—lived in South America and the southern part of North America from about 60 to only 2 million years ago. These flightless birds ranged from less than a meter to about 3 m (10 ft) tall, the bigger ones fulfilling their nicknames as top-niche predators in their ecosystems.
Among these terror birds was the Miocene Kelenken guillermoi of Argentina, which possessed the largest skull of any known bird: 71 cm (28 in) long, just smaller than that of an Allosaurus. Again, think of what it was like to be a little mammal during the Miocene in Argentina, with Argentavis in the sky and Kelenken on the land: not so different than living in the Cretaceous, but with a few big mammalian predators thrown in for good measure, too.
Other huge birds that preceded humans during the Cenozoic included Dromornis and its relatives in Australia. Like the South American Kelenken that also lived during the Miocene, Dromornis stood about 3 m (10 ft) tall but was big-boned enough to
have supported about 500 kg (1,100 lbs) on its frame. Its relatives, dromornithids, thrived in Australia until only about 30,000 years ago, including Genyornis, a bird almost as large as Dromornis. This means that people who first colonized Australia about 50,000 years ago probably saw, interacted with, and ate these birds. Scientists still do not agree on whether the extinction of dromornithids was linked to human hunting, alterations of dromornithid habitats, or climate change. But die out they did.
Other colossal flightless birds managed to evolve on islands much smaller than Australia and well into the time of humans. For example, the elephant birds of Madagascar, which consisted of species of Aepyornis and Mullerornis, were as much as 3 m (10 ft) tall and weighed about 400 kg (nearly 900 lbs). These herbivorous birds evolved to great sizes while isolated from the rest of Africa, and until about a thousand years ago the only primates they had encountered were lemurs. Sadly, all elephant birds were extinct by the end of the 18th century, likely victims of a disastrous blend of human-introduced avian diseases, too-rapid changes to their ecosystems, and hunting. Surely another factor was that their eggs were hard to ignore for anyone tempted by so much easily obtained protein in one place, as these were about 150 times more voluminous than those of chickens. Almost nothing is known about their nests, but they definitely were not located in trees, which left their eggs quite vulnerable to any creatures with opposable thumbs that were able to easily carry them away.
Until just recently—that is, in historical times—hefty birds also lived on New Caledonia, Malta, Cuba, the Hawaiian Islands, and an island of the Fiji archipelago. New Caledonia, a Pacific island with a main landmass that was connected to Australia and New Zealand during the Mesozoic, had both Sylviornis neocaledoniae—a galliform bird, related to chickens, turkeys, and other ground-dwelling fowl, but standing 1.5 m (5 ft) tall and weighing about 25 kg (55 lbs)—and Megapodius molistructor, a megapode (“large footed” bird) that weighed about 3 kg (6.6 lbs), three times larger than any living megapode. Both birds were likely mound nesters, and fortunately some trace evidence supported this idea. Archaeologists at first thought huge earthen mounds on New Caledonia—measuring as much as 40 m (130 ft) wide and 5 m (16 ft) tall—were human burial grounds, similar to those made by Native Americans. The problem with this hypothesis was that the mounds lacked an essential component of human burial grounds: bones and artifacts. This is when scientists instead thought “really big avian traces,” linking these hillocks to Sylviornis and Megapodius as more likely tracemakers for these landscape-altering features.
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