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Origin of the Konservatlagerstätten of the southern Maïder (Morocco) and gnathostome preservation


Klug, Christian; Frey, Linda; Pohle, Alexander; Rücklin, Martin (2017). Origin of the Konservatlagerstätten of the southern Maïder (Morocco) and gnathostome preservation. In: 14th International Symposium on Early and Lower Vertebrates, Chęciny, Poland, 3 July 2017 - 7 July 2017, 52.

Abstract

In the Famennian of the eastern Anti-Atlas, microremains of gnathostomes are quite common in some strata due to condensed sedimentation, particularly in the Tafilalt. In the latter region, chondrichthyan diversity can be reasonably high (up to nine species in one layer). By contrast, in the Famennian of the southern Maïder Basin, chondrichthyan diversity appears to be lower (four to five species) and genera such as Clairina, Jalodus and Protacrodus have not been found yet although they are documented from the neighboring Tafilalt Basin (Ginter et al. 2002; Derycke et al. 2008). We address the questions for the ecological factors controlling these differences in diversity and fossil preservation. The latter question is of interest because only in the Maïder Basin, chondrichthyans have been discovered preserving cartilaginous body parts as well as soft tissues. Preservation has been examined by analyzing the mineral composition of various Famennian fossils from the Maïder and the Tafilalt using XRD (X-ray Diffraction) at the Swiss Federal Institute of Technology in Zurich (ETHZ). The results show that indeed the chondrichthyan musculature is now preserved in hematite and other ferric minerals. Both placoderm bones and chondrichthyan cartilage are preserved in hydroxylapatite, fluorapatite or francolite (phosphates). Particularly the abundance of ferric oxides and hydroxides points at pyrite, which was altered due to deep weathering in the desert environment. This is corroborated by rare finds of pyritized fossils from the same strata in depths of over 10 m below today’s surface. In turn, this primary abundance of pyrite (now ferric oxides and hydroxides) in combination with the clayey facies and the scarcity of benthos in some strata suggests that the sediments containing exceptionally preserved gnathostomes were deposited under oxygenpoor conditions (Klug et al. 2016). This is supported by the palaeogeographical situation of the Maïder Basin that was closed to the south, west and north by land, while to the east, the shallower regions of the Tafilalt Pelagic Ridge limited water exchange (Wendt 1988; Kaufmann 1998). Hypoxic to anoxic conditions ultimately explain the absence of the protacrodontids, which mainly occur in shallower, better oxygenated waters (Ginter 2000). Clairina and Jalodus on the other hand likely preferred deeper environments than the one in the Maïder Basin. Following Ginter (2000), the taxa present in the Maïder (Phoebodus and cladodonts) point at an intermediate water depth.

Abstract

In the Famennian of the eastern Anti-Atlas, microremains of gnathostomes are quite common in some strata due to condensed sedimentation, particularly in the Tafilalt. In the latter region, chondrichthyan diversity can be reasonably high (up to nine species in one layer). By contrast, in the Famennian of the southern Maïder Basin, chondrichthyan diversity appears to be lower (four to five species) and genera such as Clairina, Jalodus and Protacrodus have not been found yet although they are documented from the neighboring Tafilalt Basin (Ginter et al. 2002; Derycke et al. 2008). We address the questions for the ecological factors controlling these differences in diversity and fossil preservation. The latter question is of interest because only in the Maïder Basin, chondrichthyans have been discovered preserving cartilaginous body parts as well as soft tissues. Preservation has been examined by analyzing the mineral composition of various Famennian fossils from the Maïder and the Tafilalt using XRD (X-ray Diffraction) at the Swiss Federal Institute of Technology in Zurich (ETHZ). The results show that indeed the chondrichthyan musculature is now preserved in hematite and other ferric minerals. Both placoderm bones and chondrichthyan cartilage are preserved in hydroxylapatite, fluorapatite or francolite (phosphates). Particularly the abundance of ferric oxides and hydroxides points at pyrite, which was altered due to deep weathering in the desert environment. This is corroborated by rare finds of pyritized fossils from the same strata in depths of over 10 m below today’s surface. In turn, this primary abundance of pyrite (now ferric oxides and hydroxides) in combination with the clayey facies and the scarcity of benthos in some strata suggests that the sediments containing exceptionally preserved gnathostomes were deposited under oxygenpoor conditions (Klug et al. 2016). This is supported by the palaeogeographical situation of the Maïder Basin that was closed to the south, west and north by land, while to the east, the shallower regions of the Tafilalt Pelagic Ridge limited water exchange (Wendt 1988; Kaufmann 1998). Hypoxic to anoxic conditions ultimately explain the absence of the protacrodontids, which mainly occur in shallower, better oxygenated waters (Ginter 2000). Clairina and Jalodus on the other hand likely preferred deeper environments than the one in the Maïder Basin. Following Ginter (2000), the taxa present in the Maïder (Phoebodus and cladodonts) point at an intermediate water depth.

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Additional indexing

Item Type:Conference or Workshop Item (Paper), refereed, original work
Communities & Collections:07 Faculty of Science > Paleontological Institute and Museum
Dewey Decimal Classification:560 Fossils & prehistoric life
Language:English
Event End Date:7 July 2017
Deposited On:14 Jul 2017 07:34
Last Modified:14 Jul 2017 18:30
Publisher:University of Warsaw
Series Name:Ichthyolith Issues
ISSN:1302-1314

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