Ecological change during the early Emsian (Devonian) in the Tafilalt (Morocco), the origin of the Ammonoidea, and the first African pyrgocystid edrioasteroids, machaerids and phyllocarids

Christian Klug, Bjoern Kroeger, Dieter Korn, Martin Ruecklin, Mena Schemm-Gregory, Kenneth De Baets, Royal H. Mapes

Research output: Contribution to journalReview article (Academic Journal)peer-review

Abstract

Early Emsian claystones and marls of the Tafilalt yielded two diverse and prolific faunas with nearly 5000 specimens belonging to at least 100 species being recovered and identified. The older of the two faunas contains what may be the oldest bactritids, pyrgocystid edrioasteroids, phyllocarid carapaces, complete asteropygid trilobites, acanthodian fin spines rarely preserved as pairs, and articulated machaeridians. Additionally, the lower interval yielded a diverse and largely infaunal bivalve assemblage. The younger fauna is marked by the appearance of the first ammonoids, which are represented by the genera Chebbites, Erbenoceras, Gracilites, Gyroceratites, Irdanites, Lenzites, and Metabactrites. These are accompanied by other cephalopods such as bactritoids, predominantly epibyssate bivalves, and gastropods. Based on the differences in the identified faunal elements of the two assemblages, the preservation of the fossils and the lithology from which the fossils were recovered, it can be concluded that the palaeoenvironment had more or less normal oxic conditions and a moderate water depth within the deeper part of the photic zone being below storm wave base. The high diversity of the infaunal benthonic community and the sediment itself indicate a soft bottom environment. The change in faunal composition between the two faunas strongly suggests that the palaeoenvironment of the younger fauna had decreased oxygen content in the deepest part of the water column and the sediment. It is possible that the environmental stresses created by the oxygen deprivation in the deeper part of the water column facilitated regionally the early radiation of bactritoids and ammonoids during the Emsian. Globally, the rising sea-level and the radiation of the Gnathostomata certainly played an important role in the early bactritoid and ammonoid evolution.

The majority of the macrofossils from the two faunas are systematically described; the following new taxa are introduced: the bivalve Eonuculoma babini n. gen. et sp., the orthoceratids Chebbioceras erfoudense n. gen. et sp., Infundibuloceras brevimira n. gen. et sp. and Plagiostomoceras hassichebbiense n. sp., the bactritoid Cyrtobactrites scheffoldi n. sp., the ammonoid Metabactrites ernsti n. sp., and the machaerid Aulakolepos rugatus n. sp., the phyllocarids Nahecaris jannae n. sp.and N. malvinae n. sp. as well as the edrioasteroid Rhenopyrgus flos n. sp. The latter taxa represent the first records of the families Pyrgocystidae, Lepidocoleidae and of phyllocarid carapaces from Africa.

Original languageEnglish
Pages (from-to)83-U58
Number of pages127
JournalPalaeontographica abteilung a-Palaozoologie-Stratigraphie
Volume283
Issue number4-6
Publication statusPublished - Apr 2008

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