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,
Mesangiospermae are fossil mesangiosperms from the Hauterivian stage of the Cretaceous period. Molecular clock comparisons of DNA sequences indicate that the mesangiosperms originated between 140 and 150 Mya (million years ago) near the beginning of the Cretaceous period. This was about 25 Ma (million years) after the origin of the angiosperms in the mid-Jurassic. By 135Mya, the mesangiosperms had radiated into 5 groups: Chloranthales, Magnoliids, Monocots, Ceratophyllales, and Eudicots. The radiation into 5 groups probably occurred in about 4 million years. Because the interval of this radiation (about 4 million years) is short in proportion to its age (about 145 million