Authors: Anthony J. Martin
To my wife, Ruth,
whose traces of love suffuse and nourish me every day.
Vestiges of a Cretaceous Hour
was vexed. As the largest adult male in this part of the river valley, the challenge coming from a younger male strolling through his long-established territory was intolerable. Appropriately, then, it provoked a prompt and assertive response. From a stationary start, feet planted firmly on the damp muddy sand of the floodplain, he stared at his rival, started walking, and picked up speed. As he moved, he lowered his huge, broad head while pointing his three-horned face directly at the other
, communicating his intentions unambiguously. While accelerating, his rear feet registered directly on top of where his front feet had just pressed, and then exceeded them, leaving a varied pattern of tracks behind him.
From a distance, the other male seemed to stand his ground. Yet his feet shuffled, blurring their outlines in the underlying sand, as he tried to decide whether to stand his ground or turn and flee. Regardless, he was in big trouble.
In between the two
, a group of small feathered theropod dinosaurs with stubby forearms—similar to the Asian alvarezsaur
—and a nearby bunch of slightly larger ornithopod dinosaurs (
) looked on warily. Each of these groups of dinosaurs had been striding unhurriedly across the floodplain, tolerating one another’s presence, spurred on by intriguing scents wafting down the sunlit valley. Nevertheless, a charging
provided a good reason to temporarily abandon their long-term goals and deal with this more immediate problem.
In unison, they all looked up at the advancing
, its profile and rapidly increasing pace causing it to appear ever larger as it neared. Next to them, a mixed flock of toothed birds and pterosaurs all turned and aligned themselves with the wind at their backs. They began hopping while flapping their wings, and then were aloft, chattering loudly. This was all the motivation one of the more skittish theropods needed to start running, and the rest of his group followed suit. The ornithopods only hesitated a second or two before doing the same. First, though, more than a few of both species lightened the load before taking off, involuntarily voiding their bowels and leaving variably colored and sized scat, peppered with seeds, on top of their distinctive footprints. In her haste, one
slipped on a muddy patch and fell on her side. She quickly righted herself and bolted to catch up with the others, leaving a long, smeared body impression on the sand among the tracks.
A band of
-like theropods became too crowded near the edge of the river and, in a moment of desperation, jumped into the water and began swimming. In the river shallows, their feet touched the bottom and claws on each toe gouged the sand, making parallel grooves with each stroke. Bodies buoyed by the water and swimming with the current, their tracks were nearly twice as far apart from one another as if they had been on land.
These three separate but related shockwaves—the aggressive movement of the big male
, the raucous flocks of birds and pterosaurs, and the panicked stampede of the theropods and ornithopods—triggered overt and subtle changes in the behaviors
of nearly every dinosaur nearby. In this respect, it was like many other days in the Cretaceous Period, where long periods of quiet stability were occasionally interrupted by near-chaotic commotion.
On higher ground above the floodplain, a male–female pair of predatory theropods (
) paused from digging up small mammals from out of their burrows with their rear feet. They raised their heads, looking for whatever had provoked alarm calls from the birds and pterosaurs now passing overhead. Their footprints and excavations had disturbed a considerable amount of soil in the area. But they had much more to do before leaving. Once convinced the alarming behavior below had nothing to do with them, they went back to uncovering their furry morsels, sniffing the ground and scraping with their rear feet.
couple and next to the river levee, a few other theropods (
) shifted anxiously on their sediment-rimmed ground nests. Several weeks earlier,
mothers had laid eggs in the nests two at a time, a function of their dual oviducts. Some had deposited as many as two dozen eggs, a dinosaurian form of labor that took about a week and a half to complete. After laying, each mother
vertically oriented the eggs in the center of the doughnut-shaped nest. Now it was the job of the male to sit above and otherwise guard the precious egg clutch for nearly fifty days. Almost nothing could motivate them to leave their nests, so they continued to squat above them, albeit nervously.
Two smaller feathered theropods, potential egg predators and only a few meters away, gave the
fathers further incentive to stay put. These dinosaurs gnawed on a recently dead pterosaur, scraping their teeth across its limb bones to strip whatever flesh was left. One of them, though, unsettled by the ripple effects of the dinosaur duel below, succumbed to caution and scrambled up a tree, her hands and feet imparting sets of scratch marks on the smooth bark.
The spreading disturbance initiated by the
provoked several small ornithopod dinosaurs, distantly related to
, to retreat into their burrows. The burrows, which they had
dug previously into the banks of the levee, had entrances only slightly wider than the ornithopods’ bodies, making for a tight fit as they scrambled inside. These burrows twisted to the right and then left as they descended, making S- or Z-shapes, before expanding into a main living chamber at their ends. This zigzag design effectively deterred predators while maintaining livable temperatures and humidity in each burrow, essential features for these ornithopods to safely raise their young.
Several hundred meters downstream from the two
—which were only seconds away from ramming each other—a group of ostrich-like theropods (
) walked along a gravel bar at the edge of the river. They stopped every now and then to swallow pea- and marble-sized quartz-rich rocks. These “stomach stones,” also known as gastroliths, would lodge in a muscular gizzard just above their intestines, where they served as digestive aids to grind varied omnivorous foodstuffs like internal mortars and pestles. These theropods were aware of but mostly unperturbed by what was taking place upstream, as the “
groups were still far away from them. Their tracks, accented by sharp claw marks, left vague outlines on the gravelly parts of the bar, but were crisply defined on the sandier patches. The female tracks were only slightly larger than those of the males, but otherwise identical in form.
Meanwhile, located between the
, a male–female pair of
, joined in coitus, ignored just about everything else happening around them. These heavily armored dinosaurs, each weighing about five tons, typically would have left large, deep tracks regardless of what they were doing. But the addition of the male’s weight to the rear of the female meant her hind feet sank much deeper than his into the moist sand below. Together they made a six-footed impression, the front four coming from her and two rear ones from his hind feet. The earth moved, however briefly, for both partners.
Where the “
once stood on the lower part of the floodplain, at least a few of their tracks and some of their
scat had been squashed under the thumping feet of the older
as he closed the gap between himself and the younger male. The latter could no longer stay in one place and commenced walking, then trotting, toward the other. Snails, clams, and salamanders in the floodplain sediments unlucky enough to be in the same place as a
foot were summarily crushed.
The collision between the two male ceratopsian dinosaurs, which had a combined weight of about twenty tons, was surprisingly subdued and dull. They did not hit one another at full speed, as the additive impact would have killed both instantly. Instead, they first stopped, and then clashed their mighty heads together. Their wide head shields not only served as great billboards for attracting mates and recognizing their species, but also dissipated much of the energy when they struck. Still, the left upper horn from the older male punched through the right side of the younger one’s skull, leaving a round wound about the width of the older male’s horn.
Injured badly, although not fatally, the younger male turned and began limping away. His right humerus also had been fractured by the impact and started to ache. Accordingly, his trackway acquired a new asymmetry, with both legs stepping shorter on the right side and a shallower impression in the right-front footprint as he put less weight on it. The larger male
, satisfied for now, walked away but stopped periodically to turn and look, making sure that no further bad behavior would come from this young upstart.
This was the right moment for the
. She had been standing stock-still in the higher, vegetated part of the floodplain, waiting to take advantage of the chaotic situation and get an easy meal. If she could have felt disappointment, though, she would have experienced it then as she looked down from her elevated position and saw both
walking off the floodplain, each very much alive. She used a mixture of hunting and scavenging to feed her six-ton frame, and had dined often on
that eventually died from battles with one another. Whenever she found a dead one, she would take small bites of sweet meat on the face, and then grab the head shield from the back with her teeth to tear off the
head, exposing its delectable neck muscles. It was tempting for her to follow the limping one to see if he would expire, but attacking too soon entailed much risk. Millions of years of natural selection had not resulted in one of the world’s largest land carnivores taking on prey that could also kill it.
Flexible in her menu choices, she turned her gaze toward a nearby group of hadrosaurs (
) of varying ages that were unknowingly sharing this grassy–shrubby part of the floodplain with her. Most of these dinosaurs were on all fours, grazing on the plants around them. But they occasionally reared up on their hind legs and looked down to their left, having been distracted by the battle below and its effect on the surrounding fauna. As the hadrosaurs pulled up low-lying plants and chewed, tiny bits of silica in the leaves and stems imparted microscopic scratches on their teeth. Unnoticed by either the hadrosaurs or the tyrannosaur, the herd was attracting the attention of thousands of dung beetles. These insects burrowed industriously into the feces and laid their eggs, the dung providing both a home and high-quality food for their progeny. Snails also grazed on the droppings, gaining plenty of nutrition from the digested plant material.