The debated question of asymmetrical rhynchonellids (Brachiopoda, Rhynchonellida): examples from the Late Cretaceous of Western Europe

Danièle Gaspard,Sylvain Charbonnier

doi:10.1051/bsgf/2019016

Abstract

Many Cretaceous asymmetrical rhynchonellid brachiopods (Brachiopoda, Rhynchonellida) have long been considered asRhynchonella difformis(ValenciennesinLamarck, 1819). After a revision, Owen (1962) included the Cenomanian specimens from Europe inCyclothyrisM’Coy, 1844. Later, Manceñidoet al.(2002) confirmed this decision and critically mentioned the name of another asymmetrical rhynchonellid genus from Spain,OwenirhynchiaCalzadainCalzada and Pocovi, 1980. Specimens with an asymmetrical anterior margin (non particularly ecophenotypical), from the Late Coniacian and the Santonian of Les Corbières (Aude, France) and Basse-Provence (SE France) are here compared to specimens of the original Cenomanian speciesC. difformis. They are also compared to new material from the Northern Castilian Platform (Coniacian-Santonian, N Spain) and toRhynchonella globataArnaud, 1877 (Campanian, Les Charentes, Dordogne, SW France) andRh. vesicularisCoquand, 1860 (Campanian, Charente, SW France). These observations document the great morphological diversity among all these species and lead us to erect a new species:Cyclothyris grimarginanov. sp. from the type material of Arnaud, and two new genera:Contortithyrisnov. gen. includingContortithyris thermaenov. sp.,Beaussetithyrisnov. gen. includingBeaussetithyrisasymmetricanov. sp. All of these brachiopods fundamentally present an asymmetrical state which origin is discussed.

Highlights

Rhynchonelliform brachiopods (Brachiopoda, Rhynchonellida) generally present a bivalved shell with a bilateral symmetry
Variations or deviations from the plane of symmetry are common in rhynchonelliform brachiopods
Asymmetry is an important and widespread trait, having evolved numerous times in many invertebrate and vertebrate organisms and at many levels of organisation ranging from individual cells, through organs to entire body-shapes

Summary

Introduction

Rhynchonelliform brachiopods (Brachiopoda, Rhynchonellida) generally present a bivalved shell with a bilateral symmetry. Numerous authors (e.g., Ager, 1965; Asgaard, 1968; Schumann, 1976, 1991; Asgaard and Bromley, 1991; Afanasjeva, 2014; Schrøeder et al, 2017; Berrocal-Casero et al, 2017) pointed that some variations are in relation to environmental conditions (crowded population, close attachment to a substrate preventing a normal shell growth) They correspond to an occasional modification of the normal symmetry in the shell, named dissymmetry. Other variations are related to shifted/twisted anterior commissures, i.e., shells with a bilobate anterior margin (right side-up/left side-down or the reverse), which seem rather of genetic origin (Fürsich and Palmer, 1984; Gaspard, 1991) This later case corresponds to the lack of normal symmetry in the shell, named asymmetry. Diverse examples have been observed in Jurassic and Cretaceous species (Asgaard, 1968; Fürsich and Palmer, 1984; Gaspard, 1991 and references ), but the origins of the state of asymmetry remain an open question

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