Mauisaurus ("Maui reptile") is a genus of plesiosaur that lived during the Late Cretaceous period around 80 to 69 million years ago in what is now New Zealand. 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. The only other established New Zealand plesiosaur, Tuarangisaurus keyesi, cannot be linked as a direct relative to Mauisaurus.

 Mauisaurus was similar to other plesiosaurs. It had a characteristically long neck, that grew up to 15 metres (49 ft) in length, more than half its total length. Mauisaurus' neck length makes it one of the top plesiosaurs with a large neck to body length ratio. In total, Mauisaurus grew to around 20 metres (66 ft) in length and was easily the largest plesiosaur in New Zealand waters. 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.

 Mauisaurus remains have all been found in New Zealand's South Island, near Canterbury. Altogether, around seven Mauisaurus specimens have been found in the area, most in or around the Waipara River. One Mauisaurus fossil was even found battling a mosasaur from the New Zealand region. Mauisaurus gardneri was described in 1877, but was later found to be separate to Mauisaurus haasti and is now considered a nomen nudum.

 Mauisaurus gets its name from the New Zealand Māori mythological demigod, Māui. Māui is said to have pulled New Zealand up from the seabed using a fish hook, thus creating the country. Thus, Mauisaurus means "Māui reptile". Mauisaurus gets its scientific last name from its original finder, Julius Haast, who found the first Mauisaurus fossil in 1870. The specimen was then first described in 1874.

 Mauisaurus is one of the few New Zealand prehistoric creatures, and so, has had much publicity in the country. On 1 October 1993, a set of stamps was released to the general public. Although it depicted many other dinosaurs and prehistoric life, Mauisaurus was featured hunting fish on the $1.20 stamp.

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