Geosaurus

Geosaurus is an extinct genus of marine crocodyliform within the family Metriorhynchidae, that lived during the Late Jurassic and the Early Cretaceous. "Geosaurus" was a carnivore that spent much, if not all, its life out at sea. No "Geosaurus" eggs or nests 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 "Mother of Giants lizard", and is derived from the Greek "Ge"- ("Earth", the mythical mother of the Giants) and - ("lizard"). The name "Geosaurus" was established by the French naturalist Georges Cuvier in 1824.
Description.
"Geosaurus" was a large, marine reptile of the group Crurotarsi, the line leading to modern crocodilians. Specifically, it was a "marine crocodile", or thalattosuchian. "Geosaurus" was similar in appearance to the related "Dakosaurus" with a relatively short skull and curved teeth designed for slashing, which it likely used to attack large prey.
Many early depictions of "Geosaurus" were based on a nearly complete specimen described by Eberhard Fraas, which Fraas classified as a distinct species of "Geosaurus", "G. suevicus". This specimen was found in Germany and dated to the late Jurassic (Late Kimmeridgian) period. "G. suevicus" had a distinctively long, narrow snout filled with small, pointed teeth very different from skulls belonging to the type species. Further study and a redescription of "Geosaurus" published in 2009 showed that these long-snouted forms actually represent individuals of "Cricosaurus".
Classification and species.
Genera considered junior synonyms of "Geosaurus" include "Brachytaenius" and "Halilimnosaurus ". Numerous species had been assigned to this genus since the 19th Century. However, phylogenetic analyses begun in 2005 did not support the monophyly of "Geosaurus". Although some traditional species, such as "G. suevicus" and "G. araucanensis" formed a natural group, "Enaliosuchus" is also within that group. This, as well as further study showing that traditional metriorhynchid genera were not grouped based on actual relationships, necessitated almost all traditional species being removed from "Geosaurus" and reclassified elsewhere, as well as several species previously placed in other genera to be reclassified as species of "Geosaurus".
The species included below follow this revised classification, presented by Young and Andrade in 2009.
Valid species.
A large species of "Geosaurus" is known only from a single tooth from the Nusplingen Plattenkalk of Germany.
Cladogram after Cau & Fanti (2010).
Reclassified species.
In 2009, Young and de Andrade published a re-description of "Geosaurus", examining its relationships and the validity of species lumped into the genus. After performing a phylogenetic analysis of metriorhynchids, they found that many species were grouped in a paraphyletic manner or with the wrong genera. Specifically, they found that several species formerly classified as "Geosaurus", including "G. suevicus", "G. saltillense", "G. vignaudi", and "G. araucanensis" were actually examples of the related "Cricosaurus". "Rhacheosaurus gracilis", another long-snouted species, was also at one time considered a species of "Geosaurus" (as "G. gracilis").
"Geosaurus carpenteri", also formerly referred to "Dakosaurus", is known from a partial skull. Teeth identical to the known teeth of this species are also known from Yorkshire, UK. It was assigned to its own genus, "Torvoneustes" by Andrade "et al.", 2010.
An unnamed specimen classified as "Geosaurus" was found in the Oxfordian Jagua Formation of Cuba, though further study has shown this species to be more closely related to "Cricosaurus" as well.
Additionally, "Geosaurus" fossils have been reported from the Vaca Muerta of Argentina.
Palaeobiology.
Niche partitioning.
Several species of metriorhynchids are known from the Mörnsheim Formation (Solnhofen limestone, early Tithonian) of Bavaria, Germany: "Geosaurus giganteus", "Dakosaurus maximus", "Cricosaurus suevicus" and "Rhacheosaurus gracilis". It has been hypothesised that niche partitioning enabled several species of crocodyliforms to co-exist. The top predators of this Formation appear to be "G. giganteus" and "D. maximus", which were large, short-snouted species with serrated teeth. The long-snouted "C. suevicus" and "R. gracilis" would have fed mostly on fish, although the more lightly built "Rhacheosaurus" may have specialised towards feeding on small prey. In addition to these four species of metriorhynchids, a moderate-sized species of "Steneosaurus" was also contemporaneous.