Diplotomodon is only known from a single tooth, holotype ANSP 9680, found near Mullica Hill in either the Navesink or Hornerstown Formation, marine deposits dating to the Maastrichtian stage of the late Cretaceous period.
Joseph Leidy originally described the tooth using the name Tomodon in 1865, considering it a carnivorous marine reptile, probably a plesiosaur. The generic name was derived from Greek τομός (tomos), "cutting", "sharp", and ὀδών (odon), "tooth". However, this name had already been used for the snake genus Tomodon Duméril 1853 and Leidy changed it in 1868 to Diplotomodon, adding a Greek διπλόος (diploos), "double", at that time suggesting it was a fish. The type species, Diplotomodon [=Tomodon] horrificus (Leidy, 1865) Leidy 1868 is the only species in the genus that has been described. The specific name horrificus is Latin for "dreadful".
The tooth, with a preserved length of about three inches, is very broad, flat, and symmetrical and is not recurved.
In 1870 Edward Drinker Cope concluded it was not a fish but a carnivorous dinosaur. Ralph Molnar followed this up in 1990 by suggesting that it was a synonym of the tyrannosauroid Dryptosaurus. Although from then onwards mostly seen as a dinosaur, Diplotomodon was considered a member of the Mosasauridae by Halsey Wilkinson Miller in 1955. David Weishampel thought it was an indeterminate member of the Tetanurae in 2006. Today, it is generally considered a nomen dubium.
- Leidy, J., 1865, Memoir on the extinct reptiles of the Cretaceous formations of the United States. Smithsonian Contribution to Knowledge. 14: 1-135
- Leidy, J., 1868, "Remarks on CONOSAURUS of Gibbes", Proceedings of the Academy of Natural Sciences of Philadelphia, 1868, 20: 200-202
- Weishampel, D.B., 2006, "Another look at the dinosaurs of the East Coast of North America", III Jornadas Internacionales sobre Paleontología de Dinosaurios y su Entorno, Salas de los Infantes, Burgos, Spain. Colectivo Arqueológico-Paleontológico Salense Actas, pp 129-168
- Cope, E.D., 1870, "Synopsis of the extinct Batrachia, Reptilia and Aves of North America", Transactions of the American Philosophical Society, New Series 14(1): 1-252
- Miller, H.W., 1955, "A check-list of the Cretaceous and Tertiary vertebrates of New Jersey", Journal of Paleontology, 29(5): 903-914
- Molnar, (1990). "Problematic Theropoda: "Carnosaurs"." Pp. 306-317 in Weishampel et al. (eds.), The Dinosauria. Berkeley: University of California Press.
- Holtz, T.R., 2004, "Tyrannosauroidea" In: D.B. Weishampel, P. Dodson, and H. Osmolska (eds.), The Dinosauria. University of California Press, Berkeley, pp 111-136
- The Theropod Database on Diplotomodon