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 Norway and six species are currently assigned to the genus.
Description
Nannopterygius was small for an ichthyosaur, measuring up to 3.5 metres (11 ft) long at maximum. About 1 metre (3.3 ft) of this was tail, including a deeply forked and probably homocercal caudal fin. The head is 50 centimetres (20 in; 1.6 ft) long, with a typical long narrow rostrum. The eyes are large, hence its classification as an ophthalmosaurid, and have a bony scleral 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 25–30 centimetres (9.8–11.8 in) for the forepaddles and only 10–15 centimetres (3.9–5.9 in) 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. 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). 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. 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
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