In late May 2020, 11-year-old Ruby Reynolds and her father, Justin Reynolds, drove to Blue Anchor, a seaside village in Somerset in southwest England, to hunt for fossils along the beach.
Upon arriving, they found that somebody had left a piece of fossilized bone at the top of the beach. It was about four inches long, which was "bigger than any piece of bone I'd ever found before," says Justin. "So I was very excited and sat down to have a good look at it."
Meanwhile, Ruby kept walking along the beach, eyes scanning the ground. She soon found a second piece of bone half buried in a mud slope that was twice as long as the first — and much better preserved. "It was just sort of lying there," recalls Ruby. "I was just happy, really."
Ruby and her father would later discover that they'd just chanced upon part of the largest marine reptile ever found — a giant ichthyosaur from 202 million years ago, near the end of the Triassic Period.
These animals were once the dominant predators of the world's oceans. Imagine a chunky shark with a long, toothy snout, fins, and four flippers.
"These are reptiles only very, very distantly related to things like crocodiles," says Dean Lomax, a paleontologist at the University of Bristol and the University of Manchester. And they actually resemble whales, which are mammals, in key ways: "They gave birth to live young. They committed entirely to a life at sea — they didn't come onto land."
In research published in PLOS ONE, Lomax, Ruby and Justin Reynolds, and their colleagues describe this new species of ichthyosaur, Ichthyotitan severnensis, as roughly 82 feet long. That's twice the length of a school bus.
"There were things that we can't even possibly imagine in the past," says Kelsey Stilson, a biomechanist at the National Museum of Natural History in Paris who wasn't involved in the research. "But we can get little hints. And this is one little hint at this larger picture" of how life evolved on Earth.
A prehistoric puzzle
The discovery of Ichthyotitan severnensis actually began almost eight years ago. Paul de la Salle, an avid amateur fossil collector, was combing a beach in Lilstock in Somerset when he came upon multiple chunks of fossilized bone. He showed them to Lomax, a longtime friend of his.
"They fit together perfectly like an ancient prehistoric jigsaw puzzle," says Lomax. "And we could work out that it belongs to a jawbone of a reptile called an ichthyosaur that swam in the seas at the time of the dinosaurs."
But the thing that struck Lomax was that this bone, called a surangular, was big — over three feet long. "It suggests that it was from something unusual, and something exceedingly large," he says.
Lomax, de la Salle, and their colleagues published a description of the specimen, but the bone was incomplete and partly eroded, so they had to kind of leave it at that — as tantalizing as it was to imagine what this animal may have looked like.
"What we'd hoped for — we kept our fingers crossed — we hoped that maybe more specimens would come to light in the future," says Lomax.
A couple years passed. Then, in May 2020, Lomax received an email from Ruby and Justin Reynolds who'd just concluded their trip to Blue Anchor, their two fossil treasures in hand. He recalls that the message said something to the effect of, "Hey, Dr. Lomax — we think we've found another one of your giant ichthyosaur jawbones."
Lomax downloaded the images. "Of course, they were quite right," he says. "They correctly identified these sections of bone as belonging to an ichthyosaur." Lomax was encouraged by how well preserved the Reynolds' fossils were. He organized subsequent collection trips to the beach in Somerset, which yielded additional fragments of the same jawbone.
This second specimen was nearly twice as complete as the first. Lomax and his colleagues were now confident that this was a new species, which they named Ichthyotitan severnensis after the Severn Estuary in Somerset where it was found. He cautions that without a full skeleton, he can't be sure of the size. The animal could have had a giant head and a more compact body. But if it was built like other ichthyosaurs, Lomax says it would have been massive.
"Genuinely enormous, about the length of a blue whale," he says.
This makes it the largest ichthyosaur — and therefore the largest marine reptile — ever described. In fact, Lomax says the animal may not have even been fully grown when it died.
The end of an era
The Triassic Period was, in Kelsey Stilson's words, "a really weird time."
"That's when you see the first very early mammals and the first very early dinosaurs," they say. "Everything's starting right then."
The end of the Triassic was when Ichthyotitan severnensis reigned, at a moment right before a mass extinction event that swallowed up these animals forever.
"No marine reptile ever reached such gigantic sizes ever again," says Lomax.
Not all ichthyosaurs vanished, though the ones that survived and evolved during the Jurassic and Cretaceous Periods were smaller. But they too went extinct around 94 million years ago, which created an opening in the ocean.
It was "a niche to be filled," says Lomax, "which is where eventually you had mammals — including the first whales that were walking around on land — [make] that transition into the ocean."
With the top perch in the food web now vacated, whales took over as the dominant marine predators.
And underwater reptilian rule was over.
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