Callawayasaurus is a genus of extinct plesiosaur from the family Elasmosauridae. When the first Callawayasaurus fossil was first discovered by Samuel Paul Welles in 1962, he described it as Alzadasaurus colombiensis before it was moved into its current genus by Kenneth Carpenter in 1999.
Callawayasaurus (Callaway) is named in honor of the paleontologist, Jack M. Callaway, editor of Ancient Marine Reptiles who, as Carpenter put it, "in his brief career as a vertebrate paleontologist, did much to improve our understanding of marine reptiles". The familiar suffix, -saurus comes from the Greek sauros (σαυρος), meaning "lizard" or "reptile."
The first skull of Callawayasaurus to be found was 35 cm long, while the animal as a whole grew up to 8 m long. The nares of Callawayasaurus are elongated and positioned over the maxilla, which has 3-5 teeth. The neck contains 56 vertebrae which are relatively short compared to other elasmosaurids. Callawayasaurus fossils have no pectoral bars; in common with other plesiosaurs such as Terminonatator. They also lack postaxial accessory facets.
Another nearly complete skeleton was found to be slightly more robust than the holotype specimen. This subtle change may indicate sexual dimorphism.
The fist Callawayasaurus remains were found in the Paja Formation near Leiva, Boyaca Colombia. The species name for the type, "columbiensis", means "from Columbia." Callawayasaurus are known from the Aptian faunal stage of the early Cretaceous period, which extended from 125 to 112 million years ago.