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Ichthyosaur's last meal is evidence of triassic megapredation

Some 240 million years ago, a dolphin-like ichthyosaur ripped to pieces and swallowed another marine reptile only a little smaller than itself. Then it almost immediately died and was fossilized, preserving the first evidence of megapredation, or a large animal preying on another large animal. The fossil, discovered in 2010 in southwestern China, is described in a paper published in the journal iScience.

Ichthyosaur's last meal is evidence of triassic megapredation
The stomach of this 240-million year old fossil ichthyosaur contains the mid-section of another marine reptile that
in life would have been only slight smaller. It's the first direct evidence of ancient megapredation - one large
animal eating another [Credit: Da-Yong Jiang, et al., 2020]

The ichthyosaurs were a group of marine reptiles that appeared in the oceans after the Permian mass extinction, about 250 million years ago. They had fish-like bodies similar to modern tuna, but breathed air like dolphins and whales. Like modern orca or great white sharks, they may have been apex predators of their ecosystems, but until recently there has been little direct evidence of this.

Ichthyosaur's last meal is evidence of triassic megapredation
The ichthyosaur specimen with its stomach contents visible as a block
that extrudes from its body [Credit: Ryosuke Motani]
Ichthyosaur's last meal is evidence of triassic megapredation
The ichthyosaur's teeth, with the broken white line indicating the approximate
gum line of the upper jaw [Credit: Jiang et al., 2020]

When a specimen of the ichthyosaur Guizhouichthyosaurus was discovered in Guizhou province, China in 2010, researchers noticed a large bulge of other bones within the animal's abdomen. On examination, they identified the smaller bones as belonging to another marine reptile, Xinpusaurus xingyiensis, which belonged to a group called thalattosaurs. Xinpusaurus was more lizard-like in appearance than an ichthyosaur, with four paddling limbs.

Ichthyosaur's last meal is evidence of triassic megapredation
Ichthyosaur fossil with red box around fossil in stomach
[Credit: Jiang et al., 2020]
Ichthyosaur's last meal is evidence of triassic megapredation
Detail of mid-section of thalattosaur in ichthyosaur stomach
[Credit: Jiang et al., 2020]

"We have never found articulated remains of a large reptile in the stomach of gigantic predators from the age of dinosaurs, such as marine reptiles and dinosaurs," said Ryosuke Motani, professor of earth and planetary sciences at the University of California, Davis, and coauthor on the paper. "We always guessed from tooth shape and jaw design that these predators must have fed on large prey but now we have direct evidence that they did."

Ichthyosaur's last meal is evidence of triassic megapredation
Detached tail of thalattosaur found 75 feet away from ichthyosaur fossil
[Credit: Jiang et al., 2020]
The Guizhouichthyosaurus was almost five meters (15 feet) long, while the researchers calculate its prey was about four meters (12 feet) long, although thalattosaurs had skinnier bodies than ichythyosaurs. The predator's last meal appears to be the middle section of the thalattosaur, from its front to back limbs. Interestingly, a fossil of what appears to be the tail section of the animal was found nearby.

Ichthyosaur's last meal is evidence of triassic megapredation
The quarry dig site where ichthyosaur and thalattosaur were uncovered, now part
of the Xingyi Geopark Museum in China [Credit: Ryosuke Motani]
Predators that feed on large animals are often assumed to have large teeth adapted for slicing up prey. Guizhouichthyosaurus had relatively small, peg-like teeth, which were thought to be adapted for grasping soft prey such as the squid-like animals abundant in the oceans at the time. However, it's clear that you don't need slicing teeth to be a megapredator, Motani said. Guizhouichthyosaurus probably used its teeth to grip the prey, perhaps breaking the spine with the force of its bite, then ripped or tore the prey apart. Modern apex predators such as orca, leopard seals and crocodiles use a similar strategy.

Author: Andy Fell | Source: University of California, Davis [August 20, 2020]


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