A new homolid crab, Cherpihomola italica gen. nov., sp. nov. from the Rupelian of the Ligure-Piemontese Basin (Alessandria, northern Italy)

A new genus and species of homolid from lower Oligocene (Rupelian) strata in the Ligure-Piemontese Basin (northern Italy) is introduced. Cherpihomola italica gen. nov., sp. nov. represents the first record of homolids from Oligocene deposits across Europe and extends the palaeogeographical distribution of extinct homolids.


Geological and stratigraphical setting
Lithologically, Rupelian strata in the Ligure Piemontese Basin are characterised by an alternation of greyish marls with nodular elements and silt-rich marls, occasionally sandy, resting on the "Formazione di Pianfolco", which is of Rupelian age. Macrofossils are preserved mainly within pebbles or nodules that were eroded from the highest levels exposed of this sedimentary complex, which is referred to as "Formazione di Molare". These levels overlie terrestrial units of the "Brecce di Costa Cravara and Pianfolco", studied by Charrier et al. (1964) and dated as early Rupelian (see also Gelati & Gnaccolini, 1978;Gnaccolini, 1978). The crab-bearing levels were attributed to the transition between the "Formazione di Molare" and the overlying "Marne di Rigoroso" by Allasinaz (1987) and to the biozone of the benthic foraminifer Operculina complanata (Bianco, 1985;Balossino & Bianco, 1986). Other studies on Oligocene deposits in this area were carried out by Franceschetti (1967), Gela-ti & Gnaccolini (1980) and Fantoni et al. (1983). The palaeoenvironment of the Case Cherpione area documents three Rupelian phases, from a fully terrestrial setting with forests and rivers that transported abundant plant remains (early Rupelian), to a marine, warm-water lagoon with moderate currents and coasts nearby (middle Rupelian) and finally, during the late Rupelian, different platform conditions, a bathymetric change and a different benthos/plankton ratio which led to the disappearance of the macrofauna (Gelati & Gnaccolini, 1980;Fantoni et al., 1983).
The material studied here originates from the top levels of the "Molare Formation" (middle Rupelian) at Case Cherpione (Alessandria, northern Italy); it is preserved in nodules of diagenetic origin ( Fig. 1).

Genus Cherpihomola gen. nov.
Type species: Cherpihomola italica sp. nov. Etymology: The generic name refers to Case Cherpione, the locality which yielded the type specimens.
Diagnosis: Carapace longitudinally square in outline, as long as wide; well-developed linea homolica, sinuous in outline, acute rostrum, one pseudorostral spine, one infra-orbital spine, one hepatic spine, one anterolateral spine, two posterolateral spines, regions nearly smooth and slightly raised.
Description: Carapace longitudinally square, as long as wide, well-developed linea homolica, sinuous; moderately vaulted transversely, less so longitudinally, lateral sides slanted, nearly subvertical; regions smooth well marked by grooves; triangular rostrum not sulcate axially; one pseudorostral spine, as long as the rostrum; a short infraorbital spine; anterolateral margin with one prominent subhepatic spine directed outwards; a second short spine is present ventrally, not visible in dorsal view; one prominent anterolateral spine directed outwards present between cervical and branchiocardiac grooves; posterolateral margin with two short spines; posterior margin wide, concave and rimmed; deep cervical groove, convex laterally to epibranchial lobe, strongly inclined between inferior margin of mesogastric region; branchiocardiac groove almost straight proximally, downturned posteriorly to gastric lobe, curved and continuous on branchial region; epigastric lobe defined by pair of tubercles positioned just posterior to pseudorostral spines;

PLATE 2
mesogastric lobe marked by smooth grooves laterally and well-defined cervical groove posteriorly; protogastric lobe with two tubercles; narrow mesogastric lobe; triangular cardiac lobe, with three tubercles; long, narrow and smooth intestinal lobe, slightly depressed; metabranchial lobe with two small tubercles aligned along linea ho-molica; smooth dorsal surface. Chelae with elongate palm with upper and lower margins almost parallel; outer surface of palm densely covered by punctuation, fixed finger about two-thirds of palm, long and straight. Long ambulatory legs, with denticulated upper margin.

Discussion:
The carapace of this new homolid is characterised by a well-marked linea homolica, acute rostrum, one pseudorostral spine, one infra-orbital spine, one sub-hepatic spine, one anterolateral spine and two posterolateral spines, a deep cervical groove, nearly smooth and slightly raised dorsal regions and a narrow cardiac region, with three tubercles. Although Cherpihomola gen. nov. shares features of the rostrum and pseudorostral spines with Paromola, the latter has convex lateral margins with numerous spines and tuberculated dorsal regions, delimited by shallow grooves. Paromola is known from six modern and two extinct species, namely Paromola vetula Crawford, 2008 from the Paleogene of Río Negro Province (Argentina) and Paromola roseburgensis Nyborg & Garassino, 2017 from the Roseburg Formation (lower Oligocene) of Oregon (USA).
The new genus has affinities with Latreillopsis in showing near-parallel lateral margins, a similar arrangement of the frontal and lateral spines, a near-smooth dorsal surface and a narrow cardiac region with three tubercles. However, Latreillopsis has longer pseudorostral spines and one or more accessory spines in the rostrum, an epibranchial margin without a spine, while the posterolateral margin has a single robust spine.
Of other Cenozoic genera, Prohomola has densely tuberculated dorsal regions and deep cervical and branchiocardiac grooves (see Karasawa, 1992;Blow & Manning, 1996). Dagnaudus has a triangular, acute rostrum, long pseudorostral spines with two accessory spines, lateral margins with spines and tuberculated regions bounded by shallow grooves (see Jenkins, 1977). Nogarhomola has convex lateral margins with spines, a bifid rostrum and dorsal regions with tubercles (De Angeli & Alberti, 2012), while Palehomola has an oval carapace (larger posteriorly), a long, pointed rostrum that is strongly downturned, pseudorostral spines that are slightly longer than the rostrum and with two small basal spinules, as well as a large, inflated subhepatic region, with one large triangular spine and well-developed cervical and branchiocardiac grooves (Nyborg & Garassino, 2017).

Fossil homolids from Italy
To date, only three genera are known from the fossil record. Homola Leach, 1815 (Beschin et al., 2009;Garassino et al., 2010;De Angeli & Alberti, 2012;De Angeli & Ceccon, 2013). The new genus and species erected herein represents the first record of homolid crabs from Oligocene strata in Europe, thus enlarging their palaeogeographical distribution.