I feel a certain awe in the presence of history: the Great Pyramid of Giza, Machu Picchu, Stonehenge, Chichen Itza. . . .These places feel like gifts from the past, unique and rare testaments to time periods I can't experience in any other way. But Denver, the very city in which I live, has something almost as incredible: the ground beneath our feet.
A good hundred and thirty million years ago (give or take), geologists tell us that the Farallon Plate began to subduct beneath the North American Plate. The angle was shallow, but this subduction profoundly affected the landscape above. Facilitated by a warm climate completely lacking polar ice caps, this subduction created an inland sea that, thirty million years later, stretched from the Arctic Ocean down to the Gulf of Mexico, forming the area we today call the Western Interior Seaway (or sometimes the Niobraran Sea), a two-thousand mile expanse of open water that played host to an ecosystem unlike any the earth had ever seen.
I've often said the only bad thing about Denver is that it lacks an ocean. I only missed it by about 75 million years.
The Western Interior Seaway was never particularly deep. Its greatest observed depth was only 2,500 feet (by comparison, Russia's Lake Baikal is more than twice that at 5,387 feet), with an average of only 600 feet. But its shallow waters supported a thriving tropical sea with a rich variety of life, from massive beds of calcareous algae (preserved today in places like Kansas's Monument Rock) to some of the most fearsome aquatic predators of all time.
These latter fossils draw our attention most. Mosasaurs, carnivorous reptiles attaining nearly fifty feet in length, dominated the food chain, enthroned as top predators in a world where competition for the title was fierce. Mosasaurs co-existed with dinosaurs but were only distantly related, their closest living relatives being monitor lizards (the genus including the famous Komodo dragon).
, top predator of the Western Interior Seaway--because prehistory needed a giant, swimming, Komodo dragon.
(Image by DiBgd on Wikipedia).
Typified by monsters like
, the mosasaurs of the Western Interior Seaway were indiscriminate carnivores, taking prey as diverse as fish (including large sharks), other aquatic reptiles, and on occasion, each other. At least one fossil dinosaur of an unidentifiable duck-billed species bears distinctive mosasaurid toothmarks, leading paleontologists to believe that these opportunistic predators scavenged carrion washed off the shores of the Western Interior Seaway.
Elasmosaurus platyurus were another typical sight in the Western Interior Seaway. Roughly as long as
Tylosaurus but with much of its body length taken up by neck,
and its smaller cousins sometimes became prey for larger mosasaurs. Nevertheless, it was a fearsome predator specialized in preying on fish and mollusks, including the coil-shelled cephalopods known as ammonites.
with unlucky diver for scale (Image by DiBgd on Wikipedia).
While the bodies of plesiosaurs lacked the hydrodynamic shape of mosasaurs or the dolphin-like icthyosaurs they ultimately succeeded, their long necks made them efficient hunters. Despite common portrayals, osteological studies show that plesiosaurs lacked the flexibility necessary to lift their heads above water in an s-shaped pose, and seem to have struck at prey from the side in a more snake-like fashion.
Below the plesiosaurs and mosasaurs, fish filled the middle tier of the Western Interior Seaway's ecosystem. Sharks like
, a distant extinct relative of the great white, and the impressively fanged
, a predatory bony fish capable of attaining 6 meters in length, even preyed on smaller mosasaurs.
The rather terrifying
Flying reptiles and birds were also in evidence. The most famous of these is doubtless
, an iconic pterosaur that subsisted on a diet of fish. Despite its size (wingspans sometimes exceeding eighteen feet),
was a capable flyer. While the purpose of its long head-crest is unknown, only males seem to have possessed it, perhaps indicating its use in mating rituals.
was a toothed diving bird somewhat similar to a loon, though this is due more to convergent evolution than any actual relation. At about five feet long, it was well-adapted to life in the water but likely could not stand up on land due to the positioning of its feet, necessitating movement like a seal or walrus.