Metriorhynchus


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 have given 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- and -rhynchos.

Discovery and species

Fossil specimens referrable to Metriorhynchus are known from Middle-Late Jurassic deposits of England, France and Germany.

Valid species

Only two valid species are recognized today, the type species M. geoffroyi and M. superciliosus. "Metriorhynchus" hastifer and "M." palpebrosus are generically distinct from the Metriorhynchus type species, with hastifer being recovered as a geosaurine. Species in this genus were traditionally classed into two skull groups: longirostrine and brevirostrine. However, all brevirostrine species have been transferred to the genera Purranisaurus and Suchodus.
Eudes-Deslongchamps recognized four Callovian species of Metriorhynchus, M. superciliosus, M. moreli, M. blainvillei, and M. brachyrhynchus. Later, Andrews considered there to be seven valid species: M. superciliosus, M. moreli, M. brachyrhynchus, M. durobrivensis, M. cultridens, M. leedsi and M. laeve. However, Adams-Tresman, using linear morphometrics, could only distinguish between the two skull groups, so she found there to be two species from the Oxford Clay, M. superciliosus and M. brachyrhynchus. Vignaud however, considered there to be three Callovian species: M. superciliosus, M. brachyrhynchus and M. leedsi.

Unnamed species

Fragmentary remains attributed to Metriorhynchus are known from South America during the Bajocian and Bathonian. 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.

Palaeobiology

Description

Averaging 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.

Salt glands

Metriorynchus had nasal salt glands which, like the salt glands of all other marine reptiles, were used to remove excess salt. This means that like Geosaurus it would have been able to "drink" salt-water and eat equally salty prey, such as cephalopods, without dehydrating.

Diet

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.

Palaeoecology

Predators

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 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.