Elsevier

Cretaceous Research

Volume 118, February 2021, 104661
Cretaceous Research

The ammonites of the Les Ferres Aptian Basin (Lower Cretaceous, Southeast of France): the genus Toxoceratoides (Ancyloceratina, Helicancylidae)

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Abstract

Toxoceratoides (Ancyloceratina, Helicancylidae) is a lower Aptian (Lower Cretaceous) small heteromorph ammonoid mainly known by small samples of fragmentary specimens. In the present work, we study a sample of 145 specimens of this genus from the Deshayesites forbesi, Deshayesites deshayesi and Dufrenoyia furcata zones (lower Aptian) of the vicinity of the village of Les Ferres (southeastern France). The taxa recognized (in stratigraphic order) are: Toxoceratoides sp., Toxoceratoides royerianus (d’Orbigny, 1842) and Toxoceratoides rochi Casey, 1961. Their ontogenetic sequence is described; the intraspecific variability of Toxoceratoides rochi is considerable and it concerns adult size, coiling, duration of the ontogenetic stages and ornamentation. No sexual dimorphism is recognized. The main evolutionary pattern of the genus consists in the replacement of primitive ribs (here named ‘royerianus ribs’) by derived ribs (‘rochi ribs’) on the flexus and retroversum. The genus Toxoceratoides could be used as a biostratigraphic tool.

Introduction

The Les Ferres Aptian Basin (LFAB) is a small subsident basin nowadays located in the vicinity of the village of Les Ferres, in the Estéron valley (Alpes-Maritimes department, southeastern France, Casey, 1961b, Casey, 1961a, Fig. 1). The location of the outcrops studied and the lower Aptian litho- and biostratigraphy of the LFAB have been previously described in Bersac and Bert (2019). The LFAB is characterized by relatively thick lower Aptian deposits particularly rich in macrofossils, especially ammonites (fossil cephalopods, Bersac and Bert, 2019). The age of these series extends from the Deshayesites forbesi Zone (in the sense of Bersac et al., 2012) to the late Dufrenoyia furcata Zone (Dufrenoyia dufrenoyi Subzone, Fig. 2). Its ammonite fauna is remarkable by the significant proportion of heteromorphs of the families Ancyloceratidae Meek, 1876 and Helicancylidae Hyatt, 1894 (=Acrioceratidae Vermeulen, 2004). In the LFAB, the lower Aptian Helicancylidae are only represented by the genus Toxoceratoides Spath, 1924 (Bersac and Bert, 2019), which is a small tripartite shelled heteromorph genus (the shell of Toxoceratoides is composed of a proversum, a flexus and a retroversum, Casey, 1961a; Aguirre-Urreta, 1986; Avram, 2002; Bulot et al., 2018, Fig. 3). The aim of the present work is to describe and figure the Toxoceratoides of the LFAB, based on a sample of 145 specimens from the D. forbesi-D. furcata zones interval.

Section snippets

Material

The litho- and biostratigraphic framework based on ammonites of the LFAB and the sections from which the studied material originates are detailed in Bersac and Bert (2019).

The material studied is composed of 145 specimens of Toxoceratoides from 6 sections of the LFAB. The specimens originate from: (1) the top of the Hauterivian-Aptian limestones (Deshayesites forbesi Zone, 1 specimen), (2) the Ammonitoceras Level of the Pont de la Cerise Member (Deshayesites multicostatus Subzone of the

Descriptive paleontology

  • Order Ammonoidea Zittel, 1884

  • Suborder Ancyloceratina Wiedmann, 1966

  • Superfamily Ancyloceratoidea Gill, 1871

  • Family Helicancylidae Hyatt, 1894

  • Taxonomic considerations about the family-group name Helicancylidae. The opinions of recent authors differ on the suprageneric classification of Toxoceratoides: it could belong to the family Helicancylidae (Bert, 2009), to the family Acrioceratidae (Vermeulen, 2004; Bulot in Vincent et al., 2010; Frau et al., 2017) or to the subfamily Helicancylinae of the

Conclusions

Toxoceratoides sp., T. royerianus and T. rochi have been recognized within our sample of Toxoceratoides from the D. forbesi Zone - D. furcata Zone interval of the LFAB.

The large size of our sample of Toxoceratoides rochi (133 specimens) allowed highlighting the unprecedented range of morphological disparity of this species. This disparity is so significant that it encompasses the morphology of several species of Toxoceratoides and even of some genera of Helicancylidae (Tonohamites, Helicancylus

CRediT authorship contribution statement

Stéphane Bersac: Conceptualization, Methodology, Acquisition of data, Analysis and/or interpretation of data, Writing - original draft, Writing - review & editing, Approval of the version of the manuscript to be published. Didier Bert: Conceptualization, Methodology, Acquisition of data, Analysis and/or interpretation of data, Writing - original draft, Writing - review & editing, Approval of the version of the manuscript to be published.

Acknowledgements

We warmly thank the Editor-in-Chief, the Associate Editor, Yves Dutour (Aix-en-Provence, France) and an anonymous reviewer. Their carefull reviewing and their relevant and valuable comments greatly helped to improve the manuscript.

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