A Plesiosaur That Ate Ichthyosaurs for Lunch

Finding an articulated fossil skeleton is always a delight for paleontologists. Not only do such specimens illustrate how all those bones went together, but sometimes there are little associated bonuses that could not be seen if the skeleton had been scattered. In the case of a 161- to 155-million-year-old plesiosaur recently discovered in Wyoming, the marine reptile died with its last meal preserved in its stomach.

Today Wyoming is far removed from the sea, but during the Late Jurassic a body of water called the "Sundance Sea" covered what is now the "Equality State." Numerous marine reptiles swam in those waters, including plesiosaurs and ichthyosaurs. When these reptiles were first found, they were often depicted as locked in mortal combat, two sea dragons that were destined to duel, but evidence of such awesome battles has not been found. What has now been recovered, however, is the skeleton of a plesiosaur similar to Pantosaurus striatus with the partially digested remains of a baby ichthyosaur inside.

As reported in the latest issue of the Journal of Vertebrate Paleontology, among the preserved stomach contents of the partially-complete plesiosaur skeleton there were a series of small, ring-shaped bones. At first the researchers did not know what they were, but they eventually recognized the vertebrae of a prenatal ichthyosaur (which was confirmed by the discovery of a jaw fragment from the same animal). This was very strange. Typically ichthyosaurs at so early a stage of development are found inside or in association with their mothers. What was it doing in the belly of a plesiosaur?

Exceptionally-preserved ichthyosaurs from Holzmaden, Germany might hold the answer. Some ichthyosaurs of the genus Stenopterygius have been found there with baby ichthyosaurs sticking out of what would have been their mother's body cavity. It looks like the mother ichthyosaurs were preserved in the act of giving birth, but a more likely explanation is that the babies were being expelled from the body when the mother was either dead or dying. The babies might have been pushed out during the stress of death or the buildup of gases inside their mother's body might have forced them out after death, and these babies would have been easy meals for predators that were passing by. The plesiosaur that was the subject of the new research probably scooped up the embryonic ichthyosaur after its mother had expelled it; there was probably no titanic battle like the one envisioned in Journey to the Center of the Earth.

No doubt some will find this disappointing, but it is still a significant find. This is the first time ichthyosaur remains have been found inside a plesioaur. And even though it might be difficult to full reconstruct the chain of events as they occurred this new find provides us with a little window into life and death among marine reptiles during the ancient past.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.