Eonatator

Eonatator is an extinct genus of marine lizard belonging to the mosasaur family. It is a close relative of Halisaurus, and part of the same subfamily, the Halisaurinae. It is known from the Late Cretaceous of North America, Colombia and Sweden. Originally, this taxon was included within Halisaurus, but was placed in its own genus, which also led to the subfamily Halisaurinae being created for the two genera.[1]

Eonatator
Temporal range: Santonian-Campanian
~84–72.1 Ma
Restoration of E. sternbergii
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Reptilia
Order:
Squamata
Family:
Mosasauridae
Subfamily:
Halisaurinae
Genus:
Eonatator

Bardet et al. 2005[1]
Type species
Clidastes sternbergii
(Wiman 1920)
Species
Synonyms
  • Clidastes sternbergii (Wiman 1920)

Discovery and naming

Eonatator is known from the Smoky Hill Chalk Member of the Niobrara Chalk Formation (Late Coniacian to Early Campanian) of Kansas,[1] from the Eutaw Formation (Santonian) and Mooreville Chalk Formation (Selma Group; Santonian-Lower Campanian) of Alabama (United States), from the Kristianstad Basin of southern Sweden (late early Campanian), and the unit Nivel de Lutitas y Arenas (Campanian) of the Olini Group in La Mesa, Colombia.

The name Eonatator means "dawn swimmer" (Greek eos = dawn + Latin natator = swimmer). Originally, it contained only a single species, E. sternbergii. The species is named in honour of Charles H. Sternberg and his son, Levi, who discovered the type specimen in the Niobrara Chalk during the summer of 1918. A second species, E. coellensis, was named for the town of Coello in the Department of Tolima in Colombia, near of which it was discovered.[2]

Description

Eonatator was among the smaller mosasaurs. The length of the type skeleton, which represents an adult, is only 2.65 meters in length. Like many mosasaurs, it likely fed primarily on fish and smaller marine reptiles. The length of the type specimen of E. coellensis, IGM p 881237 (which lack a complete tail), is of 2.8 meters until the last preserved caudal vertebrae, with a skull of 41,5 centimeters. The specimen of this species is remarkable for having remains of soft tissue in the ear region, the neck, thoracic and the abdominal 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 centimeters 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 the ovoviviparism previously reported in mosasauroids like Carsosaurus.[2]

Bardet et al. (2005, p. 465[1]) diagnose Eonatator 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); caudal vertebra length greater than width; fewer than four pygal vertebrae; femur length about twice distal width (convergent in Clidastes). 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 tympanic ala (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, a parietal foramen located near of the fronto-parietal suture, a triangular surface of the parietal with two medial depressions and 22 caudal vertebrae.[1]

Classification

Like many mosasaurs, this genus has a complicated taxonomic history. The type specimen (UPI R 163, Uppsala University Palaeontological Institute, Uppsala, Sweden), a nearly complete skeleton, was originally referred to the genus Clidastes by Wiman and then to Halisaurus by Russell. Hence, Clidastes sternbergii became Halisaurus sternbergii. However, by the late 1980s, some paleontologists began to suggest that H. sternbergii belonged in its own genus and that Halisaurus was polyphyletic.

In 2005, Halisaurus sternbergii was reassigned to its own genus, Eonatator by Nathalie Bardet and colleagues along with the description of Halisaurus arambourgi and the creation of the subfamily Halisaurinae.[1]

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.[3] The analysis excluded the dubious Halisaurus onchognathus and the genus Pluridens.

Halisaurinae
 

Halisaurus arambourgi

 
 
 

Halisaurus platyspondylus

 
 

Eonatator coellensis

 
 
 

Eonatator sternbergii

 
 
 

Phosphorosaurus ortliebi

 
 

Phosphorosaurus ponpetelegans

 
 
 
 
 

References
  1. Bardet, N.; Pereda Suberbiola, X.; Iarochene, M.; Baadi, B.; Amaghzaz, M. (2005). "A new species of Halisaurus from the Late Cretaceous phosphates of Morocco, and the phylogenetical relationships of the Halisaurinae (Squamata: Mosasauridae)". Zoological Journal of the Linnean Society. 143 (3): 447–472. doi:10.1111/j.1096-3642.2005.00152.x.
  2. Páramo-Fonseca, María E. (2013). "Eonatator coellensis nov. sp. (Squamata: Mosasauridae), nueva especie del Cretácico Superior de Colombia" [Eonatator coellensis nov. sp. (Squamata: Mosasauridae), a new species from the Upper Cretaceous of Colombia] (PDF). Revista de la Academia Colombiana de Ciencias (in Spanish). 37 (145): 499–518. doi:10.18257/raccefyn.31. ISSN 0370-3908. Archived from the original (PDF) on 2016-03-04. Retrieved 2014-06-28.
  3. Konishi, Takuya; Caldwell, Michael W.; Nishimura, Tomohiro; Sakurai, Kazuhiko; Tanoue, Kyo (2016-10-02). "A new halisaurine mosasaur (Squamata: Halisaurinae) from Japan: the first record in the western Pacific realm and the first documented insights into binocular vision in mosasaurs". Journal of Systematic Palaeontology. 14 (10): 809–839. doi:10.1080/14772019.2015.1113447. ISSN 1477-2019.

Further reading
  • Lindgren J, Siverson M. 2005. Halisaurus sternbergii, a small mosasaur with an intercontinental distribution. Journal of Paleontology 79 (4): 763–773.
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