Metriorhynchus
Metriorhynchus is an extinct genus of marine crocodyliform that lived in the oceans during the Late Jurassic. The type species, M. brevirostris was named in 1829 as a species of Steneosaurus before being named as a separate genus by the German palaeontologist Christian von Meyer in 1832. 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 Kimmeridgian deposits of France.The earliest discovered specimen of Metriorhynchus brevirostris is MNHN.F.RJN 116, a jaw collected from the Marnes de Villers by Charles Bacheley during the early 1770s, who believed it belonged to a fish in 1778 and then a dolphin or a sperm whale in 1808, that was described by Cuvier. The holotype of M. brevirostris is MHNG V02232, a partial rostrum from Le Havre, and Georges Cuvier mentioned that it was in the collection of the Museum of the Academy of Geneva in 1811. In 1824, Cuvier then described multiple "gavial" remains and came to the now-incorrect conclusion that he was describing two species ; Cuvier had also grouped into the remains the lectotype of Steneosaurus rostromajor.
When naming Steneosaurus in 1825, Étienne Geoffroy Saint-Hilaire created two species for the genus: Steneosaurus rostro-major and S. rostro-minor. He assigned MNHN.F.RJN 116 to S. rostro-major and Friedrich Holl created Steneosaurus brevirostris in 1829 to solely classify MHNG V02232. In 1831, John Edward Gray instead assigned the specimen to the gharial species Gavialis jurinii. The next year, Hermann von Meyer separated Cuvier's two "gharial" species and created Metriorhynchus brevirostris, Metriorhynchus geoffroyi and Streptospondylus altdorfensis. Amongst the splitting of the "gavial" species, von Meyer incorrectly placed MNHN.F.RJN 116 within S. altdorfensis.
François Jules Pictet suggested that Metriorhynchus was a synonym of Steneosaurus and in 1845 was the first author to invalidate S. rostro-minor as a species. Metriorhynchus as a genus was later reinstated, and Eugène Eudes-Deslongchamps recognized four Callovian species of Metriorhynchus: M. superciliosus, M. moreli, M. blainvillei, and M. brachyrhynchus, and he also standardised the use of the genus name Metriorhynchus.
Richard Lydekker in 1888a suggested it to be "advisable" to retain the name Metriorhynchus, and in 1889a he suggested that Metriorhynchus was similar to Cricosaurus and therefore he concluded the genus was crocodilian. Steel moved MHNG V02232 back into Metriorhynchus geoffroyii in 1973.
Young et al. was the first to restrict M. brevirostris by re-assigning specimens to other genera, and MHNG V02232 was designated as the holotype of M. brevirostris by Young et al.. Young et al. also synonymised M. geoffroyii with M. brevirostris and reassigned MNHN.F.RJN 116 to M. brevirostris.
Valid species
Only one valid species is recognized today, the type species M. geoffroyii. "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, most of brevirostrine species have been transferred to the genera Purranisaurus and Suchodus. Metriorhynchus superciliosus was also shown to be generically distinct from the type species, M. brevirostris, and now has its own genus Thalattosuchus.The genera Purranisaurus and Suchodus have been considered junior synonyms of Metriorhynchus. Recent phylogenetic analyses however, do not support the monophyly of Metriorhynchus, as believed during the 1860s-2010.
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, and a 2022 study describing a new metriorhynchid specimen advocated returning to this taxonomic system, considering Gracilineustes and Thalattosuchus junior synonyms of ''Metriorhynchus.''