Geosaurus
| Geosaurus | ||||||||||||
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Skull of G. giganteus |
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| Temporal occurrence | ||||||||||||
| Upper Jurassic (lower Tithonian) to Lower Cretaceous (lower Hauterivian) | ||||||||||||
| 152.1 to 133.1 million years | ||||||||||||
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| Scientific name | ||||||||||||
| Geosaurus | ||||||||||||
| Cuvier , 1824 | ||||||||||||
| species | ||||||||||||
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Geosaurus ( ancient Greek γεω ( geō ): "earth" and σαῧρος ( saūros ): "lizard") is a genus of extinct marine crocodiles from the family Metriorhynchidae , whose fossils were found in the Upper Jurassic and Lower Cretaceous Western Europe . Fossils of Geosaurus were among the first documented remains of extinct marine reptiles. As a result, the genus looks back on a long history of research.
description
The genus Geosaurus comprises several species of medium-sized marine crocodiles that specialized in preying on larger animals. Like its closest relatives, Geosaurus has in all likelihood had various adaptations to a marine way of life, including limbs converted into paddles, a tail fin, and the loss of the osteoderms that are otherwise typical of crocodiles .
skull
Remnants of skulls have been handed down from all known Geosaurus species. Of the type species G. giganteus two incomplete skulls are known from G. grandis a single complete skull of G. lapparenti however, only fragmentary remains. Overall, the genus Geosaurus is characterized by a rather short-nosed skull, similar to the related genus Dakosaurus . The numerous ornamental pits, such as those found in the skull bones of today's crocodiles , are only slightly pronounced in Geosaurus . Like other metriorhynchids, they are completely absent on the nasal and frontal bones . Overall, the surface of the Geosaurus cranial bones appears relatively smooth. Other features of the skull that should be emphasized include the teardrop-shaped prefrontal (a bone between the tearbone and frontal bone) and the very elongated anterior orbital window .
The type species G. giganteus is characterized, among other things, by a notch between the upper and intermaxillary bones, which when the jaw is closed provided space for a larger pair of teeth in the lower jaw as well as robust scleral rings surrounding the entire eyes .
teeth
The teeth of Geosaurus have some distinctive features compared to other metriorhynchids. The tooth crowns are pointed and strongly compressed on the sides. The two cutting edges stand out clearly and have macroscopic "denticles", small sawtooth-like structures that are lined up along the cutting edges. Overall, Geosaurus' teeth most closely resemble those of Dakosaurus . The tooth crowns in G. giganteus are round on the inside (lingual side), but form three "facets" on the outside (labial side), whereby the outside of the tooth crowns, viewed from above, has a more trapezoidal shape. Such "facets" could only be found in the closely related genus Ieldraan .
Postcranial skeleton
The postcranial skeleton of Geosaurus is only partially known. In G. giganteus it comprises the anterior and middle vertebrae with the associated ribs , the two sacral vertebrae with abdominal ribs , parts of the pelvic girdle and the two fragmentary heads of the thigh bones . By G. lapparenti remain neck and tail vertebrae and parts of the chest down to us.
Research history and systematics
Research history
Geosaurus fossils were the first remains of metriorhynchid marine crocodiles to be scientifically described, even if they were not recognized as such for a long time. The holotype and other skeletal remains of the G. giganteus type were found in the Bavarian Daiting ( Donau-Ries district ) in the limestones of the Upper Jurassic Mörnsheim Formation and handed over to the scientist Samuel Thomas von Soemmering by Count JA Reisach . He described the fossils for the first time in 1816 as Lacerta gigantea and compared them with a " Leviathan " previously found in the Netherlands (later described as Mosasaurus hoffmannii ). Among other things, due to the lack of the osteoderms that are otherwise typical of crocodiles , Von Soemmering and other authors interpreted this Lacerta gigantea in the following years as a lizard and ultimately as a young animal of the Cretaceous Mosasaurus .
Georges Cuvier finally emphasized the differences from the recent lizards genus Lacerta and introduced in 1824 Geosaurus as subgenus to which a transitional form between Cuvier to view monitor lizards had been and crocodiles. The first to consider Geosaurus as an independent genus and to introduce the combination Geosaurus giganteus was the Tübingen geologist Friedrich August Quenstedt in his work from 1854. However, the uncertainties about the systematic allocation remained until the English geologist Richard Lydekker rewrote the fossils in 1888 and wrote them to the Metriorhynchinen (at that time a subfamily of the Teleosauridae ) after he had considered Geosaurus as a close relative of the Mosasaurid Clidastes in an earlier work of the same year . Lydekker regarded the Metriorhynchiden Dakosaurus and Cricosaurus as synonyms of Geosaurus .
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In the course of the 19th century, further marine reptile finds were assigned to Geosaurus , including G. bollensis from the Posidonia schist from Bad Boll (today the teleosauroid Steneosaurus bollensis ), G. maximus from the Upper Jurassic from Schnaitheim (today the type of Dakosaurus ) or the one that is still valid today Geosaurus -species G. grandis (originally described as Cricosaurus grandis ) found in the same strata as G. giganteus . Finally, Eberhard Fraas summarized in his much-cited works from 1901 and 1902 under Geosaurus the species of the long- snouted sea crocodiles Cricosaurus and Rhacheosaurus known at the time . As a result, Geosaurus was primarily understood as a long-snouted genus in the course of the 20th and early 21st centuries , and new species were also assigned to Geosaurus due to their similarity to the original Cricosaurus material .
This interpretation of Geosaurus only changed with the work of Young & De Andrade (2009). In particular, they re-described the original Von Soemmering material and were able to show that only short-nosed species with thin "saw teeth" can be summarized under Geosaurus . Long-snouted specimens with rather small teeth aimed at catching fish should, however, be placed in separate genera such as Cricosaurus or Rhacheosaurus . According to Young & De Andrade (2009) the still valid species are G. giganteus , G. grandis , G. lapparenti and G. carpenteri . The latter was suggested by De Andrade et al. (2010) placed in their own genus Torvoneustes .
External system
According to today's view, Geosaurus belongs to the Metriorhynchidae family within the Thalattosuchia . Within this, the genus acted as the namesake of further subordinate clades , starting with the subfamily Geosaurinae . In it Geosaurus is summarized with other more advanced Geosaurinen like Dakosaurus or Plesiosuchus in the tribe Geosaurini. As a sister genus of Geosaurus currently applies Ieldraan from the Middle Jurassic of England . Together they finally form the subtribe Geosaurina.
Abbreviated cladogram according to Foffa et al . (2017)
| Metriorhynchidae |
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Internal system
Many of the species that were once assigned to Geosaurus are now assigned to other genera within the Metriorhynchidae (see research history). So now z. B. G. araucanensis , G. elegans and G. suevicus are considered species of the genus Cricosaurus . G. gracilis and G. carpenteri in turn form species of the genera Rhacheosaurus and Torvoneustes .
In addition to the type species G. giganteus , only two other species are considered valid :
- G. giganteus (from Soemmerring, 1816): Upper Jurassic (Tithonium), Germany
- G. grandis (Wagner, 1858): Upper Jurassic (Tithonium), Germany
- G. lapparenti (Debelmas & Strannoloubsky, 1957): Lower Cretaceous (Valanginium to Hauterivium), France
Paleoecology
In addition to the type species G. giganteus , a high diversity of other marine crocodiles was discovered in the type locality near Daiting . Fossils of the Geosaurus species G. grandis and the metriorhynchids Dakosaurus , Cricosaurus and Rhacheosaurus also come from there . Furthermore, von Soemmering named the species Crocodilus priscus from the Daitinger strata two years before his work on Geosaurus (today the teleosauroid sea crocodile Steneosaurus priscus ). The coexistence of several Thalattosuchia species in the southern German lagoons of the Upper Jura can be explained by a phenomenon called "niche partioning". Despite their close evolutionary relationship, the species found here differed significantly in skull shape, teeth and general body structure. G. giganteus was probably one of the top predators in the region together with Dakosaurus maximus . Differences in the tooth shape indicate that G. giganteus and D. maximus each specialized in different prey animals .
literature
- E. Fraas: The sea crocodilians (Thalattosuchia) of the Upper Jurassic with special consideration of Dacosaurus and Geosaurus. In: Palaeontographica. (1846-1933), 10902, pp. 1-72.
- ST Von Soemmering: About the Lacerta gigantea of the past. 1816.
- MT Young, MB de Andrade: What is Geosaurus? Redescription of Geosaurus giganteus (Thalattosuchia: Metriorhynchidae) from the Upper Jurassic of Bayern, Germany. In: Zoological Journal of the Linnean Society. 157 (3), 2009, pp. 551-585, doi: 10.1111 / j.1096-3642.2009.00536.x .
Individual evidence
- ↑ a b c D. Foffa, MT Young, SL Brusatte, MR Graham, L. Steel: A new metriorhynchid crocodylomorph from the Oxford Clay Formation (Middle Jurassic) of England, with implications for the origin and diversification of Geosaurini. In: Journal of Systematic Palaeontology. 2017, pp. 1–21, doi: 10.1080 / 14772019.2017.1367730 .
- ↑ a b M. B. De Andrade, MT Young, JB Desojo, SL Brusatte: The evolution of extreme hypercarnivory in Metriorhynchidae (Mesoeucrocodylia: Thalattosuchia) based on evidence from microscopic denticle morphology. In: Journal of Vertebrate Paleontology. 30 (5), 2010, pp. 1451-1465, doi: 10.1080 / 02724634.2010.501442 .
- ↑ A. Wagner: On the knowledge of the dinosaurs from the lithographic crookedness. In: Treatises of the mathematical-physical class of the Royal Bavarian Academy of Sciences. 8, 1858, pp. 415-528.
- ↑ J. Debelmas, A. Strannoloubsky: Découverte d'un crocodilien dans le Néocomien de La Martre (Var) Dacosaurus lapparenti n. Sp. In: Travaux Laboratoire de Géologie de l'université de Grenoble. 33, 1858, pp. 89-99.
- ^ MB De Andrade, MT Young: High diversity of thalattosuchian crocodylians and the niche partition in Solnhofen Sea. In: The 56th Symposium of Vertebrate Palaeontology and Comparative Anatomy. 3rd-5th September, Dublin 2008, G. Dyke, D. Naish, M. Parkes (Eds) pp. 14-15 ( online ).