Kaikaifilu is an extinct genus of large mosasaurs that lived during the Maastrichtian stage of the Late Cretaceous, in what is now northern Antarctica. The only species known, K. hervei, was described in 2017 from an incomplete specimen discovered in the López de Bertodano Formation, in Seymour Island, Antarctic Peninsula. The taxon is named in reference to Coi Coi-Vilu, a reptilian ocean deity of the Mapuche cosmology. Early observations of the holotype classify it as a member of the subfamily Tylosaurinae. However, later observations note that several characteristics show that this attribution is problematic. Based on the skull of Taniwhasaurus antarcticus, that of Kaikaifilu would have had a length estimated at 1.1–1.2 m (43–47 in). The maximum body size would be approximately 10 m (33 ft) long, making Kaikaifilu the largest mosasaur identified in the Southern Hemisphere, as well as one of the largest tylosaurines known to date, if its attribution to this group remains valid. One of the distinguishing characteristics of this taxon is that it has well-marked heterodont dentition, a trait rarely present in mosasaurs and not found in other tylosaurines. Fossil records show that Kaikaifilu lived in waters whose temperatures may have dropped below freezing. The animal would have been an apex predator who shared its habitat with a variety of other animals, including invertebrates, fish, plesiosaurs, and also other mosasaurs. Dating to the Cretaceous–Paleogene extinction event, Kaikaifilu is one of the last known mosasaurs.
Discovery and naming The only known specimen of Kaikaifilu hervei, cataloged as SGO.PV.6509, was discovered in January 2011 by the Chilean Paleontological Expedition in the Upper Maastrichtian beds of the López de Bertodano Formation, located on Seymour Island, in the Antarctic Peninsula. Fossil materials include several fragmentary parts of a skull, jawbone, 30 isolated teeth, and a partial left humerus. Ontogenetic analysis carried out on the fossils shows that they come from an adult individual. In 2012, one year after the discovery, a chapter of a scientific work was dedicated to the description of this specimen. The analysis conducted by Rodrigo A. Otero considered it as a mosasaur of the tylosaurine subgroup, but Otero remained uncertain as to whether it was a representative of a previously named taxon or not, leaving this specimen indeterminate. A new anatomical revision published in 2015 confirmed that the remains did indeed belong to a new genus and species, that Otero and his colleagues described and scientifically named in 2017. The genus name Kaikaifilu comes from Mapudungun and it is a reference to Coi Coi-Vilu, an aquatic reptilian deity from the Mapuche cosmology. The specific epithet hervei is named in honor of Francisco Hervé, a Chilean geologist who made an important contribution to the knowledge of the geology of Chile and the Antarctic Peninsula. Shortly after the article was published, Otero reported that he and his colleagues accidentally used a non-royalty-free image in their description, an error that was later changed to another, entirely original image for their work.
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