Halisaurus

H. arambourgi. Credit: Dmitry Bogdanov.

*Halisaurus* is an extinct genus of mosasaur that lived in Angola, Argentina, Republic of the Congo, Jordan, Morocco, Niger, Peru , Sweden and the United States during the Late Cretaceous.

## History

One of the smaller mosasaurs, *Halisaurus* were mid-range predators unlike the apex predators who belonged to the mosasaur family. *Halisaurus* was named in 1869 by Othniel Charles Marsh, during the Bone Wars. *H. platyspondylus*, meaning "flat-spined", was named from YPM 444, discovered from the Maastrichtian. Other referred elements are NJSM 12146 (incomplete skull from the Navesink Formation) and USNM 442450 (incomplete skeleton from the Severn Formation). Other localities are the Mont Laurel and Merchantville Formations, suggesting populations along the eastern coast during the mid-late Maastrichtian. It is differentiated by a narrow, oblique quadrate, and broadly and posteriorly rounded nares, the lack of anterior ridges on the frontal and the pterygoid having 9 teeth.

*H. arambourgi* was named after Camille Arambourg, who worked on fossil vertebrates from North Africa and the Middle East. Like the type species, it is from the Maastrichtian. The holotype is MNHN PMC 14, and incomplete skeleton with disarticulated skull and 27 articulate vertebrae from the Grand Daoui. It preserves diagnostic primitive traits, such as the V-shaped in the anterior and U-shaped in the posterior of the external nares, the ovular quadrate with a vertical stapedial notch, anterior ridges on the frontal and 12 teeth on the pterygoid.

## Description

This taxon appears early, the Santonian, in the evolutionary sequence, and retains many primitive traits (which is consistent with halisaurines, of which it is the namesake). It was medium-sized, as likely was *H. onchognathus*, which is fragmentary and dubious. Like other halisaurines, the flippers lacked hydrophalangy. Contradictory and unusual to its small size, it was poor at swimming. *Phosphorosaurus* was highly specialized to account for a lacking of hydrophalangy, and *Halisaurus* may have been as well.

## Classification

The type of *H. platyspondylus*, only an angular and basicranium fragment, provides little information. D.A. Russell finds similarity with *Platecarpus* in the angular, but is more heart-shaped to the anterior, symmetrical, has a convex and continuous anteroventral outline, an anteroexternal margin at the articular surface and very inflated in the later profile. Some of these are like *Clidastes*; and since both taxa are from different subfamilies, classification was made difficult. Russell placed it into Plioplatecarpinae based on the *Platecarpus*-like supratapedial process in *H. onchognathus*. Through the '80s-'90s, more complete specimens of the type and other halisaurines helped in classification, and Halisaurinae was validated by *Eonatator*. At the time it was believed to be a sister group to advanced mosasaurs. Recent phylogenies, such as Simões et al. (2017), suggests that Halisaurinae and Mosasaurinae are sister. It may be among the basalmost of the family. *H. platyspondylus* and *H. arambourgi* are valid, with *H. onchognathus* from the Campanian or Santonian as dubious due to being fragmentary and destroyed during WWII. Unnamed species are known from the Campanian of Texas, the Maastrichtian of California and the Santonian of Peru, suggesting a widespread distribution. Other *H.* sp. are controversial.

## Species

* *H. arambourgi*
* *H. onchognathus*?
* *H. platyspondylus*
* *H.* sp. nov. 1-3

### Reassigned Species

* *"H." sternbergii*
* "*H." ortliebi*

## References

* https://en.wikipedia.org/wiki/Halisaurus