Liodon

Gaudey (1892) listed the following unambiguous character states for the genus upon the description of species L. compressidens: "Premaxilla with small rostrum present anterior to premaxillary teeth. Thirteen to fourteen teeth in maxilla. Fourteen to sixteen teeth in dentary. Small projection of dentary anterior to first dentary tooth. Mandibular teeth become highly compressed and bicarinate posteriorly, enamel surfaces smooth".[3]

The type species of Liodon, L. anceps, is based only on a jaw fragment containing two teeth. The teeth were symmetrically bicarinate and smoothly surfaced, which lent the genus its name. Beyond the type specimen, few additional specimens have been assigned to L. anceps but it has been suggested by multiple authors that L. anceps is congeneric with Hainosaurus, a member of the Tylosaurinae, on the basis of similarities in the teeth. Liodon compressidens, known from Campanian-age deposits in France, is better known than L. anceps and clearly a mosasaurine mosasaur.[3] The species L. sectorius, known from the Navesink Formation of New Jersey, is based on fragmentary remains, primarily teeth, and seems to represent a species intermediate between L. compressidens and L. mosasauroides.[3]

Liodon anceps was originally named by Richard Owen in 1841 as "Leiodon anceps" and was upon its description based only on two tooth fragments and a minor portion of the corresponding jaw bone.[1] Owen recognised the teeth as more similar to those of Mosasaurus than to any other reptile teeth known at the time.[3] In 1846, Louis Agassiz noted that the genus name was already preoccupied by the fish Leiodon and thus replaced the genus name with Liodon.[4]

Liodon was one of the original genera included within the Mosasauridae upon its creation in 1853, along with Mosasaurus itself, Onchosaurus (later recognised to have been a batoid fish), Oplosaurus (a sauropod dinosaur), Macrosaurus (a historical mosasaur "wastebasket taxon") and Geosaurus (a thalattosuchian crocodyliform).[3]

Russell (1967)[3] considered Liodon to represent a mosasaurine and a member of the tribe Mosasaurini along with Clidastes, Mosasaurus, Amphekepubis (a dubious genus) and Compressidens (later renamed to Carinodens).

With the description of three additional species of the genus over the second half of the nineteenth century; L. sectorius, L. compressidens and L. mosasauroides, Lingham-Soliar (1993)[5] suggested that the genus of Liodon could be given a proper generic diagnosis. However, Schulp et al. (2008)[1] pointed out that the type specimen of Liodon anceps (BMNH 41639) is at present missing the teeth, in which the diagnositic features that supposedly unite the genus are found, and thus considered the genus a nomen dubium. Due to similarities with the recently described species of Prognathodon, P. kianda, the three species of Liodon other than the type species (L. sectorius, L. compressidens and L. mosasauroides) were then assigned to Prognathodon. With "Prognathodon" kianda repeatedly being recovered as a mosasaurine more basal than Prognathodon and likely a separate genus on its own,[6][7] the phylogenetic placement of L. sectorius and L. compressidens remains highly uncertain. "Liodon" mosasauroides is apparently referable to the Maastrichtian-age genus Eremiasaurus based on unpublished morphological comparisons reported in an SVP abstract by Mohr et al. (2019).[8]