Halisaurus
Halisaurus is an extinct genus of marine reptile belonging to the mosasaur family. The holotype, consisting of an angular and a basicranium fragment discovered near Hornerstown, New Jersey, already revealed a relatively unique combination of features and prompted a new genus to be described. It was named by Othniel Charles Marsh in 1869 and means "ocean lizard". It was renamed by Marsh to Baptosaurus in 1870, since he believed the name to already be preoccupied by the fish "Halosaurus". According to modern rules, a difference of a letter is enough and the substitute name is unneeded, making ""Baptosaurus"" a  junior synonym. Since its description, more complete remains have been uncovered from fossil deposits throughout the world with particularly complete remains found in Morocco and the United States. The genus remains a key taxon in mosasaur systematics due to its unique set of features and as the most complete representative of its subfamily, the Halisaurinae. With a length of , "Halisaurus" was comparatively small by mosasaur standards. Though bigger than earlier and more basal mosasaurs, such as "Dallasaurus", the sleek "Halisaurus" would have been dwarfed by many of its contemporaries, such as "Tylosaurus" and larger species of "Clidastes". Description. "Halisaurus" appeared  relatively early in the evolutionary history of the mosasaurs, during the Santonian. As such, the genus retains many primitive characteristics, as does the Halisaurinae at large. Both "H. platyspondylus" and "H. arambourgi" reached lengths of between 3 and 4 metres (9.8 - 13.1 feet). The length of the dubious "H. onchognathus" is difficult to tell due to the lack of remains, but was likely similar. As in other halisaurines, the flippers of "Halisaurus" are poorly differentiated which means that the genus lacked the hydrophalangy of more advanced mosasaurs. That "Halisaurus" was a relatively poor swimmer is relatively surprising considering its small  size, since other small and medium-sized mosasaurs were mostly adept swimmers. The description of "Phosphorosaurus ponpetelegans" revealed that "Phosphorosaurus", another member of the Halisaurinae, was highly specialized to compensate for its lack of hydrophalangy. Classification and species. The exact position of "Halisaurus" within the larger mosasaur family tree has long been considered controversial. The type specimen of "H. platyspondylus" is not comprehensive and only preserves an angular and a basicranium fragment. Though the specimen does not reveal much about the mosasaur, it was noted by D.A. Russell in his "Systematics and Morphology of American Mosasaurs" (1967) that the angular  resembles that of "Platecarpus" but is more symmetrically heart-shaped in anterior aspect and appears slightly inflated in later profile with a convex anteroventral outline that is continuous with the anteroexternal margin of the articular surface, as in the genus "Clidastes". The similarities with both "Platecarpus" and "Clidastes" were problematic, as said genera have always been classified in separate subfamilies, the Plioplatecarpinae and Mosasaurinae respectively. Russell referred the genus to the Plioplatecarpinae on the basis of a "Platecarpus"-like suprastapedial process in specimens referred to "H. onchognathus". Several discoveries throughout the 1980s and 1990s helped shed light on "Halisaurus", with more  complete specimens of the type species "H. platyspondylus" being discovered and "Phosphorosaurus ortliebi" being momentarily reassigned to the genus by Lingham-Soliar (1996). In 2005, the species "Halisaurus sternbergii" was reassigned to its own genus, "Eonatator", along with the description of the new species "Halisaurus arambourgi" by Nathalie Bardet and colleagues. With the description of "Eonatator" as a closely related genus to "Halisaurus", the two genera were grouped into the new subfamily Halisaurinae, which was then believed to be a sister-group to more advanced mosasaurs. The most recent major phylogenetic analysis of mosasaurs, conducted by Tiago R. Simões and  colleagues in May 2017, recovered "Halisaurus" and the rest of the Halisaurinae as a sister group to the Mosasaurinae. This would mean that the halisaurines are more closely related to the mosasaurines than the russellosaurines (genera such as "Tylosaurus" and "Plesiopatecarpus") are. Below is a cladogram following an analysis by Takuya Konishi and colleagues (2015) done during the description of "Phosphorosaurus ponpetelegans", which showcases the internal relationships within the Halisaurinae, showing "Halisaurus" as the basalmost genus of the subfamily. The analysis excluded the dubious "H. onchognathus" and the genus "Pluridens". Two species of "Halisaurus" are considered valid; "H. platyspondylus" and  H. arambourgi. The species "H. onchognathus", known from Campanian or Santonian deposits that were once part of the Western Interior Seaway, is considered dubious due to all the known remains of the species having been destroyed in the Second World War. Apart from the named species, fragmentary remains have been referred to the genus from across the world. Though the designation of these remains as "Halisaurus" is debatable in most cases, unnamed species are known from the Campanian of Texas, the Maastrichtian of California and the Santonian of Peru (which significantly expands the known temporal range of the genus). Section::::"Halisaurus  platyspondylus". "Halisaurus platyspondylus" is the type species of "Halisaurus", having been named by Othniel Charles Marsh in 1869. The species name means "flat-spined". Referred specimens include the type specimen YPM 444 (consisting of an angular and a basicranium fragment) from the New Egypt Formation (New Jersey), which is Maastrichtian in age. Important subsequent specimens include NJSM 12146 (an incomplete cranium from the Navesink Formation, New Jersey), USNM 442450 (an incomplete skeleton from the Severn Formation of Maryland). The species is also known from the Mount Laurel and Merchantville Formations of Delaware, implying that the species ranged across the eastern  coast of North America during the Middle to Late Maastrichtian. This species is differentiated from other species by the narrow and oblique shape of its quadrate, the broadly rounded and posteriorly rounded external nares and the lack of anterior ridges on its frontal as well as its pterygoid preserving nine teeth. Section::::"Halisaurus arambourgi". "Halisaurus arambourgi" means "Arambourg's ocean lizard" and is named in honor of Professor Camille Arambourg due to his work on fossil vertebrates in North Africa and the Middle East. Like "H. platyspondylus", "H. arambourgi" is Late Maastrichtian in age, though specimens of this species have been  found across northern Africa and potentially in the Middle East. The type specimen, MNHN PMC 14, is an incomplete skeleton that includes a disarticulated skull and 27 associated articulated vertebrae from the Grand Daoui area near Khouribga in central Morocco. The fossils of "H. arambourgi" preserve several features that distinguish it from "H. platyspondylus", among them the shape of its external nares (V-shaped anteriorly and U-shaped posteriorly), the shape of its quadrate (which has an oval vertical stapedial notch) and the presence of anterior ridges on the frontal. Its pterygoid bone also preserves twelve teeth (three more than  in "H. platyspondylus").