👉 Plesiosaurs were a group of dinosaurs that lived during the Mesozoic era, around 680 million years ago. They are named after their long necks, which resembled those of birds and allowed them to breathe through their trachea rather than lungs. Plesiosaurians had large, powerful jaws with sharp teeth, and they were known for their unique body structure that was shaped like a plexus (a type of organ), allowing them to maintain their air supply