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

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. 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). Thus, they correspond to an occasional modification of the normal symmetry in the shell, also named dissymmetry. Other variations are also related to particularly 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, also named asymmetry. Among the brachiopods, asymmetry is merely observed in rhynchonellids and affects the whole population. Diverse examples have been observed in Jurassic and Cretaceous species (Asgaard, 1968; Fürsich and Palmer, 1984; Gaspard, 1991 and references herein), but the origins of the state of asymmetry remain an open question.

Several rhynchonellids with a right and left-handed asymmetry of the anterior margin are well represented during the Cretaceous (particularly the Middle–Late Cretaceous) till the Danian (Gaspard, 1991). For a long time, most of them have been identified as Rhynchonella difformis (Valenciennes inLamarck, 1819). After a general revision of CyclothyrisM’Coy, 1844, some of these rhynchonellids have been assigned to Cyclothyris difformis by Owen (1962). This species was documented from numerous localities in the lower-middle Cenomanian of France (Normandy, Sarthe, Indre, Charente and Var), with close comparisons with the specimens of Belgium (Tourtia from Andregnies and Montignies-sur-Roc; Owen, 1962). Other specimens come from western England, particularly from Somerset, Dorset, south Devon (varieties in the sandy lower Cenomanian), southeast Devon (middle Cenomanian), Wiltshire (abundant specimens in the Upper Greensand), Isle of Wight, as well as from Northern Germany (Essen), Bohemian basin (Prague), and from the lower-middle Cenomanian of Bulgaria and from Poland (e.g., Owen, 1962, 1988; Nekvasilova, 1973; Popiel-Barczyk, 1977; Gaspard, 1991; Motchurova-Dekova, 1994, 1995; Néraudeau et al., 2013).

All the Cretaceous asymmetrical rhynchonellids should not be identified as C. difformis. Among the most common misinterpretations, specimens of Cyclothyris compressa (Valenciennes inLamarck, 1819) are often misidentified by certain authors as C. difformis, (see discussion in Gaspard, 2014). Moreover, asymmetrical rhynchonellids from the Late Cretaceous (Coniacian–Campanian) were also identified as C. difformis. These examples were already discussed (Fürsich and Palmer, 1984; Gaspard, 1991, 2017; Motchurova-Dekova, 1994, 1995). Although Rhynchonella globataArnaud, 1877 was considered as a possible variant of Rh. difformis, most of the ancient authors maintained the name, considering that this species pointed a remarkable event: its first occurrence coincides with the early Campanian. In other respects, the type series of Rhynchonella globata appears heterogeneous and needs also a revision.

The aim of the present work is to review and precise the complex situation of the Late Cretaceous (Senonian) asymmetrical rhynchonellids from Les Corbières (Aude, France), Basse-Provence (SE France), Les Charentes and Dordogne (SW France), and Castile (Spain), and to highlight the status of their asymmetry.

Specimens of Cyclothyris difformis have been sampled in the lower Cenomanian layers (glauconitic chalk) in the cliffs of Bec de Caux (Normandy, France), and in the Cenomanian outcrops of Cadeuil in Charente-Maritime, France (Néraudeau coll., Université de Rennes 1: acronym IGR). Specimens from the northeastern Pyrenees come from the Coniacian–Santonian of Rennes-les-Bains and Sougraigne (Fig. 1.1) and were observed in the collections of the Muséum national d’Histoire naturelle (acronym: MNHN.F), Paris. Those from Basse-Provence (SE France: Le Beausset, La Cadière, Les Martigues; Fig. 1.2) have been sampled or observed in MNHN collections. Comparisons of Campanian specimens of Rhynchonella globata have been made with the material from the Aquitaine Motorway A10 in Charente (Gaspard, 1985), from Tercis (Landes, France: Gaspard and Odin, 2001), from Les Charentes and Dordogne (Arnaud coll., specimens of the type series in Sorbonne-Université: acronym SU.PAL.) (Fig. 1.3). For comparisons with the Rh. globata type series from the same region, Campanian Rhynchonella vesicularisCoquand, 1860 have been observed in Sorbonne-Université (Arnaud coll.) and MNHN collections. Moreover, the original type material has been also traced in the collection of Mining and Geological Survey of Hungary (acronym MBFSZ).

For other comparisons, Coniacian specimens of Owenirhynchia have been sampled in the Nidàguila Formation at the Villamartin section from the North Castilian platform (Fig. 1.4).

The specimens sampled are imaged and observed following the external morphology: global shape, details of the anterior margin, shape and number of costae, shape and size of the foramen and details of the deltidial plates. Some of them, selected from each studied area, have been included in an epoxy resin, oriented and then cut to observe the internal characters in sets of transverse serial sections that are drawn using a camera lucida with control of the shell thickness worn out between successive sections.

Two specimens were scanned using X-ray microtomography to complete the study. The radio-transparent substrate on which the specimens are positioned is adjusted to correspond to the axis of the X-ray nano-beam (AST-RX platform, UMS 2700, MNHN, Paris). The specimens were scanned according to the following parameters: (1) specimen MNHN.F.A59944 (Cenomanian, Normandy) – voxel size: 5.977 μm³, tension: 80 kV, intensity: 120 μA, time exposure: 2000 ms, filtre: Cu 0.1 mm, 1800 images; (2) specimen MNHN.F.A26502 (Santonian, Basse-Provence) – voxel size: 9.517 μm³, tension: 110 kV, intensity: 50 μA, time exposure: 2000 ms, filtre: Cu 0.1 mm) (for videos and successive virtual transverse sections, see online Appendices A–D).

Several other scanned specimens were too recrystallized to allow a 3D reconstruction, even a clear individualization of internal characters in virtual sections (see Gaspard, 2013).

In order to highlight details at the sites of the articulation (dental sockets, dental plates), hinge plates, crura, and dorsal septum, and to precise the orientation of the fibrous elements forming the low Mg-calcite secondary layer of the shell, some selected transverse sections were observed using a scanning electron microscope (JEOL SEM; MNHN, Sorbonne Université). These sections were observed after polishing, light acid attack (a few seconds with 12.5% rapid decalcifier RDC Laboratoire Moderne^©) and gold coating (see online Appendix E).

The asymmetrical rhynchonellids from Les Corbières were first mentioned by d’Orbigny (1847–1851), d’Archiac (1854), Coquand (1860), Toucas (1880), Péron (1877), Fage (1934), Sénesse (1937) and Basse (1939). All these authors reported the presence of Rhynchonella difformis from the late Coniacian to the early Santonian.

In the present study, the asymmetrical rhynchonellids are mainly found in the Marnes à Micraster Formation at Rennes-les-Bains (Aude) and in the Marnes bleues de Sougraigne Formation at Sougraigne (Aude). The Marnes à Micraster Formation is middle Coniacian–middle Santonian in age and the Marnes de Sougraigne Formation is middle–late Santonian in age (Bilotte and Freytet, 1984). More precisely, the asymmetrical rhynchonellids of the Marnes à Micraster Formation are mainly found in the early Santonian Nowakites carezi ammonite biozone, and in the basal part of the early-middle Santonian Texanites gallicus ammonite biozone (Hancock, 1991; Kennedy et al., 1995).

The asymmetrical rhynchonellids from Basse-Provence were first mentioned by d’Orbigny (1847–1851), Toucas (1873, 1885), Vasseur (1894) and later by Babinot (1980), Tronchetti (1981), Babinot et al. (1984) and Grosheny (1986). After several discussions about the age of the outcrops, most of these authors reported the presence of Rhynchonella difformis in the Santonian of the region.

In the present study, the asymmetrical rhynchonellids have been found in La Cadière Formation at Le Moulin-de-la-Cadière (early Santonian, Grosheny, 1986) and in the Santonian at Le Beausset, Le Castellet and Les Martigues (Babinot et al., 1984).

The asymmetrical rhynchonellids from Les Charentes were collected in the Cenomanian of Cadeuil (Charente Maritime). A well-preserved specimen of Rhynchonella difformis illustrated by Néraudeau et al. (2013: pl. 12d-e), shown here, is compared with the lower–middle Cenomanian specimens from the lectotype area (Normandy).

In other respects, specimens of Rhynchonella globata were either observed in collections or collected in the Campanian from Les Charentes and Dordogne. The type series listed by Arnaud (1877) appears heterogeneous and requires an in-depth study. Specimens of Rhynchonella vesicularis from the Campanian of Les Charentes (Aubeterre-sur-Dronne) and from Dordogne (Beaufort, Issac, Puyvigier, Sourzac) were also observed, including the type material.

Asymmetrical rhynchonellid brachiopods have been found in the Upper Cretaceous of the North-Castilian Platform which geological setting is described in detail by Floquet (1978, 1991), Floquet et al. (1982) and Lamolda et al. (2002). The studied specimens were collected in the Nidáguila Formation at Villamartin section (Gaspard, 2010) and identified as Owenirhynchia Calzada inCalzada and Pocovi, 1980 by Lamolda et al. (2002), but as Cyclothyris aff. globata by Berrocal-Casero et al. (2017). More precisely, these asymmetrical rhynchonellids are late Coniacian in age (Floquet, 1991). Muñoz (1985, 1994) also recognized representatives of Owenirhynchia, later introduced by Manceñido et al. (2002) as a possible subgenus or a subjective synonym of Cyclothyris.

Brachiopoda Duméril, 1805 

Rhynchonellida Kuhn, 1949 

Hemithyridoidea Rzhonsnitskaia, 1956 

Cyclothyrididae Makridin, 1955 

Emended diagnosis afterManceñido et al. (2002). – Trilobate, sharply costate Hemithiridoidea, with anterior commissure uniplicate or sometimes asymmetrical; lamellose ornament frequently developed; squamma-glotta obsolescent. Middle Triassic–Late Cretaceous.

Cyclothyridinae Makridin, 1955 

Emended diagnosis afterManceñido et al. (2002). – Fully costate Cyclothyrididae, rarely with posterior smooth area, beak massive, with large hypothyrid rimmed foramen (i.e., deltidial plates produced into short tube around pedicle). Dorsal median septum usually very much reduced, septalium reduced or absent, crura canaliform (or at least distally concave modified raduliform). Characteristically strongly and densely costate. Middle Triassic–Late Cretaceous.

CyclothyrisM’Coy, 1844 

Type species. – Terebratula latissima J. de C. Sowerby, 1829, by subsequent designation of Buckmann (1906).

Emended diagnosis afterManceñido et al. (2002). – Medium to large, wide, with uniplication low, arcuate, commonly asymmetrical; costellae numerous, fine posteriorly, beak erect, foramen large, auriculate, deltidial plates conjunct, well exposed. Dorsal septum short or absent; hinge plates distally concave; crura long, dorsally concave.

Cyclothyris difformis(Valenciennes in Lamarck, 1819)

Figures 2 and 3 

Terebratula difformis Valenciennes inLamarck, 1819: 255.

not Rhynchonella difformis. d’Orbigny 1847–1851: 41–43, pl. 498, figs. 6–9.

Cyclothyris difformis. Owen 1962: 51, pl. 1, figs. 1–7. [lectotype selection]. – Owen 1988: 84, pl.1, figs. 13–15, pl. 2, figs. 4–6, 10–15.

Type material – Lectotype selected by Clerc and Fabre (1918). It comes from the lower Cenomanian of Normandy coast (France) and is housed at the Muséum d’Histoire Naturelle, Geneva, in the Lamarck collection (for details see Owen, 1962).

Examined material from the Cenomanian – Five specimens MNHNF.A67478–A67480, A59944, A70585 (Gaspard coll.) from the cliffs of Cap de la Hève, Pays de Caux, Normandy (Figs. 2A–2N and 3; Appendices A–C); one specimen IGR23222 (Néraudeau coll). from the lower Cenomanian, of Cadeuil (Charente-Maritime) (Figs. 2O–2Q); specimens observed in situ from the middle Cenomanian of Île Madame (Charente-Maritime).

Description of external morphology (Fig. 2) – The medium-sized to large biconvex shell is wider than long, the ventral beak is sub-erect (slightly incurved) never approaching the dorsal umbo. Sometimes, the beak seems to be more inclined and longer on one side or the other. The foramen is round with a diameter of 1–2 mm bounded by the exposed deltidial plates, not conjunct at the juvenile stage (Figs. 2A and 2D), twice large at the anterior part and whose lateral expansions form a short tube in adults (Figs. 2J, 2L, 2O). The shell is flat in juvenile stages (Figs. 2B and 2E). The lateral commissures are nearly straight with a deflection anteriorly in adults (Figs. 2B, 2E, 2H, 2M). The anterior commissure is rectimarginate in juvenile stages (Fig. 2C) then, progressively monoplicate (Fig. 2F) tending to become asymmetric in a more pronounced way in adults: right side-down/left side-up (Figs. 2I and 2N) or the reverse (Figs. 2K and 2Q). Numerous costae, small and round in juveniles then sharper and higher near the commissures (at least 30–45 in adults) ornament the shell, one to three of them take place along the shift between the side-up and the side-down of the anterior commissure.

Description of internal characters – The transverse serial sections of specimen MNHN.F. MNHN.F.A70934 (Fig. 3), and the virtual transverse and lateral views from the scanned specimen MNHN.F.A59944 (Appendices A–C) reveal: well-defined deltidial plates with the lateral expansions (Figs. 3a–3d), thick nearly parallel dental plates (Figs. 3d–3j), posterior sharp-pointed expensive dorsal septum (Figs. 3f–3j) then deceasing (Figs. 3k–3q), thick hinge teeth with small and brief lateral accessory teeth inserted in deep dental sockets and accessory counter parts (Figs. 3i–3n), slightly concave hinge plates, and raduliform to canaliform crura (Figs. 3m–3r).

Comments. – Cenomanian specimens referred to Cyclothyris difformis as reported by Néraudeau et al. (2013) present the same characteristics as those from Normandy. When well preserved, the material from the basal Cenomanian of Cadeuil is comparable in size and global shape with that from Normandy, the differences rest in the more rounded anterior part and a less deflection on one or the other side of the anterior margin (Fig. 2Q). We point out that assymmetrical specimen MNHN.F.A70585 (Figs. 2J and 2K) is also affected by a dissymmetry well visible in the dorsal and frontal views.

Cyclothyris globata (Arnaud, 1877)

Figures 4 and 5; Table 1 

Rhynchonella globataArnaud, 1877: 83, pl. 8, figs. 33–35.

not Rhynchonella globata. Arnaud, 1877: 83, pl. 8, figs. 36–38.

not Rhynchonella difformis var. globata.Fage 1934: 439, pl. 23, fig. Dg^1–5.

Cyclothyris aff. globata. Muñoz 1985: 43, pl. 1, fig. 2.

Rhynchonella globata. Gaspard 1991: pl. 1, fig. 10 (non figs. 7–9).

Owenirhynchia globata. Muñoz 1994: 154, fig. 64.

not Cyclothyris globata. Motchurova-Dekova 1995: pl. 4, 5.

Gaspard and Odin, 2001: pl. 1.1.

not Cyclothyris aff. globata. Berrocal-Casero et al., 2017: 80, fig. 4.

Type material. – The original type material of Arnaud (1877) was composed of a non-assessable number of Campanian specimens from a large set of localities from Les Charentes and Dordogne (SU.PAL.2018.0.55.9.0 to SU.PAL.2018.0.55.44.0; see Tab. 1). Among them, the specimen SU.PAL.2018.0.55.29.1 (figured by Arnaud, 1877: pl. 8, figs. 33–35) is herein selected to be the lectotype (Figs. 4A–4D). Several paralectotypes have been identified and fit the original diagnosis of Arnaud (1877). They are listed in Table 1 and some of them are illustrated in Figures 4E–4R.

Type locality. – Trélissac, Dordogne, France.

Among the paralectotypes, some localities are hand-labelled by Arnaud (1877) and others are unfortunately groups of specimens without precise locality in Charente or Dordogne (see Tab. 1).

Type age. – Campanian (levels P1-P3 of Arnaud 1877 corresponding to benthic foraminiferal biozones CI-CV of Platel, 1977).

Additional examined material from the Campanian – see Table 1.

Description of external morphology (Fig. 4) – Medium sized (length: 11.4–20.6 mm; width: 13.9–23.4 mm; thickness: 10.3–17.7 mm), rounded shells with a curved ventral beak (Figs. 4B, 4F, 4K), ornamented by about 27–30 to 38 costae (number which is not always in correlation with the shell size). The ventral umbo is curved somewhat sharp-pointed and sometimes nearly reaching the dorsal umbo (Fig. 4Q), the limit of the joined deltidial plates bounding the small foramen (diameter around 1 mm in adults, even less in some cases) is visible. Sometimes the lateral expansions of these deltidial plates are best developed, although less than in C. difformis. The dorsal area on each side of the deltidial plates is smooth except a few fine growth-lines. The lateral commissure is more or less inclined ventrally (Figs. 4B, 4F, 4K, 4N, 4Q). The shifting of the lateral lobes of the anterior margin, left-up/right-down or the reverse, is most often revealed along about one costa (Figs. 4C, 4G, 4I, 4R).

Description of internal characters (Fig. 5) – Specimen MNHN.F.A70935: pedicle collar present, thick dental plates rapidly thinning down (Figs. 5a–5f), thick hinge teeth inserted in wide dental sockets (Figs. 5g–5j), slightly concave hinge plates (Figs. 5g–5j), raduliform to canaliform crura (Figs. 5k–5m).

Comments. – Muñoz (1994) proposed the new combination Owenirhynchiaglobata and was followed by several authors until Berrocal-Casero et al. (2017). The globose shape of all the type specimens and new specimens observed in this study is not compatible with the original diagnosis of Owenirhynchia. Indeed, these last authors precised that Owenirhynchia presents shells not globose, generally larger than longer, with subtriangular outline [literal translation of “conchas no globulosas, generalmente más anchas que largas de contorno subtriangular”]. We propose an assignment to Cyclothyris based on the shape deltidial plates whose lateral expansions form a tube.

Radulović and Motchurova-Dekova (2002) identified specimens of Cyclothyris? globata from the Santonian–Campanian of southeastern Europe (Pannonides, Carpatho-Balkanides, Dinarides). We note that until now this species is restricted to the Campanian and that some identifications proposed for the Santonian outcrops should be review.

Cyclothyris grimargina nov. sp.

Figures 6–8; Table 2 

Rhynchonella difformis var. globata.Fage 1934: pl. 22, fig. Dg^1–5.

Rhynchonella globata. Gaspard 1991: pl. 1, figs. 7–9.

Etymology. – The specific epithet is a contraction of the Old Frankish grima (mask) and the Latin margo (commissure).

Type material. – Holotype SU.PAL.2018.0.55.19.1. Several paratypes are listed in Table 2.

Type locality. – Caillaud, Charente-Maritime, France.

Type age. – Campanian.

Additional examined material from the Campanian. – See Table 2.

Diagnosis. – Biconvex globose and strongly costate shell with a wide-wing shape; conjunct deltidial plates whose lateral expansions form a tube; suberect to slightly curved ventral beak, asymmetric anterior margin with a more or less inclined shift line.

Description of external morphology (Figs. 6 and 7) – Wide sized subtriangular shells (length: 11.4–26.7 mm; width: 12.1–32.4 mm; thickness: 08.0–19.3 mm) with a medium curved to suberect pointed ventral beak (Figs. 6B, 6J, 6M, 7B, 7E), ornamented by about 27 to 38 coarse costae, exceptionally 50 (Figs. 6A, 7D, 7I, 7M) round and finer in the posterior shell, sharper and wider near the margins. The limit of the joined deltidial plates is visible (Figs. 6O and 7L); these later line the round foramen (around 1.5 mm in diameter). The lateral expansions of these deltidial plates are developed to form a short tube. The dorsal areas of the ventral beak are smooth except very fine growth lines. The shifting of the lateral lobes of the anterior margin left-up/right-down, or the reverse, is most often revealed along around two to four costae (Figs. 6C, 6F, 6K, 6N, 7F, 7H, 7K, 7N). Lateral margins more often inclined ventrally in the anterior shell part (Figs. 6E, 6J, 6M). Sometimes elementary growth lines are observed at the mid-anterior shell surface (Fig. 6P). Notice that subadult specimens reveal a proportional wider foramen (Fig. 7A), are flatter with straight lateral margins (Fig. 7B), a lateral and anterior end of valves forming a sharp angle and an anterior margin slightly asymmetrical with a weak shift (Fig. 7C).

Description of internal characters (Fig. 8) – Specimen MNHN.F.A70936: pedicle collar present, ephemeral dental plates, rapidly disappearing. Deltidial plates well-marked (Figs. 8g–8j), semi-rounded to sub-flattened dorsal posterior shell (Figs. 8j–8q), nearly constricted hinge teeth closely inserted in the dental sockets (Figs. 8l–8o), slightly concave hinge plates evolving to a subhorizontal position anteriorly (Figs. 8m–8o), thick raduliform to canaliform crura (Figs. 8p–8q).

Discussion. – This species is assigned to Cyclothyris based on the following characters: asymmetrical anterior commissure, deltidial plates forming short tube around foramen, beak suberect to slightly curved. It differs from Cyclothyris difformis by a stronger asymmetry with a well-developed lateral right/left shift at the level of anterior commissure forming two distended lobes sometimes unequal. It differs from Cyclothyris globata by the wing-shaped shells (very round globose shell in C. globata), the orientation of the shifting line (more inclined) of the anterior commissure and the stronger costae. It differs from Cyclothyris vesicularis by a straight thinner ventral beak, more and finer costellae and an asymmetric anterior margin rather slightly twisted than obliquely shifted in this last species. Cyclothyris vesicularis is in a whole relatively slender than Cyclothyrisgrimargina nov. sp.

Cyclothyris vesicularis (Coquand, 1860)

Figures 9–11 

Terebratula vesicularisCoquand, 1860: 122.

Rhynchonella vesicularis. Coquand, 1862: 332, 338, pl. 34, figs. 7–9.

Type material. – The original type material was composed of a few number of specimens from Aubeterre-sur-Dronne, Charente, France. Four syntypes have been traced in the palaeontological collections of the Mining and Geological Survey of Hungary at Budapest. Among them, specimen MBFSZ K 2019.10.4.1 is here selected to be the lectotype (Figs. 9A–9C). Two paralectotypes MBFSZ K 2019.10.4.3 and K 2019.10.4.4 (Figs. 9D–9F) are also considered. The last specimen MBFSZ K 2019.10.4.2 shows strong costae and thus probably does not belong to the species.

Type locality. – Aubeterre-sur-Dronne, Charente, France.

Type age. – Campanian

Examined material. – Three specimens MNHN.F.A70583, A70584, S08809 (Péron coll.) from Aubeterre-sur-Dronne (Charente). A set of well-preserved specimens (Arnaud coll.) from Aubeterre-sur-Dronne (SU.PAL.2018.0.55.21.1-8), Puyvigier, (SU.PAL.2018.0.55.22.1-3), Beaufort (SU.PAL.2018.0.55.23.1-3), Issac (SU.PAL.2018.0.55.24.1-4), Sourzac (SU.PAL.2018.0.55.25.1-7).

Description of external morphology (Figs. 9 and 10) – Large specimens globally triangular (length: 17.0–31.5 mm; width: 19.4–40.9 mm; thickness: 11.9–20.1 mm) (Figs. 10A, 10D, 10G, 10J), with a straight suberect to slightly incurved ventral beak (Figs. 10B, 10E, 10N), the maximum width in anterior position (Figs. 10A, 10D, 10J, 10M), a variable sized foramen (diameter: around 1 mm, sometimes more) lined by well-defined lateral expansions of the deltidial plates (Fig. 10I), and a typical twisted asymmetrical anterior margin (Figs. 10C, 10F, 10H, 10L). The shell is ornamented by 75 to 98 (even 100) costellae, fine posteriorly whose size increases near the margins revealing sometimes a gerontic aspect, even exaggerated in a mid-depressed anterior margin (Fig. 10O) and an unsual gathering of two or three fine costellae in a strong one (Figs. 10O–10P). The costellae present sometimes a digitation at mid-length (see Fig. 10J). In this species a longer lateral margin is observed where the down side lobe of the anterior commissure is concerned. Often the lateral and anterior margins are slightly depressed (Figs. 10C, 10F, 10L, 10O).

Description of the internal characters (Fig. 11) – Specimen MNHN.F.A70937 (length: 22.3 mm; width: 28.0 mm; thickness: 17.6 mm): strong dental plates (Figs. 11b–11i), developed deltidial plates (stippled) (Figs. 11f–11h), an ephemeral dorsal septum (Figs. 11j–11n), dental sockets deep and slightly crenulated posteriorly and wide anteriorly (Figs. 11j–11o), constricted then thick hinge-teeth (Figs. 11k–11o), hinge plates convex dorsally (Figs. 11k–11p), and crura distally raduliform (Figs. 11p–11r).

Contortithyris nov. gen.

Type species. – Contortithyris thermae, nov. sp., by monotypy.

Etymology. – From the Latin contortus (contorted) in allusion to the shifted shape of the anterior margin.

Diagnosis. – Medium to large sized shell, anterior margin commonly asymmetrical; costae posteriorly fine and slightly sharp on the margins, medium-sized foramen, suberect beak.

Discussion. – Contortithyris nov. gen. is assigned to the Cyclothyridinae based on the asymmetrical anterior commissure and the deltidial plates forming short tube around foramen. It differs from Cyclothyris by its more massive globose shape, curved umbo, more rounded commissure and slightly finer costae, from Owenirhynchia by a wider size and thicker to globose shell, less high height of the twisted anterior margin, and from Beaussetithyris nov. gen. by the biconvex profile. It differs from all the other genera of the Cyclothyridinae by the asymmetrical anterior commissure.

Contortithyris thermae nov. sp.

Figures 12–14 

Etymology. – The specific epithet alludes to the Antique thermal baths of the type locality Rennes-les-Bains.

Type material. – Holotype MNHN.F.A59945. 32 paratypes from Rennes-les-Bains (MNHN.F. A68144–A68146, A70586, A70587, A70639–A70642, A70873-A70885, A70887–A70895, S09048), and three paratypes from Sougraigne (MNHN.F.A70643, A70644, A70597).

Type locality. – Rennes-les-Bains, Aude, Occitanie, France.

Type age. –Santonian (Marnes à Micraster Formation).

Diagnosis. – Medium to large size shell, biconvex, finely costate, posterior shell nearly smooth, medium round foramen, beak sharp, umbo suberect; anterior margin progressively to strongly asymmetrical with a pronounced high shift.

Description of external morphology (Figs. 12 and 13) – Globose shells hardly wider than long (length: 12.4–31.5 mm; width: 12.9–40.8 mm) with a curved short and massive ventral umbo (erect in juveniles) ending by a dorsally pointed beak accentuated by the lateral crests. The beak is not necessarily inclined on one side or the other (Figs. 12 and 13). The deltidial plates, 2.0–2.5 mm in height and 3–4 mm of basal width, are sometimes partly hidden, interrupted at their posterior part by the round/oval foramen of about 1.5–2.0 mm in diameter in adults. The foramen is lined by lateral expansions of the deltidial plates shaping an open tube. The dorso-lateral areas of the ventral beak are obviously smooth. The shells are ornamented by approximately 35–45 small acute angle costae (following the stage during ontogeny), not divided, of 1.0–1.5 mm at the basal part, thin, even blurred at the posterior part of valves. The lateral margins are globally straight. The anterior margin tends progressively and slightly to be asymmetrical from the juveniles (Figs. 12C and 12F) to markedly asymmetric in the adults, shifted with one side pull-down and the other one turned-up (Figs. 12I, 12L, 12O, 12Q, 13B, 13F, 13I, 13L). The deflection between the two parts is important; it follows a slight inclined line never reaching a vertical position (Figs. 12L and 13I). The specimens less than 20 mm in length, are flatter than the fully mature ones, and consequently present a smaller deflection of the lateral margin (Figs. 12B and 12E). In adults, the deflection of the anterior margin concerns 1 to 2, even 3 costae (Figs. 12I, 12L, 12O, 13B, 13F, 13I, 13L). Worn shells allow observing traces of the dental plates at the posterior ventral beak and of the dorsal septum (Fig. 12R).

Description of internal characters (Fig. 14) – Transverse serial sections in specimens MNHN.F.A70938 and A70939 reveal: strong dental plates, slightly inclined, becoming parallel and thinner anteriorly. The incidence of the curvature of the ventral umbo with its sharp beak is marked, as well as the lateral expansions of the deltidial plates on each side of the foramen (Figs. 14Ac–14Ae, 14Bb–14Bc). In specimens between 21 to 33 mm in length, the dental plates are half to one-third of the umbo shell thickness at 1.5–2.0 mm from the posterior part of the ventral valve (Figs. 14Aa–14Af, 14Ba–14Be). The posterior dorsal valve reveals the beginning of the dental sockets (Figs. 14Af–14Ag, 14Be), deepening after (Figs. 14Ah–14Aj, 14Bf–14Bj), and a faint/moderate dorsal septum (Figs. 14Ai–14Am, 14Be–14Bm). The hinge plates are sub-horizontal. The hinge teeth are inserted faintly oblique in the crenulated dental sockets (Figs. 14Ah–14Aj, 14Bg–14Bj). These teeth are constricted by the outer socket ridge and by the accessory inner socket ridge (Figs. 14Ah–14Aj, 14Bh–14Bj). The crura are canaliform to subfalciform.

Microstructure. – Details of the crural bases and/or the relative shape of hinge teeth and dental sockets are presented in Appendix E.

Comments – The specimens from Sougraigne present the same proportions with those from Rennes-les-Bains, but some are wider than the standard and are thicker (Figs. 13J–13L). Some wider specimens from Sougraigne bear stronger costae (Fig. 13J). The peculiarity of all of these shells (both Rennes-les-Bains and Sougraigne) is the high shifted right/left asymmetry of the anterior margin.

Beaussetithyris nov. gen.

Type species – Beaussetithyris asymmetrica nov. sp., by monotypy.

Etymology – From the locality of the type species, Le Beausset, Var, Provence-Alpes-Côte d’Azur, SE France.

Diagnosis. – Large shell, erected to suberected beak, costellate shell, costae sharp on the margins; foramen round to lightly oval, auriculate deltidial plates; anterior margin asymmetrical.

Discussion. – Beaussetithyris nov. gen. is assigned to the Cyclothyridinae based on the asymmetrical anterior commissure and the deltidial plates forming short tube around foramen. It differs from Cyclothyris by its dorsibiconvex shell, often more massive, from Owenirhynchia by the size and the thickness of the shell, and from Contortithyris by slightly greater size, obvious wider foramen and stronger costae.

Beaussetithyris asymmetrica nov. sp.

Figures 15–17 

Etymology. – The specific epithet alludes to the asymmetrical shape of the anterior commissure.

Type material – Holotype MNHN.F.A67492. Seven paratypes MNHN.F.A59946, A67493, A67481–A67485 from Le Beausset; one paratype MNHN.F.A26502 from La Cadière; five paratypes MNHN.F.A59948, MNHN.A70117, A70118, A70811, A70886 from Les Martigues. All are from Var department, SE France.

Type locality. – Le Beausset, Var, SE France.

Type age. – Early Santonian.

Additionalmaterial. – One specimen MNHN.F.S09235 (coll. Péron) from Le Castellet; two specimens MNHN.F.A70798, A70799 (d’Orbigny coll.) from La Cadière; two specimens MNHN.F.A61729, A70812 (d’Orbigny coll.) from Le Beausset; two specimens MNHN.F. A59947, A70645 from Les Martigues.

Diagnosis. – Medium to large shells, dorsibiconvex, conjunct deltidial plates, auriculate laterally, lateral margins slightly inclined ventrally, asymmetric anterior margin with a shift evenly high.

Description of external morphology. – (Figs. 15 and 16) Right and left asymmetric adult shells are mainly massive, dorsibiconvex, with a sub-erect ventral umbo, rarely ending by a pointed beak. The adult shells are 21.7 to 32.9 mm in length, 21.1 to 37.8 mm in width and 14.9 to 27 mm in thickness. The dorsal surfaces of the beak are smooth. The rounded foramen encroaches on the posterior part of the deltidial plates forming a short tube. The foramen is about 1.5 mm in diameter (even wider). The shells are ornamented by 29 to 36, even 39 costae. Few juvenile specimens and/or preadults (about 19.3 mm in length) show proofs of a beginning asymmetry with fine costae (1 mm at the margin). Some specimens reveal a displacement of the monoplication towards one side (Fig. 16G) rather than a displacement of the asymmetry as illustrated in Figure 16J.

Description of internal characters. – (Fig. 17) Transverse serial sections of the asymmetrical specimens from Le Beausset (Fig. 17A) and Les Martigues (Fig. 17C), and virtual transverse views from the scanned specimen MNHN.F.A26502 (La Cadière, Appendix D) reveal: globally parallel dental plates (17Ae–17Ah, 17Cb–17Cf), lateral expansions of the conjunct deltidial plates, an auriculate foramen, sometimes a more curved dorsal umbo (Figs. 17Ag–17Ai, 17Cf), a short dorsal septum becoming faintly higher anteriorly (Figs. 17Ak–17Au) compared to the more discreet one in les Martigues (Figs. 17Cf–17Cm), and hinge plates slightly undulating ventrally, then slightly oblique towards the ventral valve. The crura are raduliform (Figs. 17Ar–17At, 17Cl–17Co). A sub–asymmetrical shell from Le Beausset is illustrated for comparison (Fig. 17B).

Microstructural details. – Details of the crural bases and/or the relative shape of hinge teeth, dental sockets, and dorsal septum are presented in Appendix E.

Comments. – Some specimens herein identified as Beaussetithyris asymmetrica present subasymmetrical shells characterized by an uncommon shifted anterior margin with one side obviously longer than the other one (Figs. 16H–16J). These shells are more massive than the type material. They show a more erect umbo, and a wider foramen. They are biconvex to dorsibiconvex (length: 24.1–31.7 mm; width: 27.7–35.9; thickness: 19.4–21.7 mm). In adult specimens, the foramen is round (1.25–1.75 mm in diameter). They present longer dental plates (Figs. 17Bd–17Bq), an auriculate foramen (Figs. 17Bb–17Bg), an important dorsal umbo with an important dorsal median septum that has no equivalent in the shells previously described (Figs. 17Bh–17Bs), hidden and narrow posterior dental sockets (Figs. 17Bl–17Bm), thick hinge teeth inserted more vertically in the dental sockets than in the previous (Figs. 17Bn–17Bq), small lateral accessory teeth and dental sockets (Figs. 17Bp–17Bs) and swollen hinge plates posteriorly then forming an open angle towards the dorsal floor, subcanaliform to raduliform crura (Figs. 17Bu–17By).

Comparisons between the two sets of sections (Figs. 17A–17C) reveal differences between the asymmetrical type specimens and the subasymmetrical ones. Only new additional specimens will allow to precise if the observed differences are linked to a great variability in Beaussetithyris asymmetrica or if it is necessary to distinguish two different taxa.

Owenirhynchia Calzada in Calzada and Pocovi, 1980 

Type species. – Owenirhynchia rubra Calzada inCalzada and Pocovi, 1980 

Owenirhynchia sp.

Figures 18 and 19 

Examined material. – Specimens sampled from the Coniacian-Santonian (Nidáguila Formation) of the North Castilian Platform (Villamartin section), among them two MNHN.F. A70942, A70943 were used for transverse serial sections.

Description of external morphology (Fig. 18) – Medium sized subtriangular, equibiconvex multicostate shell, wider than long with a sub-erect ventral beak, deltidial plates shaping the typical short tube of the Cyclothyridinae around the subrounded foramen (diameter: often 1 mm, even more); slight asymmetry of the anterior margin (Figs. 18B and 18D).

Description of internal characters. (Fig. 19) – Sets of transverse serial sections of specimens from different levels reveal: joined wide deltidial plates which lateral expansions shape a tube around the foramen (Figs. 19Aa–19Ac, 19Ba, 19Bb), dental plates sub-parallel becoming slightly divergent (Figs. 19Aa–19Ae, 19Ba–19Be), supporting deeply inserted subquadrate hinge teeth, and consequently wide and deep dental sockets (Figs. 19Ad–19Ag, 19Bf–19Bh), hinge plates ventrally concave, crura canaliform to distally concave raduliform (Figs. 19Ag–19Ai, 19Bf–19Bh), reduced dorsal septum.

Comments. – The genus Owenirhynchia interpreted by Manceñido et al. (2002: 1334), as a possible subgenus of Cyclothyris or as a subjective synonym of this genus requires the observation of additional specimens for a deeper revision.

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 (e.g., brain, heart) to entire body-shapes. Geneticists try to explain this reality highlighting that the asymmetrical structure of the proteins that composed the living species are responsible of the observed asymmetry (see Flamant, 2016). Stern (2002) explained that the asymmetries between the right- and left-hand sides of the bodies are initiated at an early stage of development. In their approach of asymmetry at the level of the biomineralization process, Addadi and Weiner (2001) exposed that “understanding the formation of asymmetrical shapes during the growth of symmetrical structures is the first step towards understanding asymmetry in biology”. They reported that chiral shapes in biologically formed molecules lead to crystals involved in the formation of part of skeleton/exoskeleton of living organisms including vertebrates as well as invertebrates. Living organisms as well as fossils are concerned.

Among the invertebrates, brachiopods are concerned by asymmetry in their exoskeleton (shells). Among rhynchonelliform brachiopods, only rhynchonellids are affected by asymmetry. No modern example has been identified in the current state of our knowledge. Asymmetry is confined to rhynchonellids at certain times during evolution (Fürsich and Palmer, 1984). This observation goes against an ecophenotypical origin which logically should concern all groups of brachiopods at all times. Therefore, this rather suggests another origin, genetic, in some groups (i.e., in some rhynchonellids) as illustrated here. Several outstanding examples were found during the Mesozoic (Jurassic–Late Cretaceous: Owen, 1962; Popiel-Barczyk, 1977; Fürsich and Palmer, 1984; Gaspard, 1985, 1991; Muñoz, 1985, 1994; Motchurova-Dekova, 1995 among others).

Since then and recently, several authors (Afanasjeva, 2014; Berrocal-Casero et al., 2017; Schrøeder et al., 2017) proposed new cases of brachiopods showing asymmetry or ecophenotypical asymmetry. Careful examination of these three publications lead us to identify a confusion between asymmetry and dissymmetry. Most of the discussed modifications in the symmetry of the brachiopod shells are linked to palaeoenvironmental conditions and thus are of ecophenotypical origins. They correspond to cases of dissymmetry (see precisions in the present introduction). For instance, Berrocal-Casero et al. (2017; see their Fig. 13) hypothesized that the “functional meaning asymmetry” of rhynchonellid brachiopods from Northern Spain may be interpreted to an adaptation to sinking in soft substrates taking place as a response to changes in palaeoenvironmental conditions, and thus proposed semi-buried specimens. We remark that their hypothesis of semi-buried specimens staying till the adult stage in a position on one side or the other is not relevant. Such a position would lead quickly to the infilling of the shell by the sediment and consequently to the inefficiency of the lophophore or the ability to feed. In conclusion, we note that Berrocal-Casero et al. (2017) presented true asymmetrical specimens and true dissymmetrical specimens but the origin of the modification of the shifted commissures are different for us: genetically based in the first case and linked to the palaeoenvironments in the second one. We remark that it is also possible to observed true asymmetrical specimens affected by dissymmetry (Figs. 2J, 2K, 4M–4O).

In the present study, we identify asymmetry in four rhynchonellid genera (Cyclothyris, Contortithyris, Beaussetithyris, Owenirhynchia) at level of the anterior commissure showing a left-up/right-down shift, or the reverse. The observation of the shifted anterior commissure with approximately 50% of right-asymmetrical specimens and 50% of left-asymmetrical ones in Cyclothyris globata, and C. grimargina (see Tabs. 1 and 2) militates in favour of a genetic origin rather than an ecophenotypical expression. We point out that in Cyclothyris difformis, C. grimargina and Contortithyristhermae, this asymmetrical expression is not visible at juvenile stage and does not hit the internal hard parts of the shell, i.e., the crura.

Flamant (2016) explained that some genes (hidden or particularly associated) could be at the origin of asymmetry in different organisms. In our study, the origin of asymmetry is difficult to understand due to the lack of extant asymmetrical brachiopods. This prevents genetic observations and, as a matter of fact unable us to achieve a conclusion on the probability of a modification or the “freezing” of a morphogen. Following Fürsich and Palmer (1984), our feeling in the present cases is that the expression of asymmetry, genetically based, could emerge periodically in the course of evolution of brachiopods.

Beaussetithyris : urn:lsid:zoobank.org:act:4E5DD696-F0A0-4B2F-834B-BA7DF0635E95

Beaussetithyris asymmetrica : urn:lsid:zoobank.org:act:E057611E-0F7F-437F-925C-3F50B66D139E

Contortithyris : urn:lsid:zoobank.org:act:AB1114CE-1DA0-4336-902E-40C1508EA90A

Contortithyris thermae : urn:lsid:zoobank.org:act:814458A7-9B64-4F0E-ABE3-18E6B26C438C

Cyclothyris grimargina : urn:lsid:zoobank.org:act:5E005E69-727F-4A67-8BB3-CDDBF3B3BD1A

The authors gratefully acknowledge G. Bailly (Musée d’Angoulême), M. Bilotte (Univ. Paul Sabatier), J.F. Babinot† & G. Tronchetti† and M. Floquet (Univ. Aix-Marseille 1), D. Grosheny (Univ. Lorraine), D. Néraudeau (Univ. Rennes 1), L. Villier (Sorbonne-Université) for specimens; D. Pajaud† and S. Jouve (Sorbonne-Université) for the loan of specimens from Arnaud collection, and Palotás Klára, Makádi László, and Zoltán Lantos of the Mining and Geological Survey of Hungary who provided and authorized the use of digital images of C. vesicularis (Coquand coll.). Thanks are also due to Ph. Loubry (CR2P, UMR 7207 CNRS-MNHN, Sorbonne-Université) for help in the studio imagery, A. Lethiers, CR2P, UMR 7207 CNRS, Sorbonne-Universté, for help in achievement of drawings, M. Bellato and P. Wils (AST-RX platform UMS 2700, CNRS-MNHN, Sorbonne-Université) for help respectively for scanning the specimens and video creation. J-M. Pacaud (MNHN) is also thanked for collection numbers. We are grateful to B. Radulović for her review.