New reports of decapod Portunus monspeliensis A. Milne Edwards, 1860 from Miocene beds of eastern Slovenia with notes on palaeoecology and palaeobiogeography

In the present paper we report on several new occurrences of decapod Portunus monspeliensis A. Milne Edwards, 1860 from Miocene beds of eastern Slovenia, i. e. from the already known locality Šentilj in the northeastern Štajerska region and additional new localities in the Kozjansko and Dolenjska regions. These new reported occurrences of P. monspeliensis from the Middle Miocene (Badenian) strata of eastern Slovenia improve our knowledge of this otherwise widespread decapod crustacean. Additionally, we also re-evaluate the environmental preferences of the species and its wider palaeobiogeographical distribution during the Miocene in the Mediterranean, Atlantic and Paratethys Seas.

The remains of P. monspeliensis are one of the most abundant Miocene crabs and almost always represent the dominant decapod species in the strata where they occur. They are almost exclusively found in siliciclastic sediments such as sandstones and calcarenites, and their state of preservation is mostly not good. Due to their size, they are usually compressed, fractured and lacking exocuticle. Only recently illustrated specimens from the marly limestone of the middle Miocene Sardinia (MaranGon & De anGeli, 2009) are exceptionally well preserved and provide better insight on the species morphology of the dorsal carapace.
Despite the widespread distribution and common occurrence, this fossil portunid species has not received relevant scientific attention in the last decades. In the 19th and in the beginning of the 20th century several authors (via, 1923;veiGa Ferreira, 1954;PhiliPPe & secretan, 1971) have reported the European specimens under different, synonymous, names: Lupa hastata linnaeus, 1767; Neptunus convexus ristori, 1888; and Neptunus granulatus A. Milne Edwards,, 1860. Beside these recognized synonyms of P. monspeliensis, there are currently over 40 valid fossil species of genus Portunus weBer, 1795 , and many of them were described only from single poorly preserved specimens or chelae. Although even a superficial investigation of the fossil representatives of the genus shows great morphological variations, descriptions of fossil species usually illustrate carapaces lacking cuticle, severally fragmented specimens, and descriptions of isolated chelipeds. As it has already been pointed out by Karasawa and co-authors (2008) a re-evaluation and revision of all fossil species of Portunus would be welcome, but that is out of the scope of the present paper.
The only know occurrence of fossil remains of P. monspeliensis from the Miocene strata of Slovenia so far is the report of 27 fragments of this decapod (five carapaces) from Badenian sandstones of Šentilj (MiKuž, 2003). Interestingly, Križnar (2014)

Geology and stratigraphy of the localities
The newly presented specimens originate from various localities of the former Central Paratethys Sea that covered the eastern part of Slovenia in the Miocene period (Fig. 1). The following localities yielded new material of this widespread decapod crustacean:

Štrihovec / Šentilj
Štrihovec was until recently the only known locality with fossil remains of P. monspeliensis from the Slovenian territory (MiKuž, 2003). Štrihovec is situated near Šentilj, a town in northeast Slovenia, bordering on Austria. The fossil bearing layers were accessible in the years 1995/1996, when middle Miocene (Badenian) sandstone, marl, and lithothamnium limestone were exposed during construction works for the new highway.

Gruška / Kozje
One decapod crab specimen was recovered from the Gruška jama locality near the town of Kozje in eastern Slovenia, close to border of Croatia. The Middle Miocene (Badenian) strata exposed here are comprised of sandstone and lithothamnium limestone (aničić & juriša, 1985), where macrofossils are rare. Sporadic finds from this locality comprise mostly poorly preserved bivalves with dissolved shells and rare echinoderms (MiKuž, 2010).

Trebče / Bistrica ob Sotli
This specimen was retrieved from a road cut on local road from Trebče towards Podsreda in eastern Slovenia, in the Kozjansko region. The crab was collected in yellowish calcareous sandstone of Middle Miocene (Badenian) age. There is also an outcrop of coarser clastic rocks, i. e. conglomerates with rhodoid spheres located close by (aničić et al., 2002).

Dolnja Stara vas
A construction site behind the gasoline station in Dolnja Stara vas near Škocjan na Dolenjskem was opened in the early 2000s, which exposed a long outcrop of Middle Miocene (Badenian) strata (MiKuž & horvat, 1998). The lithologies at outcrop vary from breccia, sandstone, biocalcarenites and lithothamnium limestone. A rich macro fauna, comprising mostly of bivalve species with rare gastropods and cirripeds was described from the sandstones and calcarenites (MiKuž & Petrič, 2008), from which we also collected a specimen of the P. monspeliensis.

Šentvid / Čatež
Grey to yellowish sandstones and calcarenites interbedded with grey siltstone are exposed along the slopes of hill St. Vid, south of Brežice in eastern Slovenia. Similarly to the wider surrounding area this lithology is part of the middle Miocene (Badenian) Čatež formation sequence (rižnar et al., 2002). So far only sporadic finds of echinoderm fragments and indeterminable bivalves are reported from these beds. The crab specimens are surprisingly common in the yellowish calcarenites in this locality.

Systematic description
The systematics used herein follows . Subsection Heterotremata Guinot, 1977Superfamily Portunoidea Rafinesque, 1815 Family Description: The specimens have a medium sized hexagonal carapace, significantly wider than long (width/length ratio is about 1.75, greater in bigger individuals, up to 1.85); greatest width at the last (ninth) anterolateral spine (Tab. 1). The carapace is slightly convex in the cross section and its dorsal surface is densely covered by small granules. The front is protruding and slightly downturned, approx. 20 % of maximum carapace width, with axial notch; two forward directed short spines on either side and followed by a distinctive sharp, outward pointing, inner orbital spine. Fronto-orbital margin about 50 % of maximum carapace width. Orbits wider than front, forward directed; supraorbital margin sinuous, incised by two closed fissures, one medially and other near the outer orbital tooth. Anterolateral margin long and slightly convex with nine anterolateral spines, first spine (outer orbital) directed forward and strong, with subsequent seven subtrigonal spines each somewhat smaller and outwards directed; ninth anterolateral spine prominent, the largest and laterally directed at almost a right angle to the vertical axis; in younger specimens steeper angle with regards to anterior margin. Posterolateral margin straight with concave depression (reentrant of fifth pereiopod) in the last third of its length. Posterior margin slightly concave and rimmed, broad, width approx. 75 % of the frontoorbital margin width.
Carapace regions faintly defined; protogastric region semi-circular lobes with transverse ridge; mesogastric and metagastric regions are trapezoid shaped and separated by faint transverse ridge; mesogastric process long, ending behind frontal axial notch. Cardiac region well-defined, pentagonal, anterior borders straight, posterior borders concave, whole region somewhat swollen with longitudinally running central depression. Intestinal region faint and circular. Hepatic region flat and triangular. Epibranchial region wide, forming arcuate transverse ridge from largest anterolateral spine to mesogastric region, separating the epibranchial and mesobranchial regions. Cervical groove distinct. Branchiocardiac grooves well marked, along sides of cardiac region.
Male abdomen subtriangular with straight converging margins, abdominal somite 1 and 2 narrow and wide, only somite 2 is observable from the ventral side with a concave notch on the distal end where it interlocks with thoracic sternite 8; abdominal somites 3-5 fused in a wide, subtrapezoidal plate with slightly concave lateral margins, somite 3 widest, transversely keeled; somite 6 trapezoidal, longer than somites 3-5 with convex lateral margins; male telson subtriangular approximately as high as wide. Female abdomen much wider, about half the carapace width, semi-circular; somites 1 and 2 narrow and wide; somites 3-5 rectangular in shape with convex lateral margins; somite 6 wider posteriorly, with sinuous lateral margin, posteriorly convex and anteriorly concave, telson roundly triangular.
Merus of the third maxillipeds subrectangular and elongated. Chelipeds subequal with elongated merus; carpus short, palm rectangular and elongated, with three longitudinal ridges on outer surface. Fixed finger is triangular and elongated, as long as palm. Occlusal margin of chelae heterodontic with a clear knobstick molariform tooth in the right chela, followed by a series of tuberous teeth.

Palaeoecology and environment
As indicated in the geological setting of most of the aforementioned works and personal observations of the authors, specimens of Portunus monspeliensis are recovered from typical Miocene siliciclastic sediments. Portunus monspeliensis is almost exclusively collected from sandstone and sandy limestone that is interbedded with marl. These lithologies represent a variety of sublittoral facies comprising from inshore to offshore waters (coMaschi caria, 1956; MaranGon & De anGeli, 2009), or from estuarine and even lagoon or brackish environments with a sandy, muddy, or sea grass bottom (nichols, 2009).
Portunus monspeliensis may be considered a euryhalin species given the high salinity fluctuation existent in the different observed ecosystems it inhabits (Müller, 1993). Many of the extant portunids, such as Portunus pelagicus, are found in the Indo-Pacific waters and even in the oriental margin of the Mediterranean Sea and share similar ecological preferences and habitats as well as clear morphological similarities with species P. monspeliensis (lai et al., 2010). From this it can be inferred that other aspects of their biology and ecology, such as predation or swimming capacities, may be similar as well. Both species have similar nearly homochelic claws with a clear knobstick molariform tooth in the right chela and followed by tuberous teeth or a series of conical teeth (Pl. 1,F) which indicate the capacity of chelae for crushing and cutting its prey (sPiriDonov et al., 2014). Accordingly, P. monspeliensis must have been, similarly as Portunus pelagicus, an opportunistic predator and scavenger, depending on the availability of prey. The majority of its diet would consist of teleost fish, molluscs, crustaceans, polychaetes, and substrate debris (KunsooK et al., 2014). Judging by the shared paddle-like form of the fifth pereiopod in Portunus monspeliensis, it is supposed that the species was an active swimmer, remaining buried in the sediment during inactivity, analogous to the extant P. pelagicus.
When evaluating the depositional settings that bear fossils of Portunus monspeliensis, it can be concluded from comparison with extant P. pelagicus and fossil bearing lithologies, that they inhabited a wide range of habitats, but preferred sublittoral algal and sea grass meadows on both sandy and muddy substrata (KunsooK et al., 2014;chanDe & MGaya, 2003). The choice of habitat, ranging from shallow inshore waters to deeper offshore waters, would most likely also vary with age, sex, and season (svane & hooPer, 2004). In addition to the above mentioned similarities with the extant Portunus pelagicus which may share its habitat with genus Scylla De Haan, 1833 and with other portunids (chanDe & MGaya, 2003), P. monspeliensis species frequently occurs with less common Miocene portunids such as Scylla and Necronectes Milne-Edwards, 1881(Via, 1932Veiga Ferreira, 1954;Müller, 1993 and AO pers. obs.).

Palaeobiogeography
True portunids first appeared in the Middle-Late Eocene period according to the present fossil record (Ossó, 2016). The Portunus monspeliensis species appears to be a derived taxon from the robust stock of true portunids that dwelt in the western margin of the Tethys during the Middle-Late Eocene and Oligocene, and inhabited coasts with siliciclastic sediments that originated from the Alpine Orogeny and filled the Mediterranean and Central Paratethys basins during the Miocene period. Strikingly, we have no fossil records of P. monspeliensis from the Miocene of the Balearic Islands (J. Juárez, pers. comm.) nor in the extensive Miocene deposits of the Betic Strait (I. Bajo, pers. comm.), which would seem a natural migration path from the Mediterranean to the Atlantic coast in Portugal during the middle Miocene.
Portunus monspeliensis was widely distributed along the Paratethys, the Mediterranean and the southern Atlantic coast of the Iberian Peninsula during the Miocene. Interestingly the reported distribution of P. monspeliensis in Paratethys is heavily dominated in the Central and Western Paratethys (Fig. 3), near the hypothetical Slovenian Corridor, connecting the Central Paratethys and the Mediterranean (Bartol et al., 2014). Further fossil species of Portunus were present either eastward in the Indo-Pacific waters or westward in the Caribbean waters. Different species of Portunus have so far been reported in Miocene outcrops in Iran (Glaessner, 1928;heiDari et al., 2012;yazDi et al., 2013), India (ralte et al., 2009;tiwari & veGa, 2014), Pakistan and Burma (satsanGi & PariDa, 1980), Malaysia (collins et al., 2003), Indonesia (van straelen, 1924), Taiwan (hu, 1984), andFiji (rathBun, 1945). The genus was first recorded in Japan only from strata that date to the Pliocene period (Karasawa & noBuhara, 2008).

Conclusions
In the present paper, we described 25 new specimens of Portunus monspeliensis A. Milne Edwards, 1860 from Middle Miocene beds of eastern Slovenia from four new localities. The newly presented diversity and richness of P. monspeliensis in sublittoral siliciclastic lithologies of eastern Slovenia confirms the presence of Middle Miocene Slovenian Corridor, connecting the Central Paratethys and the Mediterranean Sea.
Considering the near shore facies and lithologies of new localities from eastern Slovenia P. monspeliensis must have been a euryhalin species, similar to some extant portunids. This widespread decapod crustacean usually has a dominant role in the ecosystems it shares with less common portunids such as Scylla and Necronectes.
The described material shows that the fossil record of P. monspeliensis in Slovenia is much more robust than previously thought. New localities in eastern Slovenia are important data points on palaeobiogeographical map of Middle Miocene, which illustrate the possible seaway connection between Paratethys and Mediterranean.