Strud: the end of a journey
The Belgian locality of Strud, between the towns of Namur and Andenne, will no longer be hosting a crowd of geologists and palaeontologists. Everything that could be excavated has been excavated, and the site seems to have given up all of its palaeontological treasures. For a few months now, several scientific articles have been completing the picture of this place as it was around 365 million years ago. The latest discovery is that it hosted a ‘nursery’ of placoderms, which are huge bony fish with powerful jaws that went extinct soon after the moment recorded in Strud.
The history of the Strud fossil site, located on the municipality of Gesves between Namur and Andenne, Belgium, starts with an insight from Gaël Clément, then a PhD student at the Museum of Natural History in Paris, France. As he was working on his thesis on primitive amphibians, he noticed a monograph from the late 19th century dedicated to fish and written by Maximin Lohest, a palaeontologist from the university of Liège. It included what Lohest described as a fish's jaw. However, there was no doubt in Gaël Clément's mind that it was actually the jaw of a tetrapod (a primitive amphibian). He then looked through the fossil collections available in Liège and eventually found the piece of jaw bone described by Lohest, which allowed him to confirm and refine his analysis: it belonged to an Ichthyostega. Gaël Clément published this discovery in 2004, with other contributors including Édouard Poty, in Nature (1). At the time, the article was something of an event in the world of palaeontology, as this type of fossil had previously been found only in Greenland. Still, the researchers had to locate the exact place where the Walloon fossil had been discovered decades earlier. It was recorded as having been dug up in the Strud locality, but this is still quite a large area! The specific location was found thanks to Jean-Marc Marion and Laurent Barchy, two geologists who were working on a geological map of Wallonia, who determined that the fossil had been found in a small quarry at the edge of the village. The quarry was then cleared, and various excavations were carried out, mostly by the university of Liège, the Paris Museum of Natural History, and the Royal Belgian Institute of Natural Sciences. The excavations ended in 2015. ‘We dug up quite a few fossils,’ remembers Julien Denayer, an postdoc researcher at EDDy Lab (Evolution and Diversity DYnamics lab) at ULg's department of geology. ‘Lots of fish material, more fragments of tetrapods, and more importantly a splendid assemblage of arthropods! Crustaceans, small shrimp, and one insect (see the article entitled ‘365 million years old and no wrinkles’), which has also caused a stir and resulted in another publication in Nature, in 2012 (2). This is because the insect is the oldest ever discovered, dating back to the Famennian age, i.e. 365 million years ago, while fossils found previously only dated back to the late Carboniferous, 300 million years ago. As a result, other scientists became interested in Strud, and the site earned a bit of fame.’
View of the Strud quarry
Strud under the tropics
How did such an accumulation of fossils end up in this remote area of the Walloon countryside? In an article (3) he published as the main author, Julien Denayer paints an accurate geological and stratigraphic picture of the region. His study confirmed that the area used to be a delta, i.e. a series of (fossil-rich) channels separated by strips of land with nearly no fossils. Julien Denayer and colleagues were able to date the strata in which the fossils were found with a high degree of accuracy, placing them in the late Famennian (365 million years ago). ‘At that time,’ he explains, ‘the delta was at the edge of a continent (the London–Brabant Massif), near the Tropic of Capricorn. However, this does not mean it was all beaches and palm trees! These only appeared much later; during the Famennian, there were few living organisms (whether plants or animals) on land, which explains why fossils are found in the channels of the delta, but seldom outside of these. The succession of strata with and without fossils is typical of deltas.’ This is also one of the reasons why the Strud site is so important: it records one of the most ancient phases in the development of tetrapods, after their appearance but before they became common on dry ground. As in all deltas, channels are created and dry up as the river changes its course. Channels that are no longer watered by the river become small lakes, where sediments settle and eventually become dry: this leads to the formation of fossils. The fossils are now visible on the surface because mountains have risen in the meantime, creating folds in the sedimentary rock, while rivers have carved valleys and rocks have been uncovered. Last but not least, during the last few centuries, humans have quarried these rocks, resulting in the fossils' discovery.
(1) Devonian tetrapod from western Europe, Nature, vol. 427, january 2004.