The marine world in the age of dinosaurs

Do you ever wonder what lived in the ocean when the dinosaurs roamed the land? Dr Anjani Ganase shares what life might have been 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.

 

 

Map of the world during the Jurassic Period, approximately 170 million years ago

 

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 % 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 to modern-day sharks. The leedsichthys is known to have grown as large as whales, up to nine metres in length and capable of filter feeding on plankton. They can open their mouths very wide to gather the plankton. Other fish species of that era were similar to species of sturgeon, gar, bowfins, salmon, trout, cod and anchovies.

 

Duria Antiquior, a more ancient Dorset by Henry de la Beche. A pictorial representation of marine life during the Jurassic period based on the fossils found by Mary Anning, a palaeontologist

Marine Reptiles

Reptiles (not dinosaurs) entered the marine world in the Triassic period. In the ocean they diversified quickly evolving to form new species to occupy the many vacant niches of the sea. Many became ocean predators. Most of these reptiles have been studied through fossils and paleo-biology. One example of the top predators in the ocean include the ichthyosaur, an ocean predator that resembles a dolphin in shape though it propelled itself through the water by moving its tails side to side like an eel.  The Plesiosaurus is a long necked marine reptile with large front flippers for thrust, two small flippers in the back and a long tail as a rudder. The mythical Loch Ness monster bears a close resemblance to the plesiosaurus. This animal grew up to three metres in length and was an ambush predator feeding on prey in shallow coastal waters but also grazing on crustaceans that grew along the bottom. The Mosasaurus, which emerged near the end of the Mesozoic from land-dwelling lizards are related to monitor lizards. These creatures fed on crustaceans and bivalves that were abundant at that time. They could grow up to 17 metres (based on fossil records) and fossils of the animal have been found worldwide. Paraplacodus is another marine reptile which resembled a giant newt that swam around the ocean also feeding on giant bivalves on the sea floor. Finally, sea turtles evolved during the Triassic Period, continuing to persist today. One ancestor species of turtle included the Archelon that grew to five metres in length; these are now extinct.

Modern-day sharks appeared near the end of the Jurassic period and rivalled the marine reptiles in size, growing up to three metres. Yet they were not the top of the food chain, being prey to dinosaurs. The most notable species was the Hybodus with razor sharp teeth for feeding on fish and other flattened teeth to break out molluscs. The hybodus also occurred in freshwater and saltwater habitats, roaming everywhere.

The Mesozoic Era ended when a meteorite hit earth and caused the loss of 80 % of all terrestrial life and 90 % of marine life. The impact resulted in tsunamis, acid rain and debris thrust into the atmosphere, enough to block out the sun and contribute to the sudden cooling of the earth’s climate. After the dinosaurs were wiped out, life took another turn, leading to the rise of the  era of mammals that grew to large megafauna both on land and in the sea. A few exceptions, such as marine turtles and crocodiles, survived from the age of dinosaurs and continue to live through the eras since then.

 

References

https://dinosaurpictures.org/

Thomas L. Stubbs et al. Ecomorphological diversifications of Mesozoic marine reptiles: the roles of ecological opportunity and extinction, Paleobiology (2016). DOI: 10.1017/pab.2016.15

Hall, Danielle, Ocean through time, Smithsonian Institution, https://ocean.si.edu/through-time/ocean-through-time

U.S Geological Survey, https://www.usgs.gov/youth-and-education-in-science/mesozoic

Science World, https://www.scienceworld.ca/stories/take-dip-prehistoric-ocean/

Stanley, George D. “The History of Early Mesozoic Reef Communities: A Three-Step Process.” PALAIOS, vol. 3, no. 2, 1988, pp. 170–83. JSTOR, https://doi.org/10.2307/3514528. Accessed 1 May 2023.

Veron J.E.N., Stafford-Smith M.G., Turak E. and DeVantier L.M. (2016). Corals of the World. Accessed 30 Apr 2023. http://www.coralsoftheworld.org/page/evolution/