Eremiasaurus

Eremiasaurus ("desert lizard") is a genus of mosasaurs, an extinct group of marine reptiles. It lived during the Maastrichtian stage of the Late Cretaceous in what is now North Africa. Only one species is known, E. heterodontus, described in 2012 from two remarkably complete fossil specimens discovered in the Ouled Abdoun Basin, Morocco. This site is known to have delivered a significant number of other related mosasaurs.

Eremiasaurus is a medium-sized representative for mosasaurs, estimated to be around 5 m long based on observations made on the syntype specimens. The skull of Eremiasaurus is robustly built and is characterized by its highly differentiated heterodonty (hence its specific name). The anatomy of the caudal vertebrae of Eremiasaurus suggests that it would have been a predator capable of swimming at high speed.

Eremiasaurus lived in the southern margin of the Mediterranean Tethys. This paleo-ocean had a significant diversity of aquatic vertebrates and had a temperate and warm oceanic climate. The fossil record also shows that there would have been niche partitioning between Eremiasaurus and the various other mosasaur species identified within the Ouled Abdoun Basin.

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, an area where other mosasaurids have been found, including Mosasaurus beaugei and Thalassotitan atrox. 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.

The genus name Eremiasaurus comes from the Ancient Greek ἐρημία (eremia, "desert") and σαῦρος (saûros, "lizard"), a portmanteau literally meaning "desert lizard", in reference to the arid climate of present-day Morocco where the marine reptile was discovered. The specific epithet heterodontus also comes from the Ancient Greek ἕτερος (heteros, "different") and ὀδόντος (odóntos, "tooth"), the two together meaning "different teeth", referring to the drastic change in the shape of the teeth at various points of the jaws.

Size
The syntypes of Eremiasaurus indicate a rather average size for a mosasaurid. The most complete specimen, UALVP 51744, is 4.5 m long, with the skull measuring 63.5 cm, but the absence of several dorsal vertebrae suggests that the actual body length would have been closer to 5 m. The second specimen, OCP DEK/GE 112, was first estimated to reach 6 m in length, based on observation of the larger skull, measuring 70 cm. However, a review of mosasaurids from Morocco conducted by Bardet and colleagues in 2015 reduced the proposed size for this specimen to 5 m.

Skull


The skull of Eremiasaurus has a robust build, but to the extreme seen in the related genus Prognathodon. The premaxillary rostrum is short and conical in dorsal view and extends forward less than the length of an alveolar space. The outline of the dorsal surface of the premaxilla is narrow and pointed forward, similar to Mosasaurus and Plesiotylosaurus. The maxilla is long and thin. Feeder foramina line the lateral surface of the maxilla and enlarge posteriorly. These foramina are located about 2 cm behind the row of teeth of the maxillae. The preorbital part is slightly larger than half the total length of the skull and the frontal region is enlarged. The lateral surface of the prefrontal is concave and has a broad, flat dorsal surface for contact with the overlapping frontal and maxilla. Posterior to the end of the premaxillary-maxillary suture, the internarial bar (the long part of the premaxilla extending behind the teeth of this bone) is laterally constricted and has a prominent median dorsal keel extending along the posterior half. A large supraorbital, rounded triangular process extends from the postero-dorsal surface of the prefrontal, similar to other mosasaurines. The infrastapedial and suprastapedial processes of the quadrate are fused, possessing a very large and rounded stapedial fossa, the latter being one of the autapomorphies of the species.

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.

Teeth
Eremiasaurus is characterized by its marked heterodonty, the teeth differing in many points in the jaws by their shape and size. The premaxillary teeth would have been pointed straight rather than projecting forward based on observations of a small anterior pit in the first tooth of the dentary bones, as well as the position of the first premaxillary socket.

Thirteen tooth positions can be counted in the left and right maxillae in Eremiasaurus. 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.

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.

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.

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.

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.

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, unlike Tylosaurus and Platecarpus, in which the fibula is the same length as the tibia. 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.

Classification
In one of the several cladograms present in the formal description of the taxon conducted by LeBlanc and colleagues in 2012, Eremiasaurus was placed as a sister group to the tribe Plotosaurini. Plotosaurini is a taxon that was first erected in 1967 by Dale Russell in his scientific work entitled Systematics and morphology of American mosasaurs, from which he revised most North American mosasaurids. The validity of this tribe begins to be questioned from 1997, the year from which the phylogenetic revision of the Mosasauroidea led by Gorden L. Bell Jr. was officially published, considering it polyphyletic and therefore obsolete. The study by LeBlanc et al. (2012) argued that, although not necessarily invalid, dropping Mosasaurini would not follow the general principle of type genus carried over to all ranks in a classification hierarchy, and that the original diagnoses of Plotosaurini are now obsolete. The authors therefore proposed to synonymize the taxon Plotosaurini to Mosasaurini, and place Eremiasaurus in a clade containing Mosasaurus and Plotosaurus based on various cranial characteristics. Characteristics justifying this classification include the presence of a keel in the internarial bar, the exclusion of the prefrontals from the narial borders, narials sunk into the frontal, and the presence of a groove in the quadrate bone. The validity of Mosasaurini will be immediately accepted, and the placement of Eremiasaurus as proposed by LeBlanc and colleagues in 2012 will be maintained in various subsequent studies.

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. Most phylogenetic trees found Eremaisaurus to be within Mosasaurini, even after refinements were made by a later study. Below is the cladogram from the most recent major phylogenetic analysis of the Mosasaurinae subfamily by Madzia & Cau (2017), which was self-described as a refinement of a larger study by Simões et al. (2017):

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 Mediterranean Tethys. Located along the equator, between 20°N and 20°S, the temperate climates were warmer than the northern margin of the paleo-ocean, which was located between 30–40°N. This place is represented by a great diversity of aquatic vertebrates, including various bony fishes like Enchodus and Stratodus to cartilaginous fishes like Cretalamna, Squalicorax and Rhombodus. The precise locality from which Eremiasaurus was discovered was already known to have a high number of other mosasaurs, with more than ten genera identified within the Ouled Abdoun Basin. The highly differentiated dental anatomy within the taxa suggests that niche partitioning occurred, in which predators occupied different niches to avoid competition with each other. For example, some mosasaurs identified within this area, such as Globidens and Carinodens, have blunt teeth for crushing shellfish, while mosasaurs such as Mosasaurus, Thalassotitan, and Prognathodon have specialized dentitions for hunting larger prey. Although the particular diet of Eremiasaurus is uncertain, its divergence from other stout-toothed mosasaurines would suggest that it specialized in food sources not exploited by its larger Moroccan contemporaries. Other marine squamates are known there, including the sea monitor Pachyvaranus and possibly the sea snake Palaeophis. Many sea turtles of the Bothremydidae family have been identified. Plesiosaurs, with the exception of Zarafasaura, are rarely present within the locality.