Necronectes michelini (A. Milne-Edwards, 1861)

Ossó, Àlex, Gagnaison, Cyril & Gain, Olivier, 2022, A re-appraisal of the middle-late Miocene fossil decapod crustaceans of the ‘ Faluns’ (Anjou-Touraine, France), Geodiversitas 44 (6), pp. 207-228 : 220-221

Necronectes michelini (A. Milne-Edwards, 1861)

( Fig. 6A-C )

Scylla michelini A. Milne-Edwards, 1861 : 137, pl. 3., figs 3-3A. — Couffon 1908: 3, 4, pl. 1, figs 6-10, pl. 2, figs 1, 2. — Schweitzer et al. 2006: 122; 2010: 110. — Karasawa et al. 2008: 106. — Emmerson 2017: 584.

Cancer macrochelus – Millet 1854: 152 (non Desmarest, 1817). Scylla cf. michelini – Glaessner 1928: 184, 185.

Scylla? michelini – Glaessner 1929: 374.

Scylla sp. (cf. michelini ) – Lőrenthey & Beurlen 1929: 178, pl. 15, figs 5, 6.

Necronectes michelini – Glaessner 1933: 5-6. — De Angeli & Marangon 1992: 178, 179. — Ossó & Gagnaison 2019: 376-378, fig. 5C-D .

Scylla michelini ? – Betancort et al. 2014: 345, pl. 1, figs K1-K2 (non I1-I2).

MATERIAL EXAMINED AND MEASUREMENTS (in mm). — Ventral carapace preserving chelipeds MHNH-2016-79-2003, W (from carpus to carpus) = 150. — Right dactylus, ULB-IV-A (51): L = 41.5, H = 16. — Right dactylus ULB-IV-A (41): L = 20, H = 9.5. — Fragment of right epibranchial portion of dorsal carapace bearing two last anterolateral teeth, ULB-IV-A (42): L = 17.5, W = 9.5.

LOCALITY AND HORIZON. — ‘La Sonneterie’ quarry, Meigné-le-Vicomte (Maine-et-Loire). ‘Savignean facies’, Langhian (middle Miocene): MHNH-2016-79-2003 and ULB-IV-A (51); and, ‘Blandinerie’ quarry, Breil (Maine-et-Loire). ‘Savignean facies’, Langhian (middle Miocene): ULB-IV-A and ULB-IV-A (42).

DESCRIPTION (EMENDED)

Large ventral carapace preserving sternum and chelipeds, surface smooth. Sternum relatively large, flattened, maximum width at level of sternite 6. Sternite 2 subtriangular; sternite 3 inverted subpentagonal; sternite 4 subtrapezoidal; sternite 5, 6 and 7 subrectangular transversely elongate, distal margin rounded; sternite 5 placed in acute angle respect the axis of sternum, sternite 6 almost in right angle, and sternite 6 in slightly obtuse angle. Episternites posteriorly directed. Suture 2/3 complete, horizontal; suture 3/4 inverted V-shaped, weakly distinguishable, only laterally. Sutures 4/5, 5/6, and 6/7 appear complete. Pleon not preserved. Sternopleonal cavity deep, narrow, almost reaching suture 3/4. Pterygostome subtrapezoidal elongate. Ischium of third maxilliped large, with medial longitudinal groove, inner margin strongly convex; exopod subrectangular elongate, narrower than ischium. Chelipeds strong, slightly heterochelous. Merus robust, subtriangular in section, outer margin strongly vaulted, wider at mid length. Carpus strong, embedded in matrix.Right chela strong, dactyli curved distally; palm about 0.70 of length of chela, subtrapezoidal, longer than high, higher distally; upper margin rounded, slightly convex; lower margin rounded, straight; outer margin strongly convex, smooth; dactylus stout, gently curved forward, strong proximal knobstick molariform tooth followed by six serial rounded conical teeth of different sizes, and rounded tip; index stout straight with rounded tip, strong flattened proximal molariform tooth, followed by three blunt teeth.Left chela strong, elongate, slightly shorter than right chela, dactyli gently curved distally; palm subrectangular, longer than high, about 0.68 of length of chela, upper and lower margins straight, rounded, outer margin strongly convex; cutting edge of dactyli with serial conical teeth.

R EMARKS

Erected by A.Milne-Edwards (1861), to place fragmentary large chelipeds found in the ‘Faluns’, Necronectes michelini , was originally placed within Scylla De Haan, 1833 due to its similarity to the strong chelipeds of the extant S. serrata ( Forskål, 1775) , albeit the author already considered the differences between the chelae of both species such as the rounded upper margin of palms, which are flattened and angled in Scylla spp. , and the absence in N. michelini of the two typical distinct spines on the distal upper part of the palm, near the insertion of dactylus (see Fig. 6B ; Couffon 1908: pl. 1, figs 6-10, t. 2, figs 1, 2; MNHN.F.B25950), which are usually present in Scylla spp. (e.g. Keenan et al. 1998; Trivedi & Vachhrajani 2013; Vincecruz-Abeledo & Lagman 2018). Glaessner (1933: 5), included S. michelini within Necronectes A. Milne-Edwards, 1881 , in view of the similarities of the chelae of both taxa. Subsequent authors assigned this species indistinctly to both genera, Scylla and Necronectes , as explained in Ossó & Gagnaison (2019: 376-378, and references therein). Since the chelipeds of the studied specimen are identical to the type series MNHN.F.B25950 described by A. Milne-Edwards (1861), the conspecificity between them is beyond doubt.

For the first time, a specimen of Necronectes michelini preserving features other than the chelipeds, is available for study, although important diagnostic features, such as the dorsal carapace is not preserved. Nevertheless, the sternum of the studied specimen fits well with the sternums of other representatives of Necronectes , such as N. proavitus ( Rathbun, 1918) and N. schafferi Glaessner, 1928 (e.g. Rathbun 1918: pl. 55; De Angeli & Berti 2017: fig. 2, 1b).

The absence of specimens formally available for study with the preserved dorsal carapace, although some of them are known in private collections (e.g. Gagnaison et al., 2020: fig. 2A), prevents us from going further in our considerations. For instance, to explore the possible conspecific relationship between the circum-Mediterranean and Paratethyan Miocene species of Necronectes , such as N. schafferi and N. batalleri ( Vía, 1941) , almost identical morphologically, and N. michelini (cf. Vía 1941: figs 3, 4, pl. 5, figs 31, 32, pl. 6, figs 33, 34; De Angeli & Marangon 1992: pl. 1, figs 1, 2, pl. 2, figs 1a, 1b; Ferratges 2017: fig. 43, pl. 29, fig. C).

Figures

FIG. 6. — Decapod from the ‘Savignean facies’ (Langhian) of ‘La Sonneterie’ quarry,Meigné-le-Vicomte (Maine-et-Loire):A-C, Necronectes michelini (A. Milne-Edwards, 1861), MHNH-2016-79-2003: A, ventral view; B, view of upper margin of chelipeds; C, frontal view of chelipeds. Decapods from the ‘Savignean facies‘ (Langhian-Serravallian) of the ‘Blandinerie’ quarry, Breil (Maine-et-Loire): D-G, Xantho moldavicus (Yanakevich, 1977): D, ULB-IV-A (6), dorsal view; E, frontal view; F, ULB-IV-A (11), dorsal view; G, frontal view. Decapod from the probable Tortonian of Doué-la-Fontaine (Maine-et-Loire): H-I, Undetermined cheliped MNHN.F.B39266: H, outer margin; I, upper margin; J, inner margin. Scale bars: A-C, 50 mm; D-G, 5 mm; H-J 20 mm. Photographs of: A-C, by P. Saulet; D-J, by À. Ossó.