Gnathomortis (meaning "jaws of death") is an extinct genus of marine lizard belonging to the mosasaur family. Fossils of Gnathomortis have been recovered from the Early Campanian Mancos Shale of Colorado. The genus contains a single species, G. stadtmani, considered a species of the related Prognathodon up until its 2020 redescription.
Gnathomortis was originally named as a species of Prognathodon in 1999. A phylogenetic analysis by Schulp et al. (2006) found that Prognathodon species coded in their analysis comprise one clade, but that "Prognathodon" stadtmani fell outside that clade, and as such did not fit in that genus. Although the holotype of "P." stadtmani consists only of a partial skull and vertebral column (catalogued as BYU 13082), further preparation of the specimen revealed unique features of the quadrate and skull roof, on the basis of which Lively moved it to the new genus Gnathomortis in 2020.
- "Meet 'Jaws of Death,' a 33-foot lizard that terrorized the seas 80 million years ago". CNN. Reuters. September 25, 2020. Retrieved September 26, 2020.
- Lively, J.R. (2020). "Redescription and phylogenetic assessment of 'Prognathodon' stadtmani: implications for Globidensini monophyly and character homology in Mosasaurinae". Journal of Vertebrate Paleontology: e1784183. doi:10.1080/02724634.2020.1784183. S2CID 224904711.
- Kass, M.S. (1999). "Prognathodon stadtmani (Mosasauridae): a new species from the Mancos Shale (Lower Campanian) of western Colorado". Utah Geological Survey Miscellaneous Publication. 99 (1): 275–294.
- Schulp, A.S.; Polcyn, M.J.; Mateus, O.; Jacobs, L.L.; Luísa Morais, M. & da Silva Tavares, T. (2006). "New mosasaur material from the Maastrichtian of Angola, with notes on the phylogeny, distribution and paleoecology of the genus Prognathodon". On Maastricht Mosasaurs. Publicaties van het Natuurhistorisch Genootschap in Limburg. 45. Natuurhistorisch Genootschap in Limburg. pp. 57–67.