Geosaurus, Dakosaurus, Metriorhynchidae, Reptile, Thalattosuchia
Metriorhynchus is an extinct genus of marine crocodyliform that lived in the oceans during the Middle to Late Jurassic. Metriorhynchus was named by the German palaeontologist Christian von Meyer in 1830. Metriorhynchus was a carnivore that spent much, if not all, its life out at sea. No Metriorhynchus eggs or nests have been discovered, so little is known of the reptile'S life cycle, 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 Metriorhynchus mated, whether on land or at sea, is currently unknown. The name Metriorhynchus means "Moderate snout", and is derived from the Greek Metrio- ("moderate") and -rhynchos ("snout").
Discovery and species
Fossil specimens referrable to Metriorhynchus are known from Middle-Late Jurassic deposits of England, France and Germany.
Species in this genus were traditionally classed into two skull groups: longirostrine (long, narrow jaws) and brevirostrine (short, broad jaws). However, there has been some contention as to how many of these species are valid, especially those from the Callovian. All brevirostrine species have been transferred to the genera Purranisaurus and Suchodus.
Two longirsotrine species, M. acutus and M. leedsi have been referred to the genus Gracilineustes.
Fragmentary remains attributed to Metriorhynchus are known from South America during the Bajocian and Bathonian (both Middle Jurassic). However, phylogenetic analysis has shown that these species cannot be referred to Metriorhynchus.
Taxonomy and phylogeny
The genera Purranisaurus and Suchodus have been considered junior synonyms of Metriorhynchus, Recent phylogenetic analyses however, do not support the monophyly of Metriorhynchus. Some of the longirostrine forms, however, do appear to form a natural group.
The cladogram presented below follows an analysis by Mark Young and Marco Brandalise de Andrade, published in November 2009.
Cladogram after Cau & Fanti (2010).
Averaging 3 metres (9.8 ft) in length, Metriorhynchus was of a similar size to modern crocodiles. However, it had a streamlined body and a finned tail, making it a more efficient swimmer than modern crocodilian species.
Recent examination of the fossil specimens of Metriorhynchus superciliosus, have shown that adults of this species had well-developed salt glands. This means that like Geosaurus it would have been able to "drink" salt-water (necessary for a pelagic animal) and eat prey that have the same ionic concentration as the surrounding sea water (i.e. cephalopods) without dehydrating.
Metriorhynchus was a versatile and opportunistic predator, predating upon both the armoured ammonites and the fast moving fish; occasionally, it was also capable of capturing flying animals such as the pterosaurs and scavenging on plesiosaur and Leedsichthys carcasses on the seafloor.
Even though Metriorhynchus was an effective predator, it was vulnerable to predation from apex predators such as Liopleurodon which could grow to 6.39 meters (21 feet) in length. Since Metriorhynchus had lost its osteoderms, "armour scutes", to become more efficient swimmers it would have had little defense against larger marine predators.