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 19^th 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.

The oldest freshwater ecosystem

This has enabled palaeontologists to find fossils of triops – and their eggs –, which are animals that can still be found today in ponds that are drying up. They reach their adult stage in a few days, then lay eggs that can resist drought for decades, until the next rain. The oldest triops found before Strud were 260 million years old. ‘We have added 100 million years to that!’ says an excited Julien Denayer. The variety of fauna and flora found in Strud is typical of a drying pond. ‘We have shown (4) that the fossils lived in a period of ecological stasis, i.e. a period where organisms and ecosystems remained unchanged for a very long time. Our evidence reveals that the Strud ecosystem was inhabited by the same organisms that can be found today in drying ponds under the tropics, for instance in Mexico or the Sahara. Over 365 million years, of course, the species have changed, but the morphological groups remain the same; the species are the oldest ever found. This makes Strud the oldest freshwater ecosystem ever identified to this day.’

Artist's depiction of the Strud channel ecosystem,used as a nursery by placorderms.

Nursery

But our small quarry – no more than six metres! – still had one last secret in store. A final publication (5), whose main author was Sébastien Olive, then a PhD student at the university of Liège and now a post-doctoral fellow at Drexel University (Philadelphia, PA, USA). In the context of his thesis on fish in the Famennian age, including those found in Strud, Sebastien Olive studied placoderms, a class of fish species whose specimens fossilised very well due to their being covered in plates of bone. Generally speaking, adults were around one metre long (though one species could reach 8 metres in length!) and juveniles 20 to 30 cm. However, Olive realised that the specimens found in Strud were very small, smaller than those found elsewhere. And this was for good reason: the Strud specimens were juveniles, as evidenced by the fact that their bone plates had not yet fused together and the absence of marks of growth. In the small Strud channel, Sébastien Olive found three different species of young placoderms, meaning the location was obviously a spawning ground. This discovery also shows that these fossils are not from a time when the channel was stagnant and drying up, but rather from a time when water was flowing and the channel was connected to the ocean. Adult fish would follow the river and spawn upstream. It is difficult to date this previous period, because it is impossible to know how long it took for water to stop flowing through the channel; probably around a century. This means the fossils are also around 365 million years old. ‘This is neither the oldest nor the largest nursery ever discovered,’ says Julien Denayer. ‘Still, Strud was home to several species of placoderms that spawned in the same place; at least, their young developed in the same place.’

(1) Devonian tetrapod from western Europe, Nature, vol. 427, january 2004.
(2) A complete insect from the Late Devonian period, Nature, vol. 488, august 2012
(3) Stratigraphy and depositional environments of the Late Famennian (Late devonian) of Southern Belgium and characterization of the Strud locality, Geological magazine, July 2015.

(4) A 365-Million-Year-Old Freshwater Community Reveals Morphological and Ecological Stasis in Branchiopod Crustaceans, Current Biology 26, 1-8, February 2016.
(5) Placoderm Assemblage from the Tetrapod-Bearing Locality of Strud (Belgium, Upper Famennian) Provides Evidence for a Fish Nursery, PlosOne, August 2016.