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Polyptychodon is a member of the plesiosaur family, which constituted a group of reptiles that were adapted to life in the shallow seaways that covered much of Texas 90 million years ago. The plesiosaurs form their own distinctive branch of the reptile family tree. Although commonly mistaken for dinosaurs, the plesiosaurs are only distant relatives. They were also very different from the mosasaurs, which formed another lineage of giant aquatic reptiles. The plesiosaurs became extinct near the end of the Cretaceous Period.
The arms and legs of plesiosaurs were modified into flippers that they used to ‘fly’ through the water, much like modern sea turtles do. Some plesiosaurs had long necks and small heads, while others had short necks and very large heads, and many grew to gigantic size. They had long, sharp teeth characteristic of animals that catch and eat fish. Together with the mosasaurs, they were among the dominant predators of the Mesozoic oceans. Although they were reptiles, they probably spent nearly all of their lives in the water.
Dr. J. R. (Bob) McDonald, an Austin dentist who was looking for shark teeth along Shoal Creek, discovered the specimen buried in the Dino Pit. He reported the find to paleontologists at the Texas Memorial Museum´s Vertebrate Paleontology Laboratory, who collected it and put it on display in the Texas Memorial Museum, in the early 1990´s.