| Halisaurinae | |
|---|---|
 | |
| Skeleton ofHalisaurus arambourgi. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Clade: | †Mosasauria |
| Family: | †Mosasauridae |
| Subfamily: | †Halisaurinae Bardet et al., 2005 |
| Genera | |
TheHalisaurinae are asubfamily ofmosasaurs, a group of LateCretaceous marinelizards. They were small to medium-sized, ranging from just under 3 meters inEonatator sternbergi to as much as 8 or 9 meters inPluridens serpentis.[1] They tended to have relatively slender jaws and small, numerous teeth, suggesting a diet of small fish and other prey. Although the skeleton is primitive compared to other Mosasauridae in many respects, halisaurines had the distinctive hypocercal tail of other mosasaurids[2] suggesting good swimming ability, and they persisted alongside other mosasaurs until the end of the Cretaceous. The earliest known remains of halisaurines occur in rocks ofSantonian age and the subfamily persists until the latestMaastrichtian. Halisaurines are known fromNorth andSouth America,Europe,Asia andAfrica,[3] indicating a more or less global distribution in theLate Cretaceous. Four genera are currently recognized:Eonatator,Halisaurus,Phosphorosaurus andPluridens.
The Halisaurinae exhibit several characteristics that suggest they are more primitive than mosasaurs of more derived subfamilies. In particular, their flippers are poorly differentiated, meaning that while better adapted to life in oceans than plesiopedal mosasaur genera likeDallasaurus andTethysaurus, they were poor swimmers compared to many of their contemporaries. Lacking swimming capabilities are quite surprising considering that most other small mosasaurs were adapted to be fast and nimble. The description ofPhosphorosaurus ponpetelegans revealed thatPhosphorosaurus was highly specialized to compensate for its lack of hydrophalangy.[4]
Despite their shortcomings, the Halisaurinae had diversified and reached a global distribution by the end of the Maastrichtian whilst other subfamilies, particularly thePlioplatecarpinae andTylosaurinae, appear to have been in decline.[5] There is also evidence supporting that they were accomplished intercontinental migrators, with a known large-scale migration event involvingEonatator during the Campanian from theWestern Interior Seaway ofNorth America to what today is southernSweden.[6]
Bardet et al. (2005, p. 464[7]) diagnosed the Halsaurinae as all mosasaurs more closely related toHalisaurus thanMoanasaurus. Unambiguous character states were listed as follows: "premaxilla-maxilla sutural contact vertical anteriorly, oblique at midpoint and horizontal posteriorly; contact plane between theparietal and the supratemporal oblique; preaxial ridge extending on two-thirds of the length of the radius;tibia andfibula long and slender with slightly expanded extremities. Ambiguous characters include "dorsal median ridge borne on the anterior two-thirds of thefrontal; frontal withventral boss; parietal foramen surrounded by a ventral boss;quadrate with large infrastapedial process; coalescent infra- and suprastapedial processes of quadrate;zygosphene-zygantrum complex absent;synapophyses of thecervical vertebrae extending ventrally to the ventral surface of the centrum; fusedhaemal spines."
Designation of this subfamily followed many decades of confusion surrounding thetype genus,Halisaurus, which had long been seen as (and remains) an important taxon in studies of mosasaurphylogeny. The confusion was especially centered onH. sternbergii, a species from theMooreville Chalk Formation ofAlabama and the Niobrara Chalk ofKansas. Originally, thespecies had been referred to the mosasaurineClidastes,[8] then toHalisaurus (Russell, 1967; p. 369[9]), which was also considered a member of theMosasaurinae at that time. Later workers[10][11] questioned thephylogenetic position andmonophyly ofHalisaurus, in part because of strikingmorphological differences betweenH. sternbergii and the other known species of thetaxon.
Several discoveries throughout the 1980s and 1990s helped shed light onHalisaurus, with more complete specimens of the type speciesH. platyspondylus being discovered andPhosphorosaurus ortliebi being momentarily reassigned to the genus by Lingham-Soliar (1996).[12] In 2005, the speciesHalisaurus sternbergii was reassigned to its own genus,Eonatator, along with the description of the new speciesHalisaurus arambourgi by Nathalie Bardet and colleagues. With the description ofEonatator as a closely related genus toHalisaurus, the two genera were grouped into the new subfamily Halisaurinae. Their study supported the model that the group was a sister-group to more advanced mosasaurs.[7] Daniel Madzia and Andrea Cau in 2017 definedHalisaurinae as "the most inclusive clade containingHalisaurus platyspondylus, but notMosasaurus hoffmannii,Tylosaurus proriger,Tethysaurus nopcsai, orYaguarasaurus colombianus".[13] Since the description ofEonatator, two more species ofHalisaurus,H. ortliebi andH. walkeri were recognised as separate genera;Phosphorosaurus andPluridens respectively.
The recentmost major phylogenetic analysis of mosasaurs, conducted by Tiago R. Simões and colleagues in May2017, recoveredHalisaurus and the rest of the Halisaurinae as a sister group to theMosasaurinae instead of the wholeMosasauridae. This would mean that the halisaurines are more closely related to the mosasaurines than therussellosaurines (genera such asTylosaurus andPlesioplatecarpus) are.[14]
The following cladogram is modified from Longrichet al. (2021) during the description ofPluridens serpentis,[15] with placement of higher taxa based on Madzia & Cau (2017).[13] In that study, Longrichet al. (2021) named the cladesHalisaurini andPluridensini to reflect the two major lineages of halisaurines, defining the former as "the most inclusive clade containingHalisaurus platyspondylus, but notPluridens walkeri" and the latter as "the most inclusive clade containingPluridens walkeri, but notHalisaurus platyspondylus".[15]
| ||||||||||||||||||||||||||||
In their 2023 description of the newHalisaurus species,H. hebae, Shakeret al. analyzed the phylogenetic relationships of members of the Halisaurinae. They suggested thatPhosphorosaurus ponpetelegans andEonatator coellensis were more closely related to the genusHalisaurus than the type species of their respective genera. They tentatively assigned these species toHalisaurus, with "Phosphorosaurus"ponpetelegans possibly representing a distinct genus in need of a new name. The results of their analyses are displayed in the cladogram below:[16]
| Halisaurinae |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Halisaurinae
Bardet N, Suberbiola XP. 2001. The basal mosasauridHalisaurus sternbergii from the Late Cretaceous of Kansas (North America): a review of the Uppsala type specimen.Comptes Rendus de l'Académie des Sciences, Série IIA332: 395-402.
