Axiidean and brachyuran decapods are reported from the middle–late Miocene pre-evaporitic deposits of several localities along the Valmarecchia (Rimini) in the Romagna Apennines (Emilia-Romagna). Goneplax rhomboides (Linnaeus, 1758), already known from the Miocene of Italy, has not been recorded previously in this area, while the record of Chlinocephalus demissifronsRistori, 1886 expands the stratigraphic range of the genus, previously known only from the Pliocene and Pleistocene of Italy. Moreover, some specimens are assigned in open nomenclature to LobocarcinusReuss, 1857, already known from the Miocene of Italy. Finally, some loose chelae are assigned to the families Callianassidae and Eucalliacidae, respectively, without generic and specific assignment. The Valmarecchia decapod assemblage enlarges the knowledge on the composition and distribution of the decapod fauna along the palaeo-Adriatic Gulf before the Messinian Salinity Crisis of the Mediterranean Basin.
1. Introduction
The Valmarecchia, located in the SW Romagna Apennines and surrounding areas, is well-known for the rich fossiliferous laminated layers ranging from the Miocene to the Pliocene, mainly for its marine vertebrate fauna (mainly fish) (Sorbini 1987; Bagli 2004). No crustacean records were previously reported from these layers, except for a single decapod caridean assigned to Palaemon monsdamarum Pasini & Garassino, 2018, from the Messinian tripoli of Mondaino (Rimini, Emilia-Romagna) (Pasini & Garassino 2018). Other previous reports of Miocene decapods from the Romagna Apennines are limited to some anomurans, axiideans, and brachyurans from two localities nearby Brisighella (Ravenna, Emilia-Romagna), which belong to the early–middle Messinian (Pasini et al. 2019). Hence, the new report from Valmarecchia outcrops enhances the scarce knowledge on the presence and distribution of crustaceans around the palaeo-Adriatic Gulf during the middle–late Miocene prior to the onset of the Mediterranean Salinity Crisis.
2. Fossiliferous localities
The Romagna Apennines are generically characterized by an outcropping succession of autochthonous early Miocene to Pleistocene mainly siliciclastic deposits that represent the infill of a fore deep basin. This part of the Apennine margin is characterized, mainly in the western area, by the strong evaporitic outcrop of Primary Lower Gypsum, the so-called Vena del Gesso romagnola, which is Messinian in age. Pre-evaporitic deposits crop out usually below the gypsum sequence. These deposits are composed by a unit straddling the Tortonian–Messinian boundary made up of finely interbedded organic and diatomite rich laminites and mudstones, informally named “euxinic shales”. These deposits span a 1.2 Ma time interval (early Tortonian–early Messinian), show a cyclic pattern and record the palaeoceanographical changes associated with the ensuing Messinian Salinity Crisis (started at 5.96 Ma) (Roveri et al. 2006). The studied specimens have been collected from the following localities located along the Valmarecchia (Fig. 1):
I Monti quarry, Campiano (Novafeltria, Rimini, 43°54′17.46″N 12°17′10.88″E), located on the left orographic side of the Valmarecchia; it consists of two distinct fronts close to each other, herein formalized as MEQ (Eastern quarry) and MWQ (Western quarry). In the MEQ quarry, the fossiliferous green-grey limestones and glauconitic yellow-greenish sandstones pertaining to the Monte Fumaiolo Formation (middle Miocene: Langhian-Serravallian) intercrops with the San Marino Formation (middle Miocene: Burdigalian–Langhian) (Bagli 2004). One single specimen was collected from these layers (MUSNAF/GEO/7188). The MWQ carves into the light blue grey sandy clays of the Argille di Casa i Gessi (Moroni 1955; Ruggieri 1970; Bagli 2004), assigned to the early Messinian (late Miocene) and which are rich in circumlittoral mollusc shells preserved as inner moulds (PIZZOLATO, pers. obs. 2019) (13 specimens: MUSNAF/GEO7183-87; 7190-96).
Montebello (Rimini, 43°58′18″N 12°22′06″E), located along the left orographic side of Marecchia River fossiliferous marly clays assigned to the middle Miocene (Langhian–Tortonian) are exposed (Moroni 1957; Bagli 2004) (two specimens: MUSNAF/GEO/7189a, b, 7197a, b).
Maciano (Rimini, 43°50′08.5″N 12°15′00″E) along the Marecchia Road (SP258), on the right orographic side of Marecchia River. Some poor, indeterminate crustacean remains have been collected from the gray-greenish clays of the Monte Fumaiolo Formation (middle Miocene: Langhian–Serravallian), associated with small incomplete echinoderms (Bagli 2004). We point out that among these not illustrated specimens there are some loose and poorly preserved straight, elongate dactyli, transversally flattened, with the occlusal margin bearing a rim of small (broken) teeth resembling the pectinate dactyli of some shrimp genera (MUSNAF/GEO/7198a–d).
3. Material
The studied specimens are mostly fragmentary and usually preserved as three-dimensionally inner moulds, due to the dissolution of the original exoskeleton, or simply as external casts. They have been assigned as follows: Callianassidae genus & species indet. (2 specimens), Eucalliacidae genus & species indet. (3 specimens), Lobocarcinus sp. (1 specimen), Goneplax cf. G. gulderi Bachmayer, 1953 (4 specimens), Goneplax rhomboides (Linnaeus, 1758) (3 specimens), and Chlinocephalus demissifrons Ristori, 1886 (1 specimen). Moreover, a single complete carapace was assigned to an indeterminate carpilioid brachyuran. The studied specimens are housed in the palaeontological collection of the Museo di Storia Naturale dell'Accademia dei Fisiocritici (Siena) (MUSNAF).
For the higher-level classification, we follow the arrangements proposed by Ng et al. (2008), De Grave et al. (2009) and Schweitzer et al. (2010).
Abbreviations: lcxp: carapace length; lpa: palm length; P1: pereiopod 1; wpa: palm width; wcxp: carapace width.
4. Systematic palaeontology
Order Decapoda Latreille, 1802
Infraorder Axiidea De Saint Laurent, 1979
Family Callianassidae Dana, 1852
Genus & species indet.
Fig. 2A
Occurrence: MWQ, Campiano (Valmarecchia, Rimini).
Material and measurements: Two poorly preserved incomplete chelae (dactyli not preserved) in lateral view (MUSNAF/GEO/7183 (left chela) – lpa: 7.5 mm; wpa: 5 mm; MUSNAF/GEO/7184 (right chela) – wpa: 6 mm).
Description: Subrectangular flattened palm, longer than wide, with upper and lower margins apparently smooth; elongate index with hook-shaped pointed tip upturned; occlusal margin of index smooth.
Discussion: Based on Poore et al. (2019), the outline shape of the studied propodi seems to fit with the diagnostic characters of the Callianassidae to which they are herein tentatively assigned. Due to their poor preservation, we cannot recognize the main generic and specific characters useful for comparison. Pasini et al. (2019: 28, fig. 1A) reported similar propodi from the early–middle Messinian (late Miocene) of Brisighella (Ravenna, Emilia-Romagna) tentatively compared with Callianassa subterranea (Montagu, 1808). The affinities of the studied propodi with the genera of the Callianassidae cannot be solved without additional well-preserved specimens. Therefore, their presence is simply recorded in the early Messinian of Valmarecchia.
Fig. 2.
A – Callianassidae genus & species indet., MUSNAF/GEO/7183, left chela, lateral view (scale bar equals 2.5 mm). B – Eucalliacidae genus & species indet., MUSNAF/GEO/7184, right chela, lateral view (scale bar equals 1.7 mm). C – Lobocarcinus sp., MUSNAF/GEO/7188, left palm and carpus (scale bar equals 30 mm). D – ?Carpilioidea, family, genus & species indet., MUSNAF/GEO/7189, carapace dorsal view (scale bar equals 7 mm).
Family Eucalliacidae Manning & Felder, 1991
Genus & species indet.
Fig. 2B
Occurrence: MWQ, Campiano (Valmarecchia, Rimini).
Material and measurements: Three poorly preserved incomplete right chelae (dactyli not preserved) in lateral view (MUSNAF/GEO/7185 – lpa: 6 mm; wpa: 6 mm; MUSNAF/GEO/7186 – lpa: 6 mm; wpa: 5.5 mm; MUSNAF/GEO/7187 – lpa: 4 mm; wpa: 4 mm).
Description: Subrectangular flattened palm, as long as wide, with upper and lower margins apparently smooth; lower part of palm with a relatively strong crest, extending medially in the index; elongate index with straight pointed tip; occlusal margin of index slightly sinuous and smooth.
Discussion: Based on Poore et al. (2019), the outline shape of the studied propodi seems to fit with the diagnostic characters of the Eucalliacidae, to which they are tentatively assigned herein. Due to their poor preservation, we cannot recognize the main generic and specific characters useful for comparison. Pasini et al. (2019: 28, fig. 1B) reported similar propodi from the early–middle Messinian (late Miocene) of Brisighella (Ravenna, Emilia-Romagna) assigned to Calliax sp. The affinities of the studied propodi with the genera of the Eucalliacidae cannot be solved without additional well-preserved specimens. Therefore, their presence is simply recorded in the early Messinian of Valmarecchia.
Infraorder Brachyura Linnaeus, 1758
Section Eubrachyura De Saint Laurent, 1980
Family Cancridae Latreille, 1802
Subfamily Lobocarcininae Beurlen, 1930
Genus Lobocarcinus Reuss, 1857
Type species: Cancer paulinowuertembergensis V. Meyer, 1847, by original designation.
Lobocarcinus sp.
Fig. 2C
Occurrence: MEQ, Campiano (Valmarecchia, Rimini).
Material and measurements: One incomplete palm and carpus three-dimensionally preserved (MUSNAF/GEO/7188 – lpa: 28 mm; wpa: 21 mm).
Description: Stout tuberculate/spinate left cheliped; palm subrectangular, with slightly convex upper and lower margins, narrowing proximally; palm ovoid in transverse section, with smooth flattened inner surface, lacking both fingers; incomplete carpus, with a pointed spine at the inner distal margin and alternate tubercles ranged in ?three longitudinal rows on outer surface.
Discussion: The big-sized incomplete P1 propodus does not preserve the original exocuticle probably covered by pointed spines as in Lobocarcinus Reuss, 1857. The outer ornamentation of the carpus (as preserved) seems to have some rows of granulate keels resembling Lobocarcinus sismondai (V. Meyer, 1843). The latter is widespread in the fossil record from the Miocene to the Pleistocene of Europe and Algeria (see De Angeli & Garassino 2006 for full references). The presence of strong tubercles on the P1 propodus resembles the ornamentation of some representatives of Xanthidae and Pilumnidae as well. However, due to the poor preservation of the studied specimen, considering the relatively big size of the studied specimen and the arrangement of the tubercles in rows (as observed), it is tentatively compared with Lobocarcinus, leaving it in open nomenclature.
?Superfamily Carpilioidea Ortmann, 1893
Family, genus & species indet.
Fig. 2D
Occurrence: Montebello (Valmarecchia, Rimini).
Material and measurements: One poorly preserved carapace in dorsal view (MUSNAF/GEO/7189a, b – lcxp: 22 mm, wcxp: 32 mm).
Description: The convex carapace is partially preserved as an internal mould, slightly compressed dorso-ventrally; carapace wider than long, ovoid in outline, domed with a finely granulate dorsal surface; narrow front; sub-elliptic orbits poorly marked; convex anterolateral margins with some spines/tubercles; posterolateral margins converging to the posterior margin; posterior margin apparently narrow; dorsal regions undifferentiated.
Discussion: The general shape of the carapace (as preserved) resembles that of some Carpilioidea Ortmann, 1893, especially that of the representatives of Palaeocarpilius A. Milne-Edwards, 1862 having a fossil range from the Eocene to the Miocene. However, the poor preservation of the carapace ornamentation and margins and the absence of the chelipeds do not allow a more confident systematic assignment of this specimen.
Superfamily Goneplacoidea Macleay, 1838
Family Goneplacidae Macleay, 1838
Subfamily Goneplacinae Macleay, 1838
Genus Goneplax Leach, 1814
Type species: Ocypoda bispinosa Lamarck, 1801 [= Goneplax rhomboides (Linnaeus, 1758)], by original designation.
Fossil species: Goneplax gulderi Bachmayer, 1953; G. rhomboides (Linnaeus, 1758).
Goneplax cf. G. gulderi Bachmayer, 1953
Fig. 3A
Occurrence: MWQ, Campiano (Valmarecchia, Rimini).
Material and measurements: Four incomplete carapaces in dorsal view (MUSNAF/GEO/7190 – lcxp: 17 mm, wcxp: 22 mm; MUSNAF/GEO/7191 – lcxp: 18 mm, wcxp: 22 mm; MUSNAF/GEO/7192 – lcxp: 18 mm, wcxp: 23 mm; MSUSNAF/GEO/7193 – not measurable).
Discussion: The studied specimens fit with the generic characters of Goneplax Leach, 1814 in showing the typical carapace dorsal proxy characters sensu Schweitzer (2003), as pointed out by Garassino et al. (2013: 357). Although the specimens are incomplete, some characters, such as the subtrapezoidal carapace; the frontal margin slightly wider than the orbits; the dorsal carapace with raised transverse ridges, and the pointed triangular extraorbital spine, suggest a comparison with G. gulderi Bachmayer, 1953 (Garassino et al. 2013: 357), previously recorded from the Miocene and Pliocene of Piedmont, Tuscany, Emilia-Romagna, and Sardinia (for an updated list see Garassino et al. 2013; Pasini et al. 2019).
Fig. 3.
A – Goneplax cf. G. gulderi Bachmayer, 1953, MUSNAF/GEO/7191, carapace, dorsal view (scale bar equals 4.5 mm). B – Goneplax rhomboides (Linnaeus, 1758), MUSNAF/GEO/7194, carapace, dorsal view (scale bar equals 4.5 mm). C, D – Chlinocephalus demissifrons Ristori, 1886, MUSNAF/GEO/7197a, b, carapace, dorsal view (part and couterpart) (scale bar equals 14 mm).
Goneplax rhomboides (Linnaeus, 1758)
Fig. 3B
1758 Cancer rhomboides. – Linnaeus, p. 626.
2013 Goneplax rhomboides. – Garassino et al., pp. 359–361, figs. 1B, C, 2 [with synonymy].
2014 Goneplax rhomboides. – Pasini et al., p. 253, fig. 8B.
Occurrence: MWQ, Campiano (Valmarecchia, Rimini).
Material and measurements: Two carapaces in dorsal view (MUSNAF/GEO/7194 – lcxp: 12 mm; wcxp: 19 mm; MUSNAF/GEO/7195 – not measurable), and one right palm (MUSNAF/GEO/7196 – lpa: 20 mm; wpa: 7 mm).
Discussion: Garassino et al. (2013) revised the fossil species of Goneplax including a full discussion and a key for the fossil species. The carapace outline and the elongate P1 right propodus fit with the main characters of G. rhomboides (Linnaeus, 1758) (Garassino et al. 2013: 359–361). According to Garassino et al. (2013) and Pasini et al. (2014), this species is widespread from the Miocene to the Pleistocene of several Italian regions, such as Piedmont, Emilia-Romagna, Tuscany, Lazio, and Sicily.
Family Euryplacidae Stimpson, 1871
Genus Chlinocephalus Ristori, 1886
Type species: Chlinocephalus demissifrons Ristori, 1886, by monotypy.
Fossil species: Chlinocephalus demissifrons Ristori, 1886.
Chlinocephalus demissifrons Ristori, 1886
Fig. 3C, D
1886 Chlinocephalus demissifrons. – Ristori, p. 101, pl. 2, figs. 5, 6.
1929 Chlinocephalus demissifrons. – GLAESSNER, p. 113.
1969 Chlinocephalus demissifrons. – GLAESSNER, p. R517.
2003 Chlinocephalus demissifrons. – KARASAWA & KATO, tab. 5.
2004 Chlinocephalus demissifrons. – Garassino et al., pp. 275–278, figs. 15, 16.
2006 Chlinocephalus demissifrons. – DE ANGELI & Garassino, p. 64.
2009 Chlinocephalus demissifrons. – DE ANGELI et al., p. 195.
2010 Chlinocephalus demissifrons. – SCHWEITZER et al., p. 133.
2012 Chlinocephalus demissifrons. – Garassino et al., p. 52.
2018 Chlinocephalus demissifrons. – BALDANZA et al., pp. 11, 27.
Occurrence: Montebello (Valmarecchia, Rimini).
Material and measurements: One complete carapace and legs in dorsal view; MUSNAF/GEO/7197a, b (part-counterpart) – lcxp: 24 mm, wcxp: 28 mm.
Discussion: According to Garassino et al. (2004: 276), the morphological characters of the studied specimen fit with those of Chlinocephalus demissifrons, to which it is assigned. The species has been previously recorded from the Pliocene of Liguria, Piedmont, Emilia-Romagna, and Tuscany (Garassino et al. 2004) and from the Pleistocene of Umbria (Baldanza et al. 2018). Based upon the fossil record of this species, it is the first record from the Miocene, expanding its stratigraphic range back to the middle Miocene (Langhian–Tortonian).
5. Conclusions
The studied assemblage result to be the first record from the Romagna Apennines of Valmarecchia (Rimini), enlarging the knowledge of the scarcely reported decapod communities living in the Mediterranean just before the late Miocene evaporitic event with dramatic reduction of the Basin and significant ecological repercussions. The ghost shrimps as burrowers are represented by loose, poorly preserved propodi, tentatively assigned to the families Callianassidae and Eucallliacidae, respectively, not assignable at specific level. Among the Brachyura, Chlinocephalus demissifrons is herein reported for the first time from the Miocene, expanding the stratigraphic range for this species. Interesting is the presence of a possible representative of Carpilioidea, and the representatives of the Goneplacidae seem to be the more common decapod crustaceans within the studied fauna. Unfortunately, as previously observed for the other studied assemblages from the Miocene of the Romagna Apennines, the usually poor preservation of the specimens consisting of isolated propodi or a single disarticulated (or incomplete) carapace, indicates that this assemblage is most probably not representative for the real original bio-community, but seems to be limited by burial events and post-mortem transportation of the specimens in a disturbed environment (Pasini et al. 2019: 32).
Acknowledgements
We wish to thank G. Manganelli (Museo di Storia Naturale dell'Accademia dei Fisiocritici, Siena, Italy), who gave us the permission to study the specimens, G. Teruzzi (Museo di Storia Naturale di Milano, Italy), for photographs of specimens, M. Mura (Museo di Storia Naturale di Milano, Italy), for the settlement of the iconographic apparatus, M. Hyžný, (Department of Geology and Palaeontology, Faculty of Natural Sciences, Comenius University, Bratislavia, Slovakia) for useful suggestions on the Axiidea specimens; H. Karasawa, (Mizunami Fossil Museum, Yamanouchi, Akeyo, Mizunami, Japan) and an anonymous reviewer for their careful reviews and criticism.
