| Kaikaifilu Temporal range: Maastrichtian,
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| Illustration of the partial left humerus of the holotype specimen | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Clade: | †Mosasauria |
| Family: | †Mosasauridae |
| Clade: | †Russellosaurina |
| Subfamily: | †Tylosaurinae (?) |
| Genus: | †Kaikaifilu Otero et al. 2017 |
| Species: | †K. hervei
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| Binomial name | |
| †Kaikaifilu hervei Otero et al. 2017
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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 López de Bertodano Formation also shows that 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.