Fossil sponges

It paints a breathtaking scene: Wallonia covered by a tropical ocean, submerged in azure blue waters populated by exotic flora and fauna. This was precisely the case some hundreds of millions of years ago in the Frasnian stage. This is a period which fascinates the geologist Anne-Christine Da Silva, who was recently awarded the Edmond de Selys Longchamps prize for two of her articles (1) in which she studied stromatoporoids, fossil sponges.

The journey through geological time is unusual because it takes place vertically. Geological archives (from the Cambrian period, some 545 million years ago, through to the Recent period) are effectively located right beneath our feet. By analyzing the strata which comprise sedimentary rocks, geologists travel through millions of years of our planet's evolution. From the most recent strata through to the oldest and from the uppermost to the most deeply buried. Geologists observe, scrutinize, and categorize these structures; sifting through the numerous fossils imprisoned within to identify the environment in which the rock was formed. Piled on top of one another in different layers, witnesses to different environments, they turn to rock through the effects of changes in pressure and temperature. In Belgium, 99% of which is covered in sedimentary rocks, the early interest in geology (as far back as the 18^th century, the rocks were studied by experienced geologists) has led to the international recognition of our rocks. Frasnian, Tournaisian, Visean, Namurian, and Dinantian - many different terms derived from the names of well-known towns in our lands and which are used today by the world's entire scientific community. Regardless of where they are found, "Frasnian" refers to rocks dating to between 376 and 382 million years ago.

Study the past, anticipate the future

During the Frasnian stage, as a result of tectonic plate movement, Wallonia was covered by a tropical ocean, located around 30° latitude south. The average temperature was around thirty degrees. A lagoon-like area sat alongside a barrier reef. The environment was similar to that which can be found today on the coast of Australia or the Bahamas. If this conjures up idyllic images, the Belgian Frasnian stage attracts researchers for other reasons. "Understanding the evolution of the planet and events which affected the surface of the earth several millions of years ago is crucial in order to better evaluate future changes, in particular in terms of an increase in CO₂ levels. In the past, CO₂ reached much higher levels than it has today and it is interesting to see what impact this had upon the flora and fauna, among other things. The Frasnian stage, for example, is a period which experienced global warming with a tenfold increase in CO₂ levels - much more significant than the rise which we face today - followed by a brutal cool-down period which led to a major extinction of more than 75% of marine species. The overall view that we have of this period may enlighten us as to what the future has in store for our planet."

Holding a post-doctoral position at the Laboratory for Sedimentary Petrology, Anne-Christine Da Silva has been interested in the Frasnian stage for several years. Two of her recent articles earned her the Edmond de Selys Longchamps prize, a quinquennial prize awarded by the Belgian Royal Academy for Sciences, Arts and Fine Arts. In particular she studies the environmental reef which developed around Belgium at this time. "We talk about a carbonate platform: with a succession of classic environments - namely a lagoon, a barrier reef and a deep, more clayey area. However, the platform which has formed in Belgium is somewhat unique," the researcher points out immediately, "because in the deep zone we find mud mounds or carbonate buildups, sort of pockets of carbonates which reach kilometric proportions. The reef is a fundamental element of the carbonate platform. Within the reef, diversity is at its greatest. Studying it provides the greatest amount of information on ancient marine fauna. But the reefs are also ecosystems which are extremely sensitive to environmental conditions, and the slightest change affects them - this can be seen today with the weakening of major coral barriers. Fossil reefs can therefore provide us with information on past climates. They are also consumers of large amounts of CO₂. Hence the value in understanding how they operate and evolve at a time when change threatens to affect us all."

The work of the young researcher focused more specifically upon the small inhabitants of reef environments: stromatoporoids, or fossil sponges. Stromatoporoids were the main reef-building organisms in the Frasnian stage. They generally take the form of laminar, branching and domical or rounded shapes. Anne-Christine Da Silva started the considerable undertaking of describing the fauna in detail and studying its distribution within Belgium's carbonate platform.

Morphology, type and distribution

Although stromatoporoids represented an enigma for paleontologists for decades - they were sorted into seven different animal kingdoms before being considered as a specific group of sponges, the calcispongiae, sponges whose skeletons have massively calcified -  they had already been studied in the 1950s, particularly in Belgium. Pioneering Belgian geologist Marius Lecomte devoted an entire book to them in 1952: Les stromatopores du Dévonien moyen et supérieur du bassin de Dinant. "His work was very important and remains a benchmark for researchers even today," says Da Silva. "It has not yet been updated, and since then, the stromatoporoids of the Frasnian stage in Belgium have not been studied on such a scale". However, some aspects are missing in this study; "Marius Lecomte draws up an inventory of stromatopores without connecting them to a particular area. There is therefore a link missing between stromatoporoids and the environment". It is this missing link which Da Silva wanted to find. She therefore went back to the field. She sampled four outcrops, mainly Tailfer (Namur), Prayon (Liège), the La Boverie quarry (Rochefort), and Villers-le-Gambon. A month of fieldwork and several hundreds of samples later, the researcher went to London to meet with renowned stromatoporoid specialist, Steve Kershaw from Brunel University. Basing themselves on the work of, among others, Marius Lecompte to identify the fossils, they described the fauna of stromatoporoids as it developed in the very varied environments in the Frasnian stage in Belgium - lagoon, reef, and deep zone - mapping out different species and their morphology as well as the condition in which they were preserved. "We then linked these data with the different environments," continues the young geologist, before going on to present their initial findings: "it appeared that the types of stromatoporoids which we had observed - thirteen in total - as well as the morphology of the fossils varied depending on the environment: stromatoporoids with laminar morphology were found in deep, calm zones characterized by the presence of significant fine particles (calcium, mud, and clay), while domical forms were found in shallow water, and the branching forms were present in most environments".

A typology of the stromatopores in terms of their environments was thus identified:
•    In the lagoonal area, which was shallow and close to the continent where there was little current and few waves, branched and domical stromatoporoids were found;
•    In the reef area, between the lagoon and the deep area where most stromatopores were concentrated, laminar or domical forms were found;
•    In the deep area, stromatoporoids were either laminar or branched in shape and were rarer: the supply of fine particles increases further from the land. Stromatoporoids, which do not like clays, are therefore few and far between. In contrast, numerous strompatoporoids can be found in mud mounds. Their distribution within these carbonate buildups follows the same general reasoning applied to isolated systems.

"Regardless of the environment, whether deep or shallow, it appears that agitation, the force of the waves, and the presence of clay controls the morphology of stromatoporoids," specifies Da Silva. She adds: "because reefs are structures which interact closely with their environment, the variations to which they are subject are recorded by the organisms which make them up. Because the external morphology of stromatoporoids varies with their environment, they can thus be used as an indicator within a comprehensive approach to how reef environments functioned and evolved."

Additional references

By proposing new connections between stromatoporoids and the environment, Da Silva's work gives yet another dimension to stromatoporoids which may now be used as environmental markers. Such results, without a shadow of a doubt, bring a wealth of precious knowledge to geologists around the world. Frasnian carbonate platforms which are similar to ours can be found in Canada, China, Spain, and Poland. Da Silva's conclusions will therefore enable many of her peers overseas to more quickly identify the type of environment with which they are dealing. It didn't take long for fellow researchers who were interested in her articles to contact her. "Researchers from Canada who work with oil companies contacted me recently. The Frasnian stage in Canada holds significant oil reserves. It is therefore of interest to them to be able to identify as easily as possible the environments corresponding to this time period." This, the first in what will undoubtedly be a long list of approaches from fellow researchers, highlights even further the recognition which this work has achieved.

(1) Da Silva, AC., Kershaw, S. and Boulvain, F. (2011) Sedimentology and stromatoporoid paleoecology of Frasnian (Upper Devonian) mud mounds from southern Belgium. Lethaia, 44, 255-274.
Da Silva, AC., Kershaw, S. and Boulvain, F. (2011) Stromatoporoid palaeoecology in the Frasnian (upper Devonian) Belgian platform, and its applications in interpretation of carbonate platform environments. Palaeontology, 54 (4), 883-905.