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Ichthyosaur: Apex predator of the dinosaur-era seas

3D science rendering of an Ichthyosaur stenopterygius, large extinct marine reptiles from Early Triassic to Late Cretaceous.
This is a 3D science rendering of an Ichthyosaur stenopterygius, a large extinct marine reptiles from the Early Triassic to Late Cretaceous period. (Image credit: Dotted Yeti via Shutterstock)

Ichthyosaurs were predatory marine reptiles that could grow to enormous sizes and ruled the seas during part of the dinosaur era. For a chunk of the millions of years when dinosaurs reigned over Earth's landmasses, ichthyosaurs claimed the title as maritime apex predators.

These massive "sea monsters" appeared on Earth about 250 million years ago, just before the emergence of dinosaurs (around 230 million years ago), and ichthyosaurs died off about 90 million years ago, just before the nonavian dinosaurs went extinct, nearly 66 million years ago. Ichthyosaurs diversified into a variety of body plans, but you could summarize their evolution as early, eel-like forms transitioning into the dolphin-like appearance typical of most later ichthyosaur species, said evolutionary biologist Ryosuke Motani (opens in new tab), a professor in the Department of Geology at the University of California Davis (opens in new tab).

Are ichthyosaurs considered dinosaurs?

Don't get fooled by that "-saur" suffix: Despite occupying the same Mesozoic era as the dinosaurs, ichthyosaurs were no dinos. Instead, alongside other such sea creatures like plesiosaurs and mosasaurs, they belonged to a separate group of archosaurs — marine reptiles.

Despite having a streamlined body, fins and an elongated head with a pointed nose, reptilian ichthyosaurs aren't closely related to dolphins or fish, either. Instead, in an example of convergent evolution, diverse and unrelated lineages of swimmers — mammals like dolphins, fish like sharks and sturgeons, and ancient reptiles like the ichthyosaurs — came to resemble one another because of similar evolutionary pressures. "Subjected to the same environmental forces, fish, aquatic reptiles and aquatic mammals independently evolved similar shapes because a streamlined body is the most efficient way of moving through the dense medium of water," wrote John Blamire (opens in new tab), a professor of biology at Brooklyn College in New York.

In "The Panda's Thumb (opens in new tab)" (W. W. Norton and Co., 1992), naturalist Stephen Jay Gould nominated the ichthyosaur as "the most astounding" example of convergent evolution on record. "This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design," Gould wrote. The ichthyosaur remained a reptile through and through, however, Gould continued, "from its lungs and surface breathing to its flippers made of modified leg bones, not fin rays."

Ichthyosaur fossil at the North Carolina Museum of Natural Sciences.

A photo of an ichthyosaur fossil at the North Carolina Museum of Natural Sciences. (Image credit: Qualiesin)

That hyperspecialization to the sea makes pinning down ichthyosaurs' lineage challenging, according to the University of California Museum of Paleontology (UCMP) (opens in new tab). They may constitute an offshoot of the diapsid, a large group of related organisms that also includes dinosaurs, birds and snakes. Or they may come from a distant turtle relative. 

However ichthyosaurs originated, recent studies have shown that they burst onto the scene, evolutionarily speaking. A 2016 study describing an early ichthyosaur suggested that the group first appeared after the end-Permian mass extinction — Earth's greatest mass extinction, which occurred about 251.9 million years ago — and then quickly diversified into new forms. The ichthyosaur described in the study, Sclerocormus parviceps, had an atypical anatomy, with a toothless, stubby snout that it may have used to suck up food. That odd design signaled that "ichthyosauriforms evolved and diversified rapidly," said Olivier Rieppel, a curator of evolutionary biology at The Field Museum in Chicago. "These ichthyosauriforms seem to have evolved very quickly, in short bursts of lots of change, in leaps and bounds," Rieppel said in a statement (opens in new tab) about the find.

Ichthyosaurs also took an evolutionary fast-track to achieve their great size, researchers reported in 2021. A newfound species, Cymbospondylus youngorum, would have stretched more than 55 feet (17 meters) long based on extrapolations from its 6.5-foot-long (2 m) skull, scientists reported in the journal Science (opens in new tab). But the enormous creature appeared only about 2.5 million years after ichthyosaurs' debut, or within 1% of their 150 million years on the planet. By comparison, whales plodded along for 90% of their 55 million years before getting so large.

How big were ichthyosaurs?

Not all ichthyosaurs got so girthy. The representative Ichthyosaurus genus, which gives the larger group its name, spanned about 10 feet (3 m) long on average and looked very fish-like, according to Britannica (opens in new tab). Most ichthyosaurs were much smaller than their biggest cousins, such as several species of Ichthyosaurus found in the United Kingdom stretching just 5 to 11 feet (opens in new tab) (1.5 to 3 m). The general ichthyosaur length ranged from 3 to 65 feet (1 to 20 m), with an average length of 6.5 to 13 feet (2 to 4 m). Some measured under 3 feet (1 m) long, while the Chaohusaurus geishanensis (opens in new tab) species was likely smaller than 28 inches (70 centimeters) long and some species could be as tiny as 1 foot (0.3 m) long.  

But by the late Triassic period, several ichthyosaur species attained great size, Britannica (opens in new tab) said. Shonisaurus popularis and others could exceed 50 feet (15 m) long, Motani wrote, while a 2004 specimen reached an estimated 69 feet (21 m) (opens in new tab) in length. A more recent discovery suggests that some ichthyosaurs rivaled blue whales in size. In 2018, scientists reported finding fragments of a massive, 3.1-foot-long (96 cm) ichthyosaur jawbone, which scales up to hint at a body that would have measured approximately 85 feet (26 m) or more. That's 25% larger than the previous, 69-foot-long record holder. By comparison, blue whales come in at 80 to 100 feet (24 to 30 m) long.

A direct comparison of two ocean giants from different epochs side by side: The Triassic Cymbopsondylus youngorum (the new species described in the paper) vs. today’s sperm whale, with human for scale.

A direct comparison of two ocean giants from different epochs side by side: The Triassic ichthyosaurs vs. today’s sperm whale, with human for scale. (Image credit: Illustration by Stephanie Abramowicz, courtesy of the Natural History Museum of Los Angeles County (NHM).)

"Biology textbook[s] have long touted the modern blue whale as the largest animal that ever lived, but this and other fascinating fossil finds hint that there may once have been even bigger creatures swimming Earth's seas," National Geographic wrote (opens in new tab) about the 2018 find.

A tooth unearthed in the Swiss Alps in 2022 hints at an even bigger ichthyosaur. At 4 inches (10 cm) long and 2.3 inches (6 cm) wide, the broken chomper is twice as wide as the previous record-holder for ichthyosaur teeth, which came from a beast nearly 50 feet (15 m) long. Because researchers have only part of a tooth to go on, however, they don't yet know if this find represents one of the biggest ichthyosaurs or just one with a particularly impressive smile.

For those wondering if they could bench press an ichthyosaur, Motani provided some rough estimates (opens in new tab) on weight, warning that estimating the body mass of extinct animals "usually involves many assumptions." Smaller ichthyosaur species like the 1.5-foot-long (0.45 m) Stenopterygius weighed in at an estimated 2 pounds (0.9 kilograms), while larger examples like the 13-foot-long (3.9 m) Ophthalmosaurus icenicus may have carried about 2,070 pounds (940 kg) of mass.

What did ichthyosaurs eat?

Carnivores to their core, ichthyosaurs fed on sea beasts large and small. Bigger species dined on squid, fish and other marine creatures (opens in new tab). Studies of stomach contents from the fish-shaped ichthyosaurs provide evidence of that diet, Motani noted (opens in new tab), with studies going back as far as 1853 documenting hooklets from squid-like animals, such as belemnites, in ichthyosaurs' stomachs. In their preference for squids, ichthyosaurs resembled modern whales, Motani said.

Exceptionally large eyes in some species may have aided in deep-sea diving to hunt squid (opens in new tab) — or in spotting other predators, like plesiosaurs. In the only modern animals with similarly gigantic peepers, the colossal and giant squids, such eyes contribute more to spying and avoiding hungry whales than hunting dinner, scientists explained in a study published in 2012 in the journal Current Biology (opens in new tab).

The ichthyosaur tooth is 4 inches long (10 cm), and missing part of its crown. The beast that bore it may be one of the alrgest sea monsters ever.

The ichthyosaur tooth is 4 inches long (10 cm), and missing part of its crown. The beast that bore it may be one of the largest sea monsters ever. (Image credit: Dr Martin Sander/ Dr Heinz Furrer)

But because ichthyosaurs ranged widely in size and existed for so long, they likely subsisted on a variety of prey gathered in a variety of ways. Big ichthyosaurs, like the large Triassic-period Thalattoarchon, boasted dangerous teeth that were likely used for catching prey the animals' own size (opens in new tab), while short-snouted beasts like the Guanlingsaurus may have relied on suction feeding (opens in new tab). Later, larger ichthyosaurs' shapes, characterized by long fins and bodies that were thick from back to belly, suggest they hunted by ambushing fish, according to Britannica (opens in new tab).

A Cretaceous ichthyosaur described in 2021 likely terrorized ancient prey with its fearsome teeth. Called Kyhytysuka sachicarum, the beast was discovered in central Colombia and wielded more and larger knife-like teeth than any other ichthyosaur species. "Whereas other ichthyosaurs had small, equally sized teeth for feeding on small prey, this new species modified its tooth sizes and spacing to build an arsenal of teeth for dispatching large prey," paleontologist Hans Larsson, an associate professor at McGill University's Redpath Museum in Montreal, Canada, said of the find (opens in new tab).

When and where did ichthyosaurs live?

As ichthyosaurs swam Earth's oceans for 160 million years (from 250 million to 90 million years ago), they had ample time to spread around the globe. And as creatures of the sea, they could range far and wide. The earliest finds from the lower Triassic have popped up in sites from Canada to China and Japan, as well as the island of Spitsbergen in Norway, Motani wrote for UCMP (opens in new tab). By the middle Triassic, ichthyosaurs had achieved global distribution, scientists reported in 2002 in the journal Paleontological Society Papers (opens in new tab), and since the late 20th century, researchers have identified and named ichthyosaurs on all seven continents.

The creatures' "very wide geographic distribution" is preserved in fossils from southern Germany, England, the western United States and Canada, according to Britannica (opens in new tab). Finds in the 21st century have cropped up from the southernmost tip of Chile's Patagonia region (opens in new tab) in 2003 to Alaska (opens in new tab) in 2014, with a find representing the "northernmost appearance of a well-preserved Triassic ichthyosaur."

The creatures' range of feeding habits helped ichthyosaur species adapt to a range of ocean environments, as well. In the 2002 study, scientists noted that ichthyosaur feeding realms varied from nearshore during the Triassic to pelagic (neither close to the seafloor nor the shore) during the Jurassic and Cretaceous. Deep-diving ichthyosaurs could plunge down to 2,000 feet (600 m).

Whereas ichthyosaurs evolved from land-based reptiles that returned to the sea, much as dolphins and whales arose from re-oceaned mammals, the ancient sea beasts were unlikely to crawl onto land, according to Britannica (opens in new tab): "If stranded ashore, they would have been as helpless as beached whales."

Additional resources

See ichthyosaur fossils and models, as the iconic marine reptiles join other "Monsters of the Deep" in a new exhibit at the Field Museum in Chicago. Learn about a pregnant ichthyosaur that was recently excavated in Chile. And did you know that ichthyosaurs and other ancient marine apex predators sometimes fed upon each other?

Original article on Live Science.

Michael Dhar
Live Science Contributor

Michael Dhar is a science editor and writer based in Chicago. He has an MS in bioinformatics from NYU Tandon School of Engineering, an MA in English literature from Columbia University and a BA in English from the University of Iowa. He has written about health and science for Live Science, Scientific American, Space.com, The Fix, Earth.com and others and has edited for the American Medical Association and other organizations.