Stromatopores
| Stromatopores | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Stromatopore colony (? Stromatopora concentrica ) |
||||||||||||
| Temporal occurrence | ||||||||||||
| Ordovician to chalk | ||||||||||||
| 488 to 65.5 million years | ||||||||||||
| Locations | ||||||||||||
|
||||||||||||
| Systematics | ||||||||||||
|
||||||||||||
| Scientific name | ||||||||||||
| Stromatoporoidea | ||||||||||||
| Nicholson & Murie , 1879 | ||||||||||||
The stromatopores , scientifically known as the stromatoporoidea , are an extinct group of sessile colony-forming marine animals that are mostly assigned to sponges today. Their fossil remains can be found today in the form of the calcareous skeletons of their colonies. The first confirmed finds come from the Ordovician . There is a gap in tradition between the Lower Carboniferous and the Middle Triassic . H. no stromatopore fossils are known from this period. At the end of the chalk they finally died out. Stromatopore colonies are typical for shallow marine deposits of tropical and subtropical waters and are often associated with corals . In the Silurian and Devonian in particular , they played a key role in reef formation . Like the "real" sponges or bryophytes , stromatopores were probably filter feeders . The diameter of a single colony is in the millimeter range up to a maximum of two meters. The internal structure of the skeletons is important for the taxomic classification of fossils in genera and species , which can only be determined by thin section analyzes . This fine structure has often been lost through the fossil-forming processes ( fossil diagenesis ). The upper parts of the colonies are usually more affected by the diagenetic processes, while the basic elements are better preserved.
construction
The stromatopore colonies are passed down today in the form of their calcareous skeletons (Coenostea). What these skeletons were originally made of is the subject of debate. Since they are often strongly diagenetically overprinted ( recrystallized ), it is assumed that they originally consisted of the calcium carbonate variants, aragonite or high-magnesium calcite, which were less stable in geological periods .
The colonies are internally structured by horizontal (laminae) and vertical (pilae, also called pillars) elements. The spaces in between are called galleries or chambers. The colony grew upwards through the formation of new horizontal layers. Units of several, more widely spaced laminae alternating with thickened layers parallel to the surface are called latilaminae and are comparable to annual rings of tree trunks. They are interpreted as growing seasons dependent on the paleoecological environmental conditions . On average, the annual height growth was probably only in the millimeter range, while the growth in width could reach up to one centimeter per year.
Some species have wart-like bumps on the surface of the colony (called mamelons or monticuli). Associated with these cusps are small, star-shaped channels (astrorhizas) that run on or just below the surface and partially open into channels (oscula) that protrude vertically into the skeleton. They probably served as inflow and outflow openings (similar to the sponges) through which the surrounding water was taken up and released again after filtration (for food intake). Stromatopore soft tissue has never been found fossilized. It probably covered the surface and was also probably in the uppermost galleries of the skeleton.
There are also differences in the internal structure of the skeletal elements, i.e. in the microstructure of the laminae and pilae. One can basically differentiate between three different types of structure: There are compact structures with no air spaces or grains being discernible within the elements. The second form is called cellular, here the structure is more or less porous. The third type has a fibrous structure; this structure is found almost exclusively in the stromatopores that appeared again in the Mesozoic era. At high magnification you can see individual fibers in the elements. There are also flowing transitions between these three different structures. However, this can be due to the diagenesis , i.e. the changes that occurred in the sediment after the death of the organism .
There are very different growth forms, whereby the living space 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 growth forms formed on the outer edge of the reef, where the sponges were exposed to the attack of the surf .
- Encrusting growth forms: The animal fills in gaps in the ground with the formation of a skeleton and forms a crust. This growth form is often found on the reef crest.
- branched ( dendroid ) forms: The skeleton is branched like a tree with more or less spread branches.
There are also laminar and cauliflower-like shapes.
Way of life
The stromatopores lived as sessile benthic dwellers in shallow and mostly warm seas. Your diet (like many other things) is still uncertain. Presumably they fed themselves as filter feeders by filtering out nutrients, trace elements and possibly also microplankton from the sea water. Most of them grew on hard ground or on the shells of other organisms. The stromatopores were instrumental in building reefs, but you can also find small, spherical colonies in deeper seas, far away from reef communities. Possibly there was a symbiosis of some representatives of the stromatopores with algae, similar to today's corals. These stromatopores usually grew in shallow water, so there was still enough light for the algae to photosynthesize. However, this assumption has not yet been proven, as fossilized algae remains have not yet been found on the stromatopores.
Stromatopores and Parasites
In the reef-forming stromatopores, vertical tubes can often be found in the skeleton (caunopores). If they are divided by floors (tabulae) and have their own wall, one can conclude that stromatopores coexist with other organisms. Usually it was the genus Syringopora , a tabular coral that lived as a commensal in the stromatopores. The coral benefited from the host, but did not cause any great damage. Traces of the Syringopora have been detected in various genera of the stromatopores from the Silurian to the Devonian. Tubes of rugose corals, especially of the Spongophyllum immersum species , have also been found. The occurrence of tabulates as commensals has been proven very frequently, at some sites up to a third of the stromatopores were affected.
Rarely, traces of worms and pit-shaped outgrowths were found in the fossils. The worm tracks, first detected at the end of the 20th century, are spiral, rolled-up tubes with diameters in the range of tenths of a millimeter. The finds have so far only been made in Central Europe and date from the Devonian period. Presumably, the tracks were of Streptindytes generated, a worm of the order Serpulimorpha of polychaete (Polychaeta). Nothing more is known about the coexistence of the two organisms and whether it is commensalism or parasitism . Such tubes have also been found in various tabulates and chaetetides.
The pit-shaped tracks resemble irregular outgrowths of the skeleton, but on closer examination it turned out that these are traces of other organisms that may have lived as parasites on the stromatopores. The diameter of these pits is in the millimeter range. Presumably the organism settled on the stromatopores and hindered growth for some time. Later the skeleton grew back on the spot and also filled the space of the pit again. It is still unknown which organism these pits were created by and whether it is actually parasitism.
The reef formation
In the early Ordovician, the bryozoa were the actual reef builders, but they did not actually form reefs in the narrower sense, because the size was very small and more comparable to hills. These types of reefs are known as bio-thermal baths or “patch reefs”. In the middle Ordovician, the stromatopores became increasingly important in the formation of reefs , at this time there were reefs formed by stromatopores and silica sponges that could reach heights of up to 10 meters and a diameter of 150 meters. Stromatopores with a size of up to two meters occurred in these societies. The reefs of the Ordovician differed markedly in the size and the composition of the community from those in the Silurian and Devonian. One example is the reef in the Crown Point Formation in the US state of Vermont . The basic structure was formed by the arm pods (brachiopods), stromatopores, sponges and bog animals contributed significantly to the further structure. Also rugose and tabulate corals were, however, only partially involved in the construction. In the final stage of development, massive stromal pores were the dominant creatures on the reef. It was not until the Silurian, with the tabular stromatopore reefs, that the tabular and rugose forms reached an important position within the reef formation.
The formation of tabular stromatopore reefs is comparable to the formation of a coral reef . Usually the basic structure of the reefs was formed by the coral-like tabulates and rugoses , as well as by stromatopores. More tabulates and stromatopores and skeletal remains, limestone sediments from dead organisms, filled the cavities of the relatively porous framework. Moss animals, algae and some stromatopore species solidified the gaps, and the reef grew upwards. When it had grown above sea level, the reef crest facing the open sea-level, above sea level, was formed by encrusting, flat stromatopores with massive skeletons that withstood the strong wave movements. On the reef sides in calm lagoons and in other shallow, calm seas, there were often thin-branched forms that often formed entire stromatopore lawns that covered the ground. The amphipora genus is a common example of this growth form, the diameter of the individual branches is usually in the range of a few millimeters. Fossils of such lawns from the Devonian can be found, for example, in the disused “Operich” quarry in Büdesheim in the Eifel. From the middle Ordovician onwards, adaptive radiation resulted in many new forms of tabulates, rugoses, stromatopores and other organisms important for reef formation. Niches were filled in the reef ecosystem, the variety of forms increased, so that the mighty tabular stromatopore reefs could finally arise in the Silurian and Devonian.
Reefs in the Silurian
The tabular stromatopore reefs of the Silurian Mountains sometimes reached a length of up to three kilometers. Important reef structures from the Silurian exist, for example, on the island of Gotland (Sweden), Anticosti (Canada) and in the Carnic Alps (Austria).
The Gotland island system in the Baltic Sea basin is an area of intensive geological research. Fossils are numerous and very often in good state of preservation. In the Silurian, Gotland was covered by a warm shallow sea and lay in a calm tectonic zone. Thus there were good ecological conditions for the reef formation. Particularly noticeable are the "Rauka", pillars made of reef limestone that protrude up to ten meters and usually have a very peculiar shape. They are the remains of the original reefs formed by erosion . Their shape is determined by the former biological composition of the reefs, largely through the shape of the stromatopores.
Reefs in the Devonian
In the Devonian the stromatopores experienced their heyday. During this period the largest tabular stromatopore reefs existed and the biodiversity of these communities was great. A well-known example is the "Devonian Great Barrier Reef" of the Pillara Formation ( Frasnium , Upper Devonian ) of the Canning Basin in Western Australia . This is a large complex of tabular stromatopore reefs with an original length of probably more than 1000 kilometers. In the Canning Basin, the conditions for the formation of this huge reef system were optimal, as the area in the Devonian was close to the equator. The fauna and flora of the reefs were very diverse and consisted of a wide variety of invertebrates such as snails , trilobites and arm pods . In addition to stromatopores and corals, reef builders were moss animals and protozoa . Fossils of free-swimming creatures such as nautilids , ammonoids and fish occur in the fine-grained sediments that were deposited in the deeper water between the reef bodies ( Gogo and Virgin Hills Formation). The finds reveal the enormous complexity of these ecosystems. In terms of the variety of shapes and colors, the tabular stromatopore reefs were probably just as unique as today's coral reefs. At the northern edge of this Canning Basin, the remains of the reef complex that still exist today reach a length of 350 kilometers and a height of over 100 meters. When the sea retreated about 250 million years ago and the reefs dried up, rivers dug deep gorges into the rock. A very well known example is the Windjana Gorge formed by the Lennard River . The geological history can be read very well on the up to 90 meter high walls of the gorge based on the sequence of layers. The name of the Devonian Great Barrier Reef refers to the recent Great Barrier Reef , which is northeast of Australia in the South Pacific. Even viewed in geological time, it was formed much later, only about 20 million years ago, and is mainly composed of modern corals ( Scleractinien and Milleporiden )
There are also Devonian reefs in the Rhenish Slate Mountains . Their rock is known as mass limestone . Mass limestone of the Middle Devonian occurs among other things in the so-called Eifeler Kalkmulden . A particularly impressive example are the dolomite rocks in the Kylltal of Gerolstein . They tower up to 100 meters in the valley. The dolomite was formed by subsequent transformation of the calcium carbonate as a result of an enrichment with magnesium. Due to this transformation, the fossils originally contained in the rock have largely been destroyed. In the Gerolsteiner dolomite rocks , however, the former reef body, the reef edge and the reef debris of the pre-reef area can still be seen. However, there are also reefs with better preserved fossils in the Eifel. The more common stromatopore genera include Stromatopora , Actinostroma and Amphipora . The stromatopores here mainly formed large biostromes , shallow reefs that grew wider in width.
Well-preserved reef remains with many fossils can also be found in the slate mountain range on the right bank of the Rhine. One example is the "Wirbelauer Reef" formed in the upper Devonian. This is a reef far from the coastal waters that lay on a volcanic threshold. It can be viewed in the disused Joerissen quarry near Wirbelau . Other excellent reef limestone outcrops of the Upper Devon can be found, again to the left of the Rhine, in the limestone quarries southeast of Aachen between Walheim and Schmithof. In addition to reef limestone, there are also large areas of tuber limestone of the frasnium .
The stromatopores in the Mesozoic Era
After the mass extinction at the end of the Devonian, the stromatopores disappeared, the coral-like tabulas and rugoses were greatly weakened. The reefs formed by organisms after the Devonian lost their size and ecological importance. Only in the course of the Triassic did reefs of similar size appear again and fossils of the stromatopores can be found again. During this time, however, the hard corals were the dominant reef builders. The forms of the stromatopores that occurred again in the Triassic (also called Sphaeractinoidea in the Mesozoic era) were no longer significantly involved in the formation of the reefs; they mostly lived as reef companions of the corals. In the Upper Jurassic, the Mesozoic stromatopores flourished again and achieved a certain importance in reef formation; they seldom formed their own small reef types again. Their largest area of occurrence was the Tethys . They were no longer found in areas with strong waves; they preferred calm and flat zones. At the end of the chalk they eventually died out completely.
Systematics
The systematic position of the stromal pores has not yet been fully clarified. In the past the stromatopores were assigned to the hydrozoans . A relationship with archaeocyathids and blue-green algae, the stromatolites , was also considered. Obviously, the stromatopores were severely affected by the mass extinction in the upper Devonian, because similar fossils can only be found again from the Triassic. Therefore one can basically differentiate between two groups:
- Stromatopores from the Paleozoic Era : The “typical” stromatopores
- Mesozoic forms: Also known as Sphaeractinoidea
The “typical” stromatopores (English: true stromatoporoids ) are regarded as a monophyletic group and are grouped with the sponges (class Stromatoporoidea). A strong indication for this are the astrorhizas, since similar structures were found in the genus Astrosclera (order Sclerospongea) of recent sponges. This systematic classification has been recognized by most scientists since the 1970s and 1980s.
Due to the gap between the two groups, the systematic position of the forms that reappeared from the Triassic and spread up to the Upper Cretaceous is still largely unclear. They are often called Sphaeractinoidea to separate them from the stromatopores. The structural structure of the skeletal elements is often cited as an important difference, because in Mesozoic fossils the elements are almost always made up of fibers. Due to the change in the fine structure through diagenesis, the taxonomic meaning is still controversial. It is therefore still controversial whether the Sphaeractinoidea and the “typical” palaeozoic stromatopores are related or whether they are convergent developments of the Sclerospongea and Hydroidea .
Some selected genera
Labechia : This genus is known from the Ordoviz to the Lower Carboniferous. Species of Labechia have been found in Europe, Asia, and North America and often lived on reefs. The labechia are among the original stromatopores in which horizontal skeletal elements are missing, instead the skeleton is constructed in the shape of bubbles . Astrorhizos are also not yet available. Often fossils were found in reefs, the growth form was mostly encrusting.
Stromatoporella : From the Ordovician to the Lower Carboniferous, the Stromatoporella also had a long "survival time". The Astrorhizas are already strongly developed in this genus and growth periods are clearly visible. At high magnification you can see a porous microstructure on the pillars.
Stromatopora : They occurred in the Silurian up to the Lower Carboniferous, were widespread worldwide and lived mainly in the reef area. The colonies were mostly solid and the latilaminae are very often clearly recognizable on the fossils, which gives the skeleton a layered shape. The astrorhizas are strongly developed and mostly easily visible. In this genus, the living tubes of the Syringopora occur very often . Stromatopora is the first genus of stromatopores to be discovered and was described by Georg August Goldfuß in 1826 . Before the investigation by thin section analysis was possible, the name was used for almost all stromatopore genera and similarly structured organisms such as the stromatolites. The taxon Stromatoporoidea was only introduced in 1879.
Actinostroma : From the Ordovician to the Lower Carboniferous the Actinostroma lived in reef areas and were widespread worldwide. The finds from the Mesozoic Era, which used to belong to this genus, are now generally assigned to the Actinostromaria (Jurassic to Cretaceous).
Amphipora : This already mentioned genus only occurred in the Middle to Lower Devonian . The thin branches show fine pores that are close to each other.
Stachyodes : The Stachyodes lived from the Lower Silurian to the Devonian. In contrast to the amphipora, they formed thick- branched structures and mostly lived in the front and rear of the reefs.
Burgundia : A Mesozoic genus occurring from the Upper Triassic to the Upper Cretaceous. They were only found in the Middle East, England, Italy and Germany.
literature
- Bernhard Ziegler: Introduction to Paleobiology, Part 2, Special Paleontology, Protists, Sponges and Coelenterates, Mollusks . Stuttgart: Schweizerbartsche Verlagbuchhandlung, 2004, ISBN 3-510-65036-0
- Ulrich Lehmann and Gero Hillmer: Invertebrates of prehistoric times: Guide to the systematic paleontology of invertebrates . Spektrum Akademischer Verlag, 1997, ISBN 3-432-90654-4
- Ulrich Lehmann: Paleontological Dictionary . Stuttgart: Schweizerbartsche Verlagbuchhandlung , 1996, ISBN 3-432-83574-4
- Lexicon of Geosciences . Spectrum Academic Publishing House. Heidelberg, 2002, ISBN 3-8274-0427-4
- Colin W. Stearn: Revision of the Order Stromatoporida . Palaeontology 36 (2), pp. 201–222 (1993), ISSN 0031-0239 Online (PDF file; 4.8 MB)
- Andreas May: Stratigraphy, stromatopore fauna and paleecology of coral limestone from the Upper Eifelian and Lower Givetian (Devonian) of the northwestern Sauerland (Rhenish Slate Mountains) . Geological Palaeontology Westphalia (24) 1993, ISBN 3-924590-34-6
- Carl W. Stock: Stromatoporoidea, 1926-2000 . Journal of Paleontology 75 (6); Pp. 1079-1089 (2001), ISSN 0022-3360
Web links
- Stromatoporoids (eng.)
- Fossil reefs worth visiting - Rheinisches Schiefergebirge (coral stromatopore reefs of the Devonian period) ( Memento from December 13, 2007 in the Internet Archive ), by Martin Nose, Winfried Werner and Günter Schweigert, contained in printed form in volume 24 of the Kleine Senckenberg series : Cities Under Water - Two Billion Years ( ISBN 978-3-510-61028-0 )