When Sharks Swam the Great Plains

Summary

The Western Interior Seaway was an ancient ocean that divided North America from 95 to 68 million years ago. Teeming with predators like sharks, mosasaurs, and plesiosaurs, the seaway was home to a fascinating array of creatures. Despite fierce competition, species coexisted through niche partitioning—evolving different hunting strategies and prey preferences. Sharks had specialized teeth, some used for hunting, others for scavenging. The seaway eventually receded as tectonic shifts raised the land, leaving behind fossil-rich limestone. This ancient ecosystem offers a glimpse into a world of survival, adaptation, and predator interactions that shaped the history of North America.

Takeaways

• 😀 The Western Interior Seaway was an ancient shallow sea that split North America in two from 95 to 68 million years ago.
• 😀 This sea was teeming with marine life, including ammonites, colossal clams, sharks, and marine reptiles like mosasaurs and plesiosaurs.
• 😀 The Western Interior Seaway hosted a diverse group of predators, including over 20 species of sharks, which coexisted for millions of years.
• 😀 Niche partitioning was a key strategy that allowed different predators to thrive in the same environment without driving each other to extinction.
• 😀 Some sharks, like Ptychodus, had specialized flat teeth for crushing shells, while others, like Cretoxyrhina and Squalicorax, had streamlined bodies for open-water hunting.
• 😀 Fossil evidence suggests that predators like mosasaurs and sharks sometimes turned on each other, with teeth found embedded in the bones of other creatures.
• 😀 Xiphactinus, a large carnivorous fish, is known for an unusual fossil where it died after swallowing a prey item too large for its digestive system.
• 😀 The rise and fall of the Western Interior Seaway were driven by tectonic activity, particularly the subduction of the Farallon Plate beneath North America.
• 😀 At its peak, the seaway extended from the Arctic Ocean to the Gulf of Mexico, covering vast portions of North America.
• 😀 Over time, geological uplift caused the seaway to recede, and fossil-bearing limestone from the seaway is now found in buildings across the Great Plains.

Q & A

What was the Western Interior Seaway and when did it exist?

The Western Interior Seaway was a shallow sea that split North America into two landmasses from about 95 to 68 million years ago during the Cretaceous period. It stretched from the Caribbean Sea to the Canadian Arctic and was teeming with marine life.

What type of predators were present in the Western Interior Seaway?

The Western Interior Seaway hosted a variety of predators, including sharks, mosasaurs, plesiosaurs, and large carnivorous fish. Some notable predators included the Squalicorax shark, the giant mosasaur Tylosaurus, and the long-necked plesiosaur Styxosaurus.

Why is the coexistence of so many large predators in the Western Interior Seaway unusual?

It is unusual because in most ecosystems, large predators competing for the same resources usually leads to extinction or displacement. However, in the Western Interior Seaway, predators coexisted for millions of years, likely due to niche partitioning, which allowed them to avoid direct competition.

How did the Western Interior Seaway form?

The Seaway formed as a result of the Farallon Plate, an oceanic crust, subducting beneath the North American plate along the Pacific coast. This caused the American Midwest to sink, and as sea levels rose, water gradually flowed into the region, forming the Western Interior Seaway.

What was niche partitioning and how did it work in the Western Interior Seaway?

Niche partitioning is when competing species evolve to occupy different ecological roles, reducing direct competition. In the Seaway, predators used different hunting methods, preyed on different sizes of animals, or adapted to different physical niches in the environment, allowing them to coexist.

How did sharks like Squalicorax and Cretoxyrhina differ in their hunting methods?

Squalicorax, known as the 'crow shark,' was likely a scavenger, relying on its keen sense of smell to find food. Cretoxyrhina, on the other hand, had large eye sockets, suggesting it relied more on eyesight to actively hunt larger fish and marine reptiles.

What did the differences in body size of predators in the Seaway allow them to do?

The differences in body size among predators likely allowed them to specialize in hunting different sizes of prey, reducing direct competition. Larger predators targeted bigger prey, while smaller ones focused on smaller animals.

How did the body structure of some plesiosaurs influence their diet?

Some plesiosaurs had long necks with small heads and teeth, limiting their ability to hunt larger prey. Their diet likely consisted of small fish, which helped them avoid competition with larger predators like mosasaurs.

What does the fossil evidence of stomach contents tell us about predator interactions?

Fossilized stomach contents provide evidence that some predators, like mosasaurs, sometimes ate other marine reptiles or sharks. For example, a Tylosaurus had the remains of a juvenile marine reptile in its stomach, indicating that predators occasionally turned on each other.

What happened to the Western Interior Seaway over time?

Around 80 million years ago, the region of the Great Plains began to rise due to geological uplift, causing the Seaway to recede. As the sea dried up, the fossils and limestone left behind became prominent in the Great Plains, visible in structures like courthouses and churches today.