Dinosaurs Without Bones (44 page)

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.

Other sizeable birds on islands included: the giant swans of Malta (
Cygnus falconeri
); the moa-nalo, consisting of four species of ducks in the Hawaiian Islands; the Viti Levu giant pigeon (
Natunaornis gigoura
), which was on one of the Fiji islands; and the giant cursorial owl (
Ornimegalonyx
sp.) of Cuba. All of these birds were flightless, and as one might guess from the adjective “giant” applied to their common names, they were significantly larger than any of their living relatives; for instance, imagine a 1.2 m (4 ft) tall owl prowling the forests of Cuba. All of these birds had something else in common, which was their rapid extinction soon after humans came in contact with them, found them delicious, introduced egg predators, and changed their habitats. Fortunately, traces again have helped fill in a few details about the behaviors of these vanished birds. For instance, moa-nola diets are known through their coprolites, which showed these were important grazers in Hawaiian ecosystems that lacked mammals as herbivores.

Although all of these recent birds were impressively sized, none were potential predators of people. So perhaps the closest situation comparable to
Jurassic Park
, in which theropods could have preyed on people only half their size on a remote island, was during the Pleistocene Epoch and on the island of Flores, Indonesia. Bones from about 12,000 to 100,000 years ago there include remains of both the largest known stork (
Leptoptilos robustus
) and a diminutive species of hominin (human relative),
Homo floresiensis
, nicknamed the “hobbit.” The size disparity between these two species was like that between
Utahraptor
and a modern
Homo sapiens
. The storks were just shy of 2 m (6.6 ft) tall, whereas adult
H. floresiensis
stood only a little more than 1 m (3.3 ft) off the ground, and stork tracks were probably twice as long as the hominin tracks.

Also, storks, despite being associated with heart-warming fables of delivering babies, are voracious carnivores. Hence, these big storks, rather than fulfilling fables, would have been more interested in consuming warm hearts. So far, though, no one has noted any trace fossil evidence of storks preying on or scavenging

H. floresiensis
, such as beak marks on bones or coprolites holding the remains of furry feet. More likely menu items for these storks would have been rodents of unusual size, such as the Flores giant rat (
Papagomys armandvillei
), which was more than double the length of the biggest urban rats.

However, Pleistocene trace fossils from South Africa do show evidence of a bird attacking a small hominin. These consist of holes in the skull of the “Taung child,” which belonged to a juvenile
Australopithecus africanus
. At first interpreted as leopard toothmarks, these holes are now regarded as beak and talon marks from an eagle, neatly matching traces left on modern monkey bones by African crowned eagles (
Stephanoaetus coronatus
). Some paleoanthropologists even speculate that frequent eagle attacks would have selected for more cooperative behavior and larger body size in hominins, thus deterring these predators and contributing to greater stature.

As one might have figured out by now, islands do funny things with the evolution of birds, especially related to body size.
Paleontologists and biologists have noted two seemingly opposite trends: smaller animals that colonize islands tend to get larger over time, whereas larger animals get smaller. Both can be summarized as the “island rule,” although this can be split into “island gigantism” and “island dwarfism,” respectively. Given the right ecological conditions and enough time, natural selection on an island can drive an animal’s lineage down either path. For instance, if what we would consider today as “normal”-sized swans, ducks, pigeons, or owls had flown to islands during the Pleistocene, were genetically isolated from others of their species over several hundred generations, had no natural predators, and plenty to eat, then their descendants might have become much larger. However, with great size came great responsibility: to gravity, that is. Part of this natural selection toward gigantism in birds would have reduced any energetically expensive traits, such as wings. Consequently, wings shrank as weight and height went up in these island birds, rendering most of them flightless.

Speaking of islands, if you’ve ever fantasized about ones where dinosaurs continued to evolve after the Cretaceous, then look no further than New Zealand. Back in the Cretaceous, plate-tectonic shifting caused the main landmasses of New Zealand to split from eastern Australia, taking its dinosaurs with it. However, very few mammals came along for the ride, so geographic isolation combined with the end-Cretaceous extinctions to favor the survival and evolution of birds over non-avian dinosaurs and mammals. As a result, New Zealand experienced a grand evolutionary experiment, answering the question: What would have happened to dinosaurs over the last 65 million years if there had been no pesky mammals competing with them? More important, as an ichnologist I might also ask what sorts of unique traces might have been made as a consequence of such evolutionary innovations?

The variety of birds and the ecological niches they filled in New Zealand were remarkable. For example, before humans showed up about eight hundred to a thousand years ago, Haast’s eagles (
Harpagornis moorei
) were the top carnivores in New Zealand, predatory theropods
that delivered death from above to other large theropods. Like some other birds, female eagles were larger than males, weighing about 12 to 15 kg (26–33 lbs) and with wingspans of 2.5 to 3 m (8–10 ft). Although the biggest eagles today have comparable wingspans, they are all less than half the weight of Haast’s eagle. And just what other large theropods were these eagles hunting in New Zealand? Moas, which were among the most famous and diverse of recent avian dinosaurs that remind us of non-avian dinosaurs.

Fortuitously enough, paleontologist Sir Richard Owen—who coined the word “dinosaur”—first studied these fabulous flightless birds in the early 19th century after British explorers brought moa bones to the U.K. Since then, paleontologists figure there were about nine species, which ranged from 1 to 3 m (3.3–10 ft) tall. Moas were grazing and browsing herbivores, basically filling the ecological role of ornithomimids, ornithopods, small ceratopsians, and other mid-sized Cretaceous dinosaurs in New Zealand, yet were alive when Geoffrey Chaucer wrote
Canterbury Tales
. Considering their sizes, abundance, and wide geographic ranges, moas must have worn deep trails throughout the landscapes of New Zealand and left easily visible browse lines along forest borders. Thanks to their coprolites, we even know what plants they ate and in which ecosystems, described in a 2013 study done by Jamie Wood and other scientists. Like the elephant birds in Madagascar, though, they fell victim to the ways of humans, and were extinct by the end of the 15th century.