Hydrobius, Leach, 1815

Fikáček, Martin, Prokop, Jakub, Nel, André & Umr, Cnrs, 2010, Fossil water scavenger beetles of the subtribe Hydrobiusina (Coleoptera: Hydrophilidae) from the Late Oligocene locality of Aix-en-Provence (France), Acta Entomologica Musei Nationalis Pragae 50 (2), pp. 445-458 : 453-454

‘ Hydrobius ’ obsoletus Heer, 1856

( Figs. 7–8 )

Hydrobius sp. : CURTIS (1829: 294, 295), BRONN (1838: 811), HOPE (1847: 250, partim), GIEBEL (1856: 53, partim), SCUDDER (1891: 534), HANDLIRSCH (1908: 765, partim).

Hydrobius obsoletus Heer, 1856: 18 , plate I: Fig. 19.

Hydrobius obsoletus: OUSTALET (1874: 128 , translated description), SCUDDER (1891: 535, catalogue), HANDLIRSCH (1908: 765, catalogue), THÉOBALD (1937: Table XI, catalogue), STATZ (1939: 76, catalogue), HANSEN (1999: 319, catalogue).

Type material. HOLOTYPE: specimen no. NHM P(IV) 39859 (badly preserved fossil of whole specimen in ventral view, originally from the Murchison collection). Deposited in Department of Paleontology, the Natural History

Museum, London.

Description. Body widely oval, for body measurements see Table 1. Head small, bearing globular eyes laterally. Prothorax rather large, pronotum ca. half as long as rest of body; pronotum wide posteriorly, strongly narrowing anteriad. Prosternum small, prosternal process slightly protruding between procoxae (but see Discussion); ventral portion of prothorax with sparse and coarse punctation. Procoxae large, globular; procoxal cavities contiguous medially. Mesocoxae widely isolated from each other medially. Mesothoracic wings thickened and without conspicuous veins (i.e., transformed to elytra or hemelytra, see Discussion). Metaventrite rather short, transverse, metanepisternum developed. Metacoxae large, transverse, separated medially from each other. Mesotibia slightly widened at distal apex, mesotarsus nearly as long as mesotibia. Metafemur robust, widened, metatibia slightly widened distally, bearing at least one longitudinal row of setae or spines.

Discussion. In spite of a bad preservation of this fossil, the species can be reliably excluded from the superfamily Hydrophiloidea based on the large globular procoxae and medially separated meso- and metacoxae. The identity of this fossil remains unknown for the time being. It partly resembles the aquatic groups of Heteroptera (especially the family Naucoridae ) by the large procoxae and an acute projection between procoxae which might be alternatively interpreted as rostrum (it is interpreted as prosternal process in the redescription above). Mesothoracic wings not overlapping posteriorly may be present in some Nepidae and Naucoridae which bear the hemelytra with reduced membrane. However, H. obsoletus disagrees with aquatic Heteroptera in the ventral morphology, especially by well divided anepisterna laying at sides of metaventrite and separated meso- and metacoxae, and the specimen is therefore considered as Coleoptera incertae sedis. The body proportions and separated mesocoxae may resemble some groups of the Scarabaeoidea, but we refrain from placing the species to this group for the time being.

Note on the records in old literature. THE HOLOTYP E OF Hydrobius obsoletus comes from the Murchison collection. For that reason it may be concluded that the record of Hydrobius sp. from Aix-en-Provence by Cur t is (1829) is most probably based on the same specimen. The record by Br onn (1838) has to be based on the Cur t is’s (1829) paper as well because Hydrobius is not mentioned by de Ser r es (1829) whose book is referred as a second source of data about fossils from Aix for Br onn’s (1838) list. Hope (1847) adopted the data from Br onn (1838) and therefore still refer to the specimen mentioned by Cur t is (1829), but he also mentioned additional specimens of Hydrobius deposited outside of Great Britain and therefore not coming from Murchison collection (‘In my late visit to the Continent [...] my attention was also directed to those [insects] which for some period or periods have been entombed in fossil state’ ( Hope 1847: 250)); his data therefore only partly refer to the holotype of Hydrobius obsoletus . Giebel (1856) refered both to Cur t is (1829) and Hope (1847) and supposed that their records of Hydrobius from Aix are based on the same specimen, which seems partly correct.

Figures

Figs. 7–8. ‘Hydrobius’ obsoletus Heer, 1856, holotype (7 – photograph in the light from above; 8 – photograph in the light from side).