Do you ever wonder what lived in the ocean when the dinosaurs roamed the land? Dr Anjani Ganase shares what life was like in the Mesozoic.
The dinosaurs roamed the earth for some 180 million years during the Mesozoic Era (252-66 million years ago). Hominids have been around for 11-16 million years; and homo sapiens (us) for only the most recent 750,000 years. The Mesozoic Era (digest that - 180 million years!) is divided into three main periods: the Triassic, the Jurassic and the Cretaceous.
The Mesozoic Era followed the splitting up of the super continent Pangaea resulting in considerable volcanic activities and climatic shifts. As the land shifted to form the Atlantic Ocean, the Gulf of Mexico and other ocean basins, the global ocean called Panthalassa shifted into the Pacific Ocean. At the time, it is thought that global climate was much warmer than today with no ice at the poles and sea levels much higher. The separation of the continents contributed to the formation of many new shallow coastal marine habitats. While many of us know something about the dinosaurs that roamed the land, much less has been shared about the oceans.
The first stony corals
Ocean plankton evolved during the Mesozoic Era and became the base of the ocean food web. Some of the phytoplankton include species that are present today, such as diatoms called coccolithophores. These diatoms produce a calcium carbonate skeleton that resemble car hub caps. They were so abundant that their skeletons settled on the ocean floor. The marine phytoplankton of that era contributed to about 60 per cent of the world’s oil reserves. Along the shallow margins of the ocean, scleractinian or stony coral reefs evolved in the mid-Triassic period forming symbiotic relationships with marine phytoplankton called zooxanthellae. The coral hosts the plankton in its cells and provide it with essential nutrients, while the zooxanthellae supplied the coral with food from photosynthesis. This relationship is responsible for reef formations. While the coral species were distinct from today’s species, some ancestors are still present. The corals were distinct in shape and size but contributed significantly to reef development in the shallow seas. By the end of the era, the corals died out and were replaced largely by rudist (tube or cone-shaped) bivalves or clams that dominated the reefs. These bivalves were a major source of food for many marine creatures.
Fish life
Across the reef and ocean habitats, fish life was abundant. The first teleost fish – aka fish with movable jaws – evolved during the Mesozoic Era, and most modern-day fish are teleost species. Some examples of large fishes at the time include the Xiphactinus Audax species which resembled a fanged tarpon and was one of the largest bony fish of the Late Cretaceous period. It could grow up to four metres and was capable of swallowing prey whole. Another fish, Leedsichthys, named after the location, where the fossils were found (Leeds UK), had a skeleton that was partly made up of cartilage, similar t
