While the age of dinosaurs on land captures immense interest, the oceans of the Cretaceous Period (145 to 66 million years ago) held a world of unique life. This era was defined not just by terrestrial giants but also by the diverse and often fearsome fish that populated the global seas. The marine ecosystems of this time were vastly different from today’s, shaping the evolution of the fish within them.
The Marine Environment of the Cretaceous
During the Cretaceous, sea levels were significantly higher than today, submerging about a third of the Earth’s current land area. This rise created vast, shallow inland seas, known as epicontinental seas. One of the most prominent examples was the Western Interior Seaway, which split North America into two landmasses and provided a warm, shallow habitat for a wide array of marine life.
Global temperatures were much warmer, creating a greenhouse climate with little to no ice at the poles. This warmth extended to the oceans, making even the polar seas relatively temperate. The circulation patterns in these warm oceans sometimes led to periods of oceanic anoxic events, where large portions of the deep ocean became depleted of oxygen, impacting marine life and seafloor chemistry.
The chemistry of the seawater supported different kinds of life at the base of the food chain. Microscopic organisms called coccolithophores thrived in these conditions. When these planktonic algae died, their tiny calcium carbonate plates sank to the seafloor, accumulating over millions of years. These deposits would eventually form the massive chalk formations that give the Cretaceous Period its name, like the famous White Cliffs of Dover.
Dominant Fish Groups of the Cretaceous
The warm, expansive seas of the Cretaceous were home to a diverse population of fish, with two major groups rising to prominence. Cartilaginous fish, which include sharks and rays, were common predators. Among them, the “Ginsu shark,” Cretoxyrhina mantelli, was a formidable species that grew to lengths comparable to a modern great white. Its teeth were sharp enough to slice through the flesh and bone of its prey.
Another common shark was Squalicorax, often called the crow shark, which was a smaller but still significant predator and scavenger. While sharks were successful, the Cretaceous was a time of major evolutionary radiation for bony fish, specifically the teleosts. This group represents the vast majority of fish species alive today and diversified rapidly, filling numerous ecological niches. By the Late Cretaceous, teleosts had become the dominant group of fishes.
Among the most impressive of these bony fish was Xiphactinus audax, a giant predatory teleost growing up to 15 feet long. Xiphactinus had a bulldog-like jaw filled with long, sharp fangs that it used to capture and consume other large fish. The most famous fossil of this creature is the remarkable “fish-within-a-fish” specimen. This fossil shows a nearly complete Xiphactinus that died shortly after swallowing a 6-foot-long fish whole.
Cretaceous Marine Food Webs
The fish of the Cretaceous were integral parts of a complex food web. Large predatory fish like Xiphactinus and the shark Cretoxyrhina occupied high positions in the marine hierarchy. They preyed on smaller fish, squid-like belemnites, and ammonites. The abundance of these mid-level predators indicates a rich ecosystem, and fossils often contain the remains of their last meals.
However, these large fish were not the masters of their domain, as the apex predators were enormous marine reptiles. Mosasaurs, giant swimming lizards that could reach lengths of over 50 feet, were dominant hunters in the Late Cretaceous. These reptiles preyed on almost anything they could catch, including large fish like Xiphactinus and sharks. Fossil evidence, such as bite marks on fish fossils, confirms these interactions.
Alongside mosasaurs, long-necked plesiosaurs patrolled the water column, using their flexible necks to snatch fish from schools. Ichthyosaurs, though declining by the Late Cretaceous, were also present as fast, dolphin-like predators. This constant threat from massive marine reptiles meant that even a hunter like Xiphactinus was not safe. This pressure shaped the behavior and evolution of the fish populations.
The K-Pg Extinction and Fish Survival
The end of the Cretaceous period was marked by the Chicxulub asteroid impact, a cataclysmic event that triggered a mass extinction. The immediate effects, including tsunamis and intense heat, were devastating to marine ecosystems. The subsequent “impact winter” caused a collapse at the base of the food chain. Dust and aerosols ejected into the atmosphere blocked sunlight, killing off the plankton that formed the foundation of the marine food web.
This collapse had a cascading effect, leading to the extinction of many marine species. Large, specialized predators like the mosasaurs and plesiosaurs, which relied on an abundant supply of large prey, disappeared. Many of the giant predatory fish, including Xiphactinus, also went extinct. Their size and specialized diets made them particularly vulnerable, and ammonites, a primary food source for many predators, also vanished.
Despite the devastation, not all fish were wiped out, as the extinction event acted as a filter. Fish that were smaller, had more generalized diets, or lived in environments like freshwater or deep-sea habitats, had a better chance of survival. The teleost fish, which had already achieved great diversity, were particularly resilient. The extinction of the giant marine reptiles and many large predatory fish opened up ecological niches. This allowed the surviving teleosts to diversify even further in the Paleogene period.