Stromatoporoidea

Stromatopore of Gotland; Diameter 45 cm.

• Worldwide

The stromatoporoids, scientifically called Stromatoporoidea, are an extinct, colony-forming animal group, which is now mostly associated with the sponges. Their fossils are found only in the form of calcium carbonate skeletons, first confirmed findings date from the Ordovician. Between the Lower Carboniferous and Triassic there is a gap, more fossils are only from the Middle Triassic in Mesozoic known. At the end of the Cretaceous they died out eventually. The sessile stromatoporoids lived exclusively in the sea, preferably in the shallow waters of the tropical and subtropical waters and were often associated with coral. Especially in the Silurian and Devonian, they were instrumental in the Riffbildung. Like the sponges and corals, these organisms were probably filter feeders. The diameter of a single colony is in the millimeter range up to a maximum of two meters. For the taxomische assignment of fossils into genera and species, the internal structure of the skeletons is important and only possible through thin section analysis. However, this fine structure is often due to the fossil -forming processes ( Fossildiagenese ) lost. The upper parts of the fossils were thus usually more damaged, while the base elements have been less affected by diagenesis.

• 6.1 Some selected genera

Physique

The stromatoporoids built fine mesh calcareous skeletons ( Coenostea ), which are rarely found today consisting of calcite and aragonite. Divided they are by horizontal ( laminae ) and vertical elements ( Pilae, also called pillars ). The spaces are referred to as galleries or chambers. Through education, more and more new horizontal layers of the organism grew in height. Units of several further apart standing laminae alternating with thickened surface layers parallel hot Latilaminae and are comparable with annual rings of tree trunks. They are seen as dependent on the environmental conditions paleoecological growth periods. On average, the rate of height growth was probably only in the millimeter range, while growth could reach in the width already up to one centimeter per year.

To wart-like bumps ( Mamelonen monticuli also called ) are located on the surface. With these small bumps asterisk -shaped channels ( Astrorhizen ) are connected run on or just below the surface and partially protrude as vertical channels in the skeleton of the. By the added water, and the ( for food intake ) was released again filtered They probably served as the inflow and outflow openings (similar to the sponges ). The living tissues has been never found petrified, it was probably on the surface and in the upper galleries of the skeleton.

Also, in the internal structure of the skeletal elements, that is, in the microstructure of the laminae and Pilae there are differences. One can basically distinguish between three different types of bodywork: There are compact structures which can be identified within the elements, no air spaces or grains. The second form is called cellular, here the structure is more or less porous. The third type is structured fibrous, this structure can be found almost exclusively in the stromatoporoids occurring again in the Mesozoic. At higher magnification one recognizes individual fibers in the elements. Between these three different structures, there are also smooth transitions. However, this can be due to diagenesis, thus taking place after the death of the organism in the sediment changes.

There are very different growth forms with the habitat of the organism ( for example, temperature and strength of the wave energy ) played a major role. In general, a distinction is made between three growth forms:

• Spherical, bulbous (massive ) or layered ( tabular ) forms: These forms of growth formed on the outer edge of the reef, where the sponges were exposed to the attack of the surf.
• Inkrustierdende growth forms: the animal fills in the gaps skeletal formation of the soil and forms a crust. This growth form is often found on the Riffkamm.
• Branchy ( dendroide ) forms: The skeleton is branched tree-like, with more or less spreading branches.

In addition there are also laminar and cauliflower-like forms.

Way of life

The stromatoporoids were living as sessile benthos in shallow and mostly warm seas. Your diet is (like many others ) is still uncertain. Presumably they subsisted as a filter feeder by filtering out nutrients, trace elements and possibly even micro plankton from the seawater. They grew mostly on hard surfaces or on shells of other organisms. The stromatoporoids were instrumental Riffbildner, but you can also find small, spherical colonies in deeper marine layers, far from reef communities. Maybe there was a symbiosis of some representatives of the stromatoporoids with algae, similar to today's coral. These stromatoporoids grew mostly in shallow water, so was still enough light for photosynthesis of the algae present. However, this assumption could not be detected because fossilized remains of algae have not been found on the stromatoporoids.

Stromatoporoids and parasites

The reef-building stromatoporoids can often be vertical tubes in the skeleton (Find Caunoporen ). When they are divided by floors ( Tabulae ) and have their own wall, you can close a cohabitation of stromatoporoids with other organisms. Mostly it was the genus Syringopora, a tabulate coral that lived as a commensal in the stromatoporoids. The coral benefited from the host, but did no great harm. Traces of the Syringopora detected in different genera of stromatoporoids from the Silurian to the Devonian. Even tubes of rugosen corals, especially immersum on the type Spongophyllum were found. The occurrence of taboo distillates as commensals was very frequently detected, in some localities were up to a third of the stromatoporoids affected.

Rare traces of worms and pit -like growths were found in the fossil record. For the first time in the late 20th century demonstrated worming is helical, coiled tubes with diameters in the range of tenths of millimeters. The finds have been made ​​only in Central Europe and date from the time of the Devonian. Presumably, the traces of Streptindytes were produced, a worm of the order Serpulimorpha of polychaete ( Polychaeta ). About the coexistence of both organisms and whether it is commensalism or parasitism, yet nothing definite is known. Such tubes have also been detected in various taboo distillates and Chaetetiden.

The pit shaped tracks resemble irregular outgrowths of the skeleton, on closer examination it turned out, however, that these are traces of other organisms that may have lived as parasites on the stromatoporoids. The diameter of these pits is in the millimeter range. Probably moved the organism to the stromatoporoids and disabled for some time the growth. Later the skeleton grew at the site after and also filled the space of the pit again. Which organism these pits were generated and whether it is in fact parasitism, is still unknown.

The Riffbildung

In the early Ordovician, the bryozoans ( Bryozoa ) were the real reef builders, but they were actually no reefs in the strict sense, because the size was very low and comparable with more hills. These types of reefs are referred to as bioherms or "patch reefs ". Were in the middle Ordovician stromatoporoids which won at the Riffbildung increasingly important, at this time there were already of stromatoporoids and siliceous sponges formed reefs heights of up to 10 meters and 150 meters in diameter reach. In these Socialization stromatoporoids came before with a size of up to about two meters. The reefs in the Ordovician were very different in size and composition of the community from those in the Silurian and Devonian. An example is the reef in the Crown Point Formation in the U.S. state of Vermont. The basic structure was formed by the brachiopods ( Brachiopoda ), for further development significantly contributed stromatoporoids, sponges and bryozoans at. Also, rugose and tabulate corals were, however, only partially involved in the construction. In the final stage of development massive stromatoporoids were the dominant creatures on the reef. Only from the Silurian, when the taboo Deferred stromatoporoid reefs reached the taboo distillates and Rugosen an important position within the Riffbildung.

The formation of the taboo Deferred stromatoporoid reefs is comparable to the formation of a coral reef. In most cases the basic structure of the reefs of the coral-like taboo distillates and Rugosen, as well as stromatoporoids was formed. On the scaffold settled more taboo distillates and stromatoporoids and the skeletal remains of carbonate sediments abgestorbenden organisms filled the cavities of the relatively porous scaffold. Bryozoans, algae and some Stromatoporengattungen solidified the gaps, the reef grew up. If it had outgrown the sea, the open sea side to facing, lying on the sea Riffkamm of encrusting, flat stromatoporoids was formed with massive skeletons that could withstand the strong wave action. At the Rückriffseiten in quiet lagoons and other shallow, Pacific, documents often lived dünnästige forms, often made ​​whole groundcovers stromatoporoids lawn. The genus Amphipora is a common example of this growth form, the diameter of the individual branches are usually in the range of a few millimeters. Fossils from the Devonian of such lawn can be found for example in the disused " Operich " quarry in Büdesheim in the Eifel. From the Middle Ordovician caused by adaptive radiation of many new forms of taboo distillates, Rugosen, stromatoporoids and other important for Riffbildung beings. In the ecosystem of the reefs niches were filled, the shape diversity has increased, so that eventually the Silurian and Devonian the powerful taboo Deferred stromatoporoid reefs could arise.

Reefs in the Silurian

The taboo Deferred stromatoporoid reefs of the Silurian achieved partly a length of up to three kilometers. Important from the Silurian reef structures exist, for example, on the island of Gotland (Sweden), Anticosti (Canada) and in the Carnic Alps ( Austria ).

Located along the Baltic Sea basin system Gotland Island is an area of ​​intense geological research. Fossils are numerous and very often in a good state of preservation. In the Silurian of Gotland was covered by a warm shallow sea and was in a quiet tectonic zone. Thus, there had good environmental conditions for reef formation. Particularly striking are the " Rauka " columns Riffkalkstein that extend up to ten feet in height and usually have a very peculiar form. They are formed by erosion remnants of the original reefs. Its shape is determined by the former biological composition of the reefs, largely determined by the shape of the stromatoporoids.

Reefs in the Devonian

In Devon the stromatoporoids had their actual flower, in this period that the greatest taboo Deferred stromatoporoid reefs and the species richness of these communities also increased. A well-known example from the Devonian is in Western Australia in Canningbecken lying, so-called " Devonian Great Barrier Reef". This is to a large system of taboo Deferred stromatoporoid reefs with an original length of probably more than 1,000 kilometers. In Canningbecken the conditions for the formation of this vast Riffkomplexes were optimal, since the area was in Devon near the equator and was covered by a shallow, tropical sea.

On the northern edge of this basin, the still standing remains of the reef can reach a length of 350 km and a height of over 100 meters. When the sea withdrew before about 250 million years ago and the weathering began, rivers dug deep gorges in the reefs. A well-known example is formed by the Lennard River Windjana Gorge. The up to 90 -meter-high walls of the gorge can be very well the temporal, geological stratification of the reef recognize and are an important reference for fossils. The name of this reef refers to the recent Great Barrier Reef, which is located northeast of Australia in the South Pacific. Was formed, even geologically speaking, a lot of time later, in about 20 million years ago.

The reef limestone is always porous, so it is often a good reservoir rock for petroleum. For industry important oil-bearing reefs from the Devonian exist, for example, in Alberta ( Canada).

Another important geological area is the Rhenish Slate Mountains. In Devon it was full of a sea, the Rhine shallow sea with depths of up to 100 meters covered. There came also to the formation of extensive stromatoporoid reefs. Particularly impressive remains of these reefs are the dolomite rocks in Kylltal of Gerolstein in the Eifel.

The remains of the resultant in the Middle Devonian reef project in the valley up to 100 meters into the air. The dolomite was formed by leaching of lime from the reef and enrichment of magnesium. This chemical process in these rocks, however, hardly any fossils to be found. However, the former reef structure, the outer edge of the reef, the Riffkern and earlier debris deposits can still see at the Gerolsteiner dolomite rocks. Good -preserving fossils are found in the environment in the so-called Eifel Kalkmulden. Commonly found in the Eifel Stromatoporengattungen include Stromatopora, Actinostroma and Amphipora. The stromatoporoids were here mainly large biostromes, flat, growing more in width reefs. Well-preserved Riffreste with many fossils can also be found in the legal Rhenish Slate Mountains. An example is the resultant in the upper Devonian " Wirbelauer Reef ". This is a far away from the coast reef, it grew up on a volcanic sill in the Rhenish shallow sea. It can be seen in the disused quarry Joerissen near Wirbelau. Other excellent outcrops in the Riffkalkfolge the Late Devonian are the limestone quarries south-east of Aachen between Walheim and Schmithof. Here the reef limestones and nodular the Frasnian are open widely.

The fauna and flora of the reefs were very diverse, they were inhabited by a variety of invertebrates such as snails, trilobites and brachiopods. On the surface, living bryozoan colonies, hydrozoans and algae. Many fossils of free-swimming creatures, such as nautiloids, fish and ammonoids one hatt found in the sediments before the Devonian Great Brarrier reef. These findings have enormous Komplexivität these ecosystems recognize. In the form of diversity and Farbenprächtigkeit the taboo Deferred Stromatoporenriffe were certainly just as unique as today's coral reefs.

The stromatoporoids in the Mesozoic

After the mass extinction at the end of the Devonian stromatoporoids disappeared, the coral-like taboo distillates and Rugosen have been greatly weakened. The reefs formed after the Devonian of organisms lost thereby greatly in size and ecological importance, emerged only during the Triassic again similar sized reefs and one also finds again the stromatoporoid fossils. During this time, however, the hard corals were the dominant reef builders. The re- occurring in the Triassic forms of stromatoporoids ( in the Mesozoic also called Sphaeractinoidea ) were no longer significantly involved in the formation of reefs, mostly they lived as Riffbegleiter the corals. In the upper Jurassic Mesozoic stromatoporoids experienced a bloom again and reached a certain importance in the Riffbildung, rarely they also formed their own small back reef types. Their biggest distribution area was the Tethys. In areas with large wave strength they never came before, she preferred quiet and shallow zones. At the end of the Cretaceous they eventually died out altogether.

System

The systematic position of the stromatoporoids is not fully understood even today. Previously, the stromatoporoids were assigned to the Hydrozoa. Also a relationship with Archaeocyaths and blue-green algae, the stromatolites was considered. Obviously, the stromatoporoids were strongly affected by the mass extinction in the upper Devonian, because only from the Triassic can find similar fossils. Therefore, one can basically distinguish two groups:

• Stromatoporoids from the Paleozoic: The "typical" stromatoporoids
• Mesozoic forms: Also referred to as Sphaeractinoidea

The "typical" stromatoporoids ( engl.: true stromatoporoids ) are considered a monophyletic group and provided to the sponges as (Class Stromatoporoidea ). A strong indication of this are the Astrorhizen because they had similar structures in the genus Astrosclera ( order Sklerospongea ) of extant sponges found. This systematic classification is recognized by most scientists since the 70 's / 80 's of the 20th century.

Due to the gap between the two groups, the systematic position of the common emerging again from the Triassic and to the upper Cretaceous forms is still largely unclear. They are often referred to as Sphaeractinoidea to separate it from the stromatoporoids. As an important difference in the structural design of the skeletal elements is often listed, because the Mesozoic fossils, the elements are almost always constructed fibrous. Due to the change of the fine structure by the diagenesis taxonomic significance is still controversial. Whether the Sphaeractinoidea with the "typical", Paleozoic stromatoporoids form a kinship group, or whether it is convergent developments Sklerospongea and hydro Idea therefore is still controversial.

Some selected genera

Labechia: This genus is known from the Ordovician to Lower Carboniferous. Types of Labechia were found in Europe, Asia and North America and often lived on reefs. The Labechia among the original stromatoporoids in which horizontal skeletal elements missing, instead the skeleton is constructed in bubble form. Astrorhizen are also not yet available. Often fossils were found in reefs, the habit was mostly inkrustierend.

Stromatoporella: From Ordovician to Lower Carboniferous the Stromatoporella also had a long " survival ". The Astrorhizen are already well developed in this genus also periods of growth are clearly visible. At higher magnification one can recognize a porous microstructure at the piers.

Stromatopora: They came in the Silurian to Lower Carboniferous ago were spread throughout the world and lived primarily in the reef area. The colonies were mostly built solid and the Latilaminae are very often clearly visible in the fossil record, thus the skeleton receives a layered form. The Astrorhizen are highly trained and usually well visible. In this genus are very common on the burrows of Syringopora. Stromatopora is the first discovered species of stromatoporoids, it was described in 1826 by Georg August Goldfuß. Before the examination by Dünschliffanalysen was possible, the name for almost all Stromatoporengattungen and organisms of similar structure as for example used the stromatolites. The taxon Stromatoporoidea was only introduced in 1879.

Actinostroma: From the Ordovician to Lower Carboniferous lived in the Actinostroma Riffbereichen and were distributed worldwide. The earlier also counted to this genus remains from the Mesozoic are now generally made to the Actinostromaria ( Jurassic to Cretaceous ).

Amphipora: This genus mentioned felt only in the medium to Lower Devonian. The thin twigs show fine pores which lie close to each other.

Stachyodes: The Stachyodes lived by Untersilur to Devon. They formed in contrast to the Amphipora dickästige structures and lived mostly in the front and back area of the reefs.

Burgundia: A Mesozoic, occurring from the upper Triassic to Upper Cretaceous genus. They were found only in the Middle East, England, Italy and Germany.