Elasmosauridae is a family of long-necked marine reptiles known as plesiosaurs that lived during the Late Cretaceous period. They are characterized by their extremely elongated necks, which could have contained up to 76 vertebrae, and their relatively small heads. These adaptations likely helped them catch fish and other small prey in the ocean. Elasmosaurids were primarily found in what is now North America and Europe. They swam using their four flippers, which provided stability and maneuverability in the water. Fossils of Elasmosaurus, one of the most well-known genera, have been discovered, giving scientists insight into their anatomy and lifestyle.