Metriorhynchids are fully aquatic
crocodyliforms. Their forelimbs were small and paddle-like, and unlike living
crocodylians, they lost their
osteoderms ("armour scutes"). Their body shape maximised
hydrodynamy (swimming efficiency), as they did have a shark-like tail fluke.[6] Like
ichthyosaurs and
plesiosaurs, metriorhynchids developed smooth, scaleless skin.[7]
Metriorhynchids were the only group of
archosaurs to become fully adapted to the marine realm, becoming
pelagic in lifestyle.[8] With tail flukes, reduced limb musculature, and long bones
histologically comparable to other obligately aquatic animals, they were almost certainly incapable of terrestrial locomotion; combined with an unusually tall hip opening, as also seen in other obligately aquatic reptiles including the
viviparousKeichousaurus, these characters suggest that metriorhynchids gave live birth.[9] A fossil of a pregnant Dakosaurus female recovered from the Late Jurassic
plattenkalk, Bavaria, preserves the complete skeleton of a neonate with small, paddle-like forelimbs unsuited for walking on land, similar to those of adults, further supporting live birth in metriorhynchids.[10][11] Recent research posits that despite their successful adaptation to a pelagic lifestyle,
basal metriorhynchids were uniquely disadvantaged among aquatic tetrapods in evolving into sustained swimmers due to little to no posterodorsal retraction of the external
nares (unlike other reptilian groups such as
mesosaurs,
phytosaurs,
thalattosaurians,
saurosphargids,
ichthyosauriforms,
sauropterygians,
pleurosaurids or
mosasauroids, as well as mammalian
cetaceans or
sirenians).[12] The family has a wide geographic distribution, with material found in
Argentina,
Chile,
Cuba,
England,
France,
Germany,
Italy,
Mexico,
Poland,
Russia,
Switzerland and
Czech Republic.[8][5][13]
Classification
Phylogenetic analyses published during the 2000s cast doubt on the idea that many traditional metriorhynchid genera formed natural groups (i.e., include all descendants of a common ancestor). The traditional species of Geosaurus,[14][15][16]Dakosaurus[14] and Cricosaurus[14] were found to represent unnatural groups, and the species traditionally classified in these genera were reshuffled in a study published in November 2009 by Mark T. Young and Marco Brandalise de Andrade.[4] The
monophyly of Metriorhynchus[14][15][17] and Teleidosaurus[14][16] is also unsupported, and the species of these genera are pending reclassification.[4]
The classification presented by Young and Andrade in 2009 was approved in later studies of the Metriorhynchidae.[5][18][19] Metriorhynchidae is a
node-based taxon defined in 2009 as the least inclusive clade consisting of Metriorhynchus geoffroyii and Geosaurus giganteus.[4] The
cladogram below follows the topology from the 2020 analyses by Young et al. and reduced to genera only.[12][20]
^Andrews CW. 1913. A descriptive catalogue of the marine reptiles of the Oxford Clay, Part Two. London: British Museum (Natural History), 206 pp.
^Alfio A. Chiarenza; Davide Foffa; Mark T. Young; Gianni Insacco; Andrea Cau; Giorgio Carnevale; Rita Catanzariti (2015). "The youngest record of metriorhynchid crocodylomorphs, with implications for the extinction of Thalattosuchia". Cretaceous Research. 56: 608–616.
doi:
10.1016/j.cretres.2015.07.001.
hdl:2318/1537833.
^Fitzinger LJFJ. 1843. Systema Reptilium. Wien: Braumüller et Seidel, 106 pp.
^Fraas E (1902). "Die Meer-Krocodilier (Thalattosuchia) des oberen Jura unter specieller Berücksichtigung von Dacosaurus und Geosaurus". Palaeontographica. 49: 1–72.
^
abAndrea Cau; Federico Fanti (2011). "The oldest known metriorhynchid crocodylian from the Middle Jurassic of North-eastern Italy: Neptunidraco ammoniticus gen. et sp. nov". Gondwana Research. 19 (2): 550–565.
Bibcode:
2011GondR..19..550C.
doi:
10.1016/j.gr.2010.07.007.
^
abYoung, M.T., Brignon, A., Sachs, S., Hornung J.J., Foffa, D., Kitson, J.J.N., Johnson, M.M., Steel, L. (November 2020). "Cutting the Gordian knot: a historical and taxonomic revision of the Jurassic crocodylomorph Metriorhynchus". Zoological Journal of the Linnean Society. 192 (2).
doi:
10.1093/zoolinnean/zlaa092.{{
cite journal}}: CS1 maint: multiple names: authors list (
link)
^Stéphane Hua (May 2020). "A new specimen of Teleidosaurus calvadosii (Eudes-Deslongchamps, 1866) (Crocodylia, Thalattosuchia) from the Middle Jurassic of France". Annales de Paléontologie. 106 (4).
doi:
10.1016/j.annpal.2020.102423.
^Buffetaut E (1982). "Radiation évolutive, paléoécologie et biogéographie des Crocodiliens mésosuchienes". Mémoires de la Société Géologique de France. 142: 1–88.
^Simonelli V (1896). "Intoro agli avanzi di coccodrilliano scoperti a San Valentino (provincial di Reggio Emilia) nel 1886". Atli della Reale Accademia dei Lincei, Series Qunita Rendiconti. 5 (2): 11–18.
^Cuvier G. 1824. Sur les ossements fossiles de crocodiles, 5. In: Dufour & D'Occagne, eds. Recherches sur les ossements fossiles, 2nd édition. Paris: 143-160