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13GEOLOGICAL AND MINERALOGICAL SCIENCES
WHY WERE REEFS AND STROMATOPOROIDS SO RARE IN THE LOWER DEVONIAN?
May A.
Dr. rer. nat. in Palaeontology, Unna, Germany ORCID ID 0000-0002-6714-3925 DOI: 10.5281/zenodo.7298618
ABSTRACT
In the middle Lower Devonian, the Pragian, reefs were very rare worldwide and stromatoporoids were rare and little diverse. As an explanation for this phenomenon, it is not sufficient that the global sea level had its low for the Devonian period during the Pragian and Lower Emsian. Therefore, three stromatoporoid-bearing reefs from the Pragian of Western and Central Europe were studied: Koneprusy in the Czech Republic, Seewarte in Austria and Zujar in Southern Spain. The following possible causes for the extreme rarity of reefs in the Pragian emerged:
1) Conspicuously high or low water temperatures that were not conducive to the growth of stromatoporoid reefs.
2) Stromatoporoid groups, which were of central importance in the Givetian and Frasnian reefs, were only at the beginning of their evolution and expansion in the Pragian - particularly mentioned are the branching stromatoporoids, the thinly encrusting stromatoporoids and the order Stromatoporellida. 3) There is evidence that the Sy-ringopora commensals increased the reef-building potential of the stromatoporoids. There seems to have been a break in the Syringopora commensalism of the stromatoporoids in the uppermost Silurian or deepest Devonian. In the Pragian, Syringopora commensals were very rare and the Devonian Syringopora commensalism began with the primitive initial stage of Syringopora praehanshanensis May, 2005.
Keywords: Pragian; reef evolution; stromatoporoid fauna; Syringopora commensalism; water temperature.
1. Introduction
While reefs built up by stromatoporoids were widespread and common in the Silurian, they became rare at the end of the Silurian due to global regressions [1, p. 113-163; 2, p. 34]. In the lower and middle part of the Lower Devonian, reefs were extremely rare worldwide [3]. When reefs did occur in the lower and middle part of the Lower Devonian, they were in many cases composed of a strikingly large proportion of algal and microbial communities [4; 5]. In parallel, Lower Devonian stromatoporoids showed drastically reduced diversity and abundance worldwide [6, p. 229; 7, p. 1013; 8, p. 257-258; 9, p. 282-285]. In the upper part of the Lower Devonian, reefs and stromatoporoids increased in abundance again. In the Middle Devonian and lower Upper Devonian, reefs and biostromes of stromatoporoids and corals were widespread and common worldwide and constituted the acme of Palaeozoic reef formation in the Givetian-Frasnian [3; 10; 11; 12). This reef-building phase was abruptly ended at the end of the Frasnian by the "Late Devonian Major Ecological Crisis" (372 Ma) [2, p. 35; 13; 14, p. 4-5; 15).
May and Rodriguez [16, p. 230-231] elaborate that the roots of the Middle Devonian reefs extend into the Pragian. They list various genera of stromato-poroids, rugose corals and tabulate corals that were important reef builders in the Givetian-Frasnian and that were already present in the Pragian. It is all the more surprising that reefs and stromatoporoids were conspicuously rare in the Pragian.
To some extent, the rarity of reefs and the low diversity of the stromatoporoid fauna in the Lower Devonian can be explained by the fact that their habitats -shallow seas without much sediment input - were reduced in extent at this time, because global sea level was at its lowest for Devonian times during the Pragian and Lower Emsian [17; 18; 19; 20]. But this alone is
not enough to explain fully why reefs and stromato-poroids were so much rarer in the Lower Devonian than in the Middle Devonian. Therefore, this short communication addresses the question of whether Lower Devonian stromatoporoids and reefs were so rare due to ecological factors or whether the causes are to be sought in the evolution of reef-building organisms.
2. Investigated reefs
The author studied three stromatoporoid-bearing reefs from the Pragian of western and central Europe. A central point in each case was the specific determination of the stromatoporoid fauna, but rugose corals, tabulate corals and microfacies were also considered to varying extents. In detail, the following occurrences were studied:
• The large reef complex in the Pragium of Koneprusy, a village about 30 km southwest of Prague in the Czech Republic (coordinates: 49° 54' 40" N, 14° 04' 40" E). This reef complex has been studied scientifically for more than a hundred years. For example, the stromatoporoids were first described by Pocta [21]. By 1995, about 500 different species of organisms had been described from the Koneprusy reef complex [22, p. 26]. It is worth mentioning that in Koneprusy are also exposed shallow marine limestones of the Eifelian and Lower Givetian, which contain stromatoporoids and corals. In this way, a direct comparison of the reef-builders is possible. The results of the author's research were published by May and Ernst [23; 24; 25].
• The "Hohe Warte Formation" at Mount Seewarte in the central Carnic Alps in Austria, close to the border with Italy (coordinates: 46° 36' 31" N, 12° 52' 15" E). These limestones comprise the Pragian and extend into the lowermost Emsian. They contain reef structures built up from stromatoporoids and corals. Information on fauna and microfacies is provided by May and Pohler [26; 27; 28].
• The locality Zújar about 35 km south-easterly of Zalamea de la Serena (Southern Spain) (coordinates: 38° 29' 30" N, 1° 46' W). Outcropping are siliciclastic sediments and limestones of Lochkovian, Pragian and Famennian age. Brachiopods and conodonts prove that the limestones with corals and stromatoporoids are Pra-gian in age [29; 30]. The stromatoporoids and rugose corals are described by May and Rodríguez [16].
Mount Seewarte is about 400 km from Koneprusy and the locality Zújar is about 2000 km from Koneprusy and about 1700 km from Mount Seewarte. Nevertheless, there are faunistic similarities between Koneprusy and the other two localities. At Mount Seewarte, the stromatoporoid Plectostroma latens (Poeta, 1894) is common, which had previously only been known from the Pragium of Koneprusy. In addition, the two tabulate corals Helioplasma aff. aliena Galle, 1973 and Scoliopora (Protoscoliopora) puberulus (Janet in Dubatolov et al., 1968), which have very closely related species in the Pragium of Koneprusy, are found at Mount Seewarte [28, p. 288). At the locality Zújar, the two rugose corals Joachimastraea barrandei Galle, Hladil and May, 1999 and Rhizophyllum ex gr. bohemi-cum Poeta, 1902 as well as the tabulate coral Remesia koneprusiana Galle, Hladil and May, 1999 are found, which had previously only been known from the Pragium of Koneprusy [16, p. 230; 31, p. 84-85; 32].
3. Discussion
The observations made at these three localities lead to three possible explanations for the fact that reefs and stromatoporoids were much rarer in the Pragian than in the Givetian-Frasnian:
1)The water temperature in the Pragian was very different from that in the Givetian-Frasnian.
2)Important elements of the Middle Devonian stromatoporoid fauna were missing in the Pragian, or the evolution of the corresponding elements of the stro-matoporoid fauna had not yet progressed far enough.
3)The evolution of the Syringopora commensal-ism was necessary.
Fig. 1: The calcimicrobe Renalcis granosus Vologdin, 1932from the Pragian of Mount Seewarte (Austria).
3.1. Very different water temperature
The Pragian reefs show peculiarities in the composition of the reef-builders, which indicate that a relevant environmental factor, which cannot be read off easily from the sedimentation conditions, was different between the Pragian and the Givetian-Frasnian. When considering this environmental factor, one must first and foremost think of water temperature. These features are:
• Stromatoporoids are only subordinate reef builders in the Koneprusy reef, whereas they are the dominant reef builders in the Middle to Upper Devonian reefs.
• Incrusting bryozoans are very common in the Koneprusy reef. In the Middle to Upper Devonian reefs, however, they are extremely rare. Apparently, the incrusting bryozoans in the Koneprusy reef take over the role of the incrusting stromatoporoids.
• The solenoporaceans are very common in the Koneprusy reef, whereas they are usually extremely rare in the Middle to Upper Devonian stromatoporoid reefs. Only in rare cases [33, p. 545] solenoporaceans are important framework builders in places in Middle to Upper Devonian reefs.
• The high abundance of the calcimicrobe Renalcis granosus Vologdin, 1932 in the Koneprusy reef is remarkable. Renalcis granosus Vologdin, 1932 also occurs in the Pragian reefs at Mount Seewarte (Fig. 1). Late Devonian limestones are described from Canada with a high amount of microbial carbonates and Re-nalcis, what is interpreted as indicators of environmental change and biotic crises in carbonate systems [34].
• Similarly, algal and microbial communities dominate in the Lower Devonian reef structures of the Urals [4] and Saudi Arabia [5].
• At Mount Seewarte, the reefs are stromato-poroid-hydrozoan buildups. Here the problematic hy-drozoan Fistulella undosa Shuysky, 1973 is very common and acts as binder and encruster in the reef community (Fig. 2) [27].
Fig. 2: The problematic hydrozoan Fistulella undosa Shuysky, 1973 from the Pragian of Mount Seewarte
(Austria).
Now, the question is whether the water temperature in the Pragian was lower or higher than in the Give-tian-Frasnian. Arguments for lower water temperatures are:
• Global sea level had its low for the Devonian period during the Pragian and Lower Emsian [17; 18; 19; 20]. This resulted in a maximum size of the dry land fraction. This in turn led, on the one hand, to a particularly large albedo of the Earth [13, p. 43; 35; 36, p. 58] and, on the other hand, to particularly large climate contrasts on the Earth. Accordingly, Boucot [37] and May [13] assume that in the Lower Devonian the climate worldwide was significantly cooler and the temperature gradient from the equator to the poles considerably greater than in the Givetian-Frasnian.
• Hladikova et al. [38, p. 239-240; 39] found that the delta-18-O values of the Pragian, Emsian and Lower Eifelian micritic limestones of Bohemia were strikingly high, but had decreased in the Upper Eifelian to values normal for Devonian carbonates. They explained this phenomenon by hypothetical barriers that hindered water exchange between Bohemia and the open ocean during this time [38]. However, this high delta-18-O values can also be explained very well by a lower water temperature [39; Hladil, oral comm.].
In contrast, modern investigations speak for strongly increased water temperatures:
• By analysing oxygen isotopes in apatite, Joachimski et al. [40] come to the conclusion that temperatures were high in the Lochkovian and Pragian, as high as they were again from the upper Frasnian onwards.
• Based on investigations in the Spanish Central Pyrenees, Slavik et al. [41] state that the essential parts of the Pragian were still a "hot and humid" period, even with the strong differences from the possibly "extremely hot" Lochkovian.
3.2. Differences in the stromatoporoid fauna
The second hypothesis is that the stromatoporoids, as the most important reef-building group of the Middle Palaeozoic reef era [11], had a low point in their evolutionary history during the Pragian and at this time the reef-building ability of the group was limited. This is supported by the following observations:
• Both in the large reef complex in the Pragian of Koneprusy and at Mount Seewarte in the Pragian, branching stromatoporoids are completely absent. Only at locality Zujar were sporadic Amphipora sp. Globally, throughout the Lower Devonian, branching stromato-poroids were rare and poorly diverse. However, they played a central role in Middle to Upper Devonian reef complexes [42; 43; 44; 45; 46; 47]. In Bohemia, this group is already found in the Acanthopyge limestone of the Eifelian with the two most important genera (Am-phipora and Stachyodes) [24, p. 129-130].
Fig. 3: The thinly incrusting stromatoporoid Syringostromella columnaris (Pocta, 1894) from the Pragian of
Koneprusy (Czech Republic) (vertical thin section).
• Thinly encrusting stromatoporoids - typically species of the genus Clathrocoilona - were very common in Middle to Upper Devonian reef complexes [43; 44; 47; 48; 49]. In contrast, thinly encrusting stromato-poroids were very rare in the Pragian: At the locality Zujar and at Mount Seewarte they are completely absent and in Koneprusy they are very rare.
The only thinly incrusting stromatoporoid in the Pragian of Koneprusy is Syringostromella columnaris (Pocta, 1894) (Fig. 3). However, S. columnaris was the only Syringostromella species to develop a thin-layered incrusting growth form; even S. zintchenkovi (Khalfina, 1960), which occurs from the Lochkovian to the Emsian and probably differs from S. columnaris only on a subspecific level, does not have a thin-layered in-crusting growth form [24, p. 200]. Instead, incrusting bryozoans were very important in the Pragian Koneprusy reef complex.
Apparently, the great rarity of thinly encrusting stromatoporoids in the Pragian is related to the fact that no stromatoporoid group was well adapted to this ecological niche. Species of the genus Clathrocoilona, which were well adapted to this ecological niche and represented the most important thinly encrusting stro-matoporoids of the Middle to Upper Devonian reefs, appeared only later - in Bohemia the first records of Clathrocoilona are from the middle Eifelian [24, p. 182].
• In the Pragian of Koneprusy, species of the order Stromatoporellida were rare and exclusively restricted to the reef core. The rarity of Stromatoporellida in the Koneprusy reef is not a phenomenon restricted to Bohemia. No Stromatoporellida at all are known from Mount Seewarte. At the locality Zujar, a total of two colonies of Stromatoporellida were found, which could
be identified as Stictostroma gorriense Stearn, 1995 and Stictostroma nunavutense Prosh and Stearn, 1996. Both species had their oldest occurrence to date in the Lower Emsian of Arctic Canada [50]. The relevant literature [7; 8; 9; 51; 52] consistently shows that the Stro-matoporellida in the Pragian were only at the beginning of their evolution. This is reflected in both the diversity and ecological range of habitats colonised by Stroma-toporellida. The increase in abundance, diversity and conquest of habitats went hand in hand for the Stroma-toporellida and enabled their flourishing in the Middle to Upper Devonian. In the Middle Devonian and Fras-nian, the Stromatoporellida in particular were also common and widespread outside the reef cores [24, p. 129130; 42; 43; 47; 48; 53].
• The four most common stromatoporoids of the central reef area of Koneprusy are endemic species: Parallelopora florida (Pocta, 1894) represents 30% of the stromatoporoids of the central reef area of Koneprusy, Plectostroma latens (Pocta, 1894) represents 16% and the above-mentioned Syringostromella columnaris (Pocta, 1894) and Schistodictyon neglectum (Pocta 1894) each represent 8.6% of the stromato-poroids of the central reef area of Koneprusy. All four species have, on the one hand, a very pronounced maximum abundance in the reef core area and, on the other hand, are not known from anywhere else in the world. There is only one exception: Plectostroma latens (Pocta, 1894) was also found on Mount Seewarte, where it is the most common stromatoporoid. This suggests that these four species were specifically adapted to the reef complex of Koneprusy. Apparently, the stro-matoporoid fauna of the Pragian lacked taxa specifically adapted to reefs in general, so that in the Koneprusy reef complex these endemic species
evolved from widespread taxa. It is noteworthy that the bryozoan fauna of the Koneprusy reef also contains numerous endemic species [25].
3.3. Evolution of the Syringopora commensalism More or less frequently, commensal organisms are found in stromatoporoids: syringoporid tabulate corals,
rugose corals or worm-like organisms. Some of them are probably true symbioses. An overview is given by Kershaw et al. [54].
In this respect, the stromatoporoids from the Pra-gian that I studied are clearly different from Middle and Upper Devonian stromatoporoids.
Fig.4: The rugose coral Fasciphyllum as a commensal within the stromatoporoid Plectostroma latens (Pocta,
1894) from the Pragian of Mount Seewarte (Austria).
Fig. 5: Tubes of the worm-like commensal Helicosalpinx in the stromatoporoid Plectostroma altum (Ripper, 1933) from the Pragian of locality Zujar (Southern Spain) (vertical thin section).
At Mount Seewarte, the only commensal organism found in the stromatoporoids was the rugose coral Fasciphyllum (Fig. 4). At locality Zújar, the stromato-poroid Plectostroma altum (Ripper, 1933) occasionally contains the worm-like commensal Helicosalpinx (Fig. 5). In addition, the rugose coral Loyolophyllum is found as a commensal in stromatoporoids at locality Zújar. Neither at Mount Seewarte nor at locality Zújar syrin-goporid tabulate corals were found in the stromato-poroids.
30% of the stromatoporoid colonies found in the Pragian of Koneprusy show tubes of worm-like commensal organisms. But only 8% of the stromatoporoid colonies found in the Pragian of Koneprusy contain sy-ringoporid tabulate commensals. May [24, p. 224-225] described these syringoporid tabulates under the name Syringopora praehanshanensis May, 2005. S. prae-hanshanensis has great similarities to the commensal Syringopora species widely distributed in the Middle Devonian. In the Middle Devonian Syringopora species, formerly called "Caunopora", the tubular coral-lites run more or less long distances parallel to the vertical skeletal elements of the stromatoporoid [24, p. 223-226; 55, p. 204-210]. But in S. praehanshanensis May, 2005 from the Pragian of Koneprusy, the tubular corallites do not run long distances parallel to the vertical skeletal elements of the stromatoporoid. Moreover, in S. praehanshanensis the colonies of the commensal Syringopora are quite small, and it can be seen that the Syringopora grew together with the stromatoporoid for only a relatively short time. Therefore, it is assumed that the Syringopora commensalism observed in the Pragian of Koneprusy is the primitive initial stage of the Middle Devonian "Caunopora" commensalism [24, p. 222].
In the stromatoporoids of the Acanthopyge limestone of the Eifelian and Lower Givetian of Koneprusy, commensalism with Syringopora is very widespread. 32 % of the bulbous to layered stromatoporoids of the Acanthopyge limestone contain Syringopora commensals. The Syringopora species here always have the typical character of "Caunopora" tubes and their tubular corallites run long distances parallel to the vertical skeletal elements of the stromatoporoid. In the stromato-poroids of the Acanthopyge limestone of the Eifelian and Lower Givetian there are no tubes of worm-like commensal organisms, but occasionally the rugose coral Fasciphyllum is found as a commensal. This picture is typical for the Middle Devonian: In the stroma-toporoids, Syringopora commensals are frequently found, occasionally commensal rugose corals and only rarely tubes of worm-like commensal organisms [55; 56].
In connection with the Devonian Syringopora commensalism, it must be mentioned that commensal Syringopora species were already present in stromato-poroids during the Silurian. These commensal Syrin-gopora species are common in the Middle and Upper Silurian and have the typical character of "Caunopora " tubes, because their tubular corallites run long distances parallel to the vertical skeletal elements of the stroma-toporoid. Illustrations and descriptions can be found in
various publications [54, p. 69-73; 57, p. 52-54; 58, p. 102-104; 59; 60; 61; 62].
Thus, it can be seen that in the Silurian and Devonian periods with frequent stromatoporoid reefs, Syrin-gopora commensalism was widespread in the stroma-toporoids and the Syringopora had the typical character of "Caunopora" tubes, as their tubular corallites run long distances parallel to the vertical skeletal elements of the stromatoporoids. In the Pragian, the reverse is true: stromatoporoid reefs were very rare, Syringopora commensalism was very rare, and Syringopora commensals grew together with the stromatoporoid for only a relatively short time. This evidence suggests that the Syringopora commensals were beneficial to the stro-matoporoid - that it was therefore a true symbiosis -and that the Syringopora commensals increased the reef-building potential of the stromatoporoid. Vinn [63, p. 147] assumes that calcareous rigid skeletons of sy-ringoporids may have reinforced skeletons of stromato-poroids.
I suspect that the Devonian Syringopora commensalism is not the direct successor of the Silurian Syrin-gopora commensalism, but that the Silurian Syrin-gopora commensal species disappeared in the uppermost Silurian or deepest Devonian. Then, in the Pragian, another Syringopora species began to reinvent commensalism with stromatoporoids and evolved into Syringopora praehanshanensis May, 2005. Syrin-gopora praehanshanensis May, 2005 from the Koneprusy Limestone bears very close resemblance to Syringopora hanshanensis Chow, 1980, from which it differs only in details of growth habit. However, S. hanshanensis is the first Syringopora species which shows the typical Middle Devonian "Caunopora" com-mensalism. Moreover, S. hanshanensis is the most common Syringopora species in the Eifelian, and the other Devonian commensal Syringopora species can be derived from it, as shown by the studies carried out by May [24, p. 221-226]. The step from the primitive initial stage of Syringopora commensalism in S. prae-hanshanensis May, 2005 to typical "Caunopora " commensalism must have occurred at about the time of the Pragian/Emsian boundary; because the holotype of S. hanshanensis showing typical "Caunopora " commensalism dates from the Lower Emsian [64, p. 131].
4. Conclusions
There are several possible reasons for the extreme rarity of reefs in the Pragian. One is water temperatures that were not conducive to the growth of stromato-poroid reefs. Another important reason was that stro-matoporoid groups, which were of central importance in the Givetian and Frasnian reefs, were only at the beginning of their evolution and expansion in the Pragian. And finally, to all appearances, there was a break in the Syringopora commensalism of the stromatoporoids in the uppermost Silurian or deepest Devonian. In the Pra-gian, Devonian Syringopora commensalism began again with the primitive initial stage of Syringopora praehanshanensis May, 2005. Which of these factors played which role cannot be judged at present. Presumably, all factors acted together. In order to better understand the interrelationships, it will be necessary to extend these investigations to other Pragian reefs and to
include reefs of the uppermost Silurian and deepest Devonian.
Acknowledgements
The Deutsche Forschungsgemeinschaft supported the research on the Koneprusy reef complex through a grant (reference: Ma 1427/3-1). The research on the locality Zújar was supported by the research project BTE2003-02065 "Bioconstrucciones del Devónico del Dominio Obejo-Valsequillo y del Carbonífero del área del Guadiato" of the Ministerio de Educación y Ciencia and GR-UCM/910231 "Estratigrafía y paleontología del Precámbrico y Paleozoico perigondwánico" of the Universidad Complutense de Madrid.
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