Eurhinosaurus And Stenopterygius NHM

''Eurhinosaurus'' (Greek for 'well-nosed lizard'- eu meaning 'well or good', rhino meaning 'nose' and sauros meaning 'lizard') is an extinct genus of ichthyosaur from the Early Jurassic (Toarcian), ranging between 183 and 175 million years. Fossils of the aquatic reptile have been found in Western Europe (England, southern and northern Germany, the Benelux, France and Switzerland).Maisch MW. 2010: Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214Fischer V, Guiomar M & Godefroit P. 2011: New data on the palaeobiogeography of Early Jurassic marine reptiles: the Toarcian ichthyosaur fauna of the Vocontian Basin (SE France). Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 261(1): 111-127Maisch MW, Matzke AT. 2000. The Ichthyosauria. Stuttgarter Beiträge zur Naturkunde, Serie B (Geologie und Paläontologie) 298: 1-159Reisdorf AG, Maisch MW & Wetzel A. 2011. First record of the leptonectid ichthyosaur ''Eurhinosaurus longirostris'' from the Early Jurassic of Switzerland and its stratigraphic framework. Swiss Journal of Geosciences 104(2): 211-224 They used to live in the deep, open sea area.Motani R. 2000. “Rulers of the Jurassic seas”. Scientific American. 283 (6): 52-59 ''Eurhinosaurus'' was a large genus of ichthyosaurs. An adult individual could reach over .

''Eurhinosaurus'' followed the regular body morphology, with a fish-like fusiform body including well developed dorsal fin, hypocercal caudal fin,Crofts S. B., Shehata R. and Flammang B. E. 2019. Flexibility of Heterocercal Tails: What Can the Functional Morphology of Shark Tails Tell Us about Ichthyosaur Swimming?. Integrative Organismal Biology 1(1): obz002 1-10 paired pectoral and pelvic fins, and remarkably large eyes. Like other ichthyosaurs, ''Eurhinosaurus'' did not have gills and used lungs for breathing. ''Eurhinosaurus'' had one distinct feature different from other ichthyosaurs: the upper jaw was twice as long as the lower jaw and covered with up and downwards-pointing teeth.McGowan. 1986. A Putative Ancestor For The Swordfish-like Ichthyosaur Eurhinosaurus. Nature. 322(31): 454-456

History and discovery

The name ''Ichthyosaurus longirostris'' was first published by Mantell in 1851 in a guide to the paleontological galleries of the old British Museum, where one of the ichthyosaurian specimens was displayed as ''Ichthyosaurus longirostris''.McGowan C. 1995. The Taxonomic Status of the Upper Liassic Ichthyosaur ''Eurhinosaurus longirostris''. Palaeontology. 37: 747-753 That specimen had exceedingly slender and elongated muzzle, but the skull was crushed. No characteristic preserved. The specific name, ''longirostris'', was just affixed to the specimen. Since the features were so unclear, scientists at that time were not able to name this specimen. The first skull of ''Eurhinosaurus longirostris'' was found by Owen and Jaeger in Switzerland in 1856, which showed clearly shortened mandibles. The genus ''Eurhinosaurus'' was erected in 1909 in a paper describing the Miocene cetacean '' Euhinodelphis cocheteuxi''. Abel noted that it was not certain whether the cetacean's mandible extended to the tip of the snout or whether it was abbreviated, which was like the case in ''Ichthyosaurus longirostris'' in 1851. He considered that the weak, attenuated mandible and some other distinguishing features of ''Ichthyosaurus longirostris'' and decided to erect a separate genus and names as ''Eurhinosaurus''. The type species by monotype, was ''Eurhinosaurus longirostris''.

Description

''Eurhinosaurus'' was a large-bodied, small-toothed, slender ichthyosaur. The vertebral column was composed of roughly 50 precaudal, 45 tail stock and less than 100 fluke vertebrae.Emily A. Buchholtz 2000. Swimming styles in Jurassic Ichthyosaurs. Journal of Vertebrate Paleontology 21, 63-71 The upper jaw was extremely long and lower jaw was weak but much shorter, which showed an extreme overbite, much as in the extant swordfish ''Xiphias''. The upper jaw was more than twice as long as the lower jaw. The orbits of ''Eurhinosaurus'' were very large and directed anterolaterally. Their huge orbits were combined with an extremely short cheek region and reduced upper temporal openings. The external naris was large and retracted.

''Eurhinosaurus'' had elongated, slender and straight teeth without distinct surface ornamentation of the crown. Their teeth were delicate, sharply pointed and the enamel was smooth. Their fenestra supratemporalis was small and external exposure of the frontal was strongly reduced. Like most post-Triassic ichthyosaurs, the parietal foramen was located on the connection point between parietal and frontal. The temporal fenestra was extremely small.

The supratemporal of ''Eurhinosaurus'' was very large and wide in the dorsal view, reaching the orbital margin. Its frontal was covered by nasals in the dorsal view. The prefrontal was very small and the postfrontal was large. Postorbital skull region was very narrow and the postorbital lacked of lamina posterior. ''Eurhinosaurus'' had the quadratojugal with long posteromedial processus quadratus and pierced by foramen of unknown function. Some small interpterygoid vacuities were found on the palate and it had short and wide pterygoid from the ventral view. Its parasphenoid endes anterolateral to the unpaired carotid foramen. The lower jaw of ''Eurhinosaurus'' had long processus retroarticularis. Atlas and axis abut very closely but were not completely fused together. No rib articulations were present on fluke vertebrae. Compared with ''Suevoleviathan'' and ''Temnodontosaurus'', the fluke of ''Eurhinosaurus'' was relatively short. The caudal fin of ''Eurhinosaurus'' was in hypocercal shape (the notochord extended into the lower lobe) with cartilaginous chevrons which could be used for swimming in a high speed.

In the vertebral column, the neural spins of the dorsal vertebrae were remarkably short, less than the height of the centrum, which was also found in other lower Jurassic large-bodied ichthyosaur such as ''Temnodontosaurus'' and ''Platypterygius''. The forefins of ''Eurhinosaurus'' offered a peculiarity: the radius was much larger than the ulna. Besides, the fins were very long and slender with four primary digits, no accessory digits and strong hyperphalangy.McGowan C. 2003. A New Specimen Of ''Excalibosaurus'' From The English Lower Jurassic. Journal of Vertebrate Paleontology 23(4): 950-956 Their hindfins were about two thirds the length of forefins.

In the shoulder girdle, the interclavicle was small and T-shaped. The scapula was elongated with a narrow, expanded dorsal blade. The postglenoidal portion of the coracoid was larger than the much reduced anterior extension. The coracoid was rounded with a notch in the anterolateral margin. The humerus had a constricted, very thickened head and expanded, flattened distal end.

The pelvic girdle was moderately reduced and also showed the fusion of pubis and ilium seen ''Stenopterygius''. The plate-like bones of the pelvis ( pubis and ischium) had modified to the elongate and waisted bones. The pubis in this form lacked an obturator foramen.

Classification

The cladogram below is based on Sander (2000),Sander,P.M.(2000). "Ichthyosauria: their diversity, distribution, and phylogeny", Paläontologische Zeitschrift 74: 1–35 Maisch and Matzke (2000), Maisch (2010)Maisch M. 2010. Phylogeny, Systematics, and Origin of The Ichthyosauria-The State of the Art. Palaeodiversity. 3: 151-214 and an analysis by Marek et al (2015).R. D. Marek, B. C. Moon, M. Williams, M. J. Benton: The skull and endocranium of a Lower Jurassic Ichthyosaur based on digital reconstructions. In: Palaeontology 58, 2015, S. 723–742.

Palaeobiology

Feeding mechanisms and diet

The postcranial morphology of ''Eurhinosaurus'' was intermediate between those super fast swimmers and slower, more flexible predators. From their extreme overbite, they probably used a predatory strategy close to today's swordfish ''Xiphias''. The elongated, densely toothed upper jaw was used as weapon to penetrate or make damage to small soft prey from the back. ''Eurhinosaurus'' belongs to the "Pierce I" predatory guild, so its dietary habits were consisted of small and soft, very delicate prey, such as small fishes, oysters and squids.

Swimming style and movement

Like other ichthyosaurs, ''Eurhinosaurus'' was a high-speed swimmer. ''Eurhinosaurus'' used the lateral oscillation of their caudal fluke on a flexible tail stock for swimming. ''Eurhinosaurus'' had a slender fusiform body with long limbs and fluke. Neural spines of fluke vertebrae were very short and almost erect. The relatively large hind limbs of ''Eurhinosaurus'' suggested their use in steering and probably propulsion at a very low speed. The swimming style of ''Eurhinosaurus'' was thunniform. Their hypocercal caudal fin, which was mounted on the narrow peduncle, moved through the water in a sinuous curve by the powerful muscles of the posterior trunk and the anterior tail region. From this motion, a strong force would be generated to pull ''Eurhinosaurus'' forward.

Like other ichthyosaurs, ''Eurhinosaurus'' probably was a deep diver. ''Eurhinosaurus'' had a very large orbit with sclerotic ring, a circular shaped bone that was embedded in their eye. The sclerotic ring was probably used to maintain the shape of their eyes against the high pressure in the deep sea while they were diving. Ichthyosaurs had the biggest eyes of any animal ever known. The big eye of ''Eurhinosaurus'' suggested that they had very good visual capacity, which helped them see clearly in the dark environment of the deep sea.

Palaeoenvironment

''Eurhinosaurus'' lived in open ocean, which was far away from the coastline. ''Eurhinosaurus'' was not like other ichthyosaurs and marine reptiles of the early Toarcian which showed a distinct provinciality. They had wide paleobiogeographic distribution in Western Europe. Fossils of ''Eurhinosaurus'' were found in the limestone and wackestone concretions in England, the Benelux, France, Switzerland and in southern and northern Germany. Limestone and wackestone were associated with the marine environment because of the aquatic formation process.“The lithostratigraphy of the Blue Lias Formation (Late Rhaetian–Early Sinemurian) in the southern part of the English Midlands”. Proceedings of the Geologists' Association. 112(2): 97-110 Besides, the fossil of the ''Eurhinosaurus'' was found with some other ichthyosaurus or marine creatures such as ammonites. This was also the evidence could show ''Eurhinosaurus'' was a creature from the marine environment.

See also

* List of ichthyosaurs
* Timeline of ichthyosaur research

References

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Early Jurassic ichthyosaurs
Ichthyosaurs of Europe
Jurassic animals of Europe
Toarcian life
Fossils of Germany
Fossils of England
Fossils of the Netherlands
Fossils of Switzerland
Fossil taxa described in 1851
Fossil taxa described in 1909
Ichthyosauromorph genera