Type species: Bathynomus giganteus A. Milne-Edwards, 1879, by monotypy.

Description: Elongate, rectangular incomplete posterior body with parallel lateral margins. Pereonites 5–7 sub-rectangular of approximately equal length, twice the length of the pleonites. Pleonites badly preserved, of approximately equal length, slightly overlapping each other with pleurae pointed posteriorly. Pleotelson sub-trapezoidal, tapering posteriorly with convex lateral pli-cate margins, longer than wide; posterior margin slightly convex. Dorsal surface convex, with minute granulated pores (in ventral view); conspicuous median longitudinal carina. Posterior pleotelsonic margin with seven strong triangular pointed spines, two most lateral one shorter, slightly curved distally towards the median longitudinal axis of the body, followed by two spines of similar length, longer than the lateral ones and nearly straight. Central spine clearly triangular, straight, simple (not bifid) and slightly longer than the other ones.

Discussion: According to the recent concept of Bathynomus (Bruce 1986; Lowry & Dempsey 2006 among others), shared also by some palaeontologists (Etter 2014; Hyžný et al. 2013, 2019, among others), the outline of the pleotelson with sinuous lateral margins and a set of large spines on its posterior margin is typical of the genus. Moreover, although the number of pleotelsonic spines is considered as variable within the genus (Lowry & Dempsey 2006; Kou et al. 2017), following Hyžný et al. (2019), we consider it a taxonomically important diagnostic character if combined with the shape of the uropod rami (Shipley et al. 2016). Unfortunately, the uropods are not preserved in the studied specimen, thus not allowing a specific assignment. However, the estimated total length of the body, based on the length of the pleotelson (approximately ¼ of the total length as noted by Magalhäes & Young 2003), suggests that the studied specimen, ranging over 140 mm in size, might be a representative of the giant or supergiant group of Bathynomus (sensu Lowry & Dempsey 2006). Indeed, we cannot clarify if the studied specimen could belong to one of these two groups, because it has a body size between the minimum size of the representatives of the giant group and the small size of the representatives of the supergiant group. Moreover, we cannot establish whether the studied specimen was an adult specimen or a juvenile.

Based on the general pleotelsonic proxy characters (mainly the number of marginal spines), apparently the closer extant species, both belonging to the giant group, are Bathynomus immanis Bruce, 1986, from the East Australian Coral Sea (Pacific Ocean), showing a pleotelson with a conspicuous median longitudinal ridge, seven pleotelsonic marginal spines with a simple (not bifid) central spine and reaching 139 mm in total length (Lowry & Dempsey 2006: 178, figs. 12, 13); and B. doederleini Ortmann, 1894, from the Western Pacific Ocean, with the same pleotelsonic characters but occasionally bearing nine marginal spines and of smaller body size (≈ 133 mm) (Lowry & Dempsey 2006: 176, fig. 10).

The studied specimen differs notably from the only known fossil species from the Plio-Pleistocene of Italy, Bathynomus sismondai (Ristori, 1891), recently revised by Hyžný et al. (2019), by an indistinct dorsal median carina and nine or eleven marginal pleotelsonic spines. Moreover, the studied specimen has been compared with Palaega scrobiculata V. Ammon, 1882, described from the Eastern Alps of Austria, sharing a very similar pleotelsonic shape (Hyžný, pers. comm. 2019), with seven posterior spines and a similar geological age (Oligocene). However, the type material of Palaega scrobiculata seems to have a straighter posterior pleotelsonic margin, with posterior serrate subequal parallel spines, less strong and distinct, not separated by a wide deep concave notch. The systematic position of that taxon has been previously discussed by other authors, and P. scrobiculata could be considered a representative of Bathynomus, based on the genus concept proposed by Hyžný et al. (2013, 2019). Unfortunately, we cannot confidently compare the Italian specimen with the Austrian type material based on the illustrations reported by v. ammon (1882, pls. 1–4), due to the different and more compressed preservation of the specimens from Austria.

Moreover, only a direct and close observation and revision of the type material of this and other doubtful Palaega species, not purpose of this paper, could confirm their potential re-assignment to Bathynomus. As previously remarked, due to the absence of the uropods in the studied specimen, a specific assignment is impossible. According to Hyžný et al. (2019) and based on the preserved generic proxy characters, we prudentially ascribe the studied specimen to Bathynomus. At the same time we cannot totally exclude that, after a revision of Palaega scrobiculata, the studied specimen could be tentatively assigned to the latter species.

Finally, based on the present state of knowledge, the studied specimen represents the oldest fossil record of Bathynomus from Italy, attesting and backdating the presence of the genus in the early Oligocene Proto-Mediterranean.

Remarks: Pasini & Garassino (2012) reported also Palaega picena from the Miocene of the Apennine Mountains. This species, for its size and shape, could be better accommodated in Bathynomus, suggesting the possible presence of the genus in the Mediterranean Basin during the Miocene (see discussion in Hyžný et al. 2019: 295). If confirmed, this record would attest an uninterrupted presence of Bathynomus in the Mediterranean Basin from the early Oligocene to the early Pleistocene.