The Cretaceous Ocean: From Apex Predators to Extinction

While dinosaurs dominate our perception of the Cretaceous period, a complex world existed beneath the ocean’s surface. This marine realm, from microscopic plankton to colossal predators, was shaped by global forces. The Cretaceous oceans were defined by a superheated climate, evolutionary arms races, and a catastrophic collapse that reset the course of life on Earth.

The Cretaceous Marine Environment

The geography of the Cretaceous world was vastly different from today. The supercontinent Pangea continued to separate, fragmenting into smaller landmasses by the mid-Cretaceous. This drift created new ocean basins and extensive shallow inland seas. At their peak, these seas reduced the planet’s land cover to about 18 percent, compared to roughly 28 percent today.

A notable feature of this geography was the Western Interior Seaway, a vast body of water that split North America in two. This seaway stretched over 3,000 kilometers long and nearly 1,000 kilometers wide. The Tethys Sea also separated northern and southern continents, creating a warm-water passage that connected to the precursor of the Pacific Ocean.

A “hothouse” climate fueled this widespread flooding. With global temperatures much warmer than today, the planet was largely free of ice, even at the poles. This warmth extended throughout the water column, creating stable oceans with less mixing between surface and deep waters.

Apex Predators and Marine Reptiles

The warm seas of the Cretaceous fostered an array of marine predators. Topping the food chain were mosasaurs, enormous marine lizards that diversified during the Late Cretaceous. Relatives of modern snakes and monitor lizards, some species like Mosasaurus hoffmanni reached lengths of up to 17 meters. Their powerful tails and paddle-like limbs made them effective ambush predators in open and shallow waters.

Other large marine reptiles shared these waters. Plesiosaurs, known for their long necks and small heads, included the Elasmosaurs, whose necks contained up to 76 vertebrae for striking at fish and squid from a distance. In contrast, the dolphin-like ichthyosaurs were built for speed but largely disappeared by the mid-Cretaceous, millions of years before the final extinction event.

Giant predatory fish also rivaled the marine reptiles. The shark Cretoxyrhina mantelli, or “Ginsu shark,” grew to over 7 meters and fed on large prey, including smaller mosasaurs and plesiosaurs. Alongside it swam Xiphactinus audax, a 5-meter-long bony fish with a bulldog-like jaw and sharp teeth. One famous fossil shows a Xiphactinus that died after swallowing a 2-meter-long fish.

Invertebrates and Seafloor Ecosystems

While giant reptiles commanded the open waters, the seafloor was dominated by invertebrates. The most common were cephalopods like ammonites, whose coiled shells are abundant in Cretaceous marine sediments. These free-swimming mollusks were a primary food source for predators like mosasaurs. Another cephalopod group, the belemnites, possessed an internal, bullet-shaped shell.

The structure of shallow marine habitats was defined by rudists, a group of bivalves. Unlike modern corals, these clams grew in dense aggregations, with one shell cementing to the seafloor and the other acting as a cap. Rudists formed extensive reef-like structures in the warm waters of the Tethys Seaway, providing complex habitats for other seafloor organisms.

Seafloor diversity also included other bivalves. Reclining forms like Exogyra and Gryphaea were widespread, as were inoceramids, giant bivalves whose shells could reach enormous sizes. These organisms filtered food from the water and contributed their shells to the sediment upon death.

The Microscopic Foundation

The Cretaceous ocean ecosystem was built upon microscopic life. The period saw a diversification of planktonic organisms floating in the sunlit upper waters, including the first major radiation of diatoms. These silica-shelled algae, along with other phytoplankton, were the primary producers, converting sunlight into energy and forming the base of the marine food web.

Coccolithophores were single-celled algae that surround themselves with microscopic calcium carbonate plates called coccoliths. During the Late Cretaceous, these organisms bloomed in immense numbers. After death, their skeletons rained down on the ocean floor, forming the massive chalk deposits that characterize the period and give the Cretaceous its name from the Latin “creta.”

The productivity of these organisms had a global impact. The accumulation of their calcium carbonate shells on the seafloor locked away vast amounts of carbon, influencing the planet’s climate. The White Cliffs of Dover and similar formations in North America and Europe are direct results of this microscopic life’s abundance.

Oceanic Impact of the Extinction Event

The Cretaceous period ended 66 million years ago with a mass extinction triggered by a large asteroid impact in the Yucatán Peninsula, creating the Chicxulub crater. The event had immediate consequences for the oceans. The impact vaporized carbonate and sulfate rocks, leading to sulfuric acid rain that altered ocean chemistry.

A prolonged effect was the dust and ash thrown into the atmosphere, which blocked sunlight for an extended period. This caused a collapse of the marine food web from the bottom up. Photosynthetic plankton, including coccolithophores, died off, starving the organisms that fed on them and causing a chain reaction of extinction.

This collapse caused the extinction of the dominant groups in the Cretaceous seas. With their food sources gone, ammonites and belemnites disappeared completely. The great marine reptiles, including mosasaurs and plesiosaurs, also went extinct. Approximately 80 percent of all marine animal species were wiped out, reshaping the underwater world and allowing for the evolution of modern marine life.