Mauisaurus ("Maui reptile") is a genus of plesiosaur that lived during the Upper Cretaceous about 80 to 69 million years ago. It was the largest plesiosaur, and perhaps the largest marine reptile in New Zealand waters at the time. Mauisaurus haasti is the only known species of the genus. A handful of specimens have been found, although only a few are well preserved and mostly complete.
With 68 cervical vertebrae, Mauisaurus had one of the longest necks of any plesiosaur. It was large, over 8 metres (26 ft) in length. Like other plesiosaurs, it had a long slender body, with numerous vertebrae, allowing flexible movement. On its underside, Mauisaurus had two sets of large flippers. These aided in swimming at high speeds, but may have also allowed the plesiosaur to venture onto shorelines for short amounts of time. Mauisaurus was a carnivore, with sharp jagged teeth that would have been used to grip fish or squid.
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- O'Gorman J.P. 2016. "A small body sized non-aristonectine elasmosaurid (Sauropterygia, Plesiosauria) from the late Cretaceous of Patagonia with comments on the relationships of the Patagonian and Antarctic elasmosaurids". Ameghiniana. 53 (3): 245–268. doi:10.5710/AMGH.29.11.2015.2928.