Ophthalmosaurus Explained

Ophthalmosaurus (Greek ὀφθάλμος ophthalmos 'eye' and σαῦρος sauros 'lizard') is a genus of ichthyosaur known from the Middle-Late Jurassic. Possible remains from the earliest Cretaceous, around 145 million years ago, are also known. It was a relatively medium-sized ichthyosaur, measuring long and weighing . 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.

Description

Ophthalmosaurus was a medium-sized ichthyosaur, growing to measure in length and weighing between . It had a robust, streamlined body that was nearly as wide as it was tall in frontal view. Like other derived ichthyosaurs Ophthalmosaurus 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 of Ophthalmosaurus 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 the anguilliform (eel-like) way more basal ichthyosaurs swam. The skull of Ophthalmosaurus 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 of Ophthalmosaurus measured 22cm-23cmcm (09inches-09inchescm) in diameter at the outer margin of the bony sclerotic ring, while the sclerotic aperture itself measured 10cm (00inches) in diameter.[1] [2]

Discovery and species

Ophthalmosaurus was first described by Harry 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 into Ophthalmosaurus.[3] Among them, Apatodontosaurus, Ancanamunia, Baptanodon, Mollesaurus, Paraophthalmosaurus, Undorosaurus and Yasykovia were all considered junior synonyms of Ophthalmosaurus in a study published by Maisch & Matzke in 2000.[4]

However, more recent cladistic analyses have contested Maisch & Matzke's conclusion. Mollesaurus periallus from Argentina was considered a valid genus of ophthalmosaurid by Druckenmiller and Maxwell (2010),[5] [6] [7] Paraophthalmosaurus and Yasykovia were both recovered as distinct genera by Storrs et al., but were later sunk into Nannopterygius[8] [9] while Undorosauruss validity is now accepted by most authors, including Maisch (2010) who originally proposed the synonymy.[6] [10] [11] [12] [13] The two other Russian taxa might be also valid.[6] [12] Likewise the Mexican ophthalmosaurid Jabalisaurus had also been referred to Ophthalmosaurus before being described as a distinct species and genus in 2021.[14]

Ophthalmosaurus natans was described as Sauranodon, then later renamed to Baptanodon by Marsh in 1880. However this decision was questioned not long afterwards with Baptanodon instead being considered an American species of Ophthalmosaurus. Recent analysis have recovered the species as closer to other ophthalmosaurines than to the Ophthalmosaurus type species,[5] [7] [15] 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 genus Arthropterygius in 2010 by Maxwell.[16]

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).[17]

Classification

Within Ophthalmosauridae, Ophthalmosaurus was once considered most closely related to Aegirosaurus.[18] 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.[6] [7]

Phylogeny

The cladogram below follows Fischer et al. 2012.[7]

The following cladogram shows a possible phylogenetic position of Ophthalmosaurus in Ophthalmosauridae according to the analysis performed by Zverkov and Jacobs (2020).[9]

Palaeobiology

Ophthalmosaurus icenicus possessed small teeth with robust tooth crowns and signs of slight wear differing notably from the robust teeth of later species of Platypterygius, known to have hunted large prey including turtles and birds, and the minute teeth of Baptanodon, interpreted to be a soft prey specialist. Fischer et al. (2016) conclude that this intermediary tooth morphology indicates that Ophthalmosaurus icenicus was most likely a generalist predator, feeding on a variety of smaller prey items.[19]

Ophthalmosaurus could likely dive for around 20 minutes. Assuming a conservative cruising speed of 1m/s (2m/s being more likely), Ophthalmosaurus could reach depths of 600m (2,000feet) or more during a dive, reaching the mesopelagic zone.[20] However, while studies on the biomechanics of Ophthalmosaurus suggests that such feats could be physically achieved, studies on the environment of the Peterborough member of the Oxford Clay suggest that Ophthalmosaurus instead inhabited relatively shallow waters there, being determined to have been just 50m (160feet) deep at a distance of 150km (90miles) from the shore.[21]

See also

External links

Notes and References

  1. Motani. Ryosuke. Rothschild. Bruce M.. Wahl. William. 1999. Large eyeballs in diving ichthyosaurs. Nature. en. 402. 6763. 747. 10.1038/45435. 1999Natur.402..747M. 5432366. 1476-4687. https://web.archive.org/web/20170118225908/http://mygeologypage.ucdavis.edu/motani/pdf/motanietal1999.pdf. 18 January 2017.
  2. Schwab. Ivan R. 2002. My, what big eyes you have . . .. The British Journal of Ophthalmology. 86. 2. 130. 10.1136/bjo.86.2.130. 0007-1161. 1771016. 11855367.
  3. H.G. Seeley . 1874 . On the Pectoral Arch and Fore Limbs of Ophthalmosaurus, a new Ichthyosaurian Genus from the Oxford Clay . Quarterly Journal of the Geological Society . 30. 1–4 . 696–707 . 10.1144/gsl.jgs.1874.030.01-04.64. 129725722 .
  4. Maisch MW, Matzke AT. 2000. The Ichthyosauria. Stuttgarter Beiträge zur Naturkunde, Serie B (Geologie und Paläontologie) 298: 1-159.
  5. Patrick S. Druckenmiller . Erin E. Maxwell . 2010 . A new Lower Cretaceous (lower Albian) ichthyosaur genus from the Clearwater Formation, Alberta, Canada . Canadian Journal of Earth Sciences . 47 . 8 . 1037–1053 . 10.1139/E10-028 . 2010CaJES..47.1037D .
  6. Fischer . Valentin . Edwige Masure . Maxim S. Arkhangelsky . Pascal Godefroit . Pascal Godefroit . 2011 . A new Barremian (Early Cretaceous) ichthyosaur from western Russia . Journal of Vertebrate Paleontology . 31 . 5 . 1010–1025 . 10.1080/02724634.2011.595464 . 2011JVPal..31.1010F . 2268/92828 . 86036325 .
  7. Valentin Fischer . Michael W. Maisch . Darren Naish . Ralf Kosma . Jeff Liston . Ulrich Joger . Fritz J. Krüger . Judith Pardo Pérez . Jessica Tainsh . Robert M. Appleby . 2012 . New Ophthalmosaurid Ichthyosaurs from the European Lower Cretaceous Demonstrate Extensive Ichthyosaur Survival across the Jurassic–Cretaceous Boundary . PLOS ONE . 7 . 1 . e29234 . 10.1371/journal.pone.0029234 . 22235274 . 3250416. 2012PLoSO...729234F . free .
  8. 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
  9. Nikolay G. Zverkov . Megan L. Jacobs . amp . First published 2020 . Revision of Nannopterygius (Ichthyosauria: Ophthalmosauridae): reappraisal of the 'inaccessible' holotype resolves a taxonomic tangle and reveals an obscure ophthalmosaurid lineage with a wide distribution . Zoological Journal of the Linnean Society . 191 . 1 . 2021 . 228–275 . 10.1093/zoolinnean/zlaa028.
  10. Book: Storrs, Glenn W. . Vladimir M. Efimov . Maxim S. Arkhangelsky . 2000 . Mesozoic marine reptiles of Russia and other former Soviet republics . The Age of Dinosaurs in Russia and Mongolia . Benton, M.J. . Shishkin, M.A. . Unwin, D.M. . Cambridge University Press . Cambridge . 140–210 . 9780521545822.
  11. McGowan C, Motani R. 2003. Ichthyopterygia. – In: Sues, H.-D. (ed.): Handbook of Paleoherpetology, Part 8, Verlag Dr. Friedrich Pfeil, 175 pp., 101 figs., 19 plts; München
  12. Michael W. Maisch . 2010 . Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art . Palaeodiversity . 3 . 151–214 .
  13. Fischer . V. . A. Clement . M. Guiomar . P. Godefroit . 2011 . The first definite record of a Valanginian ichthyosaur and its implications on the evolution of post-Liassic Ichthyosauria . Cretaceous Research . 32 . 2 . 155–163 . 10.1016/j.cretres.2010.11.005 . 2011CrRes..32..155F . 2268/79923 . 45794618 .
  14. Barrientos-Lara. Jair Israel. Alvarado-Ortega. Jesús. 2021-11-01. A new ophthalmosaurid (Ichthyosauria) from the Upper Kimmeridgian deposits of the La Casita Formation, near Gómez Farías, Coahuila, northern Mexico. Journal of South American Earth Sciences. en. 111. 103499. 10.1016/j.jsames.2021.103499. 2021JSAES.11103499B. 0895-9811.
  15. 10.1017/S0016756816000455. 2017GeoM..154..837P. The first ophthalmosaurid ichthyosaur from the Upper Jurassic of the Umbrian-Marchean Apennines (Marche, Central Italy). Paparella. Ilaria. Maxwell. Erin E.. Cipriani. Angelo. Roncacè. Scilla. Caldwell. Michael W.. Geological Magazine. 2017. 154. 4. 837. 132955874.
  16. Maxwell . E.E. . 2010 . Generic reassignment of an ichthyosaur from the Queen Elizabeth Islands, Northwest Territories, Canada . Journal of Vertebrate Paleontology . 30 . 2 . 403–415 . 10.1080/02724631003617944. 2010JVPal..30..403M . 85143039 .
  17. Fischer . V. . Maisch . M. W. . Naish . D. . Kosma . R. . Liston . J. . Joger . U. . Krüger . F. J. . Pérez . J. P. . Tainsh . J. . Appleby . R. M. . Fenton . B. . New ophthalmosaurid ichthyosaurs from the European Lower Cretaceous demonstrate extensive ichthyosaur survival across the Jurassic–Cretaceous boundary . PLOS ONE . 2012 . 7 . 1 . e29234 . 10.1371/journal.pone.0029234. 22235274 . 3250416 . 2012PLoSO...729234F. free .
  18. Fernández M. 2007. Redescription and phylogenetic position of Caypullisaurus (Ichthyosauria: Ophthalmosauridae). Journal of Paleontology 81 (2): 368-375.
  19. Fischer. V.. Bardet. N.. Benson. R.B.J.. Arkhangelsky. M.S.. Friedman. M.. 2016. Extinction of Fish-shaped Marine Reptiles Associated with Reduced Evolutionary Rates and Global Environmental Volatility. Nature Communications. 7. 10825. 10.1038/ncomms10825. 26953824. 4786747. 2016NatCo...710825F.
  20. Ryosuke. Motani. Evolution of fish-shaped reptiles (Reptilia: Ichthyopterygia) in their physical environments and constraints. Annual Review of Earth and Planetary Sciences. 2005. 33. 395–420. https://web.archive.org/web/20190707185003/https://pdfs.semanticscholar.org/ab6f/98d1a830fb355af8162d3fd513d74dcd2f1a.pdf. dead. 2019-07-07. 10.1146/annurev.earth.33.092203.122707. 2005AREPS..33..395M. 54742104.
  21. Hudson. John D.. Martill. David M.. The Lower Oxford Clay: production and preservation of organic matter in the Callovian (Jurassic) of central England. Geological Society, London, Special Publications. 1991. 58. 1. 363–379. 10.1144/GSL.SP.1991.058.01.23. 1991GSLSP..58..363H. 129388168.

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