Niobrara Formation, Nebraska, 86 Million Years Ago

    In western North America lies a vast, shallow sea. Running from the Arctic Ocean to the Gulf of Mexico, it splits the continent in two. To the west is Laramidia while the east has become Appalachia. Between these land masses, the modern states of Texas, Colorado, Wyoming, and the Dakotas lie sunken in this Western Interior Seaway. What we now call Nebraska is almost entirely engulfed by ocean water, an apt condition for a state whose name is Sioux for "broad waters."

    However, a small stretch of land remains on the coast of this widespread inland sea. Here, shallow waters flow through a series of jagged stones. Waves crash against many of the larger sharper rocks, creating explosions of foamy liquid. This disturbs the ancient sea-gull-like birds, Ichthyornis, that roost atop these pointed boulders. Their high-pitched, piercing screeches cut through the sound of the crashing tide.

What little sea water makes it through bounces on the flat, rocky outcrops on the coast's edge. These cliffs act as a border between shallows and the sandy beach above. Colonies of slimy green algae cling to the edges of these coastal rock barriers. This stone stretches across the expansive Appalachian seaboard as far as the eye can see.

    It's along this outcrop that groups of one of the seaway's strangest animals emerge from the water. Dragging themselves onto the wet stone are flocks of Hesperornis. They are an ancient lineage of flightless sea birds native to the North American waters. These 6-foot avians sport large bodies with small, barely visible wings, a short tail, and elongated, web-footed hindlegs. Their necks are slender and curved like that of a heron. The heads supported by these necks are also lengthy, ending in a dagger-like beak lined with small, sharp teeth, a rare sight among birds in our modern day.

    Their bodies are adorned with thick, water-proof plumage. Much of it is glossy, black while the fronts of their necks, breasts, and bellies are completely white. Bright yellow, eyebrow-like feather crests grow out from above their eyes. Faded pink, scaly skin coats their exposed legs in addition to the bright, orange-red of their beaks.

    Like penguins and many other modern birds of a similar niche, Hesperornis spend much of their time out at sea. It's only on rare occasions that these marine fowl drag themselves onto dry land. Unlike the penguins, ducks, or geese that we are familiar with, their feet aren't strong enough to support their weight out of the water. Instead, their terrestrial movement is much more akin to a seal. Their legs push and drag their bodies in multiple short bursts. Such motion tends to be difficult when traversing up the side of the beach stone.

Nonetheless, their efforts successfully bring them to the sands behind all the moist rocks. They're immediately greeted by the expansive, sandy shores at the edge of dry land. The soft light of the mid-morning sun illuminates the white beach sand. Here, rocks are still present, but in fewer numbers and buried by the smooth, tropical grains. Pieces of broken shells are mixed in too, brought in by much higher tides of the past. Bordering the land and the beach is a wall full of ferns and tall plants resembling today's palm trees. Amidst the warm winds, the air here is a sweltering 90 degrees Fahrenheit. Even 50 feet away from the tidal rocks, the crashing of waves can still be heard amidst the oceanic breeze.

The approaching Hesperornis are also welcomed by the honks, trumpets, and squawks of other members of their kind. Hundreds of these aquatic birds lay on bundles of brown, thin sticks across this enormous beach. These are males waiting for the arrival of their returning mates.

Upon reaching the colony, each of the females lets out calls like those of the males. Hesperornis can't identify each other by looks largely due to being identical. Instead, they recognize one another by their unique vocal characteristics. To human ears, these calls are almost indistinguishable. However, these birds can spot even the most minute differences that are unique to each individual. Such recognition among birds has become such a successful evolutionary trait that it's even seen long after the Cretaceous. It's the very thing that helps females like Jane to stand out, even amongst the calls of hundreds of others.

For a moment, Jane ceases her noises. Her head periodically darts and turns as she listens for a response. Amidst the clutter of calls, she zeroes her focus on one particular one.

Jane shuffles through the sand and pushes further into the colony. Small, yet sharp rocks and broken pieces of shells scrape across her stomach. Ancient sea fleas bounce across the sand, occasionally landing on her and causing irritation before shaking them off. She passes by many males, several of whom are already reuniting with their mates. Eventually, she reaches the one Hesperornis she had been listening for: her mate, Adam.

His calls end abruptly upon seeing the return of his partner. They then greet each other tapping their beaks against one another. Like many birds, Hesperornis mate for life, forming a connection that only ends in death. Such a strong, enduring bond is one that Adam and Jane share in stride.

Their reunion is soon interrupted by frantic yet soft chirps. Jane looks downward, finding the origin of the noises. Adam pulls his body off his stick pile to reveal the fruits of his two-month-long labor: a single chick.

Its appearance is quite different from an adult Hesperornis. While the underbelly of its plumage remains white, the black part is replaced with downy, brown feathers, blending them in with the sticks of its nest. All this feathering clings to the hatchling's thin skin. Its beak and feet also lack color, being completely black.

Only a few hours old, this tiny chick weighs no more than a large apple. Despite this, it already has a massive appetite. So, it continues rapidly chirping at its parents. In response, Jane opens her mouth and holds it to that of her baby. She then spews out a stream of ground-up fish meat for the nestling to ingest. It only takes seconds of this before she ceases filling its gullet.

As the chick grows over the coming months, so will its hunger. Thus, it will need to be fed frequently to survive into adulthood. So, one of its parents must venture into the waters of the seaway to acquire enough food. Now that Mom has returned from her trip, this responsibility falls onto Dad.

Jane drags herself atop the nest, placing herself comfortably over her youngster. They then start to communicate with one another through their sounds. This allows the two to familiarize themselves with one another and to help form the bond between mother and child.

Meanwhile, Adam and the other males trek across the shore towards the seawater. Then, for the first time in over two months, he plunges through the waves. Upon entering the liquid blue, they transform into powerful swimmers. Their large legs propel them through the water while their small wings help to direct their movement. Streams of bubbles follow in their wake as they jet through at blinding speed.

Eventually, they leave the sharp rocks guarding their coastal home and enter the deeper waters ahead. This transition is marked by a submarine cliff bordering the shallows and expansive sea. What lies below is hidden within darkness immeasurable to the naked eye.

Once here, Adam and the other Hesperornis fathers breach the surface and start floating atop it. Here, they take just one of many necessary moments on their trip to breathe. As they do, they gaze at the heavens, spotting their guides through this inland sea.

Soaring among the clouds is a flock of Nyctosaurus, members of the pterosaur order of aerial reptiles. At first glance, they are similar in appearance to Pteranodon with their slender, toothless beaks and membranous wings connected to their limbs. The biggest difference they hold from their more famous relative is their peculiar crest. It's made up of two grooved spars connected to the base of their heads, one pointing upward while the other runs backward. These 2-foot-tall structures are formed in an L shape, resembling the masts of a sailboat. By contrast, the rest of their forms are rather small for pterosaurs. The biggest individuals weigh six pounds with a wingspan of 10 feet. Their bodies are completely covered in white, hair-like pycnofibers while their crests are completely yellow with black bands. This same yellow coats their beaks with a small bit of black at the tip.

Floating and flapping in the sky, the Nyctosaurs can see far across the sea. This pterosaur's eye view serves them greatly in their search for their favorite prey: fresh fish. Hesperornis like Adam know this well too and use these flying reptiles to escort them to their mutual source of food.

However, such a trip is not without its dangers. Many predators travel the waters of the Western Interior Seaway. One of them lurks in the dark depths right below the Hesperornis. This hidden hunter is also the largest to be seen in this part of the ocean.

It's a Tylosaurus, a member of the mosasaur family of predatory marine reptiles. As a relative of snakes and monitor lizards, it retains a serpent-like shape as well as a forked tongue that periodically sticks out to smell the water. Its muscular, stream-lined body stretches nearly 50 feet, ending in a long tail with a flattened tip resembling that of a shark, but flipped upside-down. Four, paddle-like limbs help steer while its tail propels it through the liquid blue.

Its head is large, sporting a long snout with a mouth filled with sharp, conical teeth. Inside the roof of its mouth is a third jaw containing a whole nother set of the same dentition. Its skin is made up of diamond-shaped scales like those of a rattlesnake. The top is completely black, blending it in with the dark waters while the bottom is a solid white. Scarred gashes and bite marks also litter its head, evidence of past battles its kind are known to engage in.

As the top predator of the North American seaway, Tylosaurus has a wide range of common prey, including Hesperornis. However, as the marine birds rush under the ocean surface, the predator remains under. Rather than attempting a pursuit of the swimming fowl, it continues its patrol along the edge of the shallows.

***

Hours have passed since Adam and the other male Hesperornis embarked on their foraging journey. In that time, they've made it into the deepest part of the seaway. Here, what is now the driest, most barren part of the mid-western United States is engulfed by nearly 3,000 feet of salt water. At this point, even dry land remains unseen among the waves.

While they have indeed made it a great distance so far, they've already encountered their first impedance. Fog has formed in the sky above, obscuring what they would normally see right above them, including the Nyctosaurus. While their would-be guides in the seaway have vanished from sight, their various deep honks can still be heard.

Without the pterosaurs to lead them through the open waters, they're left to simply float in the middle of the vast sea. Out here, they can't afford to be stationary for long. While their purpose in the inland sea is one of hunting, they can easily become prey themselves.

Shortly, another resident of this North American sea begins to emerge from the depths. Its large frame becomes vaguely visible under the clear waters below the Hesperornis. Adam hears a breaching splash, causing him to briefly jump from fright. He turns his body around to see a head half his size poking from the surface of the water. It makes a loud, inhaling noise before its nostrils close tightly back up. The head dives back down, briefly showing a part of the neck it's attached to. Curiously, Adam dives beneath the waves to observe the rest of the creature. Upon doing so, he doesn't just find one of them, but a pod of over a dozen.

They are Elasmosaurus, members of the famous plesiosaur clade of marine reptiles. Their large, turtle-shaped bodies have four lengthy, paddle-shaped flippers and comparatively short tails that ends in a fluke. In contrast, their necks are massively elongated, reaching up to 23 feet, over half of their entire length. These necks are the second-longest of any plesiosaur, only exceeded by one Canadian relative that won't appear for another 10 or so million years. Their slender, triangular heads appear small proportionately but have several fang-like teeth that jut out from their mouths. Black spots adorn much of their smooth, dark-blue, scaleless skin aside from their much lighter blue underbellies.

Much like the Hesperornis, they too have ventured far across the seaway in search of food. This mutual search for nourishment allows Adam and the other males a new way to navigate the ocean waters. Wherever the Elasmosaurs go will inevitably lead to a massive food source. So, the entire flock dives under, following the path of the large plesiosaurs.

The further trek through the liquid blue lasts yet another hour. Yet, their persistence is soon met by the ultimate prize. Among the salty waters is a massive school of fish. Periodically, they gather in large groups to feed on small organisms such as plankton. As they do, they form living, swirling vortexes where light frequently shines off their gray scales.

With little hesitation, the Hesperornis thrust themselves into the tornado of fish. Many of them manage to swim out of the jaws of the marine birds, but others are still caught within their toothy beaks. The Elasmosaurus also acquire their share of the bounty, utilizing the long reach of their heads to scoop up their prey without alarming them with their larger bodies.

This cyclone of seafood also attracts another type of hunter, not from below, but above. Nyctosaurus start diving into the liquid blue, snapping their beaks at the fleeing fish. Some miss entirely, while others attain a decent mouthful. Either path leads them to flapping their wings till they reach the surface. There, they rest before flying back up to attempt another divebomb. Even a flock of much smaller Ichthyornis has gathered too, employing the same strategy as the Nyctosaurs.

Indeed, a variable swarm of species has gathered around this spot, almost matching the school in size. With plenty of this food source to go around, little room for competition is left among them. However, such a mass meeting of divergent fish feeders attracts a different type of oceanic hunter. It doesn't eat the fish; instead, it is a fish, but a rather different kind.

Streaking through the water and breaking through the surface is a pack of a dozen Xiphactinus. These predatory, long-bodied fish are common in this North American sea. Superficially, they resemble a tarpon, but as long as a London bus and with fangs lining their large jaws. As fish, they have dark gray scales across their body with streaks of white light shimmering across them.

They too gather around the swirl of fish, but when they're around, hunters become the hunted. Jumping Xiphactinus ambush the Ichthyornis and Nyctosaurus resting atop the surface of the seawater. While many fly away, some are swallowed whole by the predatory fish.

While the Elasmosaurs remain untouched due to their larger size, Adam and the other male Hesperornis propel themselves away from the hungry fish. In the process, a few are engulfed into the jaws of a Xiphactinus. Although, this doesn't always lead to quenching starvation. Despite being jawed fish, Xiphactinus don't chew their food but swallow it whole. By feeding in this way, they often risk death from their meals. One of the voracious fish finds out the hard way as the struggles of a Hesperornis inside it rupture its organs, effectively killing it from the inside. Now both predator and prey are doomed to float to the bottom of the sea, vanishing in the dark depths.

Having filled themselves, the remaining Xiphactinus swim away, fading into the greater blue. With the cyclone of fish severely depleted, so too do the Elasmosaurs. The Ichthyornis and Nyctosaurs propel and launch themselves to the sky in search of more food opportunities. Meanwhile, Adam and what's left of the Hesperornis flock are free to make their way to their next destination: home.

***

Late afternoon has now approached and much of the sea and shore have started to grow dark. All that remains for light is the sun slowly setting on the horizon. Its bright light progressively changes to red as day bleeds into nightfall.

In the face of this daily change, Ichthyornis have started returning to the shore rocks for roosting. The sands of the beach also remain alive with the sounds of Hesperornis calling to one another. Once soft chirps of the chicks are now the loudest, increasing in frequency with their appetite.

In just less than a day, these small youngsters have already started to show signs of change. They've shown a slight bit of growth from when they first met their mothers in addition to their upper bodies becoming wider to fit with their chubbier lower halves. No longer are their soft baby feathers clinging to their skin but are more fluffed out.

With this increase in body size also comes an ever-increasing hunger. They bray their heads and nibble at the beaks of their mothers for another meal. Unfortunately, all the females including Jane have run out of their fish reserves. Even as her youngster continues to snap and cry at her beak, she must keep pulling it out of reach. For the mother, such an effort isn't easy with how tightly the little one adheres to her body. Until her mate returns, the chick will have to go hungry.

As she continues keeping her mouth away from her nestling, Jane spots something in the waters near the horizon. A collective of small bodies jumping out and into the seawater. At last, the male Hesperornis are starting to return.

These arriving travelers are also not alone. Around the edges of the shallows, small jellyfish have gathered in mass at the surface near the cove. They feast on plankton and algae, small organisms only visible when directly exposed to the fading daylight.

Something much larger has decided to visit here too. It's an Archelon, an ancient breed of sea turtle. At up to 15 feet long and two-and-a-half tons, it's the largest turtle to ever live. Unlike many modern sea turtles, its carapace is leathery, a trait only seen today in the leatherback. It still sports four massive flippers to help it move through the liquid blue, a trait universal to all forms of sea turtles. The head of this aquatic terrapin ends with a powerful, hooked beak, adapted for crushing hard prey. Its skin and shell are all completely black with several small, white spots scattered across its form as well as a white underbelly.

Like many sea turtles, Archelons regularly feast upon jellyfish. So, this gathering of soft-bodied organisms serves as the perfect feasting opportunity for it. Although the turtle's movement in the water is slow and sluggish, many of the jellyfish still fail to escape its mouth.

This titanic terrapin's presence also has an extra, unintended benefit. Many of the jellyfish that surround the edge of the shallows have long tendrils that could sting most other animals, including the Hesperornis. Thus, their presence can make getting back to the greater colony a challenge. However, with the Archelon feasting on and chasing after them, they're forced to scatter away, allowing the sea birds to swim into the shallows and through the coastal rocks.

As the colossal turtle continues its feasting, many more Hesperornis manage to reach the shore and drag themselves to the colony. As the males approach, Jane lets out her calls to find Adam, her one distraction from her hungry chick. Amidst her own honks and cries, she listens through the clutter of hundreds of males. Unfortunately, none of them are what she recognizes as her mate.

Meanwhile, in the water, another hazard starts to present itself, one that causes the arriving Hesperornis to frantically scatter. It's a danger so great that even the Archelon is forced to retreat. Emerging from the darker waters in the distance is the newest threat to the lives of the sea birds.

It's a pack of seven Cretoxyrhina, the "Ginsu sharks." Their common name is derived from razor-like teeth resembling the Japanese Ginsu knife. As members of the mackerel shark family, their 26-foot-long bodies are similar in build and appearance to their most famous relative, the great white. Only their color is different from their deadly descendant. While their undersides are still white, their tops are much darker with faint, vertical, gray stripes along the sides of their bodies.

With speeds of 43 miles per hour, the sharks are easily able to keep up with their avian prey. Upon catching a Hesperornis in their jaws, they repeatedly bite them before swallowing them up. Some ambush their prey at such speed that they end up crashing through the water's surface, creating massive splashes as they twist and contort mid-air.

Such carnage stains the edge of the shallows in thick crimson. In these translucent, red clouds, many male Hesperornis find difficulty in navigating around the predators. Despite the danger, many escape into the shallows, some trailing small patches of blood.

One Hesperornis has yet to escape the sharks: Adam. Even as he attempts to find his way through the crimson cumulus, a Cretoxyrhina continues to chase after him. By this point, many have left having filled their stomachs, but one remains, trailing after the lone male.

This isn't the only predator left at the border. The Tylosaurus is still here, following the bird and shark from deep below. As it propels itself after Adam and his other pursuer, its tongue continues to stick out in anticipation of its meal.

Finally, the crimson starts to clear, revealing the jagged rocks of the shallows. As Adam spots this, the last shark continues to grow ever closer. Underneath, the Tylosaurus starts to jettison itself upward out of the dark depths. Between his increasing exhaustion and his belly filled with fish, he gradually starts to slow. Nevertheless, he continues to swim as fast as his body will allow. All he needs to do is swim just a few more inches and he'll be free. As he does, both predators rapidly reach within mouth's grasp of the tired Hesperornis.

At the surface of the water, all remains calm. The orange, fading daylight reflects upon the ever-darkening waters as Ichthyornis begin to slumber atop the tall rocks. Suddenly, the calm is broken by the Tylosaurus bursting from the seawater. Prey clasped in its jaws is obscured by a towering wall of white and partly red, rising liquid. Amidst this massive spout, several Ichthyornis screech and cry as they frantically fly away from the breaching predator. It gives its catch one last chomp before dropping back into the water with a thunderous splash.

This gigantic white explosion is so large that it's even visible from the beach. Upon witnessing it, Jane frantically lets out her loud cries. Her noises become so deafening they even muffle the chirps of her chick. She only pauses to scan her eyes toward the remaining males dragging across the beach. Her calls fill the air again, louder than ever before. Even so, she hears no response, not even from the other males. All she hears is the fading cries of retreating Ichthyornis.

One more, piercing cry escapes Jane's beak, the loudest she had ever produced. As this cry ends, only the echoes of her great call fill the air. Gently, Jane lowers her head to the ground, sinking it into the sand around her. Many other Hesperornis call and drag around Jane as she lies still, almost lifeless. Even as other members of the colony sound off right next to her, she hardly reacts. Not even her chirping chick gets her attention.

Abruptly, one more sound is added to the air. Jane jerks her head upward, ever alert to it. She gazes to the edge of the coast and sees a lone Hesperornis dragging itself ashore. By his calls, she immediately recognizes the lone bird as none other than Adam.

Upon seeing and hearing him, Jane frantically lets out many more of her calls. They continue for seconds until her mate finally returns to her. Immediately, they greet each other by once again touching their beaks together. Even the chick is greeted with the most important gift it could get, ground-up fish spewed into his gullet. Once fed, the nestling becomes silent as it starts to slumber. As it does, mother and father swap positions once again. This time, Adam is charged with looking after the chick while Jane heads back out to see for food. Thus, the hearty traveler gazes at the shore, observing his mate as she dives into the water with hundreds of other females.

As the leaving Hesperornis pass across the deeper waters, one animal remains underneath. At the edge of the shallows, the Tylosaurus remains, floating just above the darkness of the depths. After two more bites, it swallows up what remains of a dead Ginsu shark.

Once again, the large sea serpent doesn't follow or ambush the Hesperornis. Only after they are gone does she rise to the surface. As she does, she lets out two loud breaths.

Under the surface, a great miracle is starting to take place. A small tail is starting to poke out of the underside of the Tylosaurus. Slowly, it pushes out from the serpent's body, revealing its small, paddle-like limbs. The predator lets out one more breath before the smaller animal escapes with a burst of white.

It's the sole focus of the Tylosaurus' time here, a baby. Having just entered the world, the four-foot-long newborn's first instinct is to swim to the surface for its first taste of air. Upon emerging above water, it greets its mother by rubbing its head along her side. Soon, another youngster emerges with one more shortly after. Before long, five babies are swirling in the water around the new mother Tylosaurus.

The Western Interior Seaway is one of the most challenging places to live. Whether by raising new life or preparing for its birth, amidst this common struggle, the Hesperornis have unwittingly linked themselves to the greatest predator of this North American sea. Thus, by pure accident, the Tylosaurus has become an unexpected guardian from the deep.