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New Middle Jurassic species of Stenopterygius (Ichthyosauria)



From: Ben Creisler
bcreisler@gmail.com

This item was mentioned in a posting I did last week about
international news stories as a new ichthyosaur from Germany with a
photo of the skull (text in German). The paper came out in PLoS ONE
last week but didn't show up in my early queries to find it. So here
is the link, a bit belated:


Erin E. Maxwell, Marta S. Fernández & Rainer R. Schoch (2012)
First Diagnostic Marine Reptile Remains from the Aalenian (Middle
Jurassic): A New Ichthyosaur from Southwestern Germany.
PLoS ONE 7(8): e41692.
doi:10.1371/journal.pone.0041692
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0041692

Background

The Middle Jurassic was a critical time in the evolutionary history of
ichthyosaurs. During this time interval, the diverse, well-studied
faunas of the Lower Jurassic were entirely replaced by
ophthalmosaurids, a new group that arose sometime prior to the
Aalenian-Bajocian boundary and by the latest middle Jurassic comprised
the only surviving group of ichthyosaurs. Thus, the Middle Jurassic
Aalenian-Bathonian interval (176–165 million years ago) comprises the
time frame during which ophthalmosaurids not only originated but also
achieved taxonomic dominance. However, diagnostic ichthyosaur remains
have been described previously from only a single locality from this
interval, from the Bajocian of Argentina.

Methodology/Principal Findings

In this paper, we describe a new species of ichthyosaur based on a
partial articulated specimen from the Middle Jurassic of southwestern
Germany. This specimen was recovered from the Opalinuston Formation
(early Aalenian) and is referable to Stenopterygius aaleniensis sp.
nov. reflecting features of the skull and forefin. The genus
Stenopterygius is diverse and abundant in the Lower Jurassic of
Europe, but its presence has not previously been confirmed in younger
(Middle Jurassic) rocks from the northern hemisphere.

Conclusions/Significance

This specimen represents the only diagnostic ichthyosaur remains
reported from the Aalenian. It bears numerous similarities in size and
in morphology to the Lower Jurassic species of the genus
Stenopterygius and provides additional evidence that the major
ecological changes hypothesized to have occurred at the end of the
Toarcian took place sometime after this point and most likely did not
occur suddenly. There is currently no evidence for the presence of
ophthalmosaurids in the northern hemisphere during the
Aalenian-Bathonian interval.