Ophthalmosaurus

Ophthalmosaurus
Fossil raven bone (above) and shoulder blade (below) from the holotype of Ophthalmosaurus icenius
Temporal occurrence
Middle Jurassic to Lower Cretaceous ( Callovian to Berriasian )
166.1 to 145 million years
Locations
Oxford Clay ( UK and France ) Spilsby Sandstone Formation (United Kingdom) Sundance Formation ( United States )
Systematics
Neoichthyosauria Thunnosauria Baracromia Ophthalmosauridae Ophthalmosaurinae Ophthalmosaurus
Scientific name
Ophthalmosaurus
Seeley , 1874

Ophthalmosaurus is a genus of ichthyosaurs (Ichthyopterygia), whichgathersthe fossils of medium-sized thunnosaurs (Thunnosauria). Its representatives lived from the Middle Jurassic to the Cretaceous Cretaceous (around 166 to 145 million years ago ) on the northern edge of the Tethys Ocean and the west coast of Laurasia . Ophthalmosaurus was, in view of its body shape and its largeeyes reinforcedby bone rings , a quick hunter who hunted fish, ammonites and belemnites at great depthsand gave birth to its offspring alive. The genus represents a typical representative of Jurassic ichthyosaurs, all of which were characterized by a tuna-like blueprint and probably occupied a wide range of ecological niches. The genus is controversial withinits current taxonomic limits; the European species Ophthalmosaurus icenius and the North American Ophthalmosaurus natans do not form a monophyletic group.

The first description of Ophthalmosaurus by Harry Govier Seeley comes from 1874 and is based on material from the English Oxford Clay , from where most of the known finds come. Other material comes from France and the USA . Ophthalmosaurus is a common species in its found layers, but is often only found in fragments, especially in Europe. While the genus has been geographically and taxonomically broad in recent decades, the species O. natans has recently been separated into the genus Baptanodon , which means that O. icenius would remain the only species in a monotypical genus.

features

Skeletal reconstruction of Ophthalmosaurus icenius in the Natural History Museum . The streamlined, tuna-like silhouette enabled the animals to travel at high speeds and limited energy consumption underwater.

Ophthalmosaurus was up to 4 m long, reached an estimated weight of 930 kg and, like all other Jurassic ichthyosaurs, was characterized by a tuna- shaped shape with a long, beak-like snout: the back arched relatively steeply upwards from the head, reaching just under its apex behind the front fins and then fell slightly towards the tail. The ventral line showed a similar course, while the dorsal and caudal fins were very long, tall and narrow compared to other genera of ichthyosaurs. While many ichthyosaurs had a laterally flattened body shape, Ophthalmosaurus fossils indicate a rounder body. This was not only extremely streamlined due to its external shape , it was also strongly stiffened and stabilized due to its bone structure: The dorsal vertebrae showed the ice hockey puck shape typical of later ichthyosaurs (very flat and wide), sat close together and thus ensured a very thick one and rigid spine. As a result, the animals were no longer able to snake - neither sideways nor up and down. Conversely, the thrust of the fins enabled them to reach very high speeds because the body was compact and well stabilized against turbulence and deformation. Ophthalmosaurus probably gained its propulsion primarily from the lateral flapping of its crescent-shaped caudal fin, the lower part of which was stiffened by the caudal spine and the rest probably by cartilage tissue. The dorsal fin, which is also made up of cartilage, probably helped stabilize the position.

Artistic life reconstruction of Ophthalmosaurus icenius . The eyes of the species were relatively large and were probably used for hunting in great depths with little light.

Ophthalmosaurus had relatively long and narrow jaws at 50 cm. The teeth sat in narrow grooves in the upper and lower jaw, so they were not completely thecodont and overall only weakly anchored. A diagnostic feature of the genus is the shape of the maxilla , which forms part of the nostrils. As with all ichthyosaurs, the eyes were stabilized by a reinforced scleral ring made of 15 bone plates and were very large in relation to their body size. The scleral ring protected the eye from high water pressure and thus from deformation, which was particularly important with the pressure difference that resulted between the anterior and posterior parts of the eyes at high speeds. The shoulder blades broadened towards the body, the front fins had a single additional finger and, like the smaller rear fins, were made up of rounded bones; all three traits are diagnostic of the genus. The hip consisted of two separate parts and, as is customary for late Jurassic ichthyosaurs, was only connected to the rear extremities.

ecology

Location of the Ophthalmosaurus localities in the late Jurassic. The genus was probably less cosmopolitan than previously assumed and is likely (with the exclusion of the North American O. natans ) to have been restricted to the northern shelf seas of the Tethys .

The large, bright eyes and the streamlined body of Ophthalmosaurus paint the image of a fast swimmer who ventured into great depths to hunt his prey. Ophthalmosaurus was probably able to track down and catch prey even at a depth of over 500 m. Ophthalmosaurus thus reflects a general development of the ichthyosaurs in the Jurassic , in which tuna-like, fast forms were able to prevail and spread all over the world. However, this way of life relates primarily to the type species O. icenius . For other species, whose affiliation to Ophthalmosaurus is mainly based on the general body shape and is usually controversial, there is hardly any specific knowledge about the habitat. In other cases, the skeletal structure indicates a significantly different way of life compared to O. icenius , for example in O. ( Baptanodon ) natans , who was less fast, more agile and more likely to have been a shallow water inhabitant.

The diet of Ophthalmosaurus consisted, among other things, of belemnites and ammonites , as can be demonstrated by the hooks and shells of these animals in fossil skeletons. Fish are also likely to have come. The prey was probably grabbed with the jaws of schools of fish and cephalopods at high speed and then swallowed. As is typical for ichthyosaurs, Ophthalmosaurus was viviparous , so its young were born alive and were already fully buoyant at birth.

Fossil material and stratigraphy

Outcrop of the Oxford Clay near Peterborough . The type specimen of Ophthalmosaurus icenius comes from this area ; almost all specimens of this species were found in southern England.

The first ophthalmosaurus specimens were found in Oxford Clay , England . This rock formation comes from the middle or late Jurassic and ranges from the Callovian to the Kimmeridgian (166.1 to 152.1  mya ). The fossils recovered there are all assigned to the species O. icenius , for which evidence can also be found outside of England in northwestern France. Overall, the Western European finds of Ophthalmosaurus are numerous, but often only partially preserved. The more complete North American fossils, grouped as O. baptanodon , come from the Sundance Formation of the northwestern United States , which is dated to the Oxfordian (around 160 mya). In addition, there are various finds from the Upper Jurassic of the Volga Basin in Russia , the genus of which is controversial. The status of English ichthyosaur material from the early Cretaceous ( Berriasium , 145-139.3 mya) Spilsby Sandstone Formation of Nettleton is also unclear . It cannot be assigned to O. icenius , but probably belongs to the genus Ophthalmosaurus , which would have existed beyond the Jurassic-Cretaceous border.

Taxonomy and systematics

Ophthalmosauridae    Arthropterygius       Ophthalmosaurinae    Mollesaurus      Ophthalmosaurus icenicus      Ophthalmosaurus ( Baptanodon ) natans      Acamptonectes      Platypterygiinae

Systematic position of Ophthalmosaurus according to Fischer et al. (2013). The genus is paraphyletic with regard to Acamptonectes , but a corresponding revision has not yet been made.

The type specimen of the species Ophthalmosaurus icenius and thus of the genus Ophthalmosaurus comes from the Peterborough member of the British Oxford Clay . It includes various parts of a disarticulated skeleton, including pieces of the lower jaw and skull, parts of the forefins, several vertebral bodies and arches, and some ribs. It was excavated and prepared by Charles Leeds in the 19th century. This skeleton is now kept in the Natural History Museum in London under inventory number NHMUK R2133 . Harry Govier Seeley described Ophthalmosaurus in 1874 as the second genus of ichthyosaurs after Ichthyosaurus . The generic name is derived from the ancient Greek "ophthalmos" for "eye" (based on the large eye sockets of animals) and "sauros" for "lizard". The specific epithet is reminiscent of the Icenier , a Celtic tribe who lived in the area around the place where it was first found.

Over the decades, Ophthalmosaurus, in addition to the type O. icenius, has been assigned a number of other fossils or fossil-handed species, the locations of which are spread across North America and Eurasia: The species Baptanodon (" Sauranodon ") natans, established by Othniel March in 1880 for fossils from the Oxfordium Wyoming was placed in 1910 by Charles William Andrews to Ophthalmosaurus , as well as all other North American fossils described in this genus. The Argentine ichthyosaurs of the Bajocium ( Ophthalmosaurus periallus ) as well as the species from the Russian Jura ( O. yasikovi , O. gorodischensis , O. saveljeviensis ) established by Maxim Arkhangelsky and Vladimir Efimov were included in the Ichthyopterygia monographs by Michael Maisch and Andreas Matzke, respectively slammed by Christopher McGowan and Ryosuke Motani Ophthalmosaurus . The basis for this was initially formed by head and body shapes from fossils and skeletal reconstructions. Three-dimensional skull reconstructions by Robert Appleby , which the latter cited as an argument in favor of separating Baptanodon and Ophthalmosaurus , were rejected by later authors due to methodological doubts. The differences between the two forms were considered too small to separate the genera. Phylogenetic analyzes based on osteological features grouped the forms grouped under Ophthalmosaurus together until the beginning of the 21st century, which spoke against splitting the genus. Ophthalmosaurus was accordingly a cosmopolitan genus, for whose paleoecology O. icenius was considered exemplary.

Only with a number of ophthalmosaurid fossil finds from various Jurassic fossil deposits in Europe and comparative analyzes of the skeletal morphology did a tendency towards the division of ophthalmosaurus into different genera emerge: In the meantime, the Argentine finds have been added to the genus Mollesaurus , while a number of studies have shown consistent differences between O.-icenius -Material and North American fossils ( Baptanodon ) were able to work out. With the discovery of fossils in Great Britain and Germany that differed significantly from Ophthalmosaurus , but appeared in phylogenetic analyzes as sister taxons of O. natans , doubts finally arose in 2012 about the grouping in a common genus. While the new finds received their own genus as Acamptonectes , O. natans was initially left in the now paraphyletic genus Ophthalmosaurus . The taxonomic status of Russian and American fossils has since remained unclear, largely due to the lack of detailed osteological comparisons.

Historically, Ophthalmosaurus is a typical representative of a Central Jurassic radiation of the Thunnosaurs (Thunnosauria). This group appeared at the beginning of the Jura and was mainly characterized by their tuna-like body shape. In the Middle Jurassic, several forms emerged that further optimized this blueprint and are grouped together as Ophthalmosauridae . Within the Ophthalmosauridae belongs ophthalmosaurus together with Mollesaurus and acamptonectes (and probably Baptanodon ) to the subgroup of Ophthalmosaurinae that a commitment to its sister group Platypterygiinae distinguished by a larger average body and less specialized jaw and skull shapes. While based on the fossil record it was long assumed that the Ophthalmosaurinae became extinct at the end of the Jurassic, the genus Acamptonectes described in 2012 , which is currently still within Ophthalmosaurus , documents the continued existence of ophthalmosaurine ichthyosaurs into the Lower Cretaceous .

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literature

• Richard Ellis: Sea Dragons. Predators of the Prehistoric Oceans . University Press of Kansas , Lawrence, Kansas 2003, ISBN 0-7006-1269-6 .
• Valentin Fischer, Michael W. Maisch, Darren Naish, Ralf Kosma, Jeff Liston, Ulrich Joger , Fritz J. Krüger , Judith Pardo Pérez, Jessica Tainsh, Robert M. Appleby: New Ophthalmosaurid Ichthyosaurs from the European Lower Cretaceous Demonstrate Extensive Ichthyosaur Survival across the Jurassic-Cretaceous Boundary . In: PLoS ONE . tape 7 , no. 1 , 2012, p. e29234 , doi : 10.1371 / journal.pone.0029234 .
• Valentin Fischer, Robert M. Appleby, Darren Naish, Jeff Liston, James B. Riding, Stephen Brindley, Pascal Godefroit: A basal thunnosaurian from Iraq reveals disparate phylogenetic origins for Cretaceous ichthyosaurs . In: Biology Letters . tape 9 , no. 4 , 2013, p. 1–6 , doi : 10.1098 / rsbl.2013.0021 .
• Valentin Fischer, Maxim S. Arkhangelsky, Gleb N. Uspensky, Ilya M. Stenshin, Pascal Godefroit: A new Lower Cretaceous ichthyosaur from Russia reveals skull shape conservatism within Ophthalmosaurinae . In: Geological Magazine . 2013, p. 1-11 , doi : 10.1017 / S0016756812000994 .
• Judy A. Massare, Emily A. Buchholtz, Jennifer M. Kenney, Anne-Marie Chomat: Vertebral morphology of Ophthalmosaurus natans (Reptilia: Ichthyosauria) from the Jurassic Sundance Formation of Wyoming . In: Paludicola . tape 5 (4) , 2006, pp. 242-254 .
• Christopher McGowan, Ryosuke Motani: Ichthyopterygia . In: Handbook of Paleoherpetology, Part 8 . Friedrich Pfeil, Munich 2003, ISBN 3-89937-007-4 .
• Michael Maisch, Andreas Matzke: The Ichthyosauria . In: Stuttgart contributions to natural history. Series B (geology and paleontology) . tape 298 , 2000, pp. 1-155 .
• Ryosuke Motani, Bruce M. Rothschild, William Wahl, Jr: Large eyeballs in diving ichthyosaurs . In: Nature . tape 402 , 1999, pp. 747 .
• Harry Govier Seeley: On the Pectoral Arch and Fore Limb of Ophthalmosaurus, a New Ichthyosaurian Genus from the Oxford Clay . In: Quarterly Journal of the Geological Society . tape 30 , no. 1-4 , 1874, pp. 696-707 , doi : 10.1144 / GSL.JGS.1874.030.01-04.64 .
• Motani R .: Scaling effects in caudal fin propulsion and the speed of ichthyosaurs . In: Nature . tape 415 , no. 6869 , January 17, 2002, p. 309-312 , PMID 11797005 .

Web links

Commons : Ophthalmosaurus  - collection of images, videos and audio files

Individual evidence

1. ↑ ^{a b} Seeley 1874, p. 699.
2. ↑ ^{a b c} Motani et al. 1999, p. 747.
3. ↑ McGowan & Motani 2003, p. 47.
4. ↑ ^{a b} Maisch & Matzke 2000, p. 78.
5. ↑ Motani et al. 2002, p. 309.
6. ↑ Massare et al. 2006, p. 251.
7. ↑ Ellis 2003, pp. 92-99.
8. ↑ McGowan & Motani 2003, pp. 114-115.
9. ↑ Fischer et al. 2012, p. 17.
10. ↑ Fischer et al. 2013a, p. 4.
11. ↑ McGowan & Motani 2003, p. 110.
12. ↑ Massare et al. 2006, p. 242.
13. ↑ McGowan & Motani 2003, pp. 110-113.
14. ↑ Massare et al. 2006, pp. 245-247.
15. ↑ Fischer et al. 2012, pp. 19-20.
16. ↑ Fischer et al. 2013b, p. 10.