Early Miocene decapod Retropluma slovenica Gašparič & Hyžný , 2014 from Govce beds of Tunjice Hills ( Central Slovenia )

Increasing reports of genus Retropluma Gill, 1894 from the siliciclastic sediments of South-East Europe demonstrate the abundance and preferred habitat of this genus in Miocene seas of Central Paratethys. In the present paper we report new specimens of decapod Retropluma slovenica Gašparič & Hyžný, 2014, which extend the known palaeogeographic and stratigraphic distribution of the species to the western borders of Slovenian Basin of the Central Paratethys. The described specimens originate from the Early Miocene locality of Rovček in the Tunjice Hills in Slovenia and exhibit associated preservation, characteristic for endobenthic infaunal mode of living.


Introduction
The he rich palaeontological diversity of Tunjice Hills is known already from the early 19 th century.Most fossiliferous strata in the region are Early and Middle Miocene beds, which were investigated by Austro-Hungarian geologists and amateur naturalists of the era (Fuchs, 1875;Hilber, 1888).One of most important researchers of the region was a local priest Simon Robič who collected fossils in his daily walks through the woods and creeks of Tunjice Hills (Žalohar & Hitij, 2014).His collection, stored in Natural History Museum of Slovenia consists of about 135 fossil specimens, among others also fossil crabs Tasadia carniolica (Bittner 1884) from locus typicus within Laško Formation beds in Košiše, which are common fossils in the region.Despite frequented finds of T. carniolica in Middle Miocene layers of Tunjice Hills, other occurrences of fossil decapods are unusually rare.The oldest are the remains of crabs from Early Miocene beds south of Tunjice village in Rovček creek.Rare specimens of Chaceon sp. and Homarus sp.(Križnar & Preisinger, 2008;Gašparič & Brajkovič, 2016) occur in concretions in Early Miocene claystones.The age of fauna, containing also Teredo-bored wood remains, is questionable and concretions could have been reworked from older (Oligocene) strata (Gašparič & Brajkovič, 2016).
Herein we present new findings of Retropluma slovenica Gašparič & Hyžný, 2015 from Early Miocene clastic sequence of Rovček creek in Tunjice Hills.The family Retroplumidae Gill, 1894 consists of nine genera of brachyuran crabs, seven of which have exclusively fossil representatives (De Grave et al., 2009;Khodaverdi et al., 2016).The two extant genera, Retropluma and Bathypluma de Saint Laurent, 1989, occur in the Indo-Pacific and have been reported from muddy or sandy bottoms in the depths ranging from 70 to 600 m (McLay, 2006).

Geology and stratigraphy of the localities
Tunjice Hills belong to the westernmost part of the Tunjice Syncline of the Sava Folds (Placer, 1999(Placer, , 2008)).
Cenozoic sequence of Tunjice Hills starts with few meters of Oligocene conglomerates alternating with beds of sandstones.These are followed by grey clays and a basal conglomerate horizon of fluvial origin.Oligocene layers end with a thick horizon of grey marine clay sequence, which at its top alternates with individual lenses of fine grained clastic rocks, particularly loose sands, and conglomerates (Žalohar & Zevnik, 2006;Gašparič & Brajković, 2016).
Oligocene strata are discordantly overlain with Early Miocene Govce Formation.Lithologies of Govce Formation sequence starts with deposition of clays with lenses of sand and sandstones but it mostly consist of interchanging of conglomerates, sandstones, and fine-grained marls and clays.
Succession of retrogradational and progradational sequences implies alternation of deepening and shallowing cycles (Vrabec, 2000).According to žalohar & Zevnik (2006) Govce Formation layers consist of three separate members: the Lower Govce Member, the Middle Govce Member and the Upper Govce Member.The Lower Govce Member is represented by the succession of alternating beds of clay, siltstones, sandstone, and conglomerate.Middle Govce Member consists mainly of greenish (Glauconite rich) and brownish sandstones and marls.The Upper Govce Member begins with conglomerate, sandstones, sandy siltstones and ends with loose sands.The Rovček section, where the described specimens originate from is part of the Upper Govce Member.Total thickness of Govce beds is between 350 to 450 m (Premru, 1983).
The Early Miocene sediments in Tunjice Hills are followed, after an unconformity, by Laško Formation of Middle Miocene (Badenian).The Middle Miocene strata consist of sandstones, marls, and marly limestones, which are rich in marine macrofossil remains.In the upper part of Laško beds we recognize increasing terrestrial influence and a transition to the Sarmatian Dol Formation beds (Vrabec et al., 2014).The layers of Dol Formation are the youngest lithostratigraphic unit of Tunjice Hills.Characteristic horizons are cerithiid sandstone and Coprolitic horizon.Fossil fauna indicates marine environment still connected with Central Paratethys (Horvat, 2003).Upper most parts Dol Formation show renewed shallowing of the environment, fresh water influx and periodic tectonic isolation of the Tunjice basin (Žalohar & Hitij, 2014).

Material and methods
The studied material consists of 5 specimens of Retropluma slovenica.Original cuticle of specimens is lacking, so structures are preserved as impressions in fine grained brownish sandy siltstones.Following specimens were studied: an almost complete articulated specimen (Inv.

Description
The studied specimens exhibit subrectangular carapace, about 1.20 times wider than long, maximum width at the level of median carina.Carapace partially deformed and flattened, but appears transversely slightly convex and longitudinally nearly flat (Pl.1F).Rostrum long, with concave sides and wider distally (Pl.1A).Orbital margin sinuous with well-developed anterolateral tooth, pointing by some degrees outward; the supraorbital tooth is present, but poorly preserved (Pl.1E).Long anterolateral margin, concave till anterior carina.Lateral and posterior margins convex, with well-developed concave re-entrant for reduced fifth pereiopod at the posterolateral part.Dorsal carapace is adorned with three transverse carinae, forming blunt projections at intersection with lateral carapace margin (Pl.1A, C).Anterior carina almost straight, slightly downwards curved in last third toward lateral margin.Median carina strongly developed at flanks only, interrupted by urogastric region.Posterior carina sinuous and well formed.Dorsal carapace surface is pitted and finely granulose.
The mesogastric region pointedly pentagonal continuing over anterior carina in a long anterior process, ending behind the rostrum.All sides of mesogastric region are concave and posterior border is divided into two lobes.Protogastric regions are not clearly delimited from hepatic region, forming a sub-rectangular shape, which is intersected by the anterior carina (Pl.1D).The urogastric region is well defined and recognized as a crescent shape; its posterior border is defined by the cervical groove.The cardiac region is large and well formed; its posterior margin is long and concave and the whole cardiac region is divided by the posterior carina.The intestinal region is wide and narrow, with concave anterior margin and straight lateral margin (Pl.1G).The branchial regions are less defined, elongated and sub rectangular in outline (Pl.1C).Male sternal plate round in outline sternites 7-5 long, overlapping, with squarer termination.Sternite 8 reduced and covered by pleon.

Palaeoecology and environment
Despite the study of extant representatives, several ecological and behaviouristic aspects of crabs from genus Retropluma are still undefined, mostly due to the deep sea environments which they preferentially inhabit.De saint laurent (1989) and mclay (2006) report the occurrences of extant specimens from deep-water muddy bottom waters of up to 470 m.Similarly it can be inferred that fossil representatives have been adapted to the soft muddy or sandy bottoms on inner to outer continental shelves (Gašparič & Hyžný, 2015).As Paleogene species are known also from shallower water settings (Beschin et al., 1996;Hyžný & Müller, 2010;Khodaverdi et al., 2016), it can be concluded that the deeper water habitation preference, with depth interval of 100-450 m, was developed by the Neogene representatives of genus Retropluma.
Extant retroplumids spend most of the time buried in soft substrate (Ahyong, 2008), exposing only the most posterior part of the carapace with reduced setose fifth pereiopods, which possibly have a sensory function (McLay, 2006).Known fossil retroplumid material has so far been found exclusively in fine grained clastic rocks, which exhibit relatively fast sedimentation rates (Fraaije et al., 2005;Hyžný, 2011).This supports the hypothesis, that most of the fossil remains represent an endobenthic infaunal community that spent most of the time already buried in the sediment, which enhanced the preservation potential of otherwise fragile retroplumid cuticle.

Palaeobiogeography
The Eocene has been considered as a time of high evolution within the decapoda (SchweitZer & FelDmann, 2001).This can be also concluded for the retroplumids, which show high diversity in Eocene Tethys Ocean, with six different genera and two species of Retropluma Gill, 1894 known from Eocene strata of Europe (Vía Boada, 1959; Beschin et al., 1996;Artal et al., 2006Artal et al., , 2013;;van Bakel, et al. 2010;Hyžný & müller, 2010;Gašparič & Hyžný, 2015).It was also observed that many taxa which evolved during this period were endemic to their regions of origin (FelDmann et al., 2010).

Conclusions
New findings of Retropluma slovenica Gašparič & Hyžný, 2014 from Early Miocene siliciclastic sequence of Rovček creek in Tunjice Hills represents an important report, which further enhances our knowledge of Early Miocene decapod communities in Paratethys of Slovenia.The species was previously known from a single Early Miocene locality on northern slopes of Pohorje mountain range, so the new locality expands the geographical distribution of the species to the western part of Slovenian Basin of Paratethys which was in Early Miocene very close to the Mediterranean and most likely connected to it through the Slovenian corridor.As genus Retropluma is missing from the Miocene of Mediterranean, but occurs in its Pliocene and Pleistocene strata, it is likely the genus was reintroduced in the Mediterranean from Paratethys before the marine connection between both realms closed in Late Miocene.
Further we conclude that all fossil representatives of genus Retropluma shared the extant species preference for inhabiting a wide range of siliciclastic environments of outer continental shelves.As part of infaunal community they have a good preservation potential and are therefore likely to exhibit a robust fossil record.

Fig. 2 .
Fig.2.A detailed lithostratigraphic section of Rovček creek locality with Early Miocene marine layers from where the described specimens were recovered.