Pelagosaurus (meaning "lizard of the open sea") is an extinctgenus of thalattosuchiancrocodyliform that lived during the Toarcian stage of the Lower Jurassic, around 183 to 176 million years ago, in shallow epicontinental seas that covered much of what is now Western Europe. The systematic taxonomy of Pelagosaurus has been disputed over the years, usually being placed as a teleosauroid (gharial-like, armoured marine crocodyliformes), but with later thorough analyses consistently recovering it as a basalmetriorhynchoid (a clade containing the fully-pelagic, flippered thalattosuchians with smooth skin and caudal fins). Pelagosaurus was a small reptile, usually measuring less than 2 metres (6.6ft) long amongst the several known specimens.
Discovery
Fossil specimen (FWD 0784) of P. typus with a pathologically bent lower jaw that healed during life.
Pelagosaurus was originally described from a specimen from Normandy, but the holotype for P. typus was discovered north of the town of Ilminster in Somerset, England. Most Pelagosaurus remains have been found in the Ilminster area, but numerous other remains, predominantly skulls and articulated skeletons, have been found around Western Europe in locations such as France, Germany, and Switzerland, with a 2006 dissertation listing a total of 88 specimens. Specimens from the Somerset region come primarily from the Strawberry Bank quarry north of Ilminster; although the site had yielded other fossil remains before, the site has since been built over. Several of the specimens are those of small juveniles, providing some insight into Pelagosaurus' growth pattern.
Evolutionary relationships
Size comparison of a few P. typus specimens.
The evolutionary relationships of Pelagosaurus have historically been contentious as there have been several different interpretations of its placement in Thalattosuchia.
Pelagosaurus was initially classified as a teleosaurid, based upon anatomical similarity, by Eudes-Deslongchamps, Westphal and Duffin. It was suggested to be an early-diverging metriorhynchid by Eric Buffetaut in 1980. Phylogenetic analyses from the 1980s, 1990s and early 2000s found Pelagosaurus to be the sister taxon to both Teleosauridae and Metriorhynchidae. With subsequent studies from the late 2010s and early 2020s finding it to be a basal metriorhynchoid, whose superficial similarities with teleosauroids are actually plesiomorphies (ancestral characters) of thalattosuchia (like the presence of - albeit reduced - osteoderm coverage, a mandibular fenestra, or more than 3 premaxillary teeth).
Pelagosaurus was well-adapted to aquatic life; it had developed a long snout (between 70-76% the total length of the skull, depending on the specimen) with more than 60 needle-like teeth on the upper and lower jaws, an elongated and slender body with very small limbs, which streamlined the animal for swimming in the warm, shallow waters of its environment. It fed on fish, crustaceans and insects whilst swimming; indeed, one very young fossil specimen was found with a Leptolepis— an early teleost fish— in its stomach contents. Its morphology indicates Pelagosaurus was a pursuit predator with laterally-facing eyes, rather than a scavenger or ambush hunter like many teleosauroids or modern crocodilians with dorsally-positioned orbits. Pelagosaurus was markedly similar to modern crocodiles, and likely swam in similar manner, whipping its tail from side to side, although its veterbral structure was slightly more agile, probably allowing for more movement in the water than its modern equivalents. Pelagosaurus would have only emerged from the water to lay eggs or to rest on the banks, and would have spent the rest of its day in the water for which it was adapted.
Comparisons with other crocodilians shows it was a small prey specialist, less suited to tackle large prey than even modern gharials.
12Pierce SE, Benton MJ. 2006. Pelagosaurus typus Bronn, 1841 (Mesoeucrocodylia: Thalattosuchia) from the Upper Lias (Toarcian, Lower Jurassic) of Somerset, England. Journal of Vertebrate Paleontology26 (3): 621–635.
↑Eudes-Deslongchamps JAE. 1863. "Mémoire sur les Téléosauriens de l'époque Jurassique du Département de Calvados". Mémoires de la Société Linnéenne de Normandie13:1–138.
↑Eudes-Deslongchamps EE. 1877. Le Jura Normand, 4. F. Savy (ed). Paris. 60 p.
↑Westphal F. 1961. "Zur Systematik der deutschen und englischen Lias Krokodilier". Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen113:207–217.
↑Westphal F. 1962. Die Krokodilen des deutschen und englischen oberen Lias. Palaeontographica, Abteilung A118:1–96.
↑Duffin CJ. 1979. "Pelagosaurus (Mesosuchia, Crocodilia) from the English Toarcian (Lower Jurassic)". Neues Jahrbuch für Geologie und Paläontologie, Monatshefte1979:475–485.
↑Buffetaut E. 1980. Position systématique et phylogénetique du genre Pelagosaurus Bronn, 1841 (Crocodylia, Mesosuchia), du Toarcian d'Europe. Geobios13:783–786.
↑Buffetaut E. 1982. Radiation évolutive, paléoécologie et biogéographie des crocodiliens mésosuchiens. Mémoires de la Sociéte Géologique de France42:1–88.
↑Benton MJ, Clark JM. 1988. Archosaur phylogeny and the relationships of the Crocodylia; pp. 295–338 in MJ. Benton (ed.), The phylogeny and classification of the tetrapods, Vol. 1. Amphibians, Reptiles, Birds. Systematic Association Special Volume 35A. Clarendon Press, Oxford.
↑Clark JM. 1994. Patterns of evolution in Mesozoic Crocodyliformes; pp. 84–97 in NC. Fraser and H-D. Sues (eds.), In the shadow of the dinosaurs: Early Mesozoic tetrapods. Cambridge University Press, Cambridge and New York.
↑Buckley GA, Brochu CA, Krause DW, Pol D. 2000. A pugnosed crocodyliform from the Late Cretaceous of Madagascar. Nature405:941–944.
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