How Marine Predators Evolved with Changing Oceans

For more than 250 million years, four-limbed land animals known as tetrapods have repeatedly conquered the Earth's oceans. Now new research is revealing just how these marine predators successfully evolved amidst this ever-changing environment.

By better understanding how marine life responded to environmental change over time, Smithsonian researchers hope to shed light on how human activities will impact modern marine species in the future.

"We know from the fossil record that previous times of profound change in the oceans were important turning points in the evolutionary history of marine species," Neil Kelley, who led the study, said in a news release. "Today's oceans continue to change, largely from human activities. This paper provides the evolutionary context for understanding how living species of marine predators will evolve and adapt to life in the Anthropocene."

Marine tetrapods represent a diverse group of living and extinct species of mammals, reptiles, amphibians and birds that play a critical role as large ocean predators in marine ecosystems. This includes creatures such as plesiosaurs, penguins,and sea turtles, all which descended from separate groups of terrestrial vertebrates that convergently evolved to thrive in aquatic environments.

Now, recent investigations in the fossil record have provided new insight into the evolution of traits that allowed marine tetrapods to thrive in the sea - even during changing marine ecosystems and mass extinctions dating all the way back to the Triassic period.

"Land to sea transitions have happened dozens of times among reptiles, mammals and birds, across major mass extinctions," noted researcher Nicholas Pyenson. "You often get similar looking results but convergence is more than skin deep. It can be seen on a broad range of scales, from molecules to food webs, over hundreds of millions of years."

For example, similar anatomy evolved among lineages that adapted to marine lifestyles. Modern dolphins and extinct marine reptiles called ichthyosaurs both evolved into a similar fish-like body plan - even though they descended from distinct terrestrial species, and lived more than 50 million years apart. (Scroll to read on...)

The repeated transformation of legs adapted for walking on land into fins is another example, seen in various species such as seals and mosasaurs. This allowed them to move quickly and efficiently in the water.

In addition, in the case of deep divers such as beaked whales and seals, these species have independently evolved to have positively charged oxygen-binding proteins called myoglobin in their muscles. This allows them to survive underwater for long periods of time.

This research helps to shed light on the common and unique patterns driving the extraordinary transitions that whales, dolphins, seals and other species underwent as they moved from land to sea.

It also suggests that if these creatures could adapt and survive in the Earth's changing oceans all these years, then perhaps they have a fighting chance in the face of modern climate change.

Ocean acidification, for one, is a major concern among scientists and conservationists alike. As the oceans absorb more and more carbon dioxide that is being pumped into the atmosphere - mostly from human activities, such as the burning of fossil fuels - the water is becoming increasingly acidic.

It is already wreaking havoc on marine life across the globe, but some species are working hard to adapt. For instance, sharks are hiding out in coral reefs and crabs are going into "survival mode."

In the past, ocean acidification has triggered a mass extinction event - the greatest one ever. But marine predators have evolved in a changing world before, and this latest research provides hope that they can do it again and continue to thrive in the Earth's oceans.

The findings were published in the journal Science.

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