Temporal range: Late Cretaceous, 70–68 Ma
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.
- Leidy, 1865. Memoir on the extinct reptiles of the Cretaceous formations of the United States. Smithsonian Contributions to Knowledge. 14(6), 135 pp.
- Cope, 1870. Observations on the Reptilia of the Triassic formations of the Atlantic region of the United States. Proceedings of the American Philosophical Society. 11, 444-446.
- Cope, 1872. A description of the genus Protostega, a form of extinct Testudinata. Proceedings of the American Philosophical Society. 12, 422-433.
- Cope, 1875. The Vertebrata of the Cretaceous formations of the West. Report of the United States Geological Survey of the Territories. 2, 303 pp.
- Baird, D., 1979. Pneumatoarthrus Cope, 1870, not a dinosaur but a sea-turtle. Proceedings of the Academy of Natural Sciences, Philadelphia, 129(4):71-81.
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