Kaikaifilu is an extinct genus of large mosasaurs that lived during the Late Cretaceous (Maastrichtian) in what is now northern Antarctica. The only species known, K. hervei, was described in 2016 from a 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.
Reconstructions of the skull length of the holotype specimen are estimated at . The maximum size would be approximately 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 not found in other tylosaurines.
The fossil record shows that the animal lived in cold waters where temperatures may have dropped below freezing. The López de Bertodano Formation, from which Kaikaifilu is known, shows the presence of other aquatic vertebrates, including 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 officially named in 2016.
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.
Description
Size
As stored, portions of the holotype skull of Kaikaifilu hervei are around in length. Based on the skull of Taniwhasaurus antarcticus, a Campanian tylosaurine also known from Antarctica, the total length of Kaikaifilu'''s skull would be approximately between . This estimated length places it the largest known mosasaur in the Southern Hemisphere, as well as one of the largest tylosaurines identified to date. However, due to uncertainty regarding its attribution to tylosaurines, this latter claim remains subject to debate. Based on these observations, American paleontology researcher Gregory S. Paul estimated in 2022 that the maximum size of K. hervei would have been around long with a body mass of approximately .