| Eonatator | |
|---|---|
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| Holotype ofE. sternbergii (UPI R 163)[a] | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Clade: | †Mosasauria |
| Family: | †Mosasauridae |
| Tribe: | †Halisaurini |
| Genus: | †Eonatator Bardet et al. 2005[1] |
| Type species | |
| Clidastes sternbergii (Wiman 1920) | |
| Species | |
| Synonyms | |
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Eonatator is an extinctgenus of marine lizard belonging to themosasaur family. It is a close relative ofHalisaurus, and part of the same subfamily, theHalisaurinae. It is known from theLate Cretaceous ofNorth America,Colombia andSweden. Originally, thistaxon was included withinHalisaurus, but was placed in its own genus, which also led to the subfamily Halisaurinae being created for the two genera.[1]
Eonatator is known from theSmoky Hill Chalk Member of the Niobrara Chalk Formation (Late Coniacian toEarly Campanian) ofKansas,[1] from theEutaw Formation (Santonian) andMooreville Chalk Formation (Selma Group; Santonian-Lower Campanian) ofAlabama (United States),[4] from the Kristianstad Basin of southernSweden (late early Campanian),[5] and the unit Nivel de Lutitas y Arenas (Campanian) of theOlini Group inLa Mesa,Colombia.[2]
The nameEonatator means "dawn swimmer" (Greekeos = dawn +Latinnatator = swimmer). Originally, it contained only a singlespecies,E. sternbergii.[1] The species is named in honour ofCharles Hazelius Sternberg and his son, Levi, who discovered the type specimen in the Niobrara Chalk during the summer of 1918.[6] A second species,E. coellensis, was named for the town ofCoello in theDepartment of Tolima in Colombia, near of which it was discovered.[2]
Eonatator was a small mosasaur, with the type specimen ofEonatator sternbergii,UPI R 163, measuring approximately 2.6 metres (8.5 ft) long.[3] Bardet et al. (2005, p. 465[1]) diagnoseEonatator sternbergii as follows: "Ambiguous characters:premaxilla-maxilla lateral suture ending posterior to 9th maxillary teeth; tail about 40% of the head and trunk length (convergent in mosasaurines);caudalvertebra length greater than width; fewer than four pygal vertebrae;femur length about twicedistal width (convergent inClidastes). Autapomorphies:parietal with smooth triangular table extending very far posteriorly, bearing medium-sized circular foramen, located at distance twice its diameter from the frontal-parietal suture, and surrounded anteriorly and posteriorly by two parallel ridges; rounded quadrate with regularly convex tympanicala (wing); vertebral formula: seven cervicals, 24 dorsals, four pygals, 28 median caudals and at least 41 terminal caudals;humerus length approximately 2.5x distal width."E. coellensis is diagnosed by more retracted nostrils, between the 7 and the 17 maxillary teeth, premaxilla and maxilla with a short rostrum anterior to the first teeth; presence of a septomaxilla, a large prefrontal that makes most of the margin of the outer nostril, a short and wide frontal, aparietal foramen located near of the fronto-parietal suture, a triangular surface of the parietal with two medial depressions and 22 caudal vertebrae.[1]
The type specimen ofE. coellensis,IGM p 881237, measures 2.8 metres (9.2 ft) long as preserved, but is missing part of its tail. This specimen had a 41.5 cm (16.3 in) long skull and lacked a complete tail. Still, it is remarkable for having remains of soft tissue in the ear region, the neck,thoracic and theabdominal region. Under the pygal vertebrae and the seventeenth dorsal vertebra there is a series of 20 small vertebrae centra and a flattened bone, that together measure 25 cm (9.8 in) in length. It have features of the mosasauroids, with three vertebrae with haemal arches and procoelic centra, that suggest the possibility that these small bones belong to an embryo of this species, although the lack of diagnostic fossils like the skull or teeth prevents a complete identification. In any case, it will be consequent with theovoviviparism previously reported in mosasauroids likeCarsosaurus.[2]
Like many mosasaurs, this genus has a complicatedtaxonomic history. Thetype specimen (UPI R 163, Uppsala University Palaeontological Institute,Uppsala, Sweden), a nearly complete skeleton, was originally referred to the genusClidastes by Wiman and then toHalisaurus by Russell. Hence,Clidastes sternbergii becameHalisaurus sternbergii.[7] Although some agreed with this generic attribution, otherpaleontologists suggested that whileH. sternbergii did not belong toClidastes, its designation underHalisaurus is questionable; Lingham-Soliar (1996) referredH. sternbergii toC. sternbergii again, but this has found no acceptance in other researchers.[3]
In 2005,Halisaurus sternbergii was reassigned to its own genus,Eonatator by Nathalie Bardet and colleagues along with the description ofHalisaurus arambourgi and the creation of the subfamily Halisaurinae.[1] In the same year, Lindgren and Siverson suggested thatEonatator is an invalid junior synonym and should be classified asH. sternbergii,[8] but this has found no acceptance in other researchers who used the genus nameEonatator instead.[8][2][9]
Below is acladogram following an analysis by Takuya Konishi and colleagues (2016) done during the description ofPhosphorosaurus ponpetelegans, which showcases the internal relationships within the Halisaurinae.[9] The analysis excluded the dubiousHalisaurus onchognathus and the genusPluridens.
| Halisaurinae |
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In their 2023 description of the newHalisaurus species,H. hebae, Shakeret al. performed aphylogenetic analyses of members of theHalisaurinae. They suggested thatPhosphorosaurus ponpetelegans andEonatator coellensis were more closely related to the genusHalisaurus than the type species of their respective genera. They tentatively assigned both of these species toHalisaurus. The results of their analyses are displayed in the cladogram below:[10]
| Halisaurini |
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