Plesiosauria Plesiosauria The Plesiosauria (; Greek: πλησίος, "plesios", meaning "near to" and "sauros", meaning "lizard") or plesiosaurs are an order or clade of Mesozoic marine reptiles (marine Sauropsida), belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 203 million years ago. They became especially common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide oceanic distribution. Plesiosaurs were among the first fossil reptiles discovered. In the beginning of the nineteenth century,
South Polar region of the Cretaceous temperatures caused by the high amounts of carbon dioxide and possibly methane greenhouse gases in the atmosphere. This caused a lack of permanent ice coverage in the polar regions, though the carbon dioxide level dropped between 115 and 66 million years ago (mya), possibly allowing some permanent ice cover. It is possible that, throughout the Cretaceous, several small ice sheets developed. Increased tectonic activity, causing more geothermal heat, may have prevented glaciation and increased global temperatures. Temperatures may have been up to 15 °C (27 °F) warmer than they are in the modern age. The landscape of the Middle Jurassic