Introduction

Silurian myodocope ostracods are very abundant in the Wenlock to Pridoli series of the Armorican Massif (France) and other regions of Europe. This study is a continuation of the revision of this group, initiated during the late 20^th century by David J. Siveter, Jean Vannier and co-workers. Siveter (1984), Siveter et al. (1987, 1991), and Vannier and Abe (1992) provided evidence that, taken together, strongly suggested a pelagic lifestyle for Silurian bolbozoids and their myodocope associates. Their hypotheses were based on the functional design of the carapace, and distributional pattern and depositional environments of these ostracods in the Silurian of Europe. More recently, the discovery of soft parts in three Silurian myodocopid ostracods (Siveter et al. 2003, 2007, 2010) opened the way to new interpretations of fossil carapaces (Perrier 2007; Perrier et al. 2007, 2011). Perrier (2007) also showed the high diversity of Silurian myodocopes in the Armorican Massif (more than 12 species) and revealed the remarkable morphological stability of the group over the last 400 million years. As a follow-up to these recent revisions this paper focuses on a particularly interesting new species and examines its external carapace characters and their taxonomic importance.

Institutional abbreviations.—FSL, Faculty of Science, University of Lyon, Lyon, France; LPB, University of Brest, Brest, France; MNHN, Museum d'Histoire Naturelle de Nantes, Nantes, France.

Other abbreviations.—H, valve height; L, valve length.

Geological setting

This study is based on newly collected material from three localities in France (Fig. 1: Loc. 1, 3, 4) and also on the revision of several specimens formerly collected (Fig. 1: Loc. 2). Sineruga insolita gen. et sp. nov. occurs in these localities within the Armorican Massif and Normandie and in stratigraphical layers assigned to the Upper Silurian (late Wenlock to early Pridoli Series) based on graptolite biostratigraphy established by Sadler et al. 2009 (Fig. 1). These new specimens are deposited in the collections of the Université Claude Bernard Lyon 1 (FSL). The author also studied two collections from which Sineruga ostracods were recovered: the Racheboeuf collection (LPB; Loc. 1) and the De Tromelin collection (MNHN; Loc. 2). Details of these localities are as follows (see also Fig. 1):

Les Chevrolières.—Exposures at the intersection between the A81/E50 motorway (Laval to Le Mans section) and the departmental road D49 from St. Denis-d'Orques to Brûlon, St. Denis-d'Orques, Sarthe (= Loc. 5 in Siveter et al. 1987; and Loc. 3 in Racheboeuf 1994; herein Fig. 1). Highly deformed siltstone and mudstone rocks containing numerous partly pyritized nodules; Le Val Formation (see Paris 1977). Poorly preserved graptolites indicate the uppermost Ludlow to early Pridoli (Palmer in Siveter et al. 1987; Fig. 1C). Sineruga carapaces (3D-preserved single valves and complete closed carapaces in nodules) are associated with other myodocopid ostracods (bolbozoids and cypridinids), numerous phyllocarids (mainly tail-pieces of Ceratiocaris and Warneticaris), bivalves, orthoconic cephalopods, crinoids, gastropods, fragments of eurypterids, bryozoans, and spirorbids.

Fig. 1.

Geographic and stratigraphic position of studied localities (1–4). A. Silurian outcrops in Western Europe. B. Localities sampled in the Silurian of the Armorican Massif and Normandie, France. C. Approximate stratigraphie position of the localities. Successions after: Paris 1981 (Menez-Belair); Michel Robardet, personal communication 2005 (Normandie). Graptolite zonations from Sadler et al. (2009), not to scale.

Chemiré-en-Charnie.—Specimens from the de Tromelin collection (19^th century), housed in the Natural History Museum of Nantes (= Loc. 4 in Siveter et al. 1987; herein Fig. 1). Outcrops no longer available. Siltstone and mudstone rocks containing siliceous nodules collected by Albert Guillier in Le Val Formation (Guillier and de Tromelin 1874; Guillier 1896). Graptolites indicate the Ludlow or Pridoli (Palmer in Siveter et al. 1987; Fig. 1C). Sineruga (well-preserved and undeformed moulds of single valves) occurs with myodocopes (bolbozoids and cypridinids), phyllocarids (Ceratiocaris and Schugurocaris), orthoconic cephalopods, crinoids (Scyphocrinites?) and spirorbids.

Les Buardières.—Numerous exposures along the departmental road D104 between St. Germain-le-Fouilloux and Andouillé, Andouillé, Mayenne (= Loc. 1, Armorican Massif in Siveter et al. 1987; Fig. 1). Highly tectonised, fossiliferous, finely laminated grey to black shales in the La Lande Murée and Le Val Formations (see Paris 1981; Siveter et al. 1987). Tectonic deformation prevents accurate determination of graptolites except Colonograptus. ludensis, Monograptus haupti, and Pristiograptus dubius, that indicate a range from the late Wenlock into the early Ludlow (Palmer in Siveter et al. 1987 and Mark Williams, personal communication 2005; Fig. 1C). Sineruga (flattened single and connected valves) occurs with numerous myodocopes (seven bolbozoid and cypridinid species; Perrier et al. 2011), thinshelled bivalves (Praecardium or Dualina, Actinopteria), brachiopods, orthoconic cephalopods, crinoids, and phyllocarids (Ceratiocaris).

St. Sauveur-le-Vicomte.—Two exposures, one near the sport field, the other one near the cemetery of St. Sauveurle-Vicomte, Manche (see Robardet 1981: fig. 31, Loc. 8, 9; Fig. 1). Finely laminated and highly deformed black shales, St. Sauveur-le-Vicomte Formation (Robardet 1981). Graptolites (Neocolonograptus ultimus) indicate the early Pridoli (Jaeger in Robardet 1981; Fig. 1C). Sineruga gen. nov. (flattened and deformed) is associated with myodocopes (bolbozoids and cypridinids), bivalves (Dualina, Actinopteria, and Praecardium), orthoconic cephalopods, crinoids, and phyllocarids (Ceratiocaris).

Material and methods

Ostracod carapaces were preserved either in shales or in nodules, along with other fossils. Mechanical preparation with a “Vibrotool” and small needles was used with 3D-specimens preserved in nodules, whereas latex and silicon casts were made from external moulds of specimens present in nodules and on the surface of shale slabs. Some key specimens of both types were mounted and coated for scanning electron microscopy (SEM; Hitachi S800, Lyon). The specimens were measured under the binocular microscope using morphometric tools (Scion Image 4.0.2).

Systematic palaeontology

Fig. 2.

General views of myodocope ostracod Sineruga insolita gen. et sp. nov., Ludlow—Pridoli Silurian. A. Paratype, left valve, FSL 710893, Loc. 1. B. Smallest specimen, right valve, FSL 710897, Loc. 1. C. Left valve, FSL 710630, Loc. 1. D. Left valve, LPB 18931b, Loc. 1. E. Right valve, FSL 710895, Loc. 1. F. Holotype, right valve, FSL 710900, Loc. 1. G. Right valve, FSL 710919, Loc. 1. H. Right valve, FSL 710631, Loc. 1. I. Right valve, FSL 710601, Loc. 1. J. Right valve, MNHN 145a, Loc. 2. K. Right valve, FSL710625, Loc. 1. L. Left valve, LPB 18926b, Loc. 1. M. Right valve, FSL 710898, Loc. 1. N. Left valve, LPB 18932b, Loc. 1. O. Complete carapace, FSL 710629, Loc. 1, in oblique frontal (O₁) and oblique dorsal (O₂) views.

Fig. 3.

Details of the carapace of myodocope ostracod Sineruga insolita gen. et sp. nov., Ludlow—Pridoli Silurian. A. Holotype, right valve, FSL 710900, Loc. 1. Anterior (A₁) and anterodorsal (A₂) parts of the valve. B. Complete carapace, FSL 710629, Loc. 1, in frontal (B₁) and dorsal (B₂) views; dorsal view of the hinge (B₃); oblique left lateral view of the hinge and the dorsal part of the adductorial sulcus (B₄). C. Left valve, adductorial sulcus and ventral tubercle, FSL 710630, Loc. 1. D. Left valve, adductorial sulcus, LPB 18932b, Loc. 1. E. Right valve, MNHN 150, Loc. 2. Adductorial sulcus showing the central muscle spot (E₁), muscle spot (E₂). F. Right valve, posterior part and posterodorsal tubercle, FSL 710913, Loc. 1. G. Complete carapace, ventral view, FSL 710892, Loc. 1. H. Complete carapace, posterior view, FSL 710595, Loc. 1. Scale bars: A₁, B₁–B₄, C, E₁, F, G, 200 µm; A₂, E₂, 100 µm; D, 50 µm; H, 500 µm.

Fig. 4.

General views of flattened myodocope ostracod Sineruga insolita gen. et sp. nov., Late Wenlock—Pridoli Silurian. A. Right valve, FSL 710533, Loc. 3. B. Both valves in “butterfly position”, FSL 710924, Loc. 4. Scale bars: A, 500 µm; B, 200 µm.

Fig. 5.

Parameters used for measurements of Sineruga insolita gen. et sp. nov. H, maximum valve height; L, maximum valve length.

Fig. 6.

Measurements of Sineruga insolita gen. et sp. nov. Length-height plot of 39 specimens of S. insolita. Specimens comprise both latex casts from external moulds and 3-D preserved specimens. L, H as in Fig. 5. A = adults; A₁–A₆ = possible juvenile instars 1 to 6.

Table 1.

Characters of “biological importance” in fossil myodocopes and comparison with Recent myodocopes orders and subclass Podocopa. Data concerning Colymbosathon after Siveter et al. 2003, Nymphatelina after Siveter et al. 2007, Recent myodocopes and podocopes after Home et al. 2002 and Cohen et al. 2007.

Discussion

Characters of “biological importance”.—The systematics of fossil ostracods, especially among Palaeozoic taxa, is based on carapace features, the soft parts being unknown in most cases. Therefore, classical taxonomy often relies on a set of morphological characters such as the valve outline, the internal and external relief of the carapace (e.g., lobes, sulcus, and hinge) and ornament (e.g., spines, tubercles, reticulations, see Scott 1961). The robustness of this taxonomy is sometimes questionable; typically, however, there are no alternatives from other sources of information. As a result, the most abundant ostracod group of the lower Palaeozoic, the Palaeocopa, is classified entirely based on carapace characters. No data on the internal anatomy of these ostracods are available. Exoskeletal features provide reliable diagnosis at the specific and generic levels. However, difficulties arise in the use of carapace morphology in the systematics of higher taxa.

The discovery of ostracods with soft tissues in the Silurian of the UK (Herefordshire Lagerstätte; Siveter et al. 2003, 2007, 2010) provides important additional evidence. The myodocope body plan, as it is known in Recent forms, already existed during the early Palaeozoic. For example, the soft parts of Colymbosathon ecplecticos, clearly indicate that this species is a myodocope and probable member of the extant family Cylindroleberididae (Siveter et al. 2003). It is surprising that the carapace of these ancestral myodocopes was not ovoid as in most Recent (Cypridinidae) and fossil (bolbozoids, cypridinids) myodocopid ostracods. Two of the three species described by Siveter et al. (2003, 2007) bear a large Palaeocopid-like carapace (straight dorsal margin, lobal/sulcal morphology). The straight dorsal margin is one of the main diagnostic characters of the Palaeocopa (Benson et al. 1961). Thanks to the exceptional material from UK, we now know that this character was also shared by several Palaeozoic myodocope species. This indicates that ancestral myodocopes secreted various exoskeletons of different shape. However, this line of reasoning is not sufficient to conclude that Palaeocopa were also primitive myodocopes.

This exceptional material (Siveter et al. 2003, 2007, 2010) and Recent studies (Perrier 2007; Perrier et al. 2007, 2011) have shown that Silurian myodocopes were very diverse in their external morphology (probably more than Recent forms) even if their internal body plan showed little variation. Some Silurian forms had a carapace very close to that of Recent myodocopes, bearing an ovoid outline and a well-developed rostrum (cypridinids; see Siveter et al. 1991; Perrier et al. 2011). Other species (bolbozoids; see Siveter et al. 1991; Perrier et al. 2011) show some similarities with Recent forms (i.e., outline, rostrum), however, they display a prominent bulb unknown in Recent representatives. Finally, some groups certainly related to myodocopes (Colymbosathon and Nymphatelina) or thought to be myodocopes (entomozoids; see Gooday 1983; Perrier et al. 2007) bear a morphology distinct from all Recent forms (e.g., straight dorsal margin, lobes/sulci, complex ornament).

The new species described herein is an atypical myodocope ostracod. Its carapace morphology is very different from that of all other known Silurian species and highlights the uncertain taxonomic affinities among bolbozoids, cypridinids and entomozoids. Well-preserved specimens of Sineruga insolita gen. et sp. nov., both in three dimensions and in “butterfly position” (i.e., with both valves flattened in connection), show carapace characters of “biological importance” (see Table 1). These characters can be used to differentiate the higher myodocope taxa in the Silurian. The identification of the relationships between internal (soft) and external (hard) morphology provides insight into the taxonomy of the Silurian myodocopes and can help to differentiate myodocopes from other Palaeozoic ostracods:

Rostrum.—Sineruga gen. nov. does not bear any rostrum or rostral incisure in the frontal part of its carapace. In place of these features, this new genus bears a flattened, rounded to square projection (Figs. 2F, J, M; 3A, B) that has no equivalent in any fossil or Recent myodocope.

All swimming myodocopes, including Recent Myodocopina (Fig. 7A) and Halocypridina (Fig. 7B) as well as Silurian bolbozoids (Fig. 7K) and cypridinids (Fig. 7L), bear a well developed rostrum and incisure (Siveter et al. 1991; Vannier and Abe 1992; Siveter et al. 2010). This rostral complex (rostrum + incisure) is an important functional feature in the swimming mechanism of the animal, helping the swimming antennas (A2) to protrude (see Vannier and Abe 1993). However, even in the absence of a rostrum, the swimming antennas can simply protrude from the gaping carapace. This is the case in Recent Cladocopina (Cohen et al. 2007) and in Colymbosathon ecplecticos and Nymphatelina gravida (Siveter et al. 2003, 2007). The same assumption was made for entomozoids by Perrier et al. (2007). A similar disposition and extension of soft parts, and hence function, may also have been present in Sineruga.

Because no podocopes bear a rostrum, its presence can be used to identify myodocope ostracods. However, the absence of rostrum is not enough to classify a species within the podocopes or the palaeocopes; in fact several Recent (Cladocopina) and fossil (C. ecplecticos, N. gravida) myodocopid species lack a rostrum. Based upon this association Silurian bolbozoids and cypridinids are probably myodocopes whereas entomozoids (among them Sineruga gen. nov.) are possibly myodocopes.

Visual organs.—The anterodorsal part of the carapace of Sineruga insolita gen. et sp. nov. is often broken or altered (Figs. 2D–J, O, 3A, B) whereas the rest of the carapace is generally well preserved. This indicates that the carapace was probably thinner and less calcified at this level, allowing light to pass through it. This particular feature seems to have been directly related to vision.

Not all Recent myodocopes have lateral eyes. Halocypridina have no visual organs, whereas lateral compound eyes are frequent in Myodocopina (Land and Nilsson 1990; Cohen et al. 2007). The lateral eye is located in the anterodorsal part of the body and light is received through their translucent carapace (Vannier et al. 1998: fig. 4; Land and Nilsson 1990: fig. 1). Silurian bolbozoids, cypridinids and some entomozoids bear either a prominent unornamented bulb (bolbozoids, Fig. 7C, D, K) or an unornamented area (visual window) in the anterodorsal part of their valves (cypridinids, Fig. 7E, L). According to previous studies (Siveter et al. 1991; Vannier and Abe 1992; Perrier 2007; Perrier et al. 2007, 2011) all of these features indicate the possession of visual organs. This hypothesis is strengthened by the presence of a lateral eye in exceptionally preserved Silurian fossils Colymbosathon ecplecticos, Nymphatelina gravida, Nasunaris flata; Siveter et al. 2003, 2007, 2010), which lies exactly in the same position as the bulb and the visual window. Sineruga, with its weakened area in the anterior part of its carapace (Fig. 7N), probably displayed a different kind of visual window that had not been seen previously in the myodocope fossil record. It also suggests the presence of lateral eyes in Sineruga.

In Recent ostracods, the lateral eye is present only in myodocopes, podocopes bearing only a naupliar eye (Horne et al. 2002) as probably did palaeocopes (see Tanaka et al. 2009). The presence of symmetrical and well-developed vision-related carapace features that indicate lateral eyes, such as an eye tubercle (Colymbosathon ecplecticos), a bulb (bolbozoids) or a visual window (cypridinids, some entomozoids, and Sineruga gen. nov.), can be used to identify fossil myodocopes. However, the absence of such characters is not sufficient to discriminate podocopes or palaeocopes; in fact, Nymphatelina gravida (Siveter et al. 2007) possesses well-developed lateral eyes, although no external carapace feature indicates their presence.

Dorsal connection.—The dorsal connection is the articulation between the two valves. In “butterfly position”, the dorsal connection of Sineruga insolita gen. et sp. nov. runs the length of the dorsal margin (Figs. 2O₂, 3B₄, 4B). This dorsal connection shows no teeth or dimples and appears to be ligamentous (Fig. 3B₄).

The dorsal connection is very different among Recent myodocopes and podocopes. The podocopes have a hinge with real teeth and dimples (i.e., comparable to that of bivalve molluscs; see Scott 1961). In Recent myodocopes, the dorsal connection is not a true hinge. It consists of a flexible but strong ligament, around which the two valves revolve (Fig.7A; see Kornicker 1969). This ligamentous dorsal connection was most likely present in Silurian forms, which never show teeth and dimples in the dorsal part of their carapace (Siveter et al. 1991). The dorsal connection is shown in fossil species through the preservation of articulated valves in “butterfly position” (Fig. 7C, E, G, I), and the connection takes different positions depending on the group. In bolbozoids, the dorsal connection is located in the anterodorsal part of the valve just above the bulb, the adductorial sulcus and the muscle scar (Fig. 7C, D, K). In cypridinids it is located in the posterodorsal region of the carapace, above the scar of the adductor muscle (Fig. 7E, F, L). Finally, in Silurian entomozoids the dorsal connection is located along the entire length of the straight dorsal margin (Fig. 7G, H, N). The position of the dorsal connection in S. insolita is very close to what is observed in entomozoids (compare Fig. 7G, M and 7I, N).

Fig. 7.

Dorsal connection, sulcus and isthmus in Recent and Silurian myodocopes a key to systematic assignation. A. Dorsal connection of Vargula hilgendorfii, Recent, Japan. Soft parts removed. Frontal (A₁) and anterodorsal (A₂) views. B. Dorsal connection, isthmus and muscle position in Conchoecia atlantica, Recent, Japan. Valves opened mechanically. SEM picture showing the soft parts inside the carapace (B₁), drawing showing the position of the dorsal connection, the isthmus and the adductorial muscle (B₂). “Butterfly position” showing the dorsal connection (C, E, G, I). Dorsal (D, F, I) and posterodorsal (H) views of complete carapace. C, D. Bolbozoe anomala Barrande, 1872. C. FSL 710927, Les Buardières (Loc. 3), late Wenlock—Ludlow. D. FSL 710593, Les Chevrolières, France, Ludlow—Pridoli. E. Silurocypridina retroreticulata Perrier, Vannier, and Siveter, 2011, IRG 33016, Les Buardières (Loc. 3), late Wenlock-Ludlow. F. Silurocypridina calva Perrier, Vannier, and Siveter, 2011, FSL 710634, Les Chevrolières, France, Ludlow—Pridoli. G, H. Richteria migrans (Barrande, 1872). G. FSL 705028a, La Combe d'Yzarne, France, middle Ludlow. H. FSL 705032a, Koukolova Hora Hill, Czech Republic, middle Ludlow. I, J. Sineruga insolita gen. et sp. nov. from Loc. 1 and 4, Ludlow—Pridoli. I. FSL 710925. J. FSL 710924. K–N. Idealized dorsal and left lateral view of the different Silurian myodocopid groups showing the characters of “biological importance”. K. Bolbozoid. L. Cypridinid. M. Entomozoid. N. Sineruga insolita gen. et sp. nov. All specimens, except A_1???, have their front part on the left.

The ligamentous dorsal connection can be used to recognise myodocopes as podocopes and palaeocopes bear a real hinge (Scott 1961). In addition, the position and extension of the dorsal connection can be used to easily distinguish the three groups of Silurian myodocopes (anterior in bolbozoids; posterior in cypridinids; straight central in entomozoids and Sineruga gen. nov.). The position and length of the dorsal connection of Silurian bolbozoids and cypridinids correspond to those found in Recent Myodocopina (e.g., Vargula; see Fig. 7A). Those of Sineruga and entomozoids correspond more closely to what is observed in Halocypridina (e.g., Conchoecia, see Fig. 7B). However, these similarities likely have little- to no taxonomic value as there is no evidence for the existence of fossil halocyprids.

Adductorial sulcus and muscle scar.—Sineruga gen. nov. bears a well-developed adductorial sulcus in the middle/anterior part of the carapace. This sulcus is concave frontward in the dorsal region of the carapace and backward in its ventral part. This particular sinusoidal shape is unknown in all other fossil or Recent myodocopes. A small and simple elliptical muscle scar is present in the middle part of the sulcus.

In Recent myodocopes, the sulcus does not exist except for a slight deformation on the carapace of several forms including Halocypridina (e.g., Conchoecia imbricata, see Vannier and Chen 2000: fig. 6). This is a slight sag in the mediodorsal part of the valves, underlining the posterior part of A2 protopod. However, no Recent group bears a sulcus as marked as in the Palaeozoic forms (bolbozoids and entomozoids). The Silurian representatives of these two groups both bear a well-developed frontward concave sulcus. Because the muscle scar is always situated inside the middle part of the sulcus, the sulcus may be the imprint of the isthmus (area of attachment of the body to the carapace; Fig. 7B, K–N) on the carapace. The sulcus, like the dorsal connection, has a different position depending on the group. In bolbozoids it is located in the front half of the carapace, underlining the bulb (Fig. 7C, D, K). In entomozoids it is located in the central part of the valves (Fig. 7G, H, M). The sulcus of Sineruga is close to that of entomozoids such as Richteria migrans, the only notable difference is its sinusoidal shape. The muscle scar is the imprint of the adductorial muscle on the carapace. In Recent myodocopes and in Silurian bolbozoids (Fig. 7K) this scar consists of series of rounded or elongated patches arranged in rosette or disseminated in a small area in the centre of the valve (see Siveter et al. 1991). In Silurian cypridinids and entomozoids, this scar is very simple consisting of single crescent-shaped (Fig. 7E, L) or elliptical (Fig. 7G, M) spots. The muscle scar of S. insolita gen. et sp. nov. is comparable to that of entomozoids in its shape and position (Figs. 3D, E, 7N).

The adductorial sulcus is of no real value for recognising myodocopes. It is almost absent in Recent myodocopes whereas it is common in palaeocopes and occurs in Recent podocopes. Its position and extension, however, can be used to differentiate Silurian bolbozoids and entomozoids (including S. insolita gen. et sp. nov.). Furthermore, the morphology of the muscle scar is not typically useful for higher taxonomy. The scars of bolbozoids are close to what is observed both in Recent myodocopes and podocopes. On the other hand, the simple spot of Silurian cypridinids and entomozoids has no counterpart in any Recent forms.

Conclusions

Sineruga insolita gen. et sp. nov. is the oldest (upper Wenlock—lower Pridoli) entomozoid reported in the Armorican Massif and is always associated with other myodocopes (bolbozoids and cypridinids).

The data available suggest that Sineruga insolita was probably an atypical, non-ribbed member of the entomozoids. This assignment is based mainly on its carapace morphology (i.e., bean shaped valves, adductorial sulcus, muscle spot and dorsal connection) and indicates that early entomozoids may have had smooth representatives.

The presence of a rostrum and/or symmetrical vision related organs (i.e., indicating lateral eyes) such as the eye tubercle, the bulb, the visual window or the weakened anterodorsal carapace can be used as diagnostic features for myodocope ostracods.

The position and shape of the dorsal connection and to a lesser extent that of the sulcus and the adductorial muscle scar can be used to discriminate the higher groups of Silurian myodocopes (bolbozoids, cypridinids, and entomozoids).