Monday, 26 March 2012

A fresh look at the Albian Ichthyosaur Platypterygius hercynicus.

The Ichthyosaurs were a group of marine tetrapods that resembled dolphins. They appear in the fossil record in the mid-Triassic about 245 million years ago, and survive till the mid-Cretaceous, about 90 million years ago. During the Jurassic they appear to have been the top marine predators, but in the Cretaceous they were overshadowed by other groups, most notably the Plesiousaurs. They are generally referred to as 'marine reptiles', but their relationship to other tetrapods is unclear, beyond the fact that they were clearly descended from terrestrial vertebrates. They bore live young, were fully aquatic, and may have been warm blooded, suggesting that they were as distinct from the Reptiles as Birds and Mammals are, and should probably just be called Ichthyosaurs.

Early Cretaceous Ichthyosaurs are not well understood, due to a small number of specimens, and the fragmentary nature of these specimens. One of these fragmentary specimens is a fragmentary Ichthyosaur skull found in the mid 1970s by amateur palaeontologist Jean−Pierre Debris at Saint−Jouin in the Seine−Maritime Department of northwestern France, and donated by him to the Muséum d’Histoire Naturelle du Havre.

Map of northern France, showing where Platypterygius hercynicus was originally discovered (star). Fischer (2012).

In a paper in the April 2012 edition of the journal Acta Palaeontologica Polonica, Valentin Fischer of the Geology Department at the University of Liège redescribes the Saint−Jouin Ichthyosaur, and uses it to draw conclusions about the classification of Early Cretaceous European Ichthyosaurs.

Fischer found that the Saint-Jouin specimen closely resembled the holotype (and only specimen of) Platypterygius hercynicus, and since in taxonomy a specimen is named as the holotype for each species, then any other specimen deemed to be similar enough belongs in the same species, the Saint-Jouin specimen was also allocated to the species Platypterygius hercynicus. Fischer also found individuals of the ammonite Callihoplites gr. strigosus in the rock-matrix in which the Ichthyosaur was preserved, dating the specimen to the Mortoniceras inflatum Zone or early Callihoplites auritus Subzone of the Albian age (the last subdivision of the Early Cretaceous), making it roughly 106.4 million years old.

The skull from Saint-Jouin newly assigned to Platypterygius hercynicus. Top, photograph. Middle, interpretive drawing. Bottom, reconstruction. Fischer (2012).

The new material is more extensive than the previous material assigned to Platypterygius hercynicus, allowing a better comparison to other members of the genus Platypterygius and other Ichthyosaurs within the Ophthalmosauridae, the group of Ichthyosaurs within which the genus Platypterygius is placed. Based upon this Fischer observes that some of the features originally assigned as definitive of the genus Platypterygius are not present in all the members of the genus, and that others are present in members of the Ophthalmosauridae not currently included within the genus. Based upon this he concludes that the genus Platypterygius as it currently stands is not a good taxonomic unit, and is in urgent need of revision.

The original German specimen of Platypterygius hercynicus from the late Aptian of Salzgitter. Right, photograph. Center, interpretative drawing by Kolb and Sander (2009). Left, new interpretative drawing, taking into account data from the Saint-Jouin specimen. Fischer (2012).

No comments:

Post a Comment