|Holotype vertebra ANSP 9225 seen from three different angles|
Pneumatoarthrus is an extinct genus of sea turtle known from the Late Cretaceous (early Maastrichtian) Mount Laurel Formation of Monmouth County, New Jersey. Only a single species, P. peloreus, is known.
The holotype of Pneumatoarthrus, ANSP 9225, was originally identified as a sacrum belonging to Hadrosaurus by Joseph Leidy in an 1865 monograph on Cretaceous reptiles from the US. Edward Drinker Cope later identified it belonging to a dinosaur more closely related to Anchisaurus, Efraasia, and Clepsysaurus than to Dryptosaurus and Ornithopsis, and in his 1872 description of the sea turtle Protostega he decided that Pneumatoarthrus was likely a sea turtle as well, which he reiterated in his 1875 monograph on Cretaceous vertebrate fossils from the Western Interior. Later authors overlooked Cope's 1875 monograph and considered it either a theropod or a hadrosaur (Huene 1932 considered Pneumatoarthrus the sacral vertebrae of Dryptosaurus). Baird (1979) confirmed the protostegid identification of Pneumatoarthrus by Cope (1872, 1875) based on examination of ANSP 9225.