Eremiasaurus
Eremiasaurus | |
---|---|
Life restoration | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Order: | Squamata |
Clade: | †Mosasauria |
Superfamily: | †Mosasauroidea |
Family: | †Mosasauridae |
Tribe: | †Mosasaurini |
Genus: | †Eremiasaurus Leblanc et al., 2012 |
Type species | |
†Eremiasaurus heterodontus Leblanc et al., 2012
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Eremiasaurus ("desert lizard") is a
Eremiasaurus is a medium-sized representative for mosasaurs, estimated to be around 5 metres (16 ft) long based on observations made on the
Eremiasaurus lived in the southern margin of the
Research history
In 2012, paleontologists Aaron R. H. LeBlanc, Michael W. Caldwell and Nathalie Bardet officially described Eremiasaurus heterodontus on the basis of two specimens discovered within the phosphate deposits of the Ouled Abdoun Basin in Morocco, and more precisely in the area of Sidi Daoui, located near the town of Oued Zem. The syntypes, cataloged UALVP 51744 and OCP DEK/GE 112 respectively, are two more or less complete specimens representing almost the entire known skeleton of the genus. The rationale for using syntypes rather than a single holotype comes from the fact that UALVP 51744, the most complete specimen, is derived from the commercial field without precise locality data. However, OCP DEK/GE 112 was exhumed by one of the describers, Nathalie Bardet, allowing its detailed geographic and stratigraphic position to be recorded. The precise zone concerning this discovery is located at the level of Upper Couche III, dating from the Upper Maastrichtian of the Late Cretaceous,[1][2] an area where other mosasaurids have been found, including Mosasaurus beaugei[2] and Thalassotitan atrox.[3] Referred specimens of Eremiasaurus have been also discovered in geological formations of Brazil and Israel, at the same latitude and time period as those in the Ouled Abdoun Basin.[4][2]
The
Description
Size
The syntypes of Eremiasaurus indicate a rather average size for a mosasaurid. The most complete specimen, UALVP 51744, is 4.5 m (15 ft) long, with the skull measuring 63.5 cm (25.0 in), but the absence of several dorsal vertebrae suggests that the actual body length would have been closer to 5 m (16 ft). The second specimen, OCP DEK/GE 112, was first estimated to reach 6 m (20 ft) in length, based on observation of the larger skull, measuring 70 cm (28 in).[1] However, a review of mosasaurids from Morocco conducted by Bardet and colleagues in 2015 reduced the proposed size for this specimen to 5 m (16 ft).[2]
Skull
The
The lower jaw is thin compared to the size of the teeth lodged along the dentary. The dorsal margin of the dentary is slightly convex in lateral view. This convexity is not as pronounced as in most Prognathodon species and rather resembles that in Mosasaurus and P. kianda. The coronoid bears a large posterior process oriented vertically, which gives the dorsal margin of the bone an angle of nearly 90° between the horizontal anterior end and the vertical hind wing. The articular bones form broad rectangular extensions of the lower jaws behind the glenoid fossae. An exceptional case among mosasaurs, Eremiasaurus seems to have a hyoid bone, an element rarely found in the fossils of representatives of this group. This bone is slightly widened in its posterior part.[1]
Teeth
Eremiasaurus is characterized by its marked
Thirteen tooth positions can be counted in the left and right maxillae in Eremiasaurus.[a] The three anterior teeth of UALVP 51744 resemble those of the premaxilla and are long, straight, and conical, teardrop-shaped to round in cross-section. These teeth possess only serrated anterior carinae (ridges). When closing the jaw, its premaxillary and dentary teeth lodge in small pits between the teeth. This type of contact is also found along the dental margin of the maxilla. The crowns are blade-shaped, bicarinate (two-keeled), and are much longer from front to back than in the anterior dentition. The enamel surfaces are smooth with faint traces of facets on the dental crowns. In addition, the anteroposterior expansion of the dental crown leaves little space between the adjacent teeth, and at the seventh tooth of the maxilla, the interdental pits are absent. From this precise point, the teeth of the maxillae and dentaries no longer interdigitate, but shear against each other so that the teeth of the maxillae hide the lower teeth in lateral view when the jaws are closed. The fourth to ninth maxillary teeth become progressively more laterally compressed and slightly curved. The tenth to thirteenth teeth of the maxillae are also bicarinate, but show much less lateral compression and are also asymmetrical and widened anteriorly. As a result, the anterior surfaces of these teeth are convex in lateral view. These crowns are also flared at their bases and have smooth enamel surfaces.[1]
The dental bones have fifteen teeth on each side. The pattern of heterodonty in the lower dentition mirrors that of the premaxilla and maxillae. The first five teeth are straight and conical and each has only one dentate tooth and an anterior keel. The sixth through ninth teeth are more laterally compressed and blade-like, and each has a serrated carina on the anterior and posterior edges. These dental crowns are also more convex along their lingual surfaces (the side facing the tongue) than along their labial sides (the side facing outwards). The tenth to fifteenth teeth are more bulbous and curved, and still have two carinae with an intercarinal angle of 180°. There are small depressions along the tooth margin between the adjacent teeth, which disappear posteriorly from the eighth position of the teeth of the dental bones.[1]
A row from a pterygoid bone consists of five teeth and one isolated but associated tooth. The isolated tooth belongs to the same row of teeth of the pterygoids, on the basis of an identical curvature of the crowns. The teeth of the pterygoids are long, approaching the sizes of the posterior marginal teeth, but are significantly smaller than the medial marginal crowns. The two front teeth are thin, straight and tapered, while the last three become progressively smaller and more curved. The isolated tooth is smaller than the marginal dentition and has an enlarged base of the crown. The marked degree of heterodonty in the dentition of the pterygoid bones reflects similar changes in the marginal teeth, where the crowns become smaller, more bulbous, and curved further back. Furthermore, the cross-section of the areas of the bases of the pterygoid teeth in Eremiasaurus do not enlarge forward, contrary to what one would expect for all species tentatively classified within the genus Prognathodon.[1]
Postcranial skeleton
The exact number of vertebrae in Eremiasaurus is unknown. The most complete specimen, UALVP 51744, lacks many of the dorsal and caudal vertebrae but preserves the cervical column. But excluding this, some vertebrae and other postcranial parts of the animal are well preserved and allow to visualize it as a whole. The ribs get smaller posteriorly, with the ribs numerous and wide on the first and second dorsals before becoming short and thin. Several large thoracic ribs hide the dorsal margin of the scapular blade in lateral view, obscuring the arch formed by the dorsal edge of the scapula.[1]
Eremiasaurus' caudal vertebrae are most similar to Plotosaurus. Among the most notable features are the presence of a ventral deviation of the tail, a fanning of the caudal neural spines, and an unusually long series of pygal (modified sacral) vertebrae. The large number of pygal vertebrae, accompanied by a reduced number of intermediate caudals is unique to Eremiasaurus. The presence of a ventral tail deflection, as in Plotosaurus, and a wide range of neural spines in the same region of the caudal vertebral series suggest a similar development of an incipient dorsal fin lobe in Eremiasaurus, though less developed. These changes in proportions, such as an increase in the number of pygal vertebrae, suggest a high-speed pursuit predator, converging with the vertebral proportions of Plotosaurus.[1]
The scapula and coracoid appear to be sutured tightly anterior to the glenoid fossa. This same pit is also slightly domed, another distinctive feature of the taxon. Posterior to this fossa the posterior edge of the scapula extends dorsally before extending to form the posterior margin of the scapular blade, similar to Clidastes. The lateral aspect of the scapula is wide, smooth and flat. The humerus is subequal in height and width, unlike Mosasaurus and Plotosaurus, where they are wider than they are tall. Like all mosasaurids, the iliac crest is reduced to a forward-leaning cylindrical process. The distal end of the ilium is enlarged and bears facets for articulation with the pubis and ischium. The tibia is a rectangular element, longer proximally than anteroposteriorly. The fibula is bell-shaped, with the distal end being much wider than the proximal end, unlike Mosasaurus and Plotosaurus, where both ends of the fibula are of less width. This bone is also about three-quarters the length of the tibia,[1] unlike Tylosaurus and Platecarpus, in which the fibula is the same length as the tibia.[5] The largest of the known elements of the tarsus is interpreted to be an astragalus. This bone is kidney-shaped, with a pedunculated fibular facet on the dorsal side. The phalanges are elongated spindle-shaped with moderately enlarged epiphyses, different from the stout, block-like proportions seen in Mosasaurus and Plotosaurus.[1]
Classification
In one of the several
A 2017 phylogenetic analysis of the mosasauroids used several analyses to find the most valid classifications, as if a grouping was consistently recorded it was likely a true one.[11] Most phylogenetic trees found Eremaisaurus to be within Mosasaurini,[12] even after refinements were made by a later study.[11] Below is the cladogram from the most recent major phylogenetic analysis of the Mosasaurinae subfamily by Madzia & Cau (2017),[11] which was self-described as a refinement of a larger study by Simões et al. (2017):[12]
Paleoecology
The deposits of the Ouled Abdoun Basin in Morocco constituted during the Maastrichtian a large part of what was once the southern margin of the
Notes
References
Further reading
- Paulina Jiménez-Huidobro; Tiago R. Simões; Michael W. Caldwell (2017). "Mosasauroids from Gondwanan Continents". S2CID 89780058.