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A Day in the Life of a Mosasaur

Copyright 2001-2009 by Mike Everhart

Last updated 08/10/2009

02/09/01 - Translated into Dutch

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Tylosaurus painting copyright Carl Buell; used with permission of Carl Buell

A shorter version of this story was published in The West Australian Dinonews, January, 2000, 15:3-6. Illustrations linked within this page are used with permission of and are copyright by the artists.

The bright, mid-day sun glinted off the calm waters of the Inland Sea and silhouetted the long, sinuous form of a huge mosasaur lying motionless amid the floating tangle of yellow-green seaweed. Twenty years old and over 10 meters in length, the adult mosasaur was almost full grown, and was much larger than any of the fish or sharks that lived in the shallow seaway. A powerful swimmer, the mosasaur used the thrust of his muscular tail to outrun his prey with a short burst of speed. His jaws were more than four feet long and were lined with sharp, conical teeth that he used to seize and hold his prey. Special adaptations in his lower jaws allowed them to flex in the middle and enabled him to swallow large fish and other animals that he caught. This ability was essential to the mosasaur because he had to hold on to his prey or risk losing it. If he let go of his prey in the middle of the ocean, there was a good chance it would sink to the bottom and be lost.

He was floating at the surface with his eyes and nostrils just above the water. His dark upper body absorbed hot rays of the late Cretaceous sun as dozens of tiny fish emerged from hiding in the seaweed and darted cautiously around his submerged bulk. They were feeding on parasites and other small invertebrates that had attached themselves to his scaly hide. He breathed slowly and quietly through his nostrils as his ears and other senses remained on the alert for the tell-tale signs made by approaching prey. He was a patient hunter, preferring to let his victims come to him instead of wasting energy swimming around the vast seaway in search of food.

Overhead, pteranodons of various sizes floated lazily through the cloudless sky, riding thermal currents rising off the warm water while looking for schools of small fish feeding near the surface. Occasionally, one would skim the surface of the water and grab an unwary fish with it’s narrow beak. He had recently eaten the floating carcass of a dead pteranodon, but the living ones could see him clearly from above and avoided feeding near him.

Amid an ever changing mixture of background noises made by a variety of creatures in the ocean, he noticed that a faint buzz of clicking sounds were getting louder, alerting him to a group of hard shelled ammonites that was feeding nearby. Though not his favorite prey, they were all that had approached him since he had surprised and taken a large fish early in the morning. Even with that recent meal, his appetite was still unsatisfied.  Just after sunrise, a large squid had evaded his attack and now hunger was beginning to gnaw at him.

Exhaling most of the air from his lungs, he slowly submerged his head below the surface, leaving behind only the faintest of ripples. His large eyes immediately found the brightly colored, coiled shells of the ammonites as they approached, bobbing and darting below him. Jet propelled by forcing water through their internal siphons, they moved generally backwards through the water, with their short tentacles trailing behind them. Instinctively, he knew that their large shells would hide him from their view until they had moved well past him. He would attack from above, long before they had a chance to sense the danger.

Using his four large paddles, the mosasaur carefully maneuvered his long, snake-like form into an attack position, watching intently for any indication that the ammonites had detected the danger from above. Singling out a slightly larger ammonite in the middle of the group, he dove downward with a slash of his long, broad tail. The ammonites reacted quickly and instinctively to the disturbance, scattering in all directions below him, but not before his heavy jaws closed across the soft body of his victim. His sharp teeth shattered the front edge of the ammonite’s shell, destroying the buoyancy of the shell and rendering the ammonite helpless.

Without this captive pocket of air, the ammonite would sink swiftly to the bottom of the seaway. With practiced ease, the mosasaur flexed his body upward and raised the ammonite toward the surface. Then he released it and grabbed the tentacles of the immobilized creature with his teeth as it began to sink. Far too late, the ammonite released a cloud of jet black ink into the water. He ignored the bitter taste of the ammonite's last defense as he gave a quick jerk of his head to pull the ammonite’s soft body from its shell. The heavy shell and several fragments slipped sideways through the water, and then quickly disappeared into the murky depths. Opening and closing his jaws rapidly, he swallowed the fleshy morsel in a single gulp.

Looking around for more prey, he saw another ammonite swimming in confused circles nearby. A swift lunge and his sharp teeth crunched through the ammonite’s shell. Moments later, the soft body of the second ammonite followed the first into the mosasaur's stomach. The rest of the group of ammonites had jetted away as fast as they could and were no longer in view. His hunger briefly satisfied, he rose slowly to the surface to breathe, and resumed his ambush position rather than chasing after the fleeing shellfish.

He had hardly settled into waiting when he sensed the noises made by the approach of another mosasaur. Female mosasaurs tended to band together in pods for the protection of their young, while the males were solitary and very territorial. The approaching mosasaur was probably a young male searching for his own place in the expanse of the Inland Sea. In one swift, fluid motion, the older mosasaur turned and began to swim rapidly toward the sound of the approaching intruder. With his flippers held tightly against his body, the mosasaur moved quickly through the water just beneath the surface. His tail broke through the water’s surface repeatedly as he intentionally made as much noise as possible. He wanted to sound as threatening as possible to the other mosasaur. Although he was prepared to fight for his territory, he would first try to frighten other males away with his size and ferocity. Long healed scars on his body showed that even the winners in such fights could be badly hurt. He had been very lucky several times earlier in his life and had survived injuries that easily could have been fatal. As he got older, he learned to avoid such battles whenever possible.

His course intercepted the other mosasaur broadside in a patch of open water. Turning quickly to face the threat, the smaller animal displayed a mouth filled with sharp teeth. Despite of being nearly ten feet shorter, and much less massive, the invader refused to turn and flee. The big mosasaur circled warily around his now stationary foe, watching intently as the other animal almost doubled back upon itself as it continued to show its open jaws. Trying to appear as threatening as possible, the younger animal still refused to turn and run.

The big mosasaur was in no mood for such tactics. Making a large splash with his tail to distract the intruder, he surged forward and seized the smaller animal across the throat and back of the head. For a moment, the smaller mosasaur struggled helplessly as the powerful grip of the larger animal threatened to crush his skull. Then the big mosasaur moved his head quickly, snapping the other mosasaur’s neck. The smaller mosasaur gave a brief shudder, and then went limp. Angrily, he shook the slender body again, making certain that his foe was no longer a threat.

Realizing that his victim was too large for him to swallow, the mosasaur released his grip and moved away. The body of the dead mosasaur rose slowly toward the surface and floated there until most of the remaining air had escaped from it’s lungs. Then it began to sink head first toward the bottom. Still enraged by the intrusion of his territory, the big mosasaur searched about for any other interlopers as he swam in a large circle back to his ambush site. The commotion caused by the brief battle had frightened any other prey away and would certainly draw sharks to the area to feed on the remains of the dead mosasaur. Sharks also seemed to be attracted the movement of a mosasaurs tail. Although he was too large for them to be much of a threat to him, any shark bite could cause a wound that could become seriously infected. He already had several healed scars from past bites on his tail and flippers.

Later in the afternoon, he sensed the noisy approach of a group of swimming birds. Large and wingless, these birds migrated through the seaway on the way to and from their nesting grounds to the north. They were fast swimmers and fed on the abundance of small fish and squid that lived in the sea, catching them in their toothed beaks.

He submerged quietly until he was well below the surface, and swam slowly toward the birds. From the sounds that he heard, he could tell that they were feeding. In the past, he had been able to ambush careless stragglers from below as they rested between dives for food. Nearing the flock, he could see the darker bodies of the birds silhouetted against the sun lit surface as they dove and fed on a school of silvery fish. Slashing his powerful tail from side to side, he surged upward toward the body of the nearest bird. His mouth opened just before he reached the surface and quickly closed on the bird as his momentum carried his upper body several feet out of the water. Crushed by his powerful jaws, the bird struggled briefly and died. When he was certain his prey would not escape, he moved the limp body around in his mouth until it was pointed head first into his throat. Then he lifted his head out of the water and allowed gravity to help him swallow the bird. The noises made by the rest of the retreating flock was already fading in the distance.

The hours passed by and dark clouds of an approaching storm covered the sun as it sank toward the horizon. Driven by the changing weather, the ocean swells became larger and larger. The mosasaur found it increasingly difficult to maintain his stationary position and nearly impossible to sense the approach of possible prey against the increasing background noise caused by the wind and rain. Instinctively, he knew it was time to move to open water. Moving forward with rhythmic undulations of his tail, he headed toward the edge of the seaweed mat.


For more about Tylosaurs, visit the virtual Tylosaur Museum here.

For a Tylosaurus dig, check out this site: Tylosaurus proriger

For more on mosasaur eating habits, see the stomach contents of a Tylosaur specimen at the South Dakota School of Mines.

For more about Hesperornis, go here.

For a page of Dan Varner's paleo-life art, GO HERE

For a page of Doug Henderson's paleo-life art, GO HERE

Vladimir Krb's art work is copyright by and used with the permission of the Royal Tyrrell Museum of Paleontology

More paleo-fiction by Mike Everhart here:   A Day in the Western Interior Sea     and      A Moment in Time