Pelagosaurus typus Bronn, 1841 (Mesoeucrocodylia: Thalattosuchia) from the Upper Lias (Toarcian, Lower Jurassic) of Somerset, England

Abstract

ABSTRACT The thalattosuchian crocodyliform Pelagosaurus typus Bronn, 1841 is fully documented and described from the Upper Lias (Toarcian, Lower Jurassic) of England. The material under study is part of a historical collection made by Charles Moore (1814–1881) at Strawberry Bank (Ilminster, Somerset, England) around 1848. Apomomorphic features of the genus include: sculpturing on almost the entire skull and mandible with extensive sculpturing on the prefrontal, lachrymal, frontal, parietal, temporal arcade and the posterior extent of the mandible; 30 piercing teeth on each side of the upper and lower jaws; small, shallow, egg-shaped antorbital fenestra present within the lachrymal and maxilla; supratemporal fenestrae short anteroposteriorly; paired frontal; anterior margin of internal choana tapers anteriorly between the paired palatines; and presence of a choanal septum on the palatine and pterygoid. Pelagosaurus was a small, exceedingly long-snouted, gracile crocodyliform whose diet probably consisted of small fishes, crustaceans and possibly insects. The small-bodied fish Leptolepis is confirmed as part of its diet as a specimen is found in the rib cage of a small juvenile Pelagosaurus. Laterally placed eyes suggest that this species actively pursued its prey rather than sitting and waiting at the water surface. The phylogenetic position of Pelagosaurus has been debated. Pelagosaurus possesses the majority of teleosaurid apomorphies, including: four premaxillary teeth; small prefrontal; lachrymal visible in dorsal aspect; presence of mandibular fenestrae; dermal armor; and a straight tail; however, these are all plesiomorphic for the Crocodyliformes in general. There also appear to be problems when defining the proposed metriorhynchid features of Pelagosaurus (i.e., broad nasal, large frontal, lateral orbit, sclerotic ring and arrangement of prefrontal-lachrymal). Moreover, characters that have been used in past cladistic analyses are either incorrect or too simplistic. Accordingly, Pelagosaurus is considered to belong to the Thalattosuchia incertae sedis until a more thorough phylogenetic investigation is conducted.