Sineruga insolita, Perrier, 2012

Perrier, Vincent, 2012, An atypical Silurian myodocope ostracod from the Armorican Massif, France, Acta Palaeontologica Polonica 57 (2), pp. 363-373 : 365-368

Sineruga insolita gen. et sp. nov.

Figs. 2–5 , 7I, J, N.

Etymology: From Latin insolita , unusual.

Type material: Holotype: FSL 710900 ( Figs. 2F , 3A ) right valve preserved in 3D (L = 2.91 mm; H = 1.75 mm) . Paratype: FSL 710893 ( Fig. 2A ) left valve preserved in 3D (L = 3.19 mm; H = 1.96 mm) .

Type locality: Les Chevrolières , St. Denis−d’Orques, Sarthe, France (Loc. 1) .

Type horizon: Highly deformed siltstone and mudstone beds containing numerous partly pyritized nodules; Le Val Formation ; uppermost Ludlow to lower Pridoli .

Referred material.—Hundreds of valves from four localities in France. 3D preserved specimens: the largest and the smallest specimens (respectively FSL 710630, Fig. 2A, L = 3.20 mm, H = 2.03 mm and FSL 710897, Fig. 2A, L = 1.33 mm, H = 0.87 mm) come from Loc. 1. Flattened specimens: the largest specimen (FSL 710533, Fig; 4A, L = 3.96 mm; H = 2.60 mm) comes from Loc. 3, the smallest (FSL 710924, Fig; 4B, L = 1.11 mm; H = 0.70 mm) from Loc. 4.

Diagnosis.— Entomozoidae with reniform outline, rounded to square anterior margin, rounded ventral margin, slightly pointed posterior margin and straight dorsal margin. Anterior half of valves less prominent than posterior. Dorsal connection centred above sulcus. Sinusoidal adductorial sulcus with simple elliptical muscle spot. Ventral and posterodorsal tubercle often present. Valves completely smooth.

Measurements.—See scatterplots diagrams ( Fig. 6 ). Length to height ratios (L:H) were calculated for 39 specimens from four localities. Five to six ontogenetic stages are probably represented ( Fig. 6 ).

Description.—Symmetrical reniform valves. Maximum valve length (L) across the posterior tubercle; maximum valve height (H) at approximatively mid−length (ventral tubercle) and maximum width across the centre of the posterior half of the carapace. The lateral outline tapers anteriorly. Valves divided by a subcentral sulcus; anterior part less prominent than posterior part (respectively 1/3 and 2/3 of valve area). Straight dorsal margin showing the dorsal connection (more than half of valve length) centred above the sulcus and the muscle spot. Anterodorsal part of the carapace flattened, rounded to square without rostrum or rostral incisures. Ventral margin perfectly rounded. Posterior margin slightly pointed (tubercle) or rounded when tubercle absent. The entire lateral surface is more or less deeply (deeper in the dorsal part) incised by a narrow sinusoidal sulcus. This sulcus originates in a triangular area (centre of the dorsal connection) and extends from dorsal to ventral margins. The sulcus is concave frontward in the dorsal part, and backward in the ventral part. The sulcus broadens in the muscle spot region, which is located at the inflection point of the sulcus. The muscle spot consists of a single reniform scar. Two small tubercles are often present; they are situated posterodorsally and ventrally. External surface of the valve completely smooth.

http://dx.doi.org/10.4202/app.2010.0006 http://dx.doi.org/10.4202/app.2010.0006

Remarks.— Sineruga insolita gen. et sp. nov. is difficult to place within a previously described group of Palaeozoic myodocopid ostracods. Sineruga definitely belongs neither to the cypridinids (no rostrum, but a sulcus and a straight dorsal margin; compare Fig. 7L and N) nor to the bolbozoids (no rostrum and no bulb, but simple muscle scar and straight dorsal margin; compare Fig. 7K and N). Nevertheless, Sineruga shares several characteristics with Silurian and Devonian entomozoids. For example, S. insolita gen. et sp. nov. shares several morphological features with Richteria migrans (middle Ludlow, Fig. 7G, H; see Perrier et al. 2007), such as a bean−shaped outline, a straight dorsal connection, and a well−developed adductorial sulcus (compare Fig. 7M and N). However, Sineruga bears two tubercles and a small frontal projection. These characters are absent in Richteria , which displays the typical entomozoid ribbed ornament and an “eye tubercle”. In some features, Sineruga is also close to the Devonian Rhomboentomozoidae Franklinella lysogorensis from Givetian/Frasnian boundary interval ( Olempska 2002). Both the Silurian and the Devonian species display a straight dorsal connection with a small anterodorsal projection and two tubercles situated posteriorly and ventrally. However, Franklinella has a ribbed ornament and a nearly circular outline very different from the elongated bean shape of S. insolita gen. et sp. nov.

The biggest problem with the entomozoid assignment is that Sineruga lacks the typical external ribbed ornament characteristic of the entomozoaceans (see Gooday 1983). However, when Barrande (1872) described the first known entomozoid “ Entomis migrans ” (now known as Richteria migrans ), he included in this genus three smooth species that clearly belong to the entomozoaceans (see details in Perrier et al. 2007). Their carapaces are smooth, bean shaped with a more or less developed adductorial sulcus, and one species bears an anterior tubercle. Other smooth species were described as Entomis by Canavari (1900; Ludlow, Sardinia). Within these new Canavari (1900) forms, Perrier et al. (2007) suggested that six of these non−ribbed taxa should be included in the entomozoaceans (see also Gnoli et al. 2009). Among the smooth forms described by Barrande (1872) and Canavari (1900) several poorly preserved specimens may have lost their external ornament. However, some specimens clearly show an unornamented carapace.

The carapace of Sineruga insolita gen. et sp. nov. also resembles that of Colymbosathon ecplecticos (Wenlock of England; Siveter et al. 2003) in its general outline and shares with this species the possession of a deep dorsal sulcus. However, S. insolita gen. et sp. nov. has a much longer S−shaped adductorial sulcus that reaches the anteroventral margin, two tubercles located both ventrally and posteriorly ( Colymbosathon has only an eye tubercle), and a less prominently developed anterior lobe. C. ecplecticos was described with its soft parts and placed within the Cylindroleberididae (Myodocopa) . In the absence of preserved soft parts the author cannot be precise with the assignment of S. insolita , however its general outline shows many similarities with Richteria migrans and herein, S. insolita is placed within the Entomozoidae where it possibly deserves its own subfamily. Perrier et al. (2007) showed that R. migrans has a very similar valve outline and lobal/sulcal morphology to that of C. ecplecticos . Moreover, these two taxa lack a rostrum and a rostral incisure. These authors suggested that entomozoaceans may have had strong affinities with myodocopes and perhaps Cylindroleberididae . The same suggestion can be made for Sineruga gen. nov. Stratigraphic and geographic range.—Silurian (late Wenlock to early Pridoli Series), Armorican Massif ( France).

Figures

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.