Episode 231 – Back to the Water (Secondarily Aquatic Vertebrates)

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Ever since an unusual group of fish gave rise to the first land-dwelling vertebrates, their descendants have been returning to the water. This episode, we explore the many evolutionary paths of Secondarily Aquatic Vertebrates.

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Back to the Water

Nearly 400 million years ago, a group of fish gave rise to the first land-dwelling vertebrates. Since then, many of their descendants have made the transition back to aquatic lifestyles. These secondarily aquatic vertebrates include some of the most successful groups of animals in Earth history.

The shift from land to water involves a host of challenges for which these groups have evolved remarkably similar solutions. Paddle-like flippers and tails allow these animals to swim efficiently, while dense fur or blubber help them retain heat in the water. Salt glands prevent the body from taking in too much salt from seawater, and various changes to the respiratory system allow for deep breaths between dives.

Some swim primarily using undulations of their bodies, such as cetaceans, crocodilians, and (of course) sea snakes. On the other hand, rigid-bodied animals like sea turtles and penguins propel themselves mainly using their flippers. Due to the differing structure of their spinal columns, aquatic mammals tend to undulate up-and-down while reptiles swim side-to-side like their distant fish ancestors.

The fossil record is full of extinct examples among mammals, birds, and reptiles. In some cases, aquatic lifestyles are easily inferred from the flipper-like shape of the limbs, the density of the bones, and the location of the fossils in sediments that formed underwater. But this lifestyle is so variable that the details – such as how these ancient species swam, reproduced, or kept warm – can be difficult to determine.

Some fossils present a confusing mixture of signals, leading to lots of debate about just how aquatic they really were. Famous examples include the Triassic tanystropheids, the mammalian desmostylians, and of course the dinosaur Spinosaurus.

Fully terrestrial vertebrates began evolving back into the water almost as soon as they had left it. Semi-aquatic reptiles are known from the Late Carboniferous Period, mere millions of years after the origins of their ancestors who had left behind amphibious habits. The oldest fossils considered to be fully aquatic reptiles are the mesosaurs of the Early Permian Period. Then, in the Triassic, several more lineages of marine reptiles spread through the seas.

Learn More

The locomotion of extinct secondarily aquatic tetrapods (technical, open access)
The Role of Ancestry on Secondarily Aquatic Transitions (technical, open access)
Sensory biology of aquatic mammals (technical, open access)

Was Mesosaurus a Fully Aquatic Reptile? (technical, open access)

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 70 – Convergent Evolution
• Episode 77 – Fins to Feet: the Fish-Tetrapod Transition
• Or the many episodes we’ve done on secondarily aquatic tetrapod groups: marine crocs, sea snakes, turtles, ichthyosaurs, plesiosaurs, mosasaurs, pinnipeds, penguins, whales, and specifically toothed whales

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