Sunday, 7 June 2015

Redescription of a Thalattosuchian Crocodyliform from the Jurassic-Cretaceous boundary of Patagonia.

Purranisaurus potens is known from a single partial specimen from the left margin of the Arroyo del Cajón Grande in southeast Malargüe Department, Mendoza Province, in the Neuquún Basin of west-central Argentina. It was first described in 1948 as a Plesiosaur, then reclassified as a Nothosaur in 1967. Since this time it has generally been considered to be a Metriorhynchid Thalattosuchian, a sort of Mesozoic Marine Crocodyliform, and a close relationship to two Chilean species, Metriorhynchus  casamiquelai and Metriorhynchus westermanni, and it has been suggested that the three species be placed in a single genus, though these analyses have been based upon photographs and descriptions from the original papers describing the specimen, rather than re-examination of the material.

In a paper published in the Journal of Vertebrate Paleontology on 20 February 2015, Yanina Herrera, Zulma Gasparini and Marta Fernández of the División Paleontología Vertebrados at the Museo de La Plata at the Universidad Nacional de La Plata redescribe the Purranisaurus potens, following repreparation of the original specimen at the Museo de La Plata during which more of the skull was exposed from the matrix.

The single known specimen of Purranisaurus potens comprises a three-dimensionally preserved skull with damage to the orbital region (area around the eyes) caused by post-mortem compression, as well as a partial left lower jaw. The original description also included several ribs and vertebrae, however these could not be located and appear to have been lost.

Purranisaurus potens in left lateral view. (A) Photograph; (B) line interpretation. Abbreviations: eor, external otic recess; fr, frontal; gd, nasal gland duct; j, jugal; la, lacrimal; ls, laterosphenoid; mx, maxilla; na, nasal; or, orbit; pa, parietal; pfr, prefrontal; po, postorbital; pro, prootic; q, quadrate; sq, squamosal; sqs, squamosal flat surface. Scale bar equals 10 cm. Herrera et al. (2015).

Based upon the new description Herrera et al. were able to re-analyse the phylogenetic position of Purranisaurus potens. This new analysis suggests that the species belongs firmly within the Geosaurine group of Metriorhynchids, and shows no close relationship with Metriorhynchus  casamiquelai and Metriorhynchus westermanni, and the true relationships of these two species remains unclear, with Herrera et al.’s analysis suggesting they occupy a basal position within the Geosaurinae, and are not closely related to other members of the genus Metriorhynchus.

Strict consensus topology of the unordered phylogenetic analysis. Herrera et al. (2015).

See also…

The Rosso Ammonitico Veronese is a red nodular limestone found on the Trento Plateau in northern Italy, and Middle to Late Jurassic in age. It is noted for its numerous Ammonite fossils, but also produces occasional Marine Reptiles, notably Thalattosuchian Crocodylomorphs and Plesiosaurians. The earliest recorded discovery of such a Reptile is a...

The Thalattosuchians were a group of marine Crocodiliformes (the group of Archosaurs that includes modern Crocodiles and a diverse range of Mesozoic animals) that were found in seas around the world from the Early Jurassic to the Early Cretaceous. They...

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