A new diverse turtle fauna in the late Kimmeridgian of Switzerland
Talk given at the 12th Swiss Geoscience Meeting in Fribourg, Switzerland, November 22nd, 2014.
Abstract:
During the Kimmeridgian and the Tithonian (Late Jurassic), Europe was the theater of the diversification of numerous coastal eucryptodiran turtles (Plesiochelyidae, Thalassemydidae, and Eurysternidae). Most turtle assemblages were discovered during the 19th century. The best localities and horizons include the Kimmeridge Clay of England, the Turtle Limestone of Solothurn, Switzerland, and the lithographic limestones of Bavaria, Germany. Despite the abundance of material, these faunas have been scarcely studied during the 20th century. Historically, Solothurn is the richest turtle locality for the Kimmeridgian (Bräm, 1965; Meyer and Thüring, 2009), with several hundreds shell remains and six valid species (Anquetin et al., 2014). The presence of six more or less closely related, relatively large coastal marine turtles in the same paleoenvironment is remarkable in terms of paleobiodiversity. For comparison, there are currently only eight species of marine turtles worldwide.
Here, we report an even more diverse turtle assemblage from late Kimmeridgian deposits in the vicinity of Porrentruy, Canton Jura, Switzerland. These turtles were discovered by the PAL A16 team during controlled excavations along the course of the A16 Transjurane motorway. The PAL A16 Kimmeridgian turtle collection includes about 80 sub-complete shells, five crania, four mandibles, and thousands of isolated remains. The description of this rich assemblage is still ongoing, but the presence of at least eight distinct taxa has already been recognized. Three of these are also present in the Solothurn assemblage: Plesiochelys etalloni, Thalassemys moseri, and Tropidemys langii. As in Solothurn, Plesiochelys etalloni is the most abundant species in Porrentruy. Interestingly, the second most abundant species is Thalassemys moseri, for which only two partial shells are known in Solothurn. Tropidemys langii is represented by several articulated shells, providing new insights into the anatomy of this characteristic plesiochelyid turtle (Püntener et al., 2014).
Remarkably, the rest of the Porrentruy assemblage differs from that of Solothurn. Thalassemydidae are represented by a new species of Thalassemys (see Poster by Christian Püntener). At least two species of Eurysternidae are present: Solnhofia aff. parsonsi and Eurysternum sp. Solnhofia aff. parsonsi is notably characterized by a short snout, whereas the Tithonian holotype of Solnhofia parsonsi is characterized by an elongated snout. Whether this is a distinct species or a morph of the same species will be further investigated. A skull from Solothurn initially referred to Solnhofia parsonsi may actually belong to this new form. The presence of numerous remains of Eurysternum sp. in Porrentruy is remarkable as eurysternids are rarely abundant in plesiochelyid-dominated assemblages. Plesiochelyidae are further represented by several additional taxa. A large, complete shell that exhibits several unusual features probably represents a new taxon. A few specimens, including a previously reported trampled shell (Billon-Bruyat et al., 2012), may be provisionally assigned to Craspedochelys sp. And finally, two additional taxa can be identified based on cranial material. One is a distinct, undetermined plesiochelyid that might correspond to one of the shell-based taxa. The other is identified as a new species of Portlandemys, a form previously only known in the Tithonian of the Isle of Portland, England.
The turtle localities around Porrentruy are slightly older (end of Cymodoce zone and Eudoxus zone) than the Turtle Limestone of Solothurn (Autossiodorensis zone). This short difference in age may explain some discrepancies between the two faunas, but other factors also probably played a part. The paleoenvironments must have been different, as apparent from the lithology and the presence of eurysternids. Some ammonites and sharks, as well as ostracods, also reveal the existence of a boreal signal in the Porrentruy fauna. Turtles may also reflect this influence, as suggested by the presence of Portlandemys sp.
The description of this rich new turtle assemblage in months and years to come, coupled with the long overdue taxonomic reassessment of Late Jurassic eucryptodires at the European scale, will provide a better understanding of the diversity and paleobiogeography of these coastal turtles, which represent the oldest documented radiation of these reptiles into marine environments. The first results suggest that European turtle faunas were more homogeneous than previously thought.
REFERENCES
Anquetin, J., Püntener, C. & Billon-Bruyat, J.-P. 2014: A taxonomic review of the Late Jurassic eucryptodiran turtles from the Jura Mountains (Switzerland and France). PeerJ, 2, e369.
Billon-Bruyat, J.-P., Marty, D., Bocat, L. & Paratte, G. 2012: Under the feet of sauropods: a trampled coastal marine turtle. Abstract, Fourth Symposium on Turtle Evolution 2012, University of Tübingen, Germany.
Bräm, H. 1965: Die Schildkröten aus dem oberen Jura (Malm) der Gegend von Solothurn. Schweizerische Paläontologische Abhandlungen, 83, 1–190.
Meyer, C. A. & Thüring, S. 2009: Late Jurassic marginal marine ecosystem of the Southern Jura Mountains. In: Billon-Bruyat, J.-P., Marty, D., Costeur, L., Meyer, C. A. & Thüring, B., eds. Abstracts and Field Guides, 5th International Symposium on Lithographic Limestone and Plattenkalk, Actes 2009 bis de la Société jurassienne d’émulation. Porrentruy, Switzerland, 130–141.
Püntener, C., Billon-Bruyat, J.-P., Bocat, L., Berger, J.-P. & Joyce, W. G. 2014: Taxonomy and phylogeny of the turtle Tropidemys langii Rütimeyer, 1873, based on new specimens from the Kimmeridgian of the Swiss Jura Mountains. Journal of Vertebrate Paleontology, 34, 353–374.
