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Ontogenetic sequence comparison of extant and fossil tadpole shrimps: no support for the “living fossil” concept

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Abstract

The tadpole shrimp Triops cancriformis (Branchiopoda, Eucrustacea) is often referred to as a “living fossil.” This term implies that the morphology of a species has barely changed for hundreds of millions of years; in the case of T. cancriformis, for about 200 million years. In 1938, Trusheim documented fossil notostracans from the Upper Triassic of southern Germany (237–200 million years) and named them T. cancriformis minor due to their small size compared to modern forms of T. cancriformis. We compared the ontogenetic sequence of the fossil forms to that of modern forms. Fossil material came from the Museum Terra Triassic in Euerdorf and originated from the same geological formation (the Hassberge Formation) as the Trusheim material, which is considered to be nearly entirely lost. The specimens were documented using cross-polarized light and processed into high-resolution images. Fluorescence microscopy was used to document exuviae and carcasses of extant representatives of T. cancriformis. Both forms showed an elongation and similar trends in the length/width ratio of the shield during ontogeny. However, differences were found in the starting point of the developmental processes. Fossil forms start out with a more roundly shaped shield, which becomes more elliptical, while extant forms already start with a more elliptical shield shape. Further differences between extant and fossil forms were found upon comparing shield to trunk ratios. All differences are highly significant statistically. These differences in ontogeny cast severe doubt on the interpretation that T. cancriformis has been static for 237 million years. While the term “living fossil” is misleading and its use should be discouraged in general, it seems to be especially inappropriate to apply it to T. cancriformis.

Kurzfassung

Triops cancriformis (Branchiopoda, Eucrustacea) wird oft als „lebendes Fossil“ bezeichnet. Dieser Terminus impliziert, dass sich die Morphologie einer Art seit einigen hundert Millionen Jahren nicht verändert hat, im Falle von Triops cancriformis seit rund 200 Millionen Jahren. 1938 untersuchte Trusheim fossile Notostracen aus der Oberen Trias von Süddeutschland (237–200 Millionen Jahre) und verwendete für diese den Namen Triops cancriformis minor, da sich diese für ihn nur durch ihre geringere Größe von modernen Vertretern von Triops cancrifomis unterscheiden ließen. In der vorliegenden Studie wurde die Ontogenese fossiler und rezenter Vertreter verglichen. Die untersuchten Fossilien stammen aus dem Museum Terra Triassica in Euerdorf und wurden in derselben geologischen Formation (Hassberge-Formation) wie die Trusheim-Fossilien gefunden. Das Trusheim-Material gilt, bis auf ein einzelnes Stück, als verschollen und konnte somit nicht untersucht werden. Die Fossilien wurden hochauflösend unter kreuz-polarisiertem Licht fotografiert. Zum Vergleich wurden Exuvien rezenter Exemplare von Triops cancriformis fluoreszenzmikroskopisch dokumentiert. Beide Formen zeigten eine Verlängerung und eine ähnliche Veränderung des Längen-Breiten-Verhältnisses des Schildes während ihrer Ontogenese. Allerdings unterschied sich der Zustand zu Beginn des Entwicklungsprozesses. Die fossilen Vertreter weisen zunächst einen eher runden Schild auf, welcher während der Ontogenese eine elliptischere Form annimmt. Rezente Vertreter hingegen beginnen bereits mit einem elliptischen Schild, welcher schließlich noch länglicher wird. Weitere Unterschiede wurden im Verhältnis von Schild zu Rumpf gefunden, wobei alle Ergebnisse hoch signifikant sind. Diese ontogenetischen Unterschiede ziehen die morphologische Stasis von Triops cancriformis seit rund 200 Millionen Jahren stark in Zweifel. Daher erscheint der Begriff „lebendes Fossil“, welcher auch generell sehr kritisch gesehen werden sollte, unpassend.

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Acknowledgements

We would like to thank all the people without whose help this study would not have been possible. Our acknowledgements go to the people supporting the Museum Terra Triassica. We thank all the private collectors who helped to collect and provide all examined specimens. Tom Hegna, Macomb, and Mike Reich, Munich, provided helpful comments on the manuscript. We are grateful to J. Matthias Starck for interesting discussions and support of this study. Furthermore, we thank all of the people who spent their time programming the open source or open access software that was used during this study, such as Open Office and Combine ZM. CH was supported by the Bavarian Gender Equality Grant; JTH was supported by the German Research Foundation.

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CH initiated the study. PW documented the specimens, performed the measurements and the statistical analyses, and drafted the manuscript with input from JTH and CH. JS collected and prepared the fossil specimens and wrote the geological section. All authors discussed the results and approved the manuscript.

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Correspondence to Philipp Wagner.

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Handling editor: Mike Reich.

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Wagner, P., Haug, J.T., Sell, J. et al. Ontogenetic sequence comparison of extant and fossil tadpole shrimps: no support for the “living fossil” concept. PalZ 91, 463–472 (2017). https://doi.org/10.1007/s12542-017-0370-8

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