From Massive Eyes to Shark-Like Tails, Seven Amazing Adaptations That Helped Prehistoric Reptiles Thrive at Sea
The creatures cruised the world’s oceans with features we often associate with marine mammals, such as coats of blubber and the ability to birth live young
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The Mesozoic wasn’t just the age of dinosaurs. Reptiles of all sorts thrived through the Triassic, Jurassic and Cretaceous at the same time our favorite dinosaurs were roaming on land. Long-necked plesiosaurs, shark-shaped ichthyosaurs, toothy mosasaurs and many more saurians were swimming through the ancient seas, their success underwritten by a suite of anatomical specializations.
From the time the four-paddled, long-necked Plesiosaurus was named in 1821, paleontologists generally thought of marine reptiles as comparable to lizards, snakes and crocodiles in the terrestrial realm. Marine reptiles were thought to be cold-blooded, their body temperature determined by the surrounding seawater. And scientists didn’t know whether the ancient creatures had blubber, scales, smooth skin or some other sort of body covering. During the past three decades, however, a new wave of paleontological interest has fundamentally changed how experts think about the likes of Mosasaurus and Ichthyosaurus. The repeated evolution of reptiles that lived entirely at sea was possible thanks to an array of adaptations that are still being uncovered by researchers. This list highlights the features that allowed Mesozoic marine reptiles to thrive beneath the surface.
Blubbery bodies
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Many millions of years before whales and seals would evolve blubber, marine reptiles were already enjoying the benefits of the thick subcutaneous fat. In 2018, paleontologists described a fossil of the Early Jurassic ichthyosaur Stenopterygius that preserved parts of the marine reptile’s skin as well as remnants of fats and proteins that made up the animal’s flesh.
By 180 million years ago, the fossil revealed, ichthyosaurs had smooth skin. Clues from those fossil tissues also hinted that Stenopterygius was darker above and lighter below, a look biologists call countershading, which helps organisms blend in when seen from above or below in the water. But the findings also revealed that ichthyosaurs evolved blubber beneath their skin, an insulating layer of fat that helped them maintain warm, constant body temperatures.
A shark-like tail
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The largest lizards of all time didn’t live on land. They swam in the seas. Mosasaurs, like the famous Mosasaurus itself, were seagoing lizards related to today’s monitor lizards that proliferated through Cretaceous seas between 66 million and 94 million years ago. Naturally, living in the seas required that mosasaurs evolve some traits better suited to the water than land. The shape of their limbs left no doubt that the hands and feet of mosasaurs evolved into broad paddles that helped them steer, but only recently have paleontologists recognized mosasaurs had shark-like tail fins, too.
First described in 2010, a stunning fossil of the mosasaur Platecarpus embodies how the reptile’s tail vertebrae fit together when the lizard was alive and swimming. The bones lined up such that the end of the mosasaur’s tail is turned downward. Experts have seen this change in tail angle before, both in ichthyosaurs that had crescent-moon-shaped tail fins and in modern sharks, in which the end of the tail angles upward to support the broad tail below. Mosasaurs that lived farther from shore and cruised the seas before speeding up to nab prey, especially, seem to have had modified spines to support such tails.
Born into the sea
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Some marine reptiles alive today lay their eggs on land. Sea turtles, for example, haul themselves out onto beaches during breeding season to dig nests and plop dozens of eggs into the sandy divots before returning to the waves. Paleontologists used to wonder if some marine reptiles did the same, awkwardly dragging themselves onto beaches to lay eggs rather than birthing their young at sea like whales do.
But paleontology was still a new science when experts began to find clues that marine reptiles were more like their marine mammal counterparts than they first assumed. In 1852, paleontologists began to report on exceptional Jurassic ichthyosaurs preserved with embryos near the base of their tails. Experts have found that the babies were pushed out of their mothers by gases during decomposition before preservation. Over time, evidence for live birth in other marine reptiles has piled up. A fossil of the long-snouted, short-necked plesiosaur Polycotylus, for example, was found with the bones of an embryo inside, and mosasaurs evolved the ability to give live birth, as well. While many more forms of marine reptile lived during the Mesozoic, the abundant ichthyosaurs, plesiosaurs and mosasaurs all spent their entire lives in the water.
Streamlined scales
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Varied lineages of marine reptiles all underwent different modifications as they evolved in the aquatic realm. The physical disparities among ichthyosaurs, plesiosaurs, mosasaurs and other marine reptiles underscore the fact that there was no single, optimal way to be a saurian in the water. Body coverings are one example. While ichthyosaurs slid through the water with slick skin, other marine reptiles evolved streamlined scales.
A fossil of the large mosasaur Plotosaurus described in 2009 preserved fossil soft tissues along with the bones, including scaly skin. The scales of Plotosaurus were small and roughly similar to those of modern lizards, but they possessed an important specialization. These scales were keeled in such a way that they streamlined the lizard’s body and would have allowed it to swim with less effort, a critical adaptation for a predator thought to have cruised open waters. Plesiosaurs had more of a mix-and-match of smooth skin and scales. A 183-million-year-old plesiosaur fossil described earlier this year displayed smooth skin on the body and small scales on the flippers, which could have helped the appendages cut through the water and also given the plesiosaur better grip when dipping down to the sea bottom to trawl for morsels in the sand.
The eyes have it
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All marine reptiles breathed air. Despite their amazing array of other adaptations, they were always tied to the surface. But some may have gone deep. The big eyes of the ichthyosaur Ophthalmosaurus offer a clue that some marine reptiles dove into darker waters in search of food.
Many ichthyosaur species had large eyes, supported inside by thin bones arranged in a ring. The eyes of Ophthalmosaurus, especially, were large both for its body size and in absolute terms, measuring more than nine inches across. Given the relationship between the size and proportions of eyes and how they interact with light, paleontologists have ascertained that Ophthalmosaurus was able to see in dim, low-light conditions. Estimates of how deep Ophthalmosaurus could dive exceed 2,000 feet, deep enough that low-light vision would have been incredibly useful in finding squid-like cephalopods and other grub to dine on.
Running hot
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One of the most persistent mysteries paleontologists face about extinct reptiles is what their physiology was like. During the past half-century, in particular, paleontologists have realized that dinosaurs and other prehistoric reptiles once thought to be “cold-blooded” and similar to living reptiles actually maintained high, often constant body temperatures. The same realization has bubbled up around marine reptiles.
A 2010 study of extinct marine reptiles compared geochemical proxies for temperature in fossil teeth with those found in the fossils of extinct fish, generally regarded as having body temperatures heavily influenced by ocean temperature. The researchers found that both ichthyosaurs and plesiosaurs had elevated body temperatures compared with the fish. The reptiles were likely warm-blooded, which is consistent with the idea that they actively pursued prey instead of hunting by ambush as a crocodile does. Mosasaurs have been more difficult to assess, but a 2016 study using similar methods found evidence that the sea lizards, too, evolved to have elevated body temperatures that helped make them faster and more persistent in pursuit of their meals.
Growth spurt
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Prior to the Triassic, no large marine reptiles existed. Only after the worst mass extinction of all time, as Earth’s ecosystems struggled to recover from intense global warming spurred by volcanoes, did reptiles began to live by the shoreline and become ever more at home in the water. Ancestral ichthyosaurs were among these early swimmers, and the “fish lizards” got big fast.
In 2021, paleontologists described the skull of the Triassic ichthyosaur Cymbospondylus youngorum. Based upon the skull, the living reptile exceeded 50 feet in length and was comparable in size to large whales today. Most striking of all, however, is that the giant ichthyosaur lived 244 million years ago, just five million years after the first, tiny ichthyosaurs. Ichthyosaurs evolved to giant size at a faster rate than early whales did. The reason why, like so many other facets of marine reptile history, is still being investigated for experts. Oceans cover most of our planet, after all, and paleontologists have only found a fraction of the marine reptiles that have lived among the ancient seas. Researchers will undoubtedly keep delving deep to investigate how so many real-life sea serpents thrived on our world.
Illustration by Emily Lankiewicz / Lindgren et al. PLOS One, August 2010 via Wikipedia under CC By 2.5 / Captmondo via Wikipedia under CC By-SA 3.0 / Kim Alaniz - Paleo Hall at HMNS Uploaded by FunkMonk via Wikipedia under CC By 2.0 / Mariolanzas via Wikipedia under CC By-SA 4.0
