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.’

(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.