Liocarcinus sp., Stimpson, 1871

Ossó, Àlex & Gagnaison, Cyril, 2019, An appraisal of the Middle-Late Miocene fossil decapod crustaceans of the ‘ Faluns’ (Anjou-Touraine, France), Geodiversitas 41 (9), pp. 367-383 : 374-376

Liocarcinus sp.

( Fig. 5A, B )

MATERIAL EXAMINED AND MEASUREMENTS (in mm). — Partially preserved carapace, MS2012 0 172b: L = 10.5, W = 12, FOW = 8.5, F = 4.

LOCALITY AND HORIZON. — ‘La Sonneterie’ quarry, Meigné-le- Vicomte (Maine-et-Loire). ‘Savignean facies’, Langhian-Serravallian (Middle Miocene).

DESCRIPTION

Small sized carapace, subhexagonal, smooth, slightly wider than long. Regions weakly defined. Three longitudinally aligned tubercles at each side of carapace. Front incomplete, straight, but appears to be slightly produced medially. Orbits relatively large; supraorbital margin with two closed fissures. Anterolateral margin slightly arcuate, with four subtriangular teeth (excluding the outer orbital tooth), the last of them (epi- branchial tooth) the sharper. Posterolateral margin straight; reentrant of fifth pereiopod well marked. Posterior margin straight.Frontal region smooth, flattened.Gastric process poorly defined. Epigastric lobes slightly marked. Protogastric lobes slightly swollen with faintly marked transverse ridge at each lobe; marked tubercle near gastrohepatic groove. Mesogastric region weakly marked. Urogastric region depressed. Cardiac region large, slightly swollen. Intestinal region depressed. Hepatic region depressed. Epibranchial region sigmoidal, weakly marked, with medial tubercle. Mesobranchial region slightly swollen, with marked tubercle, Metabranchial region depressed. Cervical groove faintly marked. Branchiocardiac groove weakly marked. Ventral parts and appendages not preserved.

REMARKS

The features of the studied specimen fit with those of Liocarcinus , and its general outline matches well with the known Miocene Liocarcinus species (see Schweitzer et al. 2010). However, they differ in having a most decorated or rough surface of the carapace, as for instance L. rakosensis Lőrenthey in Lőrenthey & Beurlen, 1929 and L. kuehni (Bachmayer, 1953), whereas in the studied specimen the carapace surface is completely smooth; or in having more marked and raised regions as in the case of L. oroszyi (Bachmayer, 1953) and L. praearcuatus Müller, 1996 (see Bachmayer 1953a: pl. 2, figs 2, 5-5a; Müller 1984a: 83-84, pl. 69, figs 2-6, pl. 71, figs 1-4; 1996: 10, pl. 2, figs 2-3). Liocarcinus ottnangensis (Bachmayer, 1953) differs clearly in having a trifid front and a characteristic prominent transverse ridge running across the mesogastric and epibranchial regions, which is absent in the studied specimen (see Hyžný et al. 2015). Liocarcinus oligocenicus ( Paucă, 1929) and L. lancetidactylus ( Smirnov, 1929) present a different pattern of anterolateral margins and front, furthermore, their poor preservation makes difficult an accurate comparison (e.g. Jerzmańska 1967: fig. 1; Schweitzer et al. 2009: fig. 6; and Garassino & Novati 2001, respectively).

Regarding the extant species of Liocarcinus (see Ng et al. 2008), they have usually a more arched anterolateral margins than the studied specimen, and most of them have a trifid front, and/or a rough surface of carapace, which is smooth in the studied specimen. Liocarcinus pusillus ( Leach, 1815) and L. navigator (Herbst, 1794) have a similar blunt front medially produced, similar to the studied specimen, but they differ in having a rougher dorsal surface, and by the lack of the conspicuous protogastric ridges and three characteristic tubercles, longitudinally aligned, present in the French specimen possess (e.g. Froglia & Manning 1982; Koch & Ďuriš 2016)

The general outline and dorsal sculpture of the studied specimen also resemble that of Miopipus pygmaeus ( Brocchi, 1883) ( Brocchi 1883: 1, pl. 5, figs 4-4a). However, Müller (1984a: 84, pl. 72, figs 1-4), described the front of M. pygmaeus as trilobed, narrow and prominent, and the carapace regions as strongly inflated, which precludes a congeneric relationship with the specimen described herein.

Therefore, in spite that the aforementioned differences could lead us to propose a new species of Liocarcinus for this specimen, based on the poorly preserved sole specimen, we report it as Liocarcinus sp., until more specimens are available for study.

Figures

FIG. 5. — A, B, Liocarcinus sp., MS2012 0 172b: A, dorsal view; B, frontal view; C, D, Necronectes michelini (A. Milne-Edwards, 1861): C, ULB-SO-9, right dactylus; D, ULB-SO-10, left dactylus; E, Paguristes gagnaisoni Gagnaison, 2012, MS2006 FS 134b, right chela; F-I, Xantho cf. moldavicus (Yanakevich, 1977): F, ULB-SO-11, left chela, inner lateral view; G, outer lateral view; H, ULB-SO-12, left chela, outer lateral view; I, frontolateral view; J-M, undetermined chelae: J, ULB-SO-13, lateral view of right chela; K, frontal view; L, ULB-SO-14, lateral view of left chela; M, close-up showing an encrusted colony of bryozoans. Scale bars: A, B, 5 mm; C-M, 10 mm. Photographs by À. Ossó.