Cricosaurus

Cricosaurus is an extinct genus of marine crocodyliform belonging to the family Metriorhynchidae. The genus was established by Johann Andreas Wagner in 1858 for three skulls from the Tithonian (Late Jurassic) of Germany.[3] The name Cricosaurus means "Ring lizard", and is derived from the Greek Cricos- ("ring") and σαῦρος -sauros ("lizard").

Fossil specimens referrable to Cricosaurus are known from Late Jurassic deposits in England, France, Switzerland, Germany,[4] Argentina (Vaca Muerta),[5] Cuba,[6] and Mexico.

History and classification
Cricosaurus was first named by Wagner in 1858,[3] as a re-classification of a specimen he had previously described in 1852.[8]

Several other species have since been named, including C. suevicus by Fraas in 1901 (originally as a species of Geosaurus.[9] One former species, C. medius (named by Wagner in 1858) has since been reclassified as a junior synonym of Rhacheosaurus gracilis.[10]

The original three skulls (all assigned to different species) were poorly known, and the genus had been considered a junior synonym of Metriorhynchus, Geosaurus or Dakosaurus by different palaeontologists in the past.[4] Some phylogenetic analysis did not support the monophyly of Cricosaurus,[11] However, a more comprehensive analysis in 2009 showed that the species contained in Cricosaurus were valid, and furthermore that several long-snouted species formerly classified in the related genera Geosaurus, Enaliosuchus and Metriorhynchus were in fact more closely related to the original specimens of Cricosaurus, and thus were re-classified into this genus.[10]

Cladogram after Cau & Fanti (2010).

Paleobiology
All currently known species would have been three metres or less in length. When compared to living crocodilians, Cricosaurus 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.

Salt glands
Recent examination of the fossil specimens of Cricosaurus 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.[14][15] Adult specimens of Metriorhynchus also have these well-developed salt glands.

Niche partitioning
Several species of metriorhynchids are known from the Mörnsheim Formation (Solnhofen limestone, early Tithonian) of Bavaria, Germany: Dakosaurus maximus, Geosaurus giganteus, 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 Dakosaurus and G. giganteus, which were large, short-snouted species with serrated teeth. The long-snouted Cricosaurus species would have feed 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.[17]

From the slightly older Nusplingen Plattenkalk (late Kimmeridgian) of southern Germany, both C. suevicus and Dakosaurus maximus are contemporaneous. As with Solnhofen, C. suevicus feed upon fish, while D. maximus was the top predator.