Dinosaurs Without Bones (53 page)

Flowering plants do. The huge difference between “pre-birds” and “post-birds” for flowering plants was in rates of long-distance dispersal, which with the assistance of birds became hundreds of times faster and much more regular than relying on mere chance. Multiply these faster rates by the cumulative effects of generations of birds and angiosperms, and then add the effects of migrations to this equation. For instance, if Cretaceous birds started to move great distances annually, including over mountains and seaways that previously were barriers to plants and their seeds, the geographic spread of angiosperms would have accelerated dramatically. Birds aiding the long-distance dispersal of flowering-plant seeds—through eating fruit, carrying seeds, and defecating—must have transformed landscapes in an astonishing way over the last
half of the Cretaceous Period. This Mesozoic crap was evolutionary gold.

How do ingested seeds resist digestion? Most have a hard coat that enables them to pass through the harshest of acidic digestive systems unscathed. This even happens in the guts of alligators and crocodiles, some of which eat a surprising amount of fruit. The bonus for these seeds is that they temporarily stay in a warm, moist place—namely, an animal’s digestive tract—exit that place with a good amount of high-quality plant food on top, and grow up in a place different from where their parents lived. Birds make this happen more than any other animals. Granted, mammals do their part in playing the role of Johnny Appleseed, too, as a huge number of mammals are fruit eaters and very good at taking seeds to new places. For instance, fruit bats, justifying their common names, eat fruits, fly to other places, and defecate seeds covered with wondrous bat guano.

However, the big difference between birds and most mammals (including bats) is in their evolutionary histories. Throughout much of the Mesozoic Era, mammals were subordinate to dinosaurs in their shared ecosystems, and as far as we know no mammals evolved powered flight until nearly 15 million years after the end of the Cretaceous. On the other hand, the earliest birds that evolved from non-avian dinosaurs likely witnessed the first flowers. Although flying insects were also around then, they played more of a role in eating other plant parts and pollinating; in most instances, they would have been too small to carry seeds elsewhere. In other words, plants and birds have had a much longer time to get to know one another in an evolutionary sense, and this co-dependency is so deeply rooted that mammals have only added to the apple cart, not yet upsetting it.

Like all co-dependencies, though, a dark side emerges when one asks: What if I am a bird who does not carry out a plant’s wishes? For plants, revenge is a dish best served fruity, as some flowering plants discourage seed eating by poisoning animals that dare to digest their seeds. For instance, apples and cherries are perfectly fine foods for humans and many other animals. But do not chew
and swallow, say, a cup of apple seeds: every seed contains a small amount of cyanide, a very nasty toxin that interferes with oxygen absorption. Likewise, cashew nuts, which are the seeds inside the fruits of cashew trees (
Anacardium occidentale
), have poisonous shells, so every nut must be extracted from its shell before enjoying them. Hence, flowering plants used a “carrot and stick” approach in their co-evolution with birds. First, reward animals that eat your fruit, carry your children to a new, far-away land, and “plant” it with droppings there. Second, punish animals that try to take their hunger one step further by eating your children.

The role of birds in specially delivering plant seeds to novel places is now well documented, particularly for islands. Charles Darwin even thought of this, as he wondered how the isolated Galapagos Islands off the coast of South America had managed to acquire such thriving plant communities. Ocean currents and winds—which can transport seeds long distances—certainly played a role. But this assumes too much: after all, not every seed floats, nor do all seeds survive being immersed in salty water for long journeys, nor does each seed stay aloft once airborne. They needed help, and birds stepped in to do this, probably starting in the Cretaceous.

Yet birds did not just ferry about the potential offspring of plants to islands and other places: they also carried animals. Given this thought, it might be tempting to conjure an image of an avian airlift in which squadrons of cranes and storks on a continental landmass each grasp a small mammal or lizard, and one by one fly them to the nearest island and drop them off. They would then repeat this over generations, but taking these vertebrates to different islands, and farther afield. Assuming that each payload included both males and females of those animals, the islands eventually all got colonized. End of story.

Well, not quite. If birds want to go long distances without exhausting themselves, they are much better at handling very small passengers, ones they do not even notice as stowaways. These hitchhikers also can ride in larger numbers, which improves their chances of reproductive success when they arrive at their new destinations. Animal immigrants could include members of these
birds’ microbiomes, such as lice and other skin parasites, but other accidental tourists—such as snails, larval insects, or larval crusta-ceans—also can attach to bird feet. The best feet for these animals to latch onto are webbed ones, which have the most surface area; moreover, webbed feet tend to step into environments with lots of aquatic larvae. This means that ducks, geese, seagulls, pelicans, and other birds with webbed feet (palmate and totipalmate) are among the best candidates for taking off with the highest number of inadvertent travelers. The likelihood of this scenario is further improved if those bird feet step into mud, which acts as a temporary glue for sticking seeds and tiny invertebrates onto their pilots.

Based on fossil tracks from Korea, we know that palmate bird feet had evolved by about 120
mya
(Early Cretaceous). Moreover, webbed bird tracks became more common and bigger throughout the rest of the Cretaceous. This meant that more birds evolved to shoreline habitats, and hence were more capable of picking up little invertebrates on their feet, taking them to ecosystems and places their ancestors had never before experienced. All of this implies that perhaps the largest living traces of dinosaurs, and ones we still live with every day, are these bird-assisted patterns of biogeography.

Darwin, Hitchcock, and the Dinosaur-Bird-Trace Connection

The idea of modern avian dinosaurs and their predecessors dispersing both plants and animals seems so brilliantly modern, one might wonder how scientists figured this out. Perhaps they tagged birds with GPS chips and then tracked their movement in real time with satellites, mapping and otherwise analyzing their routes with computers. Even better, the invertebrates were probably identified from afar by scanning the birds with lasers, and the scans were then converted to 3-D images, enlarged, and reproduced on a 3-D printer. Or maybe the researchers used other high-tech tools, all of which made science reporters giddily compare these to something they once saw on
Star Trek
, regardless of whether it was the original series,
The Next Generation
,
Deep Space Nine
, or
Voyager
. (We will not speak of
Enterprise
.)

But if you ever play a word-association game and the words “brilliantly modern” are referred to a concept associated with evolution and island biogeography, it is best to just answer “Darwin.” Yes, that’s right, Charles Darwin thought of flighted birds taking both seeds and small invertebrates to new homes, and repeating these actions over many generations. He even set up a few experiments to test this idea, none of which involved using GPS-enabled devices, satellites, computers, lasers, 3-D printers, or emergency medical holograms. For one experiment, he simply dipped duck feet into a pool of water with aquatic snails, watched them crawl onto the feet, then took the duck feet out of the water to see how long the snails stayed attached. In short, he just observed, questioned, tested, observed more, and then wrote a carefully worded conclusion based on the preceding. From such methods, he not only devised some of the most important tenets of modern evolutionary theory, but also explained how flowering plants, freshwater invertebrates, and marine invertebrates were able to travel to faraway places through the power of birds.

What Darwin did not know, though, was that through his study of birds and their long-distance movement of seeds and animals, he was also studying the ichnology of dinosaurs. In the late 19th century, Darwin was aware of the few Jurassic and Cretaceous non-avian dinosaurs that had been discovered in the United Kingdom and elsewhere, as well as the Late Jurassic
Archaeopteryx
from Germany. But he did not know, nor could he have suspected at the time, that birds are dinosaurs.

Interestingly, fossil theropod and ornithopod tracks were already known during Darwin’s lifetime, but their makers had been misidentified. For instance, in 1845 he corresponded with ichnologist Edward Hitchcock of Amherst College in Massachusetts, who surmised that Late Triassic and Early Jurassic dinosaur tracks from the Connecticut River Valley were oversized bird tracks. In that letter to Hitchcock, in which Darwin refers to these dinosaur tracks as “footsteps,” he mused about the meaning of these tracks with relation to birds:

In my opinion these footsteps (with which subject your name is certain to go down to long future posterity) make one of the most curious discoveries of the present century & highly important in its several bearings. How sincerely I wish that you may live to discover some of the bones belonging to these gigantic birds: how eminently interesting it would be (to) know whether their structure branches off towards the Amphibia, as I am led to imagine that you have sometimes suspected
.

Had both Hitchcock and Darwin lived to today, they would have been disappointed to learn that this wistful hope of finding bones of those “gigantic birds” (dinosaurs, as it were) would have been largely unfulfilled. As of this writing, despite continued discoveries of theropod and ornithopod tracks, almost no dinosaur bones are known from the Connecticut River Valley. Furthermore, body fossils of birds are unknown from rocks older than the Late Jurassic anywhere in the world.

Nonetheless, even though Darwin and Hitchcock missed the connection between dinosaur tracks, the evolution of birds, and biogeography, they each in their own way linked these seemingly disparate subjects. What Hitchcock got right, but without knowing it, was how the close resemblance of theropod tracks to those of modern birds did indeed reflect their evolutionary relatedness. What Darwin also got right was how birds changed the face of our modern ecosystems. Darwin had also very likely seen tracks of rheas (
Rhea pennata and R. pennata
) during his travels in South America, which closely resembled the fossil tracks Hitchcock described. Most important, though, both of their initial studies cleared intellectual trails, on which subsequent generations of biologists and paleontologists, likewise making careful observations of tracks and other traces, led to broader and more eclectic views of how the earth’s surface has been and will continue to be shaped by the traces of dinosaur behavior.

Regardless of how quickly our technological tools might serve us in such endeavors, the preceding methods and concepts can
be done and understood by using the greatest scientific tools we already possess—our senses and cognition.

Dinosaurs Without Bones: Trace Fossils of the Future

So now that we know that we are living with dinosaur traces, both ancient and modern, we can also better appreciate the role of individual dinosaur trace fossils in our understanding of these iconic animals. But what then to do with this ichnologically bestowed enlightenment, a newly realized super-power that allows for recognizing how dinosaur trace fossils superbly augment and oftentimes surpass the paleontological worth of dinosaur bones in interpreting dinosaur behavior?

Obviously, we must apply what we’ve learned about dinosaur trace fossils of our paleontological past to those of the future. With that goal in mind, here are words of advice and predictions, given with full awareness that my own knowledge of dinosaur trace fossils, much like a landscape occupied by dozens of small burrowing ornithopods, is full of holes.

We’ll find more dinosaur trace fossils, and they’ll be different.
One of the most excitedly received dinosaur books in recent years was
All Yesterdays
(2012), coauthored by paleontologists Darren Naish and C.M. Kosemen and co-illustrated by paleoartists John Conway and Scott Harman. Although only 100 pages long, and with its illustrations taking up more space than its words, its inventive artistic renderings and descriptions of unexpected behaviors in dinosaurs—such as tree-climbing ceratopsians and mud-wallowing sauropods—wowed many of its fans. (The book’s subtitle very easily could have been
Dinosaurs Gone Weird
.) That book’s popularity then led to a sequel published in late 2013, titled
All Your Yesterdays
, bearing contributions by a variety of artists in which they portrayed yet more unconventional dinosaur behaviors. This sort of cooperation between paleontologists and visual artists—including those who create computer-generated imagery—points toward a potential fountain of creativity that can expand our perception of dinosaurs as real, living animals.

The main point of these books was to promote thinking a little differently about dinosaur behaviors. In that spirit, I will now take it a step further in this book and ask us to think differently about trace fossils that could be made by different behaviors and different dinosaurs. Furthermore, because dinosaur trace fossils are so much more common than their bones in most places, finding odd trace fossils should be easier than finding bones of odd dinosaurs. Sure, the usual theropod, ornithopod, and sauropod tracks will continue to be discovered nearly every day. Yet I have a more ambitious wish list of trace fossils, some of which I sincerely hope will be uncovered by ichnologically adept paleontologists in upcoming years. These are a few of my favorite things: