Deadly Prehistoric Sea Monster Found Inside Second, Slightly Larger Sea Monster

Credit: Ryosuke Motani, UC Davis

Paleontologists have made a unique find: A 5-meter Triassic ichthyosaur with a 4-meter Triassic thalattosaur jammed down its gullet. By all appearances, the former literally bit off more than it could chew and choked on it.

One of the challenges of studying prehistoric ecosystems is figuring out what, exactly, was eating what. Coarse-grained distinctions and broad inferences can be made based on factors like body plan, relative size, dentition (when present), limb structure, and known characteristics of the prehistoric environment. But these are generalities: Knowing that a bird’s beak is well-adapted to catching insects does not automatically tell you which insects it ate, while the limited fossil record may not preserve evidence of predators and prey in the same geographical area.

The best way to figure out what a species ate is for said species to die and fossilize in a particularly useful manner immediately after eating. (Keep that in mind if you want to make a particularly useful donation to the future of archaeology and plan your funeral arrangements accordingly). Such finds are rare — and even when they happen, the resulting material may be no more than a fragment of dis-articulated bone, with no clues as to whether the creature was a predator, scavenger, or both.

Every now and then, however, you get lucky. Prior to this discovery, it was thought that ichthyosaurs fed on fish and cephalopods, and this particular species was not believed to be an apex predator. When the fossil was excavated in 2010, however, a mass of bone protruded upwards. Detailed examination revealed the remains of a thalattosaur, missing both its head and tail, and the bones were aligned within the fossil in a manner that made it possible to identify the orientation of the prey’s body as the predator ate it. There is no indication that the bones were exposed to stomach acid, suggesting death occurred shortly after ingestion. A nearly complete thalattosaur tail was found near the would-be predator, implying that it and the unfortunate prey had been previously acquainted.

This is as smoking-gun as it gets for proof that ichthyosaurs ate more than just squid and fish. While it’s still possible that the ichthyosaur scavenged the thalattosaur, existing models of water decay do not support this hypothesis. There are flippers inside the guts of the ichthyosaur, while the tail is lying a short distance away. In humans, the first body parts to rot off are the hands and feet. The vertebral column is reinforced with connective tissue that is much more resistant to decay.

Second, it would have taken time for the ichthyosaur to swallow prey very nearly its own size, and Guizhouichthyosaurus (that’s our predator) was an air-breather. Most carrion consumption in marine environments, the authors note, occurs on the seafloor, as in modern “whalefalls.” There’s no plausible mechanism for Guizhouichthyosaurus to have eaten a lot of thalattosaur while bouncing up and down off the sea bottom.

There are some interesting implications of this find. Guizhouichthyosaurus has a type of tooth structure called the “Smash” sub-type (you can check the paper) and is thought to have fed on very different types of food. The authors postulate that the creature may have torn the head and neck off the thalattosaur via the “grip and tear” method, which is the only method we see in modern creatures. The way the bones are packed inside the creature suggests the prey was eaten as a solid piece, but the vertebrae were broken into 3-4 “strings” of approximately 10 vertebrae each. The authors also postulate that an odd gap between the head of the ichthyosaur and the rest of its skeleton may have been somehow related to the way the creature died.

The ichthyosaur specimen. The raised stomach contents can be seen, as can the dis-articulated head.

This is less crazy than it might seem. Some 14 years ago, a python appears to have eaten an alligator before exploding some period of time later. This mostly decapitated the python. While it is not known exactly why the python died, one theory was that gas buildup inside the python related to the alligator’s decomposition might have blown it apart. If the ichthyosaur died immediately after eating the thalattosaur — and based on the nearby location of the tail, it did — then it’s also possible that gas buildup decapitated the creature post-mortem. Finally, and most boringly, it’s also possible that ordinary fossilization processes moved the head.

These sorts of “last battle” moments are rare in the fossil record, but not unknown. The most spectacular I’m aware of is the “Fighting Dinosaurs” fossil, which preserves the last moments of a battle between a velociraptor and a protoceratops. Both animals are believed to have been buried in a dune collapse or rapid sandstorm. The reconstructed posture shows the protoceratop’s jaws clamped around the velociraptor’s forelimb, while the velociraptor’s rear claw is buried in the protoceratops’s throat.

Feature image by Ryosuke Motani, UC Davis

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