Abstract
Albian sedimentary successions of northwestern Canada have yielded a diverse assemblage of Mesozoic marine vertebrates, and ichthyosaurs form an important component of these faunas. Here, we describe a partial postcranial skeleton of a small (estimated at less than 3 m total body length) ichthyosaur from the Wabiskaw Member of the Clearwater Formation (lowermost Albian). The semi-articulated specimen includes much of the presacral vertebral column, dorsal ribs and gastralia. Most significantly, it possesses an articulated pectoral girdle and humerus, and also preserves the pelvic girdle, allowing new insights into girdle evolution in ichthyosaurs. Whereas both sets of girdles are thought to exhibit large amounts of intraspecific variation, the pectoral girdle of ophthalmosaurids appears to evolve very slowly, remaining essentially unchanged from the Middle Jurassic onwards. In contrast, the pelvic girdle shows taxonomically informative changes within Ophthalmosauridae. The variable and poorly known nature of girdle morphology in Cretaceous ichthyosaurs precludes generic referral of the specimen.
Kurzfassung
Die sedimentären Ablagerungen des Alb von Nordwest-Kanada haben eine mannigfaltige Ansammlung von mesozoischen marinen Wirbeltieren hervorgebracht und Ichthyosaurier stellen eine wichtige Komponente dieser Fauna dar. Wir beschreiben hier ein unvollständig erhaltenes Postkranium eines kleinen Ichthyosauriers (geschätze Körperlänge von weniger als 3 Metern) aus der Wabiskaw Einheit der Clearwater Formation (unterstes Alb). Das semi-artikulierte Exemplar umfasst einen Großteil der Präsakralwirbelsäule, der dorsalen Rippen und Gastralia. Von besonderem Interesse ist der artikulierte Schultergürtel und Humerus sowie der Beckengürtel, die neue Einblicke in die Entwicklung der Extremitätengürtel von Ichthyosauriern bieten. Während allgemein angenommen wird, daß die Extremitätengürtel große intraspezifische Variation aufweisen, scheint der Schultergürtel der Ophthalmosaurier nur sehr langsam zu evolvieren und bleibt im wesentlichen ab dem mittleren Jura unverändert. Im Gegensatz dazu weist der Beckengürtel taxonomisch informative Veränderungen innerhalb der Ophthalmosauridae auf. Die variable und unzureichend bekannte Anatomie der Gürtelmorphologie kreidezeitlicher Ichthyosaurier schließt eine generische Zuordnung des Exemplars aus.
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Abbreviations
- BMNH:
-
Natural History Museum, London UK
- PMAG:
-
Peterborough Museum and Art Gallery, Peterborough UK
- TMP:
-
Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada
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Acknowledgments
The authors thank W. Brevant, who discovered the specimen. Kind acknowledgements are extended to mine geologists at Syncrude Canada, Ltd., D. Heyser, C. Logan, D. McIntosh, J. Hudecek and D. Karmen; D. Thompson, R. Barnes and B. Shumsky provided financial and logistical support. Thanks to N. Dodsworth at the Oil Sands Discovery Centre, S. Chapman (BMNH) and G. Wass (PMAG) for access to the specimen and comparative material. The authors acknowledge the assistance of P. Currie, E. Koppelhus, T. Russell, and M. Caldwell; N. Fröbisch translated the abstract. Comments from two reviewers (J. Massare and V. Fischer) improved the MS. This project is an extension of a doctoral dissertation by P.S.D., which was supported by an NSERC Discovery grant to his advisor, A.P. Russell, and an I.W. Killam postdoctoral fellowship to E.E.M.
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Maxwell, E.E., Druckenmiller, P.S. A small ichthyosaur from the Clearwater Formation (Alberta, Canada) and a discussion of the taxonomic utility of the pectoral girdle. Paläontol Z 85, 457–463 (2011). https://doi.org/10.1007/s12542-011-0106-0
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DOI: https://doi.org/10.1007/s12542-011-0106-0