| Ophthalmosaurus | |
|---|---|
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| Composite skeleton (NHMUK PV R3702, R3893, R4124) ofOphthalmosaurus icenicus at theNatural History Museum, London, with the forelimbs mounted backwards[1] | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Ichthyosauria |
| Family: | †Ophthalmosauridae |
| Subfamily: | †Ophthalmosaurinae |
| Genus: | †Ophthalmosaurus Seeley,1874 |
| Type species | |
| †Ophthalmosaurus icenicus | |
| Species | |
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| Synonyms | |
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Ophthalmosaurus (Greek ὀφθάλμοςophthalmos 'eye' and σαῦροςsauros 'lizard') is a genus ofichthyosaur known from the Middle-Late Jurassic. Possible remains from the earliestCretaceous, around 145 million years ago, are also known. It was a relatively medium-sized ichthyosaur, measuring 4 m (13 ft) long and weighing 940 kg (2,070 lb).[2][3] Named for its extremely large eyes, it had a jaw containing many small but robust teeth. Major fossil finds of this genus have been recorded in Europe with a second species possibly being found in North America.
Ophthalmosaurus was a medium-sized ichthyosaur, growing to measure 4 m (13 ft) in length and weighing between 930–950 kg (2,050–2,090 lb).[2][3] It had a robust, streamlined body that was nearly as wide as it was tall in frontal view. Like other derived ichthyosaursOphthalmosaurus had a powerful tail ending in a pronounced bi-lobed caudal fluke whose lower half was formed around the caudal spine whereas the upper lobe was made up entirely from soft tissue. The limbs ofOphthalmosaurus were short and rounded with the forelimbs being noticeably larger than the hind limbs. The combination of rather inflexible trunk, powerful caudal fluke and reduced limbs suggests a tail-propelled mode of locomotion with the limbs helping with steering, differing from theanguilliform (eel-like) way more basal ichthyosaurs swam. The skull ofOphthalmosaurus was long with a slender, toothed rostrum and an enlarged posterior portion of the cranium. The dentition was relatively small with robust tooth crowns and the lateral area of the cranium was almost entirely occupied by the animal's massive eyes that gave the genus its name. The proportionally large eyes ofOphthalmosaurus measured 22–23 centimetres (8.7–9.1 in) in diameter at the outer margin of the bony sclerotic ring, while the sclerotic aperture itself measured 10 centimetres (3.9 in) in diameter.[2][4]
Ophthalmosaurus was first described byHarry Seeley in 1874 with particular focus on the morphology of the clavicular bones. Over the years following its description a variety of genera have been sunk intoOphthalmosaurus.[5] Among them,Apatodontosaurus,Ancanamunia,Baptanodon,Mollesaurus,Paraophthalmosaurus,Undorosaurus andYasykovia were all consideredjunior synonyms ofOphthalmosaurus in a study published by Maisch & Matzke in 2000.[6]
However, more recentcladistic analyses have contested Maisch & Matzke's conclusion.Mollesaurus periallus fromArgentina was considered a valid genus of ophthalmosaurid by Druckenmiller and Maxwell (2010),[7][8][9]Paraophthalmosaurus andYasykovia were both recovered as distinct genera by Storrset al., but were later sunk intoNannopterygius[10][11][12] whileUndorosaurus's validity is now accepted by most authors, including Maisch (2010) who originally proposed the synonymy.[8][10][13][14][15] The two other Russian taxa might be also valid.[8][14] Likewise the Mexican ophthalmosauridJabalisaurus had also been referred toOphthalmosaurus before being described as a distinct species and genus in 2021.[16]
Ophthalmosaurus natans was described asSauranodon, then later renamed toBaptanodon by Marsh in 1880. However this decision was questioned not long afterwards withBaptanodon instead being considered an American species ofOphthalmosaurus. Recent analysis have recovered the species as closer to other ophthalmosaurines than to theOphthalmosaurus type species,[7][9][17] suggesting that the previous name should be reinstated. Similarly,Ophthalmosaurus chrisorum, whose holotype has been recovered in Canada and described by Russell in 1993, was moved to its own genusArthropterygius in 2010 by Maxwell.[18]
While primarily known from the Jurassic, material from theSpilsby Sandstone dating to the earlyBerriasian stage of theLower Cretaceous has been referred to cf.Ophthalmosaurus (i.e., eitherOphthalmosaurus or a closely related species).[19]
WithinOphthalmosauridae,Ophthalmosaurus was once considered most closely related toAegirosaurus.[20] However, many recentcladistic analyses foundOphthalmosaurus to nest in a clade withAcamptonectes andMollesaurus.Aegirosaurus was found more closely related toPlatypterygius, and thus does not belong to theOphthalmosaurinae.[8][9]
The cladogram below follows Fischeret al. 2012.[9]
| Thunnosauria |
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The following cladogram shows a possible phylogenetic position ofOphthalmosaurus in Ophthalmosauridae according to the analysis performed by Zverkov and Jacobs (2020).[12]
| Ophthalmosauria |
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Ophthalmosaurus icenicus possessed small teeth with robust tooth crowns and signs of slight wear differing notably from the robust teeth of later species ofPlatypterygius, known to have hunted large prey including turtles and birds, and the minute teeth ofBaptanodon, interpreted to be a soft prey specialist. Fischeret al. (2016) conclude that this intermediary tooth morphology indicates thatOphthalmosaurus icenicus was most likely a generalist predator, feeding on a variety of smaller prey items.[22]
Ophthalmosaurus could likely dive for around 20 minutes. Assuming a conservative cruising speed of 1 metre per second (3.3 ft/s) (2 metres per second (6.6 ft/s) being more likely),Ophthalmosaurus could reach depths of 600 metres (2,000 ft) or more during a dive, reaching themesopelagic zone.[3] However, while studies on the biomechanics ofOphthalmosaurus suggests that such feats could be physically achieved, studies on the environment of the Peterborough member of theOxford Clay suggest thatOphthalmosaurus instead inhabited relatively shallow waters there, being determined to have been just 50 metres (160 ft) deep at a distance of 150 kilometres (93 mi) from the shore.[23]
