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Questioning the living environment of early tetrapods: A tale of delta(s).

The story began in 2014, when Pr. Christophe Lécuyer and Dr. Romain Amiot, both of them geochemists working at the Laboratory of Geology of Lyon, offered me to start a PhD with them. It was about developing the analyses of sulfur stable isotopes in biogenic apatites (the minerals that constitute bone or tooth) in order to question the living environments of the first tetrapods, which originated during Devonian time, at least 365 million years ago. The idea immediately seduced me and so began the adventure.

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May 30, 2018
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The paper in Nature is here: https://go.nature.com/2JeYBV0

Starting from the isotopic delta…

During my first year of PhD, I spent a long time with Dr. François Fourel, trying to tame an elemental analyzer called VarioPyrocubeTM, and his faithful associate, an isotope ratio mass spectrometer called IsoPrime 100TM (Photo 1). With a lot of care and patience, we finally achieved to tune these two guys to measure the sulfur isotope composition, of a few bones and teeth of present-day and fossil vertebrates. Sulfur isotopic composition is denoted using the delta notation: δ34S. The first results were very promising. Indeed, the δ34S values were in good agreement with the ecology of the vertebrates we analyzed. For instance, both present-day and fossil vertebrates living in marine environments had clearly distinct δ34S values than those living in freshwater or terrestrial environments.

Photo 1 The VarioPyrocubeTM elemental analyzer (on the right side) interfaced to an IsoPrime 100TM isotope ratio mass spectrometer (on the left side) hosted at the Laboratoire d’Ecologie des Hydrosystèmes Naturels et Anthropisés(Villeurbanne, France).

Therefore, the next step of my PhD was to collect some present-day vertebrate samples covering a wide range of their phylogenetic lineage (fish, birds, snakes, lizards, crocodiles, turtles, mammals, amphibians) and having different ecologies (freshwater, seawater, terrestrial). We then analyzed their δ34S to build up a present-day isotopic referential on which paleontological studies could rely on.

Photo 2 A skull of Nile Crocodile (Crocodylus niloticus), one of the specimen sampled in the collection of the Musée des Confluence in Lyon, France. Behind the skull, you can see the Dremel® system used to collect bone powders.

The next episodes was to collect some fossils representing those early tetrapods and their associated fauna. One of the most important collection of those fossils is held in the collections of the Museum of Geology of Copenhagen. Pr. Gilles Cuny, who was the ancient curator of those collections, was just recruited as a professor in the Laboratory of geology of Lyon… and his office was assigned just next of mine! Taking advantage of this fortunate coincidence, I came to Copenhagen with him. And here I am (Photo 3), in the collection of the Museum of Geology of Copenhagen, surrounded by the fossils of those Devonian vertebrates. Among them some placoderms, dipnoans and early tetrapods. Let’s collect some bone powders using the Dremel® system!

Photo 3 On the left side, the author, standing in the collection of the Museum of Geology of Copenhagen, holding a reconstitution of one of those early tetrapods. On the right side, an hemi-mandible of early tetrapods collected in Greenland and hosted in one of the numerous drawers of the collection.

In the quest of Devonian fossil, I also had the opportunity to come to Beijing, at the Institute of Vertebrate Palaeontology and Paleoanthropology (Photo 4) to meet a Chinese colleague, Pr. Zhu Min, specialist of the vertebrates from Devonian times. Once again, I took my Dremel® and collect some bone powders from these old fossils.

Photo 4 On the left side, the frontage of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China. On the right side, the author and Dr. Romain Amiot making a detour to the Great Wall of China.

…and finishing with environmental delta

Since the discovery of the first early tetrapods in the Devonian sediments of East Greenland in the 1930s, it has been assumed that they were living in freshwater environments. However, these freshwater paradigms have been questioned since a few decades. Indeed, many sedimentary deposits which have yielded early tetrapod remains are now interpreted as being deposited in transitional environments, between seawater and freshwater. However, the question was still pendent, especially because sediments do not necessarily correspond to the living environment of fossil embedded on. Indeed at their death, organism can be transported before their burial and their eventual fossilization.

The great advantage of measuring the δ34S of bones is that it is a direct tracer of the living environment. Indeed, environmental sulfates have very different δ34S values between freshwater and seawater, and are recorded as such in vertebrate bones. Our isotopic analyses confirms that the freshwater paradigm does not hold water. The combined analyses of both sulfur and oxygen isotope analysis demonstrate that some early tetrapods (from Devonian deposits of Greenland) and their associated fauna were living in transitional environments where freshwater were mixing with seawater like deltas (Image 1).

Image 1 Two early tetrapods Acanthostega exploring every nook and cranny of a Devonian (~365 Ma) deltaic environments, where seawater (at the background) invaded land and mixed with fluvial freshwater (at the forefront). The drawing is from Mazan (mazan.dethan@wanadoo.fr; http://www.mazanonline.fr/)


Go to the profile of Jean Goedert

Jean Goedert

Dr, UMR CNRS 5199 PACEA, Université de Bordeaux

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