Ophthalmosaurus

Ophthalmosaurus (meaning "eye lizard" in Greek) is an ichthyosaur of the Jurassic period (165–160 million years ago). Possible remains from the Cretaceous, around 145 million years ago, are also known. Named for its extremely large eyes, it had a graceful long dolphin-shaped body, and its nearly toothless jaw was well adapted for catching squid. Major fossil finds of this genus have been recorded in Europe and North and South America.
Description.
"Ophthalmosaurus" had a body shaped like a tear-drop and a caudal fin like a half-moon. Its forelimbs were more developed than the hind ones, which suggests that the front fins did the steering while the tail did the propelling. "Ophthalmosaurus"' chief claim to fame is its eyes (some reaching about in diameter) which were extremely large in proportion to its body. The eyes occupied almost all of the space in the skull and were protected by bony plates (sclerotic rings), which most likely assisted to maintain the shape of the eyeballs against water pressure at depth.
The size of the eyes and the sclerotic rings suggests that "Ophthalmosaurus" hunted at a depth where there is not much light or that it may have hunted at night when a prey species was more active.
Discovery and species.
The genera "Apatodontosaurus", "Ancanamunia", "Baptanodon", "Mollesaurus", "Paraophthalmosaurus", "Undorosaurus" and "Yasykovia" were all considered junior synonyms of "Ophthalmosaurus" by Maisch & Matzke, 2000. However, all recent cladistic analyses found that "Mollesaurus periallus" from Argentina is a valid genus of ophthalmosaurid. "Ophthalmosaurus natans" has been recovered as closer to other ophthalmosaurines than to the "Ophthalmosaurus" type species, necessitating resurrection of "Baptanodon" Marsh, 1880 as the only available generic name. "Undorosaurus"'s validity is now accepted by most authors, even by Maisch (2010) who originally proposed the synonymy. and the two other Russian taxa might be also valid. "Ophthalmosaurus chrisorum" Russell, 1993 was moved to its own genus "Arthropterygius" in 2010 by Maxwell. On the other hand, "Paraophthalmosaurus" and "Yasykovia" are apparently valid and distinct from "Ophthalmosaurus". While primarily known from the Jurassic, material from the Spilsby Sandstone dating to the early Berriasian stage of the Lower Cretaceous has been referred to cf. "Ophthalmosaurus" (i.e., either "Ophthalmosaurus" or a closely related species).
Classification.
Within Ophthalmosauridae, "Ophthalmosaurus" was once considered most closely related to "Aegirosaurus". However, many recent cladistic analyses found "Ophthalmosaurus" to nest in a clade with "Acamptonectes" and "Mollesaurus". "Aegirosaurus" was found more closely related to "Platypterygius", and thus does not belong to the Ophthalmosaurinae.
The cladogram below follows Fischer "et al." 2012.
Palaeobiology.
Like other ichthyosaurs, "Ophthalmosaurus" gave birth to its pups tail-first to avoid drowning them. Skeletons of unhatched young have been found in over fifty females on fossil finds, and litter sizes ranged from two to eleven pups. 
"Ophthalmosaurus" could likely dive for around 20 minutes. Assuming a conservative crusing speed of ( being more likely), "Ophthalmosaurus" could reach depths of or more during a dive, reaching the mesopelagic zone.
Scientists have found evidence of decompression sickness (the bends) in the bone joints of "Ophthalmosaurus" skeletons, possibly caused by evasive tactics. Modern whales have been known to get the bends when they ascend rapidly to escape predators.