The oldest ammonoids of Morocco (Tafilalt, lower Emsian)

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.

Morphological disparity trends in Devonian trilobites from North Africa

“…However, this hypothesis seems unlikely because some potential predators, such as ammonoids and chondrichtyans, appeared only from the Emsian (Becker et al . 2019) or developed in the end of the Devonian, like fishes (Derycke 2017). In addition, the morphologies with spines disappeared in the Middle Devonian and Frasnian, in contrast to other shapes, which still existed in the Famennian (Fig.…”

Special issue: Cephalopods through time

“…Following the tradition of the Cephalopods-Present and Past-symposia, this volume contains also several papers on ammonoids. Becker et al (2019) present new record of the oldest ammonoids from Morocco, possibly even from the whole of Gondwana. Klug and Vallon (2019) interpret organic structures from the Famennian Hangenberg Black Shales as regurgitalites of predators that fed on ammonoids and regurgitated the shell fragments.…”

“…On Saturday, we began with the stratigraphically important locality Jebel Mech Irdane, where we visited the Eifelian/Givetian boundary GSSP. Before, however, we looked at early Emsian strata on the quest for the possibly oldest ammonoids of Gondwana and succeeded by finding reasonably well-preserved specimens, which are described in this volume (Klug 2001;Becker et al 2019). In the afternoon, we visited the adjacent synclines of Jebel Ihrs and Jebel Mdouar, where strongly condensed cephalopod limestones of Eifelian to Frasnian age are exposed.…”

Is the relative thickness of ammonoid septa influenced by ocean acidification, phylogenetic relationships and palaeogeographic position?

“…They are widely used index fossils and usually record their ontogeny in their aragonitic shell. They also represent excellent model organisms to examine the effect of oceanic environmental change from their origin in the Early Devonian (Becker et al, 2019 , 2020 ; Klug et al, 2015 ) to their demise at the end of the Cretaceous (De Baets et al, 2016 ; Landman et al, 2015 ; Tajika et al, 2018 , 2020 ). During this interval, ammonoids experienced dramatic diversity-fluctuations due to several major extinction events (end-Devonian, end-Permian and end-Triassic; Arkhipkin & Laptikhovsky, 2012 ; Veron, 2008 ) as well as numerous smaller scale extinctions (Becker et al, 2020 ; Korn et al, 2015 ).…”