Paraophthalmosaurus

''Nannopterygius'' (meaning "small wing/flipper" in Greek) is an extinct genus of ophthalmosaurid ichthyosaur that lived during the Middle Jurassic to the Early Cretaceous (Callovian to Berriasian stages). Fossils are known from England, Kazakhstan, Russia, and NorwayMcGowan, C. & Motani, R. (2003). ''Ichthyopterygia''. ''In'' Sues, H.-D. Handbook of Paleoherpetology, vol. 8. Verlag Dr. Friedrich Pfeil, Munich, 175 pp., 19pls. and six species are currently assigned to the genus.

Description

''Nannopterygius'' was small for an ichthyosaur, measuring up to long at maximum. About of this was tail, including a deeply forked and probably homocercal caudal fin. The head is long, with a typical long narrow rostrum. The eyes are large, hence its classification as an ophthalmosaurid, and have a bony sclerotic ring inside the eye socket. There are at least 60 disc-shaped vertebrae, although owing to the condition of the fossil it is not possible to tell exactly how many there were, showing that ''Nannopterygius'' was flexible, agile and probably a fast swimmer. The ribs are long and curved, but do not quite join up. Most of the features are very similar to the close relative ''Ophthalmosaurus.'' However, its paddles are much smaller, around for the forepaddles and only for the hindpaddles. This gave it a very streamlined, torpedo-shaped look, but would have made it quite difficult to generate much lift or to turn quickly, making it an inefficient long-distance swimmer but speedy over short distances. It is therefore possible that it was an ambush predator which plunged into shoals of fish quickly in the shallow seas where it lived.

Discovery and Classification

The first specimen was found in the Kimmeridgian Kimmeridge Clay Formation of Kimmeridge Bay, Dorset, UK and described by Hulke in 1871, who named it ''Ichthyosaurus enthekiodon''.Hulke, J. W. Note on an ''Ichthyosaurus'' (''I. enthekiodon'') from Kimmeridge Bay, Dorset. ''Quarterly Journal of the Geological Society'', 27, 440–441, pl. 17. This referred to its teeth being 'sheathed' in cementum and less likely to fall out than those of other ichthyosaurs. A year earlier, Hulke had described some remains from the same horizon and locality that he thought were ichthyosaurian, naming them ''Enthekiodon'' (no species given).Hulke, J. W. Note on some teeth associated with two fragments of a jaw from Kimmeridge Bay. ''Quarterly Journal of the Geological Society'', 26, 172–174. These are now lost, but Hulke considered them sufficiently similar to demote the name to species level. In 1922, Friedrich von Huene separated this species into the new genus ''Nannopterygius'', named for the small fore- and hindpaddles.Huene, F. F. von 1922. ''Die Ichthyosaurier des Lias und ihre Zusammenhänge''. Verlag von Gebrüder Borntraeger, Berlin, 114 pp., 22 pls. The first fossil is the most complete, but is flattened. All subsequent fossils are fragmentary. In 2020, several more species, including ''N. borealis'', and the species once contained in the genera ''Paraophthalmosaurus'' and ''Yasykovia'' were named based on remains found in Norway and Russia. The existence of two more species from Kazakhstan, ''N. mikhailovi'' and ''N. yakimenkae'', was confirmed by Yakupova & Akhmedenov (2022).

The following cladogram shows a possible phylogenetic position of ''Nannopterygius'', which was found to be the sister taxon to '' Thalassodraco'', in Ophthalmosauria according to an analysis performed by Zverkov and Jacobs (2020).

Paleoecology

The type species is known from the Kimmeridge Clay Formation in Dorset, England. The Kimmeridge Clay hosts a wealth of marine fossils dating to the Kimmeridgian and Tithonian stages of the Late Jurassic, many of which are incredibly well preserved, however unfortunately very little has been published. At the time, Europe represented an island archipelago which was closer to the equator than it is today, surrounded by warm, tropical seas. The Kimmeridge Clay specifically represents an offshore marine environment, in which the seafloor was far enough below the surface not to be disturbed by storms. It was home to a great variety of marine life, including many cephalopods, fishes such as ''Thrissops'' and the early ray ''Kimmerobatis'', and remains of occasional dinosaurs like ''Dacentrurus'' which had been washed out to sea. It is most famous for diversity of marine reptiles, such as the metriorhynchids ''Metriorhynchus'' and '' Plesiosuchus'', the plesiosaurs ''Colymbosaurus'' and ''Kimmerosaurus'', and the ichthyosaurs '' Grendelius'' and ''Thalassodraco''. The apex predators of the Kimmeridge ecosystem would have been the several species of the pliosaurid ''Pliosaurus'' which have been recovered there, as well as large metriorhynchids like ''Plesiosuchus''. Additionally, the pterosaurs ''Cuspicephalus'' and ''Rhamphorhynchus'' are also known from the Kimmeridge Clay.

The species ''N. yasykovi'' and ''N. saveljeviensis'' are known from the Volga region of Russia, which gives the Volgian stage its name. Though very little is known or published about the fossils of these localities, fossils of a number of marine animals have been recovered, including several species of the ichthyosaurs ''Arthropterygius'', ''Grendelius'', and ''Undorosaurus''. In addition, fossils of the pliosaurid ''Pliosaurus rossicus'' and indeterminate remains belonging to a metriorhynchid, as well as a high diversity of ammonites including the large-bodied taxon ''Titanites'', are also known from the Volgian-aged sediments of this region.N. G. Zverkov, M. S. Arkhangelsky and I. M. Stenshin (2015) A review of Russian Upper Jurassic ichthyosaurs with an intermedium/humeral contact. Reassessing Grendelius McGowan, 1976. Proceedings of the Zoological Institute 318(4): 558-588

Additionally, the species ''N. borealis'' is known from earliest Cretaceous sediments of the Slottsmøya Member of the Agardhfjellet Formation. The Slottsmøya Member consists of a mix of shales and siltstones and was deposited in a shallow water methane seep environment. The seafloor, which was located about below the surface, seems to have been relatively dysoxic, or oxygen-poor, although it was periodically oxygenated by clastic sediments. Despite this, near the top of the member, various diverse assemblages of invertebrates associated with cold seeps have been discovered; these include ammonites like ''Lytoceras'' and '' Phylloceras'', lingulate brachiopods, bivalves like '' Myophorella'' and '' Laevitrigonia'', rhynchonellate brachiopods, tubeworms, belemnoids, tusk shells, sponges like ''Peronidella'', crinoids, sea urchins like ''Hemicidaris'', brittle stars, starfish like '' Pentasteria'', crustaceans like ''Eryma'', and gastropods, numbering 54 taxa in total. Though direct evidence from Slottsmøya is currently lacking, the high latitude of this site and relatively cool global climate of the Tithonian mean that sea ice was likely present at least in the winter. In addition to ''Nannopterygius'', the Slottsmøya Member presents a diverse assemblage of other marine reptiles, including the ichthyosaurs ''Undorosaurus gorodischensis'', several species belonging to the genus ''Arthropterygius'', and a partial skull attributed to ''Brachypterygius sp.'' The presence of these taxa indicates that there was significant faunal exchange across the seas of Northern Europe during this time period. Additionally, 21 plesiosaurian specimens are also known from the site, including two belonging to the large pliosaur ''Pliosaurus funkei'', three to '' Colymbosaurus svalbardensis'', one to '' Djupedalia engeri'', one to ''Ophthalmothule cryostea'', and one each to '' Spitrasaurus wensaasi'' and '' S. larseni''. Many of these specimens are preserved in three dimensions and partially in articulation; this is correlated with high abundance of organic elements in the sediments they were buried in.

See also

* List of ichthyosaurs
* Timeline of ichthyosaur research

References

{{Taxonbar, from=Q1028280

Fossil taxa described in 1922
Middle Jurassic ichthyosaurs
Jurassic reptiles of Europe
Late Jurassic ichthyosaurs
Ichthyosaurs of Europe
Jurassic United Kingdom
Fossils of Great Britain
Tithonian life
Fossils of Svalbard
Agardhfjellet Formation
Early Cretaceous ichthyosaurs
Berriasian life
Cretaceous Norway
Ophthalmosauridae
Ichthyosauromorph genera