Geosaurus

Taxobox
name = "Geosaurus"
fossil_range = Fossil range|157|143Oxfordian - Berriasian

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image_width=250px
image_caption = Restoration of "Geosaurus suevicus"
regnum = Animalia
phylum = Chordata
classis = Sauropsida
subclassis = Diapsida
infraclassis = Archosauromorpha
unranked_ordo = Mesoeucrocodylia
subordo = Thalattosuchia
familia = Metriorhynchidae
genus = "Geosaurus"
genus_authority = Cuvier, 1824 Cuvier G. 1824. Sur les ossements fossiles de crocodiles, 5. In: Dufour & D'Occagne, eds. "Recherches sur les ossements fossiles, 2nd édition". Paris: 143-160]
subdivision_ranks = Species
subdivision =
* "G. giganteus" (von Sömmerring, 1816) Sömmerring ST von. 1816. Ueber die "Lacerta gigantea" der Vorwelt. "Denkschriften der Königlichen Akademie der Wisseschaften zu Münch" 6: 37-59.] (type)
* "G. gracilis" (von Meyer, 1831)Meyer H von 1831. Neue fossile Reptilien aus der Ordung der Saurier. "Nova Acta Academiae Leopoldino-Carolinae Curios" 15 (2): 173-184.]
* "G. suevicus" Fraas, 1901 Fraas E. 1901. Die Meerkrokodile (Thalattosuchia n. g.) eine neue Sauriergruppe der Juraformation. "Jahreshefte des Vereins für vaterländische Naturkunde, Württemberg" 57: 409-418.]
* "G. vignaudi" Frey "et al.", 2002 Frey, E., Buchy, M.-C., Stinnesbeck, W. & López-Oliva, J.G. 2002. "Geosaurus vignaudi" n. sp. (Crocodylia, Thalattosuchia), first evidence of metriorhynchid crocodilians in the Late Jurassic (Tithonian) of central-east Mexico (State of Puebla). "Canadian Journal of Earth Sciences" 39: 1467–1483.]
* "G. araucanensis" Gasparini & Dellapé, 1976 Gasparini ZB, Dellapé D. 1976. Un nuevo cocodrilo marino (Thalattosuchia, Metriorhynchidae) de la Formación Vaca Muerta (Jurasico, Tithoniano) de la Provincia de Neuquén (República Argentina). "Congreso Geológico Chileno" 1: c1-c21.]
* "G. saltillense" Buchy "et al"., 2006 Buchy, M.-C., Vignaud, P., Frey, E., Stinnesbeck, W. & González, A.H.G. 2006. A new thalattosuchian crocodyliform from the Tithonian (Upper Jurassic) of northeastern Mexico. "Comptes Rendus Palevol" 5 (6): 785-794.]
synonyms =
* "Halilimnosaurus" Ritgen, 1826 Ritgen CH. 1826. Becken der krokodilartigen Salzumpf-eideche. "Nova Acta Academiae Leopoldino-Carolinae Curios" 13 (1): 331-358.]
* "Rhacheosaurus " von Meyer, 1831
* "Brachytaenius" von Meyer, 1842 Meyer H von 1842. "Brachytaenius perennis" aus dem dichten gelben Jurakalk von Aalen in Würtembertg. In: Meyer H von, Althaus GB, Münster G, eds. "Beiträge zur Petrefacten-kunde". Bayreuth: 22-23, & plate 7.]

"Geosaurus" was a small, gracile genus of marine crocodylian within the family Metriorhynchidae that lived during the Late Jurassic to the Early Cretaceous. "Geosaurus" was a carnivore that spent much, if not all, its life out at sea. No "Geosaurus" eggs or nest have been discovered, so little is known of the reptile's lifecycle, unlike other large marine reptiles of the Mesozoic, such as plesiosaurs or ichthyosaurs which are known to give birth to live young out at sea. Where "Geosaurus" mated, whether on land or at sea, is currently unknown. The name "Geosaurus" means "Earth lizard", and is derived from the Greek "Ge"- ("Earth") and Polytonic| -"sauros" ("lizard"). The name "Geosaurus" was established by the French naturalist Georges Cuvier in 1824.

Discovery and species

Fossil specimens referrable to "Geoaurus" are known from Late Jurassic deposits from England, France, Switzerland, Germany,Steel R. 1973. Crocodylia. "Handbuch der Paläoherpetologie, Teil 16". Stuttgart: Gustav Fischer Verlag,116 pp.] Argentina, Cuba,Gasparini ZB, Iturralde-Vinet M. 2001. Metriorhynchid crocodiles (Crocodyliformes) from the Oxfordian of Western cuba. "Neues Jahrbuch für Geologie und Paläontologie, Monatshefte" 9: 534–542.] and Mexico.

Valid species

The type species "Geosaurus giganteus", meaning "giant Earth lizard", is known from Western Europe (Germany) of the Late Jurassic (Early Tithonian). Originally named "Lacerta gigantea" by von Sömmerring.

"Geosaurus gracilis", meaning "gracile Earth lizard", was originally the type species of the genus "Rhacheosaurus". It is known from Western Europe (Germany) of the Late Jurassic (Early Tithonian).

"Geosaurus suevicus" is known from Western Europe (Germany) of the Late Jurassic (Late Kimmeridgian).

"Geosaurus araucanensis" was the first species of "Geosaurus" to be described from specimens outside of Europe. A complete skull and mandible (along with numerous other specimens) are known from Argentina of the Late Jurassic (Early Tithonian).

"Geosaurus vignaudi", meaning "Vignaud's Earth lizard", was the first species of "Geosaurus" to be described from Mexico. Its fossils are Late Jurassic (Middle Tithonian) in age

"Geosaurus saltillense", meaning "the Earth lizard from Saltillo", was the second species of "Geosaurus" to be described from Mexico. Its fossils are Late Jurassic (Early Tithonian) in age.

Unnamed species

An unnamed specimen of "Geosaurus" has been found in Oxfordian age rocks of Cuba.

Taxonomy and phylogeny

The modern Chinese term for "Geosaurus" is Dilong 地龍 "earth dragon" (地龙 in simplified Chinese).Carr M. 1990. Chinese Dragon Names. "Linguistics of the Tibeto-Burman Area 13.2":87-189.] This contrasts to the dinosaur genus "Dilong" which was named from Chinese "dilong" 帝龍 "emperor dragon".

Genera considered junior synonyms of "Geosaurus" include: "Brachytaenius", "Rhacheosaurus" and "Halilimnosaurus ".Steel R. 1973. "Crocodylia. Handbuch der Paläoherpetologie, Teil 16". Stuttgart: Gustav Fischer Verlag, 116 pp.] Recent phylogenetic analyses however, do not support the monophyly of "Geosaurus".Young MT. 2007. The evolution and interrelationships of Metriorhynchidae (Crocodyliformes, Thalattosuchia). "Journal of Vertebrate Paleontology" 27 (3): 170A.] Mueller-Töwe IJ. 2005. Phylogenetic relationships of the Thalattosuchia. "Zitteliana" A45: 211–213.] Although, the species "G. suevicus" and "G. araucanensis" are within a natural group.Gasparini Z, Pol D, Spalletti LA. 2006. An unusual marine crocodyliform from the Jurassic-Cretaceous boundary of Patagonia. "Science" 311: 70-73.]

Palaeobiology

Morphology

All currently known species would have been three metres or less in length, which when compared to living crocodilians, "Geosaurus" can be considered moderate to small-sized. Its body was streamlined for greater hydrodynamic efficiency, which along with its finned tail made it a more efficient swimmer than modern crocodilian species.Massare JA. 1988. Swimming capabilities of Mesozoic marine reptiles; implications for method of predation. "Paleobiology" 14 (2):187-205.]

Salt glands

Recent examination of the fossil specimens of "Geosaurus araucanensis", have shown that both juveniles and adults of this species had well-developed salt glands. This means that it would have been able to "drink" salt-water from birth (necessary for a pelagic animal) and eat prey that have the same ionic concentration as the surrounding sea water (i.e. cephalopods) without dehydrating.Fernández M, Gasparini Z. 2000. Salt glands in a Tithonian metriorhynchid crocodyliform and their physiological significance. "Lethaia" 33: 269-276.] Fernández M, Gasparini Z. 2008. Salt glands in the Jurassic metriorhynchid "Geosaurus": implications for the evolution of osmoregulation in Mesozoic crocodyliforms. "Naturwissenschaften" 95: 79-84.] Adult specimens of "Metriorhynchus" also have these well-developed salt glands.Gandola R, Buffetaut E, Monaghan N, Dyke G. 2006. Salt glands in the fossil crocodile "Metriorhynchus". "Journal of Vertebrate Paleontology" 26 (4): 1009-1010.]

Niche partitioning

Several species of metriorhynchids are known from the Mörnsheim Formation (Solnhofen limestone, early Tithonian) of Bavaria, Germany. Alongside "Dakosaurus maximus", three species of "Geosaurus" are known, including "G. giganteus", "G. suevicus" and "G. gracilis". It has been hypothesised that niche partitioning enabled several species of crocodilians to co-exist. The top predators of this Formation appear to be "Dakosaurus" and "G. giganteus", which were large, short-snouted species with serrated teeth. The long-snouted "G. suevicus" and "G. gracilis" would have feed mostly on fish, although the more lightly-built "G. gracilis" may have specialised towards feeding on small prey. In addition to four species of metriorhynchids, a moderate-sized species of "Steneosaurus" was also contemporaneous.Andrade MB, Young MT. 2008. [http://www.svpca.org/general/pages/abstractPage.php?i=1378&r=talksAndPosters.php&y=2008| High diversity of thalattosuchian crocodylians and the niche partition in the Solnhofen Sea] . The 56th Symposium of Vertebrate Palaeontology and Comparative Anatomy]

From the slightly older Nusplingen Plattenkalk (late Kimmeridgian) of southern Germany, both "G. suevicus" and "Dakosaurus maximus" are contemporaneous. As with Solnhofen, "G. suevicus" feed upon fish, while "Dakosaurus" was the top predator.Dietl G, Dietl O, Schweigert G, Hugger R. 2000. "Der Nusplinger Plattenkalk (Weißer Jura zeta)" - Grabungskampagne 1999.]

ee also

*List of marine reptiles

References