The oldest ammonoids of Morocco (Tafilalt, lower Emsian)
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REGULAR RESEARCH ARTICLE
The oldest ammonoids of Morocco (Tafilalt, lower Emsian) Ralph Thomas Becker1 Ahmed El Hassani4
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Christian Klug2 • Till So¨te1 • Sven Hartenfels3 • Zhor Sarah Aboussalam1
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Received: 11 December 2018 / Accepted: 18 February 2019 Akademie der Naturwissenschaften Schweiz (SCNAT) 2019
Abstract During a fieldtrip of the 10th International Cephalopod Symposium to the Tafilalt (SE Morocco), a fauna with the so far oldest ammonoid of the region was discovered at the top of the basal Emsian Deiroceras Limestone of Jebel Mech Irdane. The new material confirms the record of a single, poorly preserved specimen of the same age from Ras El Kebbar in the northwestern Tafilalt. All specimens are assigned within the Chebbitinae n. subfam. (Mimosphinctidae) to Praechebbites debaetsi n. gen. n. sp., which differs from the slightly younger Chebbites reisdorfi mostly in the lack of a dorsal lobe and dorsal imprint zone. In the conodont succession, the new species and genus falls in the top of the Eolinguipolygnathus excacatus M114 Zone, which has been proposed to define in future the base of the Emsian. Since the new taxon is morphologically already advanced, it is likely that even older ammonoids with loosely coiled conchs will be discovered in the future. A block from a Lower Carboniferous olistostrome of the adjacent Tinerhir region contained the gyroconic Ivoites n. sp. only. The global comparison of oldest ammonoid faunas shows the rarity of assemblages assigned to Lower Devonian (LD) III-B, often dated by conodonts and dacryoconarids. These faunas display a high level of endemism, which suggests that ammonoid origin and early evolution took place in regional populations with a restricted dispersal of genera and species. Keywords Ammonoids Emsian Taxonomy Stratigraphy Morocco
Introduction Ammonoids are among the fossil groups with the richest, in terms of abundance and palaeobiodiversity, and stratigraphically most refined global fossil record. Despite this, the ancestry and early evolution of the superorder is still enigmatic, mostly because of a surprisingly incomplete
knowledge of transitional forms from the ancestral bactritids. The morphological changes that occurred around the evolution from slender orthoconic, compressed Lobobactrites with ventral siphuncle and ventral lobes of septa via cyrtocones and gyrocones ultimately to fully coiled goniatites are well documented (e.g., Schindewolf 1932; Erben 1964, 1966; De Baets et al. 2012; Klug et al. 2015),
Editorial Handling: D. Fuchs. 1
Christian Klug [email protected]
Institut fu¨r Geologie und Pala¨ontologie, Westfa¨lische Wilhelms-Universita¨t, Corrensstrasse 24, 48149 Mu¨nster, Germany
2
Till So¨te [email protected]
Palaeontological Institute and Museum, University of Zurich, Karl Schmid-Strasse 4, 8006 Zurich, Switzerland
3
Sven Hartenfels [email protected]
Institut fu¨r Geologie und Mineralogie, Universita¨t zu Ko¨ln, Zu¨lpicher Strasse 49a, 50674 Cologne, German
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