Pelagosaurus (meaning "lizard of the open sea") is an extinctgenus of thalattosuchian crocodyliform that lived during the Toarcian stage of the Lower Jurassic, around 183 Ma to 176 Ma (million years ago), in shallow epicontinental seas that covered much of what is now Western Europe. The systematic taxonomy of Pelagosaurus has been fiercely disputed over the years, and was assigned to Thalattosuchia after its systematics within Teleosauridae were disputed. Pelagosaurus measured 2–3 m (6.6–9.8 ft) long.[2]
Discovery
Fossil of Pelagosaurus typus.
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. 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. One of the specimens was that of a small juvenile, providing some insight into Pelagosaurus' growth pattern.[3]
Evolutionary relationships
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.[4][5][6][7][8] Its position as a basalmetriorhynchid was suggested by Eric Buffetaut in 1980.,[9][10]Phylogenetic analyses from the 1980s, 1990s and early 2000s found Pelagosaurus to be the sister taxon to both Teleosauridae and Metriorhynchidae.[11][12][13] Some later phylogenetic analyses in the 2000s found Pelagosaurus to be a basal teleosaurid,[14][15][16] while other subsequent studies from the late 2010s and early 2020s have found it to be a basal metriorhynchoid.[17][18]
Palaeobiology
Restoration
Pelagosaurus was well adapted to aquatic life; it had developed a long, streamlined snout, a tail with fin-like attributes and paddle-like limbs for swimming in the warm, shallow waters of its time. Pelagosaurus had 30 teeth suitable for hunting and grasping fish, crustaceans and insects whilst swimming; indeed, one fossil specimen was found with a Leptolepis— an early teleost fish— in its stomach contents. Its forward-facing eyes and streamlined body suggest that Pelagosaurus was a pursuit predator, rather than a scavenger or ambush hunter. Pelagosaurus was markedly similar to modern crocodiles, and would have swum in much 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.[3]
Comparisons with other crocodilians shows it was a small prey specialist, less suited to tackle large prey than even modern gharials.[19]
^ abPierce 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.
^Mueller-Töwe IJ. 2005. Phylogenetic relationships of the Thalattosuchia. ZittelianaA45: 211–213.
^Gasparini Z, Pol D, Spalletti LA. 2006. An unusual marine crocodyliform from the Jurassic-Cretaceous boundary of Patagonia. Science311: 70-73.
^Young MT. 2007. The evolution and interrelationships of Metriorhynchidae (Crocodyliformes, Thalattosuchia). Journal of Vertebrate Paleontology27 (3): 170A.
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