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,
Swithland Wood and The Brand high mountain range, the processes of erosion began. By the Triassic Period, (240 million years ago) the mountains were eroded down to something like their current height. There then began a new deposition, this time of desert sand and fine dust, that produced a new layer of soft red marl, creating the gentler rolling appearance of the land, with just the tips of the older rocks reaching the surface. Over most of Swithland Wood the marl is the surface rock type (although the slate beds will occur underneath that). Finally, during the ice age, glacial erosion stripped back the Triassic