Oosternum Sharp, 1882

Hebauer, Martin Fiká Č Ek Franz & Hansen, Michael, 2009, Taxonomic revision of New World species of the genus Oosternum Sharp (Coleoptera: Hydrophilidae: Sphaeridiinae) I. Definition of species groups and revision of the Oosternum aequinoctiale group, Zootaxa 2054, pp. 1-37 : 6-7

Genus Oosternum Sharp, 1882

Oosternum Sharp, 1882: 112 — Type species: Oosternum costatum Sharp, 1882 [= O. sharpi Hansen, 1999b: 242 , replacement name] (monotypy). Gender: neuter.

= Crypteuna Motschulsky, 1863: 448 — Type species: Cryptopleurus? aequinoctialis Motschulsky, 1855 (monotypy) – Syn.: Hansen 1991: 304 (suppressed name: ICZN 1998: 58). Gender: feminine.

= Pemelus Horn, 1890: 309 — Type species: Megalosternum costatum LeConte, 1855 (monotypy) – Syn.: Hansen 1999b: 241. Gender: masculine.

Crypteuna Motschulsky, 1863 : The genus was proposed by Motschulsky (1863) for his species Cryptopleurum aequinoctiale (using the incorrect subsequent spelling C. aequinoxialis ). This generic name is older than Oosternum Sharp, 1882 and therefore has priority over it. However, the name was never used by authors other than Motschulsky (1863, 1868) or mentioned in any catalogue, except for short notes by d’Orchymont (1924, 1928), Méquignon (1942) and Hansen (1991) who tentatively considered Crypteuna as a synonym of Oosternum . During the preparation of the taxonomic revision of the genus Oosternum, M.

Hansen reexamined the type species of Cryptopleurus? aequinoctialis (see below under O. aequinoctiale for details), confirmed the synonymy of Crypteuna with Oosternum and proposed the conservation of the name Oosternum ( Hansen 1996) . Subsequently, Crypteuna was suppressed and Oosternum was conserved by the Commission of the Zoological Nomenclature (ICZN 1998).

Pemelus Horn, 1890 : This genus was proposed by Horn (1890) for the aberrant species Megalosternum costatum LeConte, 1855 (Megalosternum is an unjustified emendation for Megasternum Mulsant, 1844 , see Smetana (1978)). Later, Oosternum sculptum Bruch, 1915 was transferred to Pemelus in the unpublished thesis by Malcolm (1981) and this transfer was followed by Hansen (1991). The attribution of both mentioned species to the separate genus was based on an aberrant morphology of both species, characterized by pronotum with longitudinal furrows, highly costate elytral intervals and morphology of the median portion of the prosternum. During the work on the taxonomic revision of Oosternum, M. Hansen realized that there are “intermediate forms” between the “normal” Oosternum and O. sculptum : (1) some undescribed species of the genus (belonging to the Oosternum group B defined below) share all characters with usual Oosternum but bear weak furrows on pronotum as Pemelus ; and (2) Pemelus costatus ( LeConte, 1855) resembles the species mentioned under (1) by low pronotal ridges, but otherwise it bears all diagnostic characters of Pemelus . Based on these “intermediate forms”, Hansen (1999b) placed Pemelus in synonymy with Oosternum , considering Pemelus as a subordinate, highly derived form of Oosternum . In the same paper ( Hansen 1999b), he also proposed the replacement name O. sharpi Hansen, 1999 for Oosternum costatum Sharp, 1882 due to the newly created homonym to Oosternum costatum ( LeConte, 1855) after the synonymization of Pemelus with Oosternum .

Detailed comparison with other representatives of Oosternum revealed many additional differences between Oosternum and the species attributed originally to Pemelus (see the definition of the Oosternum sculptum species group for some of them). Most of the differences are, however, parallel to the characters delimiting the megasternine genus Emmidolium d’Orchymont, 1937 and some highly sculptured species of Cryptopleurum Mulsant, 1844 , although Emmidolium and Cryptopleurum are not closely related to Oosternum (see Fikáček (2007) for details and list of shared characters). Emmidolium and highly sculptured Cryptopleurum species also seem to be highly derived forms of “usual looking” relatives. For this reason, the Hansen’s (1999b) concept of Pemelus as a subordinate derived form of Oosternum seems to be justified and is followed here. The taxa attributed originally to Pemelus are treated here as Oosternum sculptum species group. A phylogenetic analysis is needed to confirm the position of this species group in respect to other Oosternum species; this analysis will be performed after finishing the taxonomic revision of Oosternum and related genera.