''Eardasaurus'' is a genus of

thalassophonea Thalassophonea is an extinct clade of pliosaurids from the Middle Jurassic to the early Late Cretaceous (Callovian to Turonian) of Australia, Europe, North America and South America. ''Thalassophonea'' was erected by Roger Benson and Patrick D ...

pliosaurid Pliosauridae is a family of plesiosaurian marine reptiles from the Latest Triassic to the early Late Cretaceous ( Rhaetian to Turonian stages) of Australia, Europe, North America and South America. The family is more inclusive than the archetypa ...

from the middle Jurassic

Oxford Clay Formation The Oxford Clay (or Oxford Clay Formation) is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset and as far north as Yorkshire. The Oxford Clay Formation dates to the Jurassic, specifical ...

. The animal would have measured over long (the tail was not fully preserved) and possessed a high amount of teeth relative to other pliosaurs. Its teeth show distinct ridges formed by the tooth enamel, some of which are very pronounced and similar to carinae, giving the teeth a cutting edge.

History and naming

The remains of ''Eardasaurus'' were discovered H. Philip Powell in the

near

Yarnton Yarnton is a village and civil parish in Oxfordshire about southwest of Kidlington and northwest of Oxford. The 2011 Census recorded the parish's population as 2,545. Archaeology Early Bronze Age decorated beakers have been found in the par ...

in 1994. The only known fossil and

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

, specimen OUMNH PAL-J.2247 (previously listed as OUMNH J.02247), was unearthed from the Peterborough Member of the formation, which indicates that it lived during the

Callovian In the geologic timescale, the Callovian is an age and stage in the Middle Jurassic, lasting between 166.1 ± 4.0 Ma (million years ago) and 163.5 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the ...

stage of the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

. The specimen is fairly complete and partially articulated, preserving both the cranium and mandible, large parts of the

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...

as well as ribs and gastralia as well as the remains of the left fore- and hindlimbs. The

genus name Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

is composed of the

Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...

word "eard", which means "home" or "dwelling", and "saurus", Greek for lizard. The name of the nearby town of Yarnton is likewise derived from "eard". The species name on the other hand honors H. Philip Powell for discovering the fossil.

Description

The holotype specimen measures approximately long, however as the tail is only incompletely preserved the animal would have been larger in life.

Skull

''Eardasaurus'' was a longirostrine animal with a roughly triangular skull in dorsal view. The skull measures long, with 54% of the length composed of the section in front of the eyes. Among the Oxford Clay pliosaurs this ratio is closest to

Liopleurodon ''Liopleurodon'' (; meaning 'smooth-sided teeth') is an extinct genus of large, carnivorous marine reptile belonging to the Thalassophonea, a clade of short-necked pliosaurid plesiosaurs. ''Liopleurodon'' lived from the Callovian Stage of the Mi ...

with a preorbital skull length that makes up 55% of the total length. While the rostrum itself is well preserved, the back of the skull is much more fragmented, rendering it difficult to identify the bone sutures of the region. The anterior-most tip of the rostrum is made up of the paired

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...

, which bear a series of foramina along their surface. The premaxillae contain five and six teeth respectively, in line with the other pliosaurids it shared its habitat with. The teeth themselves are irregular (anisodont), with the largest being the penultimate alveolous. The smallest on the other hand are the first and last teeth, which show a notable size decrease compared to the other teeth around them. Still, the last tooth in the premaxilla is not as much reduced as observed in other pliosaurs, which may show a size decrease of over 50%. All teeth of the premaxilla are closely spaced except for the area where the premaxilla contacts and interlocks with the maxilla. Here a toothless area (diastema) is present. Such a diastema is also present in ''Liopleurodon'', but absent in most other middle Jurassic pliosaurs. More widespread is the presence of a constriction in the same area. The dorsal contact between the premaxillae is almost straight but interrupted by a slit-like opening surrounded by rugose texturing. While such an opening is present in a more pronounced fashion in

rhomaleosaurids Rhomaleosauridae is a family of plesiosaurs from the Earliest Jurassic to the latest Middle Jurassic (Hettangian to Callovian stages) of Europe, North America, South America and possibly Asia. Most rhomaleosaurids are known from England, many sp ...

, in the case of ''Eardasaurus'' it is more than likely a pathology. The premaxillae stretch over the skull deep between the maxillae, only ending at approximately the same level as the anterior edge of the orbits, where they meet the

parietal bones The parietal bones () are two bones in the skull which, when joined at a fibrous joint, form the sides and roof of the cranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named ...

. While the

premaxillae The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...

form the tip of the snout and front of the tooth row, the sides of the rostrum is formed by the

maxillae The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

, which contain the majority of teeth. Based on the better preserved maxilla, ''Eardasaurus'' may have possessed thirty-four to thirty-five teeth. This is a high number of teeth, far exceeding ''Liopleurodon'' (20 teeth) and even derived Cretaceous taxa including ''

Stenorhynchosaurus ''Stenorhynchosaurus'' is an extinct genus of pliosaurid plesiosaurs which lived in the Early Cretaceous of South America. The type species and only known is ''Stenorhynchosaurus munozi''. It was a medium-sized pliosaur, reaching an adult body l ...

'' (30 teeth). Like the premaxillary teeth, the teeth of the maxillae are anisodont, reaching their largest size with the fifth tooth before gradually decreasing in size towards the back of the toothrow. The maxillae extend far back, but do not contribute to the margins of the orbit. The nares are placed between the maxilla and the

frontal bones The frontal bone is a bone in the human skull. The bone consists of two portions.''Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, part ...

. The

jugal The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Anatomy ...

is unusually large for a plesiosaur and forms the posteroventral edge of the orbits before extending back to meet the

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal The quadratojugal is a skull bone present in many vertebrates, including ...

. The jugal is covered by a series of fluted grooves which are typically found around the orbits in other

ns, but ''Eardasaurus'' is the only member of the group with the ornamentation present on the jugal bones. The contact between the jugal and squamosal is step-like, with the later forming a process that overlays the jugal bone. This differs from the contact in other pliosaurs, which is typically interlocking. ''Eardasaurus'' may have possessed a distinct bone similar to the

lacrimal bone The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks of ...

observed in other animals, even if the two bones are not homologous with one another. Such a bone is present in other pliosaurs as well, however it is unclear if this structure is an entirely unique bone or a process of the jugal. The poorly preserved nature of this region of the skull leaves the bones origin ambiguous. The mandible is preserved almost in its entirety, only missing small sections at its anterior-most tip. It measures long and . The dentary forms the symphysis of the mandible and houses thirty-eight teeth before terminating just behind the end of the toothrow. The symphysis is relatively short compared to other pliosaurids, only containing the first nine teeth, including the largest dentary teeth (tooth three - tooth seven). These teeth correlate with the widest point of the symphysis and are followed by a significant size decrease. The eight dentary alveolous is only 60% the diameter of the one preceding it and is followed by continuously smaller alveoli. Besides being anisodont, the teeth towards the tip of the snout are generally large and conical with a slight curvature. All surfaces of the tooth bear distinct vertical riges formed by the

tooth enamel Tooth enamel is one of the four major Tissue (biology), tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the Crown (tooth), crown. The ...

, however they are most densely packed on the inner side of the tooth. Most of these ridges originate at the base of the crown, but some begin higher up and take on a carinae-like appearance. While the enamel ridges continue to be present on the posterior teeth, the carinae-like ridges are only found on the teeth at the front of the skull.

Classification

The phylogenetic analysis conducted for ''Eardasaurus'' is generally consistent with other analysis regarding the internal relationships within Pliosauridae. ''Eardasaurus'' was recovered as an early-diverging thalassophonean, more basal than ''Liopleurodon'', ''Simolestes'' and the Brachaucheninae.

Paleobiology

The enamel ridges present on the teeth of ''Eardasaurus'' are thought to have formed carinae-like cutting edges that likely held a significant ecological importance. The enhanced cutting ability given to the teeth by these ridges is commonly thought to be indicative of a diet consisting of large prey items, and such ridges are observed in other pliosaurids like ''Pliosaurus'' and ''Liopleurodon'', which at times bear additional serration. Compared to ''Liopleurodon'' however, the carinae-ridges of ''Eardasaurus'' are much finer with weaker serration. Nevertheless, the distinction between regular enamel ridges and carinae-like ridges is still highly pronounced. Ketchum and Benson subsequently suggest that such enamel ridges may be much more widespread among plesiosaurs than previously assumed.

References

{{Taxonbar, from=Q112128618 Pliosaurids Callovian life Middle Jurassic plesiosaurs of Europe Jurassic England Fossils of England Oxford Clay Fossil taxa described in 2022 Sauropterygian genera