Fossil in a Fossil Offers Evidence of Mega Predators in Triassic Oceans

Friday, August 21, 2020

(Jiang, Motani, et al. / iScience)

Paleontologists have made the unusual discovery of a near perfect ichthyosaurus skeleton with another impressive skeleton residing within its stomach, proving that the prehistoric reptile was in fact an apex predator.

Details of this study, conducted by Ryosuke Motani, a professor of paleobiology at the University of California, Davis, Da-Yong Jiang, a paleontologist at Peking University in China, and colleagues, are published Thursday in the journal iScience.

Found during a dig in a quarry in southwest China, this two-in-one fossil is the oldest of its kind to show such clear evidence of ichthyosaurs’ predatory diet. Also unique is the size of the stomach contents, as the second fossil was a 13-foot-long aquatic reptile known as the thalattosaur. 

Although they lived side by side with them, ichthyosaurs were not dinosaurs and were most closely related to lizards and snakes with the relative shape like a dolphin. This animal was nearly the size of the modern blue whale, able to reach up to a staggering 85 feet long, though most grew to an average length of five to 11 feet.

This ichthyosaurus’ last meal, the thalattosaur or “ocean lizard,” was another species of marine reptile and was among the first land-dwellers to return to the ocean. 

Thalattosaurs were generally nine to 13 feet long with a long, pointed, tweezer-shaped nose that paleontologists suspect was good for finding food in small crevices of coral reefs. Their fossils are normally quite rare, but to find one in the stomach of another creature is a massive stroke of luck.

“If you look across all the similar marine reptiles that lived in the age of dinosaurs, we’ve actually never found something articulated like this in the stomach,” said Motani. “Our ichthyosaur’s stomach contents weren’t etched by stomach acid, so it must have died quite soon after ingesting this food item. At first, we just didn’t believe it, but after spending several years visiting the dig site and looking at the same specimens, we finally were able to swallow what we were seeing.” 

Paleontologists usually deduce an animal’s diet based on the size and shape of their teeth and jaws, making this discovery even more special as they could actually see what was on the marine creature’s menu. 

It is generally accepted that apex predators, like the notorious Tyrannosaurus Rex, don large, razor-sharp teeth for cutting and tearing food, though modern day predators like hippos and crocodiles challenge this belief. What they lack in sharp teeth they make up for with jaw strength, able to crush their prey for consumption.

Ichthyosaurs share these same dental characteristics with crocodiles, but the authors say that with no solid evidence of its prey of choice, they were led to believe that the predator fed exclusively on smaller aquatic animals, specifically cephalopods and fish. This new evidence, however, completely changes the game.

“Now, we can seriously consider that they were eating big animals, even when they had grasping teeth,” said Motani. “It’s been suggested before that maybe a cutting edge was not crucial, and our discovery really supports that. It’s pretty clear that this animal could process this large food item using blunt teeth.”

The authors remain unsure whether the giant thalattosaur was preyed upon by the prehistoric sea monster or scavenged, but Motani notes that judging by the state of the skeleton, there are little to no signs of scavenging. 

Previous research has shown that decomposition of this reptile would have at least resulted in detached limbs, which was not the case. Instead, its limbs were still intact and only the tail was separated and found nearby, likely lost in a fight with a predator.

“We now have a really solid articulated fossil in the stomach of a marine reptile for the first time,” Motani said. “Before, we guessed that they must have eaten these big things, but now, we can say for sure that they did eat large animals. This also suggests that megapredation was probably more common than we previously thought.”

Another noteworthy ichthyosaurus fossil was found in May of 2016 by archaeologist Paul de la Salle at a beach at Lilstock, Somerset. This find was particularly significant because it confirmed the baffling size these sea reptiles could reach. 

The team recovered a 3.2-foot jaw bone that challenged any size estimations of the past, but after comparing it with a much more complete ichthyosaur fossil from 2004, they concluded that the fossil in question was a staggering 85 feet long.

The authors note that the dig site is still active and has been made into a museum. They hope to find more storytelling fossils as the work continues.

“We’ve been digging in that particular quarry for more than 10 years now, and still, new things are coming out,” Motani added. “At this point, it’s beyond our initial expectations, and we’ll just have to see what we’ll discover next.”