The Last Day of a Doomed Dinosaur

Itch. The annoying impulse repeats itself over and over again through the young hadrosaur’s body. Itch itch itch, a terrible tingle between the dinosaur’s toes and along his scaly flanks.

There’s only one thing for the young Edmontosaurus to do. Sheltered by the shade of the forest, on the edge of the dinosaur’s small herd, the shovel-beaked herbivore stops to relieve the tingling burn. A beech tree will suit the situation nicely. The snarl of trunks sprouting low to the ground creates a series of great riblike scratching posts, each wrapped in sufficiently coarse bark. Tilting his thick, hefty tail backward to seesaw the front half of his body up, the dinosaur slowly steps and scrapes and rubs against the rough trunks, the friction sending momentary relief over the pebble-like scales covering his body.

This particular Edmontosaurus, still a young male at about eighteen feet long, is not the first to visit this particular post. The bark of the beech and the surrounding trees—sycamore, dogwood, laurel—have been polished, rubbed, and broken open by dinosaurs desperate for some form of relief. Gooey sap has oozed here and there, collecting small flies and other unlucky invertebrates before hardening into resin. Someday, perhaps, these globs will be amber. For now, they’re just a mess and a mark left by the megaherbivores who call this part of the floodplain home. Generations of dinosaurs have marked their patch of forest in just this way, each scratch prompting the same behavior until the herds of Edmontosaurus know their own favorite spots.

The juvenile lets out a satisfied honk as he goes back down on all fours again, mitten-like gloves of flesh around his hands touching the ground as the great, three-toed hind feet start to move the dinosaur along again. Satisfying as the scratch was, lagging behind for too long is a risky move. While the Edmon­tosaurus is large enough to avoid the jaws of the smaller predators like Acheroraptor—and perhaps even ward them off with a great swing of his muscular tail—he is still small enough to be a day’s supply of food for a large T. rex. Best not to tempt a sudden bite to the rump. Step by step, the Edmontosaurus catches up with the rest of the herd and wiggles his way back into the crush of dinosaurian bodies as the duckbills trample their way through the woodland.

The healthy dread of sharp teeth and watching eyes in the forest shadows has led Edmontosaurus to unintentionally reshape the environments around them. Theirs is a landscape of fear, and the movement of the herd outlines the boundaries of this invisible, shifting territory.

A lone Edmontosaurus does not have any extravagant defenses. These dinosaurs lack the horns of Triceratops or the hard scutes embedded in the skin of the Ankylosaurus. A large adult can kick, swing its tail, and flail about if attacked, but by then the damage might already be too catastrophic to survive. By adulthood, every Edmontosaurus has seen this happen—a sudden lunge from the edge of the forest, a blur of great jaws erupting from the shadows to crush and consume.

And so Edmontosaurus have become wary of the forest’s edge. The scores of tree trunks can conceal a stalking tyrannosaur all too easily, with perhaps only the snap of a branch or the screech of an alarmed bird to provide any warning. Instead, the open meadows and floodplains feel more comfortable for Edmontosaurus herds. Here, there are no surprises. Any stalking T. rex will have to step out of the shadows to be seen, by which time the hadrosaurs will have ample time to use their one advantage in such a face-off—speed.

For all their terrible chomping power, adult tyrannosaurs can’t move very fast. Most of the time they amble along at a slow stroll. In a rush, they can run at speeds up to fifteen miles per hour. But they don’t need to move fast. They did not evolve to chase down prey. Instead T. rex—like the other great tyrannosaurs before it—evolved to hunt by ambush, waiting until just the right moment to put the dinosaur’s leg power into one great lunge. If the tyrannosaur misses and the prey takes flight, or if the tyrannosaur is spotted before this critical moment, the hunt ends and the great carnivore moves off. And over the millions of years that tyrannosaurs and hadrosaurs have coexisted, this hunting strategy has molded the attributes of Edmontosaurus. Given room to move, adult edmontosaurs can rear up on their hind legs and take off at about 28 miles per hour. That’s far from being a dinosaurian land speed record, but it’s enough. If one Edmontosaurus in a herd spots a tyrannosaur and honks in alarm, the whole herd stands upright and bolts, leaving the tyrannosaur to try to dine another day.

Naturally, Edmontosaurus are not on their toes at all times. Some of them do get caught, especially when having to cross through patches of forest to get at their preferred grazing spots. Much like carnivores of any era, the tyrannosaurs intentionally target the old, the sick, and the young, largely leaving healthy adults alone. While there’s nothing on an Edmontosaurus that can impale or bash a tyrannosaur, hadrosaurs can still deliver powerful kicks and thrashes that can break bone. So T. rex often tries to pick out younger animals when they can, creating generation after generation of evolutionary pressure to grow up faster. By growing rapidly, Edmontosaurus race toward a body size that’s less likely to be targeted by carnivores. For the most part, if edmontosaurs can survive their first year their chances of survival steadily increase until they reach old age and their joints begin to creak with arthritis.

The nature of the Hell Creek landscape is pruned and shaped by these interactions. While it’s certainly true that plants form the foundation of any food web, ecosystems are also affected by top-down forces between the animals. An ecosystem does not simply grow from the ground up but is somewhat squeezed between various pressures—landscapes evolve just as animals do. And so the Edmontosaurus herds are unknowingly helping to ensure their own survival by maintaining open fields of low-lying vegetation.

Without the Edmontosaurus or Triceratops, much of Hell Creek would likely be forest. Young trees would compete with each other for space and sunlight, but the Cretaceous woods would grow far thicker with a greater number of saplings allowed to seek their dendrological potential. With such ever-hungry megaherbivores around, however, the grazers pluck up many of the low-lying plants and young trees before the shoots and saplings have much of a chance to become established. The Edmontosaurus herds are effectively weeding a Cretaceous garden, maintaining the open spaces that they prefer. More than that, the repeated movements of these dinosaurs create game trails and gradually depress the soil. Season by season, divots in the dirt have become puddles, which have become ponds, altering the habitats through little more than force of dinosaurian habit.

All the same, even idyllic habitats carry dangers and annoyances. T. rex isn’t the only creature in Hell Creek that feeds on dinosaurs. In fact, the young male edmontosaur is already being eaten. The itch is a feeling, a sensation to be relieved, but the herbivore has no idea what causes it—lice.

An Edmontosaurus is a cozy home for a louse and its prolific, extended family—foot after foot of supple dinosaur flesh just waiting for tiny piercing mouthparts. While the dinosaur’s skin is tough overall, and very good for avoiding abrasions and scratches and bruises that come from wandering through forests, the great swaths of skin also create folds and indentations where the limbs meet the body and along the neck. These hot, dark places are comfy for lice and other biting arthropods, single-minded parasites nibbling away at the dinosaur dermis until they break the skin open just enough to have a capillary offer up some blood. All those blood meals fuel the next generation, which continue in the same tradition as their parents. And given the bumping, thumping, humping, and other forms of skin-to-skin contact among the edmontosaurs as they move along, the lice have plenty of opportunities to spread. To the adults, the lice are primarily annoyances. The insects itch and create sores, but they are as much a fact of life as the sun overhead or the breeze that wafts through the magnolia blooms. But hatchlings might be overrun by lice if their parent carries a heavy load, and all the effort required by the immune system to fight off the bugs can stunt the growth of the younger members of the herd. The best the hadrosaurs can hope for is trees to scratch on, mud to roll in, or a night that is cold enough to kill off some of the lice.

The herd has moved out into the open now, leaving the sheltering tangle of forest as they enter a broad clearing dotted with ferns and cycads, interrupted here and there by palms jutting straight into the air. This is a saturated, turned-over place, once a great green lunchbox but now a stopover snack station that has yet to fully reseed. Edmontosaurs and Tri­ceratops have changed the landscape itself. The herbivores eat seedlings, push over trees, and churn the soil with their steps, keeping the Cretaceous meadow sparser than it used to be. The hungry dinosaurs munch anyway, lowering their long, square-muzzled mouths to the ground to nip and crunch at the vegetation. Despite the name they’d take on millions of years in the future, these are no duckbills. They are more like shovel cows, beak and teeth working together to break down even the coarsest vegetation. The dinosaurs’ long snouts are tipped in a square, ridged beak that juts downward—a shape adapted to plucking up lots of low-lying vegetation quickly. And, unlike the actual ducks that would eventually evolve among the avian dinosaurs, every Edmontosaurus has an impressive battery of more than a thousand teeth crammed together. Individually, each fresh tooth looks roughly diamond-shaped. But put together and worn down by plants that are packed with dirt and have themselves begun to evolve defenses against herbivores, the teeth make great flat grinders that work like massive molars—pulverizing and pulping whatever is placed into those cheeks. In cooler months, a hadrosaur might chaw rotten logs to get their daily fiber—with mushrooms and insects adding a little protein to the mix. But the teeth of these hungry hungry herbivores are only part of their impressive oral apparatus. The skull bones of these dinosaurs have give and flex to them. They are not locked in place as if the whole skull is a big set of bony scissors. As the lower jaw comes up with each chewing stroke, the bones holding the upper teeth are flexed out and to the side, smearing and grinding plant matter against the teeth. These teeth go back to their resting position when the jaw opens again, ready for the next round of crunching. It’s an incredibly impressive anatomical setup, the majesty of which is somewhat offset by the fact that every Edmontosaurus has a small, thin dome of flesh on top of its head to help herd members know who’s who. It’s a ridiculous ornament, but one that these dinosaurs seem to find quite fetching.

The herd is calm as they snort and graze. There is no whiff of carnivore on the air, no stink of carrion to make them grow wary. The day is bright. A soft breeze momentarily mitigates some of the day’s heat. The dinosaurs are left to munch undisturbed. Each and every one of them, all thirteen members of the small herd, is totally unaware of what just happened.

There was no impending sense of doom. There was no shift to the wind, or darkening of the clouds. No lightning, no thunder. In this little patch of Hell Creek, Montana, all is as it ever was as far as the dinosaurs are concerned. But more than two thousand miles away, a chunk of extraterrestrial stone more than seven miles across just slammed into the Earth. In the hours, days, weeks, and years that follow, the consequences of the impact will wipe out about 75 percent of all species on the planet. This is how the end of the world starts.

From The Last Days of the Dinosaurs by Riley Black, copyright © 2022 by the author and reprinted by permission of St. Martin’s Press. On sale April 26, 2022.

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Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.