Dinosaurs Without Bones (52 page)

Similarly, another review done in 2008 by Graeme Lloyd and eight other paleontologists examined how what they called the “Cretaceous Terrestrial Revolution” implicated many groups of animals—not just dinosaurs—in the evolution of angiosperms. They also found that the supposed “boom” in the biodiversity of dinosaurs toward the end of the Cretaceous was likely a consequence of paleontologists looking
more closely for dinosaur fossils in Late Cretaceous rocks; after all, the more you look, the more you find. (Granted, the Late Jurassic Morrison Formation has been studied intensively too, but its dinosaurs are still not as biodiverse as those from the Late Cretaceous.) They also pointed out that direct evidence of dinosaurs eating fruits or other parts of flowering plants is actually quite rare, and only represented by a few trace fossils such as dental microwear in hadrosaurs, an ankylosaur cololite, and ornithopod and sauropod coprolites.

So what evidence would be needed for those who find themselves romantically attached to the idea that dinosaurs had somehow contributed to Valentine’s Day festivities and were behind the evolution of roses, tulips, petunias, azaleas, and other gorgeous flowers we see, smell, and appreciate today? As just mentioned, some Cretaceous dinosaur trace fossils gave paleontologists specific examples of how these dinosaurs interacted with plants in their ecosystems. Also, in places where herbivorous-dinosaur tracks are common but their bones are rare or absent, these trace fossils help paleontologists to at least say whether or not these dinosaurs were in the same places as flowering plants, which they could have eaten. But these few trace fossils are like single frames taken from a 135-million-year-long movie, not showing the fuller connections between herbivorous dinosaurs and flowering plants throughout the Cretaceous Period. As a result, more comprehensive trace fossil studies are needed, and perhaps from Cretaceous rocks in one area, better enabling paleobotanists, vertebrate paleontologists, and ichnologists to assess what happened over time.

Still, flowering plants as dinosaur fodder are only one factor to consider when thinking about their evolution. Sure, if angiosperms grew and reproduced quicker than gymnosperms after being chomped by sauropods, hadrosaurs, ankylosaurs, and ceratopsians, that was an important trait in their favor. Yet two other dinosaur behaviors must have affected plants and are related to dinosaur traces: stomping and pooping.

For the former, whatever low-lying plants dinosaurs were not eating, they were stepping on. Hence, flowering plants may have
had traits that allowed or promoted their survival after dinosaur feet had compressed them. Some of these traits might include different root systems, which allowed for more clonal growth into new shoots post-trampling, or increased selection for
vegetative growth
, in which pieces of a flowering plant take root when dropped somewhere other than their original home. Dinosaur feet also might have carried flower pollen or seeds to other places, helping plants as either sex surrogates (pollination) or scattering their babies (seed dispersal), something today’s animals still do
en masse
for flowering plants. Dinosaurs trampling around freshwater environments also would have disturbed those sediments sufficiently that some gymnosperms were excluded, but made conditions perfect for angiosperms to take hold as “weedy” species that thrived in mixed-up soils.

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?