The Great Barrier Reef on the northeast coast of Australia is a spectacular sight from the air, stretching over a distance of some 2000 kilometres. From its southernmost parts among the coral cays of the Capricorn Group to its northern limit near the Murray Islands in the Torres Strait, this giant among organic structures is a changeable creature. In places there are widely scattered reefs, about 2500 in all; elsewhere it grows as a nearly continuous wall of coral. The reef builders, mostly colonial corals and their millions of animal and plant neighbours, have slowly fashioned the bioengineering marvel we see today.
But if we could travel 160 million years back in time, we would see another reef in an area that occupied most of what is now Europe. At first sight this reef and its communities have striking similarities to the Great Barrier Reef. But this ancient reef structure is unique; its main architects were not corals, but multicellular marine sponges, many of which have no match today. And this reef was even bigger than the Great Barrier Reef. Its fossil remains stretch about 2900 kilometeres from southern Spain to eastern Romania, making it one of the largest living structures ever to have existed on Earth.
This reef is exposed today in a vast area of central and southern Spain, southwest Germany, central Poland, southeastern France, Switzerland and as far as eastern Romania, near the Black Sea. Despite the scale of this buried structure, until recently researchers knew surprisingly little about it. Individual workers had seen only glimpses of reef structures that formed parts of the whole complex. They viewed each area separately rather than putting them together to make one huge structure. The problem was compounded by the lack of scientific cooperation mand exchange of information between European adversaries during and after the First and Second World Wars. The geological technology was certainly available to assemble the pieces of this palaeontological puzzle into one, but knowledge was lagging behind.
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As a doctoral student at the University of Tubingen, in Germany between 1980 and 1982 I had the opportunity to observe some of these reefs in the field. We had a scientific exchange programme between Tubingen and the University of Warsaw, so when some of the Polish geologists visited Tubingen I began to learn more about the exposures of the reef in Poland. There are plenty of articles in the geological literature form the early part of this century about the Jurassic sponges and their sediments from various regions in Europe. But again, there were no studies on a regional scale sufficiently detailed to make it clear that sponges or algae could form such a huge reef. In fact, siliceous sponges have formed reefs before, but they played only a minor role. The only prominent reefs built by lithistids and hyalosponges were found in the Upper Jurassic some 150 million years ago. In contrast, today’s reefs are almost wholly the product of corals.
In 1984, I flew to Germany to meet German and Polish scientists, observe possible localities in Germany and start to discuss a major international, multidisciplinary project on these reefs. The National Geographic Society paid for field work across Europe from 1986-1988. Discovering the extent of the sponge reef was the highlight of this three-year expedition, taking 3 months each year, to study the palaeontology and general ecology of this ancient reef’s unique geological structure. To cover the huge territory where good fossilised reef exposures abound, we needed to be an international and multidisciplinary team of palaeontologists and field geologists. You can imagine the logistical problems involved in such a field project. Our assorted group had to tramp across the highlands and roadways of Europe and cope with the different language and cultures of each particular region. More importantly, we needed to date quickly and accurately the rocks that we found and build up a picture of the reef.
With this information we could trace the reef’s outline at particularly times in the Jurassic, which lasted from 213 to 144 million years ago. One reason for our success is the excellent timescale during the Jurassic, based on a now extinct group of marine organisms known as ammonites, chambered shellfish similar to modern day squid. Through careful identification of particular ammonite species, we can identify where rocks are formed in space and time.
About 200 million years ago the sea level rose throughout the world. A huge ocean known as the Tethys Seaway expanded to reach almost around the globe at the Equator. Its warm, shallow waters enhanced the deposition of widespread lime muds and sands which made a stable foundation for the sponges and other inhabitants of the reef. The sponge reef began to grow in the Late Jurassic period, between 170 and 150 million years ago, and its several phases were dominated by siliceous sponges.
A reef is a complex living system similar to a large city; tall buildings shape the general outline of the metropolis, and less conspicuous dwellings occupy the spaces left between the skyscrapers. In the Late Jurassic of Europe, sponges were the major reef builders, unlike today when corals dominate. In fact, since their evolution on Earth sometime in the Cambrian (about 600 million years ago), sponges were at their most diverse in Jurassic times.
In the fossil reef, certain sponges are especially conspicuous. One of the first things that you see as you look at an outcrop are shapes like vases and saucers, 1 or 2 metres across.
These strange shapes are responsible for their name – mummies. They are so conspicuous because the process of fossilisation has replaced the original silica skeleton with calcium carbonate. The silica that originally made the sponges’ skeletons has largely been dissolved and replaced by secondary crystals of silica, or, more frequently, by crystals of calcite. This process does not affect the shape of the skeletal meshwork, so that the sponge’s canals, pores and cavities are always clearly visible. The decomposition of the soft sponge tissue generally favours precipitation of fine calcite within and around the sponge skeleton. These so-called ‘mummies’ or ‘lime mummies’ stand out by their darker colour from the surrounding white, fine carbonate muds and sands. But no one is sure what creates the mummies. Not every fossil sponge in the reef is surrounded by a mummy-like casing. Geologists often assume that the mummies developed very soon after a sponge died, perhaps through the chemical activity of cyanobacteria (formerly known as blue-green algae) covering the sponges. Perhaps the coating of algae contributed to the rigidity and growth of a sponge’s body in life, and protected it after death. Another suggestion is that soon after the sponge died, its skeleton acted as a trap for fine sediments in the surrounding waters, quickly burying the organism. The rapid burial sealed away the body from corrosive chemicals, but kept the shape of the sponge as it was in life, until it was fossilised and later replaced by calcium carbonate. But both these processes depend on chemical processes in sea water. As researchers investigate the mummification process, they will find out more about the chemistry of the Late Jurassic oceans.
Sponges are one of the most colourful and ubiquitous components of today’s shallow-water marine communities. Their range of shapes and tolerance of a variety of ecologies make them an important member of the marine and animal family. Palaeontologists can start to find out more about life when the fossil reef formed by cataloguing the types of sponges that built it. But here the fossil sponge are of little help in classifying it. Many sponges can only be truly assigned to a species or genus level on the basis of the internal structure of their skeletons. The most reliable internal features are the arrangement of the intricate water canals that the sponges use for feeding and the different types of spicules, the tiny rods and knobs that are the framework of the skeleton. At first sight, most of these fossils are preserved just as outlines. To see the important details, researchers have to etch specimens in weak acetic acid to separate the spicules and show their distinctive shapes. In other cases they slice whole specimens in half, and make thin sections to reveal the organisation of the canal and the arrangement of the spicules.
Once we had identified the different species, they revealed yet more detail about the structure of the reef. As on Australia’s Great Barrier Reef, sponge communities vary from region to region of the European Reef. We found patterns that changed both in space and time. In Poland the most conspicuous sponges are lithistids and hyalosponges. Lithistids were common in the early stages of the reef’s formation, but their dominance decreased dramatically as the number of hyalosponges increased later. The changes may have come from gradual shifts in the climate, perhaps in sea temperature. By examining the differences in types of other fauna and flora present, such as species of forams or ammonites, we can detect changes in sea temperatures, because some species are adapted to cooler seas than other types. Sediment type may also be used to gauge sea temperature change. In Germany, the reef was mainly made of hexactinellid sponges represented by the genera Stauroderma, Tremadictyon, Cypelia, Rhopalicus, and Linosoma among many others. Lithistids appear less frequently here. In Spain the reef is made of a similar assemblage of sponges, but here there are almost no lithistids. The hexactinellid sponges have pronounced shapes in the Spanish outcrops, with prominent dish, tube and vase forms. Such shapes tend to form in very still, deep water conditions. The absence of flat encrusting sponges – typical of fast currents – is extra evidence that these parts of the reef lay in deep water.
The depth of water is not the only factor that could explain the distribution of different types of sponges. The shape of the seafloor beneath the reef may also have had a profound influence on sponge settlement and growth. Because the carbonate shelf or platform upon which the reef grew was not completely horizontal, the development of the reef and probably the settlement of particular sponge communities, would have been governed by water depth and temperature as the reef grew laterally along the sloping shelf. So, the very position of the shelf relative to sea level may have influenced local climatic conditions on the reef, and so controlled where sponges grew.
Caribbean cousins
It is very difficult to imagine how these sponge communities functioned on the reef because we have so few modern examples for comparison. There are concentrated areas of sponge communities along ocean shelf walls in very deep waters off the Cayman Islands, but they are very small and patchy compared with reefs in the Jurassic. Most modern reefs are calcareous, and grow in shallow water. There are siliceous lithistid sponges, but many of these thrive at great water depths – typically between 250 and 400 metres – making investigations only possible with deep-diving submersibles. I have collected a few isolated individuals in the Caribbean using a submersible, and they look remarkably similar, internally and externally, to their fossil ancestors. But nowhere could we see signs of reef building from the submersible window.
During Jurassic times, the chemistry of the ocean was right for the deposition of fine calcium carbonate sediments over the wide shelf of the Tethys Seaway. In many areas of southern Germany, where the sediment accumulation is especially thick, the term ‘White Jurassic’ has been coined because the rocks are chalky white. European landscapes have been sculptured largely by their rocks. The carbonates formed during the Upper Jurassic are more resistant to erosion than the older Jurassic rocks lying below them, so they can create high plateaus with steep escarpments. Many of the individual reefs began to develop on such carbonate muds. The reef grew so quickly that the mud deposits around it could not keep pace. The living structure gradually built up above the seafloor. This elevation was important; without it the sponges would have suffocated in the soft thick sediments that settled around them on the seafloor.
Sponges are among the simplest of multicellular organisms. They do not swim or move about the seafloor, but stay in one place, and feed by filtering food from sea water, utilising cells with tiny ‘arms’ known as flagellae, to pump water through the system of canals within their bodies. Though sponges do this in a variety of ways, the general mechanism involves drawing sea water into the sponge’s body through tiny openings, called ostia, found on the sponge’s outer surface. The water then flows through tubes – canals where special feeding cells extract the tiny food particles. The unwanted waste water is passed into a large central cavity where currents, created by cells with flagellae, move it out through a large, exhalant opening at the top of the sponge.
Living in soft muds sets a particularly nasty problem for the successful sponge. If the sponge grows at about the height of the surrounding mud deposits, the delicate ostia will soon be clogged with sediment and the water that carries the sponge’s food would not reach the canals. Also, the thicker accumulation of fine muds would act as a baffle and slow down the currents in the sea around the sponge that are so critical in bringing it food. So, the more sediment that is deposited, the more important it is for the reef to grow upwards to keep free of the mud.
But what started the sponge reef on its upward growth? We found that it was the tiny cyanobacteria. There were traces of algae in the calcareous crusts that covered the fossil sponges and other inhabitants of the reef. The algae precipitated calcium carbonate sediments from the sea water and cemented the structure. This provided additional platforms for new sponges. Ironically it was these very cyanobacteria that formed stromatolitic reefs, the earliest known organisms to exist on Earth, some 3.5 billion years ago. The combination of building by sponges and algae resulted in massive individual reefs that initially spread across the seafloor, then grew upward as new reef patches developed upon existing ones. Building new sponges and algal communities on the shoulders of abandoned ones created a structure that was higher than the surrounding seafoor, and could capture sunlight and nutrients for the sea dwellers near the top of the reef. This binding, rigid framework is still characteristic of today’s coral reefs.
In addition to the sponges and algae, the European reef was teeming with a host of other marine life forms. Ammonites, common in the rocks formed on the seafloor around the reef, are now extinct, and the only living relatives we an compare them with are the nautiloids and octopuses. The ancient animal had a coiled outer shell with many small and medium size compartments or chambers. By regulating the amount of body gases it released into these hollow chambers, the ammonite could regulate its buoyancy, like a submarine. By controlling its depth, the organism could avoid predators or seek food resources in areas other reef dwellers could not reach.
A host of bivalves and brachiopods flourished on the reef surface. These clam-like creatures could attach themselves firmly to hard surfaces by secreting cements. They fed by filtering water from currents. But the most abundant and varied organisms appear within and on the sponges. Many fossil sponges carry other creatures that used the surface of the sponge like piggy-back riders. One of the most important are the cyanobacteria, and the crustal mats that they formed. Not only did these algae bind neighbouring sponges together and make the reef more compact by growing on inner and outer surfaces of the sponge, but they also protected the fragile body of the sponge from immediate disintegration after death. There were also the tiny foraminifera that attached themselves to the interior of the sponge; there can be thousands within the walls of a single sponge. Foraminifera are single-celled animals which can secrete an outer shell. They are very choosy about the temperature and depth of the water they live in, so as fossils they are good indicators of conditions in the past. Bryozoans, or moss animals, often lived on the underside of sponges. Though many bryozoans are capable of standing erect within the current, forms living at the base of the sponge were mainly encrusting and they, too, were filter feeders, passively taking food from the moving water of the currents.
Each living component, no matter how small, ultimately contributes to the general ecology and wellbeing of the reef. When paleontologists are lucky enough to observe a fossil outcrop with excellent exposures and continuous sequences of rock, they have a rare glimpse into the past and future. For unlike the biologist, who can rarely witness the evolutionary changes of an organism generation after generation, paleontologists can monitor a group of species through its history, and see its beginnings and its ends. This gives them a better chance of predicting the fate of modern species that face extinction. The European sponge reef is a useful example of this, because it represents a relatively closed ecosystem that persisted for several million years and is well preserved within the rock record.
The sponge complex began about 160 million years ago. It continued to expand across the seafloor for between 5 and 10 million years until it occupied most of the wide sea shelf that extended over central Europe. This shelf was open to the Tethys Seaway, which undoubtedly influenced the reef’s development. The Tethys Seaway contained warmer, shallower seas than those in which the reef started to grow, so only some of the species of the reef thrived as time went on. The sponges and their neighbours lived in relatively deep, quiet waters at the height of the reef’s growth. At no time during this period (about middle Late Jurassic, 155 million years ago) did we see any evidence that the seas around the complex grew shallower, although some German geologists believe otherwise. Roman Koch and co-workers at the University of Heidelberg believe on the basis of Late Jurassic rocks in Germany, that the sea level dropped so far that the reef had emerged repeatedly, and there were even influxes of freshwater from the nearby shore. Our investigations indicate that the complex was covered by at least 150 metres of water during its peak growth.
The deepest areas were in the Spanish region where the reef was farthest from shore. Here the reef was unaffected by wave action; there was no sign of erosion or rubble from breaking waves, and the large forms such as hyalosponges survived intact. In addition, there were no signs of sediment washed in from the land. As we tracked the reef to the East, towards Germany and Poland, we picked up some signs of shallow seas, but at no time were the waters so shallow that the reef was exposed.
At the end of the Late Jurassic a drastic change came to the creatures of the reef. The seas shallowed considerably, and sponges became rare in outcrop on the reef. Instead, a familiar reef dweller appears – the corals. Perhaps one of the causes of the demise of the sponge reef was the lowering of the sea level itself attributed to global climatic cooling, which enlarged both polar ice caps at this time. The shallower waters were havens for corals because they depend on photosynthesising algae living inside their tissue. Once the sponges succumbed, there was a vacancy for the corals to replace them on the reef.
We do not know whether the demise of this fossil sponge reef was caused by an environmental change to shallower waters, or from the competition for growing space with corals. What we do know is that such a structure never appeared again in the history of the Earth. Had it been successful enough to span the millions of evolutionary years into today’s world, perhaps structures such as the Great Barrier Reef would be awash in sponges.
Over the past century the national and political boundaries that have carved up Europe have hindered detailed studies of this fossil monolith. Areas in Romania, where we know the reef existed, and regions of what was East Germany, where fossil remains may abound, have yet to be investigated. Now that these political borders have finally been removed, we look forward to discoveries that will no doubt aid our understanding of this once sleeping giant.
SPONGE SHAPES AND STREAM SPEEDS
The shapes of sponges on a fossil reef are all you need to find the speed of the sea current. We can do this if we assume that Jurassic sponges worked in similar ways to their modern counterparts. All the evidence says they do.
Sponges face a vexing problem as they filter the surrounding sea for food. They continually push out their waster water through a large exhalant opening on their topmost side. So, the flow through this opening must be powerful enough to throw the waste material some distance away from the sponge. If it is not, the sponge faces the unpleasant task of re-ingesting the very waste material it has just released, through the hundreds of tiny, inhalant pores that cover the outer walls of its body.
This is especially annoying when the exhalant opening is very wide. To understand this, think about the nozzle of a common garden hose as an illustration. If you open the nozzle of the garden hose to its maximum diameter, the jet of water would be very weak and not travel very far. But if you slowly close the opening in the nozzle, the velocity of flow would increase noticeably, and the jet of water would travel much farther away from the nozzle. The simple hydrodynamic principles that control the velocity of flow in the hose affect sponges as well, apart from the fact that a sponge’s exhalant opening cannot be adjusted like the nozzle of a garden hose.
Here, the speed currents in the sea is the variable factor. Thus, a fossil sponge with a large exhalant opening probably lived in turbulent waters because there was not sufficient water pressure from the sponge itself to push the waste material away from the body. In contrast, sponges with narrow openings could live in quiet waters as they did not need the flow of sea current to help remove their wastes.
These principles of water pressure and sponge types can be taken one step further by observing the overall spong outline. Flat-lying encrusting sponges are adapted to rapid sea currents, because they are highly resistant to overturning – a hazard in such high energy flows. But they can not easily survive in less turbulent seas because the debris that they eject from the top of the sponge would simply resettle on its surface. Solid cylindrical and solid branching types that stand upright cannot survive in very swift currents because they can easily topple over and be buried. But they do need reasonable water movement or they would be feeding on their own wastes.
Tubular and vase-shaped sponges are adapted to slow moving, quiet waters because there they can effectively separate their inhalant and exhalant water flows and need no assistance from the surrounding ocean currents. They cannot live in high energy flows, however, as their body shape is not very resistant to current surges – they would be dislodged. These two sponge shapes were very common in Late Jurassic seas.
Joe Ghiold is a research associate of the Department of Earch and Planetary Sciences, Western Australian Museum, Perth.
