Taphonomy and Palaeoecology of the Green Devonian Gypidulid Brachiopods from the Aferdou El Mrakib, Eastern Anti-Atlas, Morocco
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Swiss J Palaeontol (2013) 132:23–44 DOI 10.1007/s13358-012-0050-y Taphonomy and palaeoecology of the green Devonian gypidulid brachiopods from the Aferdou El Mrakib, eastern Anti-Atlas, Morocco Lorena Tessitore • Mena Schemm-Gregory • Dieter Korn • Ferdinand R. W. P. Wild • Carole Naglik • Christian Klug Received: 27 August 2012 / Accepted: 11 December 2012 / Published online: 12 February 2013 Ó Akademie der Naturwissenschaften Schweiz (SCNAT) 2013 Abstract On Aferdou El Mrakib, a large reef mound in thickest parts of the shells appear green and thin-shelled the Maı¨der region (Anti-Atlas, Morocco), thick-shelled forms never display the green colour. There is also some gypidulids of two genera are locally very abundant. Like indication that the Fe content increases towards deeper shell Stringocephalus in the shallow water limestone formations layers (further away from the surface). In addition, we in Germany, these Moroccan brachiopods of the genera examined the quality and spatial distribution of sublethal Devonogypa and Ivdelinia often display greenish shells. By injuries in over 200 specimens of Devonogypa and Ivdelinia. analysing these shells by EDX, it turned out that the colour Shape, spatial distribution on the shells, and abundance of was possibly caused by impurities of Fe2?-ions. The con- the sublethal injuries support the hypotheses that (1) the centration varies, indicating that the colour is less dependent injuries had several causes, (2) some of these were inflicted on the concentration than on shell thickness, because only the by predators, probably cephalopods, and (3) many fractures and deformations might have been caused by the brachiopod Electronic supplementary material The online version of this shells hitting each other in dense populations in agitated article (doi:10.1007/s13358-012-0050-y) contains supplementary water. The existence of dense clusters, built by the associa- material, which is available to authorized users. tion of members of both genera or of only one taxon, is L. Tessitore (&) Á C. Naglik Á C. Klug corroborated by the patchy occurrence of these brachiopods. Palaeontological Institute and Museum, University of Zurich, Karl Schmid-Strasse 4, 8006 Zurich, Switzerland Keywords Brachiopods Á Taphonomy Á Palaeoecology Á e-mail: [email protected] Reef-mounds Á Devonian Á Morocco C. Naglik e-mail: [email protected] C. Klug Introduction e-mail: [email protected] M. Schemm-Gregory The Devonian succession of sedimentary rocks in Morocco Centro de Geocieˆncias da Universidade de Coimbra, Largo has become famous in the past century mainly for its Marqueˆs de Pombal, 3000-272 Coimbra, Portugal richness in fossils but also for its remarkably well-exposed e-mail: [email protected]; [email protected] carbonate build-ups (e.g., Roch 1934; Massa et al. 1965; Hollard 1974;To¨nebo¨hn 1991; Brachert et al. 1992; Wendt D. Korn 1993; Belka 1994, 1998; Kaufmann 1997, 1998; Kaufmann Museum fu¨r Naturkunde, Leibniz-Institut fu¨r Evolutions- und et al. 1999; Bultynck and Walliser 2000; Aitken et al. 2002; Biodiversita¨tsforschung, Humboldt-Universita¨t, Invalidenstraße 43, 10115 Berlin, Germany Cavalazzi et al. 2007; Berkowski and Klug 2012). One of e-mail: [email protected] these, Aferdou El Mrakib, is a conspicuous structure in the Maı¨der region in the eastern Anti-Atlas, about 70 km SW F. R. W. P. Wild of Rissani (Fig. 1). It is a reef mound of early Givetian age, Anorganisch-chemisches Institut, University of Zurich, Winterthurerstrasse 190, 5087 Zurich, Switzerland which measures ca. 900 m in diameter and ca. 100–130 m e-mail: [email protected] in elevation above the underlying strata and hence is the 24 L. Tessitore et al. largest carbonate build-up in the Maı¨der Basin (Wendt This reef mound contains the most diverse fauna among 1993; Kaufmann 1996, 1997, 1998). Its geometry is almost the carbonate build-ups in the Maı¨der (Kaufmann 1998). In circular in outline with a truncated cone-shape (Kaufmann addition to the usual Devonian reef fauna, the two large 1997, 1998). gypidulid brachiopods Ivdelinia pulchra Franchi et al. Fig. 1 Map showing the localisation of the reef mound Aferdou El Mrakib and other carbonate build-ups in the Maı¨der (modified after Klug 2002) Taphonomy and palaeoecology of the green Devonian gypidulid brachiopods 25 (2012) and Devonogypa sp. are particularly abundant in the In order to draw conclusions on the taphonomy of these sediments of this mountain (Fig. 2). Their greenish shell two gypidulids, we also examined the thick-shelled bra- colour and patchy mass occurrences have already attracted chiopod Stringocephalus from Germany, which apparently the attention of some palaeontologists (e.g., Franchi et al. underwent a similar kind of taphonomy (as reflected in 2012; Z. Belka, Posnan, and A. T. Halamski, Warszawa are local occurrence of greenish shells) and which had habitat currently preparing comprehensive papers on Aferdou El preferences similar to those of Ivdelinia and Devonogypa, Mrakib and its brachiopod fauna). i.e. shallow, agitated water and a reef or reef-like Although mentioned by Kaufmann (1998), the mass environment. occurrences of the two rather large gypidulids Ivdelinia Institutional abbreviation—PIMUZ, Pala¨ontologisches pulchra (40 9 45 mm) and Devonogypa sp. (70 9 77 mm) Institut und Museum, University of Zurich, Switzerland had not been studied in detail until recently. In a first study, (PIMUZ 30018–30213, PIMUZ 30214–30218). Franchi et al. (2012) described a new species of Ivdelinia based on material from the Aferdou El Mrakib. In addition, the entire brachiopod fauna of this reef mound is still a Methods present work subject of different authors. It will also be the subject of a project, which has much broader scope, dealing Geochemical analyses of the shells of Ivdelinia pulchra with several sedimentological and palaeontological aspects and Devonogypa sp. from Aferdou El Mrakib (Morocco) of this prominent geological feature. Nevertheless, the and of Stringocephalus sp. from Warstein (Germany) were peculiar taphonomy of these thick-shelled brachiopods, performed (Tables 1, 2, online material) with a Jeol JSM- especially the origin of the green colour of their shells, has 6060 scanning electron microscope (SEM) equipped with not been assessed before (compare Klug et al. 2009). In Bruker Quantax EDX detector, having an energy resolution addition, palaeoecological aspects of these mass occur- of 126 eV in the Institute of Inorganic Chemistry of the rences of gypidulids were also never examined in detail. In University of Zurich. Element concentrations were deter- this study, we discuss the injuries that left traces in most of mined with the help of the Bruker Esprit 2.1 software. The the shells we have collected. Sublethal injuries and other principles of the ‘‘standard free’’ determination of elements shell deformities in brachiopods have repeatedly been by EDX are described in Eggert (2005). For preservation of described by various authors (some Palaeozoic examples: the samples, these were not coated but wrapped in alu- Brunton 1966; Elliott and Bounds 1987; Elliott and Brew minium foil exhibiting small windows for the measure- 1988; Bordeaux and Brett 1990; Brett and Walker 2002; ments. In addition, braided copper wires grounded the Balinski 1993; Balinski and Biernat 2003). As in molluscs, samples, preventing static electricity build-up. The con- it is difficult to find evidence for the cause of these injuries, centration of the elements Cu and Al may be biased by the which are also discussed. fact that they were used as foils wrapped around the Therefore, the aims of this study were (1) to examine the samples to make them sufficiently conductive for the SEM preservation of these large, thick-shelled gypidulid bra- analysis. The findings for both elements are not discussed chiopods, and (2) to document and evaluate, both statistically for this reason. and qualitatively, the sublethal injuries found in these bra- Many of the brachiopod shells display injuries. chiopods. These results might help to answer the question Remarkably, the majority of injuries were found on the whether their mass occurrences are caused by taphonomic ventral valve (an exception is visible in Fig. 4: 1d, e, on shell accumulations or whether they represent more or less the right). We think that this might have been caused autochthonous accumulations of the two species. partially by the fact that the ventral valve has a much larger surface area and by the fact that the dorsal valve pointed more or less upward syn vivo and was thus more protected. Material In order to enhance contrasts and make the injuries better visible, the ventral valves of all specimens were first The material contains specimens belonging mainly to the whitened by NH4Cl and then photographed approximately two brachiopods Ivdelinia pulchra (Fig. 3) and Devono- in the same position. Second, these images and the actual gypa sp. (Fig. 4), which attained large sizes on the reef specimens were examined for healed injuries. Third, the mound Aferdou El Mrakib. In addition, the accompanying angular distance of all injuries from the plane of symmetry fauna was sampled, which is present throughout the mound of the specimens was measured (Fig. 5; Table 2). We and which contains various species of other brachiopods, measured the angle with a thin plastic foil, which displays trilobites (in association with the gypidulids, phacopids and lines arranged radially, separated by an angle of 10° each. scutellids are the most common groups), crinoids, gastro- This sheet was wrapped around the injured specimens to pods, corals, stromatopores, bryozoans, and cephalopods. measure the angular distance of the injuries from the plane 26 L. Tessitore et al. Taphonomy and palaeoecology of the green Devonian gypidulid brachiopods 27 b Fig. 2 Gypidulid brachiopod-occurrences on Aferdou El Mrakib, There are two possible biases in this analysis: (1) some Maı¨der, Morocco.