Stenopterygius

Stenopterygius is an extinct genus of thunnosaur ichthyosaur known from Europe (England, France, Germany, Luxembourg and Switzerland). This genus of ichthyosaur grew to a maximum length of .
History.
"Stenopterygius" was originally named by Quenstedt in 1856 as a species of "Ichthyosaurus", "I. quadriscissus". Otto Jaekel in 1904 reassigned it to its own genus, "Stenopterygius". The type species is therefore "Stenopterygius quadriscissus". The generic name is derived from "stenos", Greek for "narrow", and "pteryx" (πτερυξ), Greek for "fin" or "wing".
Description.
"Stenopterygius" was physically similar to the better known "Ichthyosaurus", but had a smaller skull and narrower flippers. Beautifully preserved fossils of "Stenopterygius" have been found in Germany. Its skull was extended into a kind of a beak and was armed with a quantity of large teeth. The limbs had been transformed to fin-like structures. The tail terminated in a large, semicircular, leathery, vertical caudal fin and even a triangular dorsal fin was present. One well-preserved fossil of "Stenopterygius" preserves traces of skin, from which the animal's coloration was discovered to be countershaded (darker on the back than the underbelly).
Classification.
Most of the known specimens of "Stenopterygius", more than 100, were recently redescribed by Michael W. Maisch. He found that "S. quadriscissus" (the type species) also includes "S. eos", "S. incessus", and "S. macrophasma", as well as specimens previously referred to "S. hauffianus" and "S. megacephalus". Maisch followed Woodward (1932) and considered "Ichthyosaurus triscissus" to be a valid species of "Stenopterygius". The type specimens of "S. longifrons", "S. megacephalus", and "S. megalorhinus" were all referred to this species, as the name "I. triscissus" has a priority over them. Some specimens previously referred to "S. megalorhinus", as well as the holotype of "S. cuneiceps", were found to belong to a species of their own for which the binomen "Stenopterygius uniter" can be used.
As the holotype of "S. uniter" was destroyed in World War II, Maisch proposed a neotype. Maisch also found that "S. promegacephalus" is a "nomen dubium", as it is based on a juvenile specimen, and that the lectotype of "S. hauffianus" can be determined as "Stenopterygius" cf. "S. quadriscissus" at best, so this species should be considered a "nomen dubium". He found out that most specimens previously referred to "S. hauffianus" can be referred to "S. quadriscissus", while the rest belongs to a highly distinctive new taxon that cannot be referred to any valid species of "Stenopterygius". This species was reassigned to its own genus, "Hauffiopteryx".
"Stenopterygius" is known from the lectotype GPIT 43/0219-1, articulated complete skeleton which preserved a very large embryo. The animal is about in length. It was collected from the "Harpoceras elegantulum-exaratum" ammonoid subzones (more specifically Lias ε II3-4), "Harpoceras falcifer" zone, of the famous Posidonien-Schiefer lagerstätte (Posidonia Shale) of Holzmaden, dating to the early Toarcian stage of the Early Jurassic, about 182 million years ago. Maisch referred to the type species 30 additional specimens, all came from Dobbertin of Mecklenburg-Vorpommern and Holzmaden, Germany and Dudelange, Luxembourg. They were collected from the "Harpoceras palum" to "H. falciferum" ammonoid subzones (Lias ε I2-II11, lower-middle early Toarcian), "Harpoceras tenuicostatum-falcifer" zones, of the Posidonia Shale. "S. triscissus" is known from the holotype GPIT 12/0224-2, articulated almost complete skeleton. The animal is a young adult about in length. It was collected from the "Harpoceras exaratum-elegans" ammonoid subzones (more specifically Lias ε II6), "Harpoceras falcifer" zone, of the Posidonia Shale in Ohmden, dating to the middle Early Toarcian stage of the Early Jurassic. Maisch referred to this species 13 additional specimens, all came from various localities in England, France, Germany, Luxembourg and Switzerland. They were collected from the Lias ε II1-III, dating to the middle-late Early Toarcian. 
"S. uniter" is known from the holotype SMNS 14216, articulated complete skeleton which was destroyed in World War II. The animal is an adult about in length. The proposed neotype is GPIT 1491/10, articulated almost complete skeleton. The animal is a young adult about in length. It was collected from the "Harpoceras falcifer" ammonoid subzones (more specifically Lias ε II10), "Harpoceras falcifer" zone, of the Posidonia Shale in Holzmaden, dating to the middle Early Toarcian stage of the Early Jurassic. Maisch referred to this species 10 additional specimens, all came from Holzmaden. They were collected from the "Harpoceras exaratum" to "H. falciferum" ammonoid subzones (Lias ε II6-II11, middle early Toarcian), "Harpoceras falcifer" zones, of the Posidonia Shale.
Additional materials were described by Hannah Caine and Michael J. Benton in 2011, from the early Toarcian Beacon Limestone of Strawberry Bank, Ilminster of England. The specimens are all juveniles or infants, which were preserved mostly by almost complete skeletons and some skulls. They include BRLSI M1405, BRLSI M1407, BRLSI M1408, BRLSI M1409. Caine and Benton referred these specimens to "S. triscissus".
A new Middle Jurassic species from southwestern Germany, "Stenopterygius aaleniensis", was described in 2012.
Maisch and Matzke (2000) and Maisch (2010) regarded "Chacaicosaurus" and "Hauffiopteryx" to be stenopterygiids. However, they didn't perform any cladistic analyses to confirm these claims. Fischer "et al." (2011) performed a cladistic analysis that found "Chacaicosaurus" to be a basal thunnosaur which is placed outside both Stenopterygiidae and Ophthalmosauridae. Both Maisch (2008) and Caine and Benton (2011) performed cladistic analyses that found "Hauffiopteryx" to be either a basalmost member of Eurhinosauria or a basalmost member of Thunnosauria (which is an equivalent position to a basalmost member of Stenopterygiidae sensu Maisch [2008] with exclusion of "Ichthyosaurus"). These results mean that the Stenopterygiidae are a monotypic family that includes only the type genus "Stenopterygius".
The cladogram below follows the topology from a 2010 analysis by Patrick S. Druckenmiller and Erin E. Maxwell.
Palaeobiology.
The habits of "Stenopterygius" spp. were similar to those of present-day dolphins. They spent most of their lives in the open sea, where they hunted fish, cephalopods, and other animals. The abdominal cavities of skeletons of this ichthyosaur often contain the remains of such food.
One famous fossil is that of a mother and baby that died in childbirth (ichthyosaurs were viviparous). This proved that ichthyosaur infants were born tail-first, just like cetaceans, to prevent them from drowning before fully clearing the birth canal.
"Stenopterygius" was a very fast swimmer, with a cruising speed similar to that of tuna, which is among the fastest of all living fishes.
In 2018, a "Stenopterygius" specimen was reported with evidence of having had blubber, which indicates that other ichthyosaurs and it were homeothermic ("warm blooded").