Dinosaurs Without Bones

by Anthony J. Martin

  Lastly, with regard to dung and its effect on flowering plants, also think “fertilizers,” and on an immense scale. As a hint of how dinosaur feces might have affected soils and plants during the Mesozoic, paleoecologists, in a 2013 study done on extinctions of large herbivorous mammals in the Amazon River Basin at the end of the Pleistocene Epoch (about 12,000 years ago), found that after these mammals died out, the soils there never quite recovered. The paleoecologists concluded that the magic ingredient missing from these soils was large-mammal excrement. The big herbivores—which included giant ground sloths, glyptodonts (armadillo-like animals the size of a compact car), and elephant relatives—had acted as agricultural agents, spreading the wealth (so to speak) and donating nutrients to soils wherever their droppings dropped. Also, because they were big animals, they traveled greater distances, meaning their dung was distributed far and wide. However, once this copious supply of enriched organic matter was gone, soils suffered, which accordingly meant plant communities grew less exuberantly and became less diverse.

  The main implication of this research was that the presence of large defecating herbivores was very important for maintaining plant communities during the Pleistocene in that part of South America. Furthermore, their long-time presence probably affected the evolution of angiosperms there, and few places in the world are more famous for their floral diversity than the Amazon basin. Likewise, whenever great poopers die out, one should also mourn for dung beetles, which die with them; this meant that insect biodiversity also declined with the demise of the megafauna.

  So now take this concept much further back into the geologic past, such as during the Cretaceous, and ponder the effects of herbivorous dinosaur feces on enriching nutrients in soils and plant growth. Although not all dinosaur feces survived to become coprolites, these traces surely contributed to the fruition of flowering plants, and thus the evolutionary legacy of flowering plants embodies these fecal traces.

  Scared Green? The Possible Effects of Predatory Theropods on Vegetation

  What about large predatory theropods, those poor neglected dinosaurs that almost no one seems to care about, nor remember? How did these big carnivores relate to the evolution of land plants, including flowering ones? Putting oneself in their places, thick vegetation either would have served as great cover for ambush predation or gotten in their way when chasing down their prey. But that’s thinking too small. When viewed from an ecological perspective, one should rather imagine how abundant, large, healthy theropods probably maintained river–floodplain plant communities and facilitated the growth of low-lying vegetation and forests wherever they lived. In other words, large theropods, such as Allosaurus, Acrocanthosaurus, Gigantosaurus, and Tyrannosaurus, might have been the original “green” dinosaurs, saving plants wherever they stalked, and hence helping the evolution of their ecological communities.

  This seemingly incongruous leap of logic is loosely based on recent research into the effects of apex predators on riverbanks (called riparian zones) and forest ecosystems, exemplified by wolves in the vicinity of Yellowstone National Park (Wyoming). Since the 1990s in Yellowstone, wildlife biologists and ecologists have examined the ecological effects of wolf reintroductions, in which wolf packs were put back in places where they had been locally extinct for a while. One of the most surprising results of these reintroductions was how riparian ecosystems in Yellowstone improved, with greater and more vigorous plant growth that approached their recent, pre-colonial state, when wolves naturally inhabited this area. Rapid stream erosion and flooding also lessened, both direct results of more plants growing along stream banks.

  What did this have to do with wolves, or even carnivorous dinosaurs? First, with regard to wolves, their favorite item on the menu—one that they will pick nearly every time if given a choice—is elk (Cervus canadensis). With no major threat from predators in Yellowstone ecosystems, elk ran wild (more so), overpopulating and eating much of the vegetation, including young tender plants along riparian zones. Reduced numbers and heights of plants along streams meant fewer plant roots holding down the soils, which led to accelerated erosion and flooding around Yellowstone streams, making it tougher for new plant growth to take hold. But once wolves were back in the neighborhood, salads that were once taken leisurely plummeted, riparian plant communities bounced back, and streams no longer lost so much sediment or flooded with quite so much ferocity.

  Part of this situation was because wolves killed and ate some of these gluttonous plant munchers, but they also managed to exert a sort of mind control over their prey. For instance, once wolf packs had hunted elk over several generations in this area, these herbivores restricted themselves to eating vegetation only in certain places, and skittishly, with the threat of death as a big motivator for not hanging out in any one place and browsing too long. This fear factor even caused elk to have smaller families, as the added stress of possible predation triggered hormones that decreased female-elk fertility. With fewer elk, and elk eating less, plant communities expanded and became more contiguous. This effect even helped wolves’ super-friends, grizzly bears, which then had lots more berries to eat during lean times. In short, wolves helped to change the ecosystems around and in Yellowstone National Park for the better, and as a result riparian plants there breathed a sigh of oxygen-laden relief.

  Now imagine this situation with theropods as the predators—whether as pack hunters, or carrying the biomass of a dozen wolf packs in a single body—and herds of big herbivores as prey. Transfer these same concepts to their Mesozoic ecosystems, in which certain predators kept certain herbivores in check, preventing them from staying in any one place and eating too many plants. Consider all of the healed bite marks, other toothmarks, gut contents, coprolites, and other trace fossils that tell us about predator–prey relations between dinosaurs at different times during the Mesozoic. Then multiply these trace fossils by millions to recreate what must have happened over more than 150 million years, and envisage the aggregate effects of predators on herbivores in their plant-filled ecosystems.

  Also contemplate how Mesozoic streams may have changed their dynamics—flow patterns, erosion rates, and flooding—according to a presence or absence of predatory theropods. Lastly, visualize how riparian plants, flowering or otherwise, then thrived and were more able to pass on their genes to future generations. If any or all of these scenarios happened, then these are additional subtle but large-scale dinosaur traces, ecological echoes of the interplay between dinosaurs seeking their respective sustenance.

  Worldly Traces: Birds, Pollination, Seed Dispersal, and Hitchhiking Animals

  As modern dinosaurs, birds have surely changed the world in small ways through their extremely varied behaviors and their resultant traces. Bird tracks, nests, burrows, beak probes, drillholes, cough pellets, gastroliths, feces, and tools certainly constitute bird calling cards, letting you know that individual birds have visited almost everywhere you look. Yet bird behaviors and their vestiges have also changed the face of terrestrial environments, resulting in the ultimate trace of their reign as Cenozoic dinosaurs.

  Glance at nearly any landscape, and then look more carefully for a flowering plant in it, which you will likely find without much searching. (Hint: all grasses are flowering plants.) Chances are good that a bird was somehow involved in the evolutionary heritage of that plant for at least the past 65 million years. Now think about how flowering plants range in habitat from seashores to mountains, from deserts to freshwater ponds, and from Arctic tundra to tropical rainforests. Also consider how flowering plants often dominate those ecosystems through sheer numbers, or play integral roles as keystone species: remove certain flowering plants, and some ecosystems become ghosts of their former selves.

  How did birds influence this situation, helping flowering plants to live almost everywhere on the land? Much of this world-altering activity came about through the special relationship between birds and these plants. Coincidentally (or not), early birds and flowering plants
expanded and diversified at about the same time, which was in the middle of the Cretaceous Period (about 100–125 mya). Although paleontologists cannot say for sure that birds helped with the spread and evolution of flowering plants during the last half of the Cretaceous, or that flowering plants aided bird evolution, or that a combination of the two happened, the ecologically tight relationship we see today between these two suggests that they did indeed co-evolve.

  To be sure, insects—especially pollinators, like bees, wasps, beetles, and others—were a big part of this picture, too. But once theropods, both non-avian and avian, began climbing trees and using powered flight, they must have also sought food resources in those trees, which surely included fruit. In some of the earliest studies done of this phenomenon, ecologists estimated that about half to 90% of all fruited trees of modern forests are adapted for birds and mammals to eat them. Similarly, dinosaurs, including birds, must have been powerful change agents, spreading flowering plants to places they never could have reached through other means. These actions even brought about changes that later benefited the evolution of tree-dwelling mammals, including those in our own lineage. Look at a friend or relative, then yourself, and thank a dinosaur for helping to shape the ecosystems that aided your shared ecological and evolutionary legacy.

  The way Mesozoic non-avian dinosaurs and birds accomplished this momentous task, which was carried on as an evolutionary tradition by birds throughout the Cenozoic Era, was through their poop. Very simply, flowering plants produce seeds covered by delicious and nutritious fruit. Birds are among the animals that eat these fruits, seeds and all, which they carry in their bellies. Birds then later deposit the seeds somewhere else, while helpfully covering them with a nice mix of nitrogen- and phosphorus-rich fertilizer.

  For an extreme example, recall the previously mentioned cassowaries of Australia and New Guinea, big flightless birds that eat the fruits of more than a hundred species of flowering plants and later dump the seeds of these plants, which are ensconced in voluminous piles of feces. Now apply this same thought to small, flight-worthy birds that ingest seeds in berries or other fruits, then fly away from those plants to drop seeds tens of kilometers away. No big deal, you might think: gravity would have done the same thing, through fruit just falling off plants, rolling a little bit downhill, or perhaps was aided by wind or water, which, after tens of millions of years, would have very gradually extended the geographic range of those plants. The same would have happened with islands, in which ocean currents or storms would have given these seeds a one-way ticket to a new home. Who needs birds?

  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 likel
ihood 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.)