Introduction
With more than 1500 species within 117 extant genera placed in seven extant tribes, the subfamily Omaliinae is a relatively large and widespread group of rove beetles (Newton et al. Reference Newton, Thayer, Ashe and Chandler2000; Herman Reference Herman2001). This group is usually recognisable by possession of a pair of ocelli near the hind margin of the head, well-developed pronotal postcoxal process, five tarsomeres, a pair of paratergites on abdominal segments III or IV to VII, and a well-developed defensive gland complex associated with an anterior projection of abdominal sternum VIII (Newton et al. Reference Newton, Thayer, Ashe and Chandler2000). In addition, omaliine rove beetles usually have a broader body form than other typical staphylinids, a shorter and less flexible abdomen, and in some genera, relatively long elytra (Newton et al. Reference Newton, Thayer, Ashe and Chandler2000). To date, eight genera belonging to Omaliinae (as tribe incertae sedis, summarised in Herman Reference Herman2001) have been reported from the Mesozoic era: Archodromus Tikhomirova, Globoides Tikhomirova, and Porrhodromus Tikhomirova from the Upper Jurassic of Karatau, Kazakhstan (Tikhomirova Reference Tikhomirova1968); Daiodromus Ryvkin and Prodaia Ryvkin from the Upper Jurassic of Daya, Russian Transbaikalia, Mesodeliphrum Ryvkin from the Lower Cretaceous of Turga, Russian Transbaikalia (Ryvkin Reference Ryvkin1990); Eophyllodrepa Ryvkin from the Middle-Upper Jurassic of Novospasskoe, Russia, and Morsum Ryvkin from the Middle Jurassic of Kubekovo, Russian Siberia (Ryvkin Reference Ryvkin1985). No definitive fossil omaliine rove beetles have been described from China.
The discovery of a new omaliine fossil species from the Middle Jurassic of northeastern China (∼165 million years ago) reported here is of significance for understanding the origin and early evolution of the extant subfamily Omaliinae. The sole specimen was collected from a locality near Daohugou Village, Inner Mongolia, China. The Daohugou biota has yielded a number of plants, abundant and diverse insects, abundant conchostracans, anostracans, spiders, salamanders, a pterosaur, and intriguing mammals (Huang et al. Reference Huang, Nel, Shen, Selden and Lin2006).
Sinanthobium Cai and Huang, new genus
Coleoptera: Staphylinidae: Omaliinae: Anthophagini
Type species
Sinanthobium daohugouense new species, here designated.
Etymology
Derived from the prefix Sino-, referring to China, and the genus name Anthobium; it is neuter in gender.
Diagnosis
The new genus can be distinguished from extant omaliines and other staphylinids by possessing the combination of: body oval-shaped, small-sized (shorter than 2 mm long); antenna moderate, extending to the posterior margin of pronotum; antennomeres 8–10 nearly quadrate, not transverse or elongate; pronotum very transverse, having a well-developed pronotal hypomeron with a subtriangularly produced process; elytra relatively long, exposing apical four abdominal segments (tergites V–VIII).
Sinanthobium daohugouense Cai and Huang, new species
Sinanthobium daohugouense, holotype, NIGP156219: (1) General habitus; (2) enlargement of left antenna; (3) enlargement of left metatarsus; a, antennomere; t, tarsomere. All moistened with 70% alcohol. Scale bar = 500 μm in 1; 100 μm in others.
Line drawing of Sinanthobium daohugouense. Scale bar = 500 μm.
Material examined
Holotype, NIGP156219, sex unknown; part only, without counterpart, showing more ventral structures than dorsal structures. The holotype is housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, Jiangsu, China.
Occurrence
Middle Jurassic Jiulongshan Formation; Daohugou Village, Ningcheng County, Inner Mongolia, China.
Etymology
Derived from the fossil locality, where the type specimen was collected.
Description
Body small, 1.93 mm long, broad, oval shaped. Head small, somewhat distorted, apparently transverse, retracted under prothorax, widest across eyes, 0.38 mm wide, at least 0.22 mm long. Ocelli not visible. Gular sutures moderately separated, diverging anteriorly and posteriorly. Antenna 0.66 mm long, extending to posterior margin of pronotum, slightly broadened towards the apex; with 11 antennomeres, antennomere 1 elongate, second shorter, antennomeres 3–7 distinctly longer than wide, antennomeres 8–10 nearly quadrate, antennomere 11 largest, conical.
Pronotum transverse, widest at about posterior third, narrowed anteriorly and posteriorly. Lateral margin slightly expanded. Prosternum strongly transverse, prosternal process subtriangular and acutely pointed at the apex. Pronotal hypomeron developed, subtriangularly produced inwards. Procoxal cavities open behind.
Elytron 0.95 mm long, widest slightly before the apex, slightly wider than pronotum, combined width 1.05 mm, about 2.5 times as long as pronotum. Mesoventrite transverse. Mesoventral posterior process produced. Mesocoxal cavities slightly separated by mesoventral and metaventral processes. Metaventrite very large.
Legs moderately long. Procoxae contiguous, transverse; profemur robust, elongate; protibia (only partly visible) thin. Mesocoxae narrowly separated. Metacoxae contiguous, transverse, subtriangular; metafemur robust, fusiform, distinctly longer than profemur; metatibia thin and elongate, apex of metatibia with at least two spurs; metatarsus elongate, with five tarsomeres, tarsomeres 1–4 combined longer than tarsomere 5.
Abdomen relatively broad, gradually tapered towards the apex, with six visible sternites. Abdominal segments III–VII each with a pair of narrow paratergites.
Discussion
Among the 32 extant staphylinid subfamilies within four groups of subfamilies, the genus Sinanthobium can be easily placed in the Omaliine group of subfamilies on the basis of the following characters in combination: the ovoid body shape; the relatively long elytra; the contiguous and broad metacoxae; each abdominal segment with one pair of paratergites (Newton and Thayer Reference Newton and Thayer1995). The Omaliine group currently comprises 10 extant subfamilies: Dasycerinae, Empelinae, Glypholomatinae, Micropeplinae, Microsilphinae, Neophoninae, Omaliinae, Proteininae, Protopselaphinae, and Pselaphinae (Newton and Thayer Reference Newton and Thayer1995). Sinanthobium is readily separated from Dasycerinae and Empelinae by its body form and having shorter elytra, not covering the whole abdomen as in the latter two subfamilies; from Micropeplinae by bearing 11-antennomere, non-clavate antenna (not with nine antennomeres as in Micropeplinae) and lacking sculpture of ridges and pits on dorsal surface; from Glypholomatinae and Neophoninae by lacking 9 or 10 rows of punctures on elytron and clubbed antenna and from the latter by the body form; from Protopselaphinae and Pselaphinae by having a broader and less compact body, transverse pronotum, five metatarsomeres (not three metatarsomeres), and relatively long elytra. The new genus displays superficial resemblances to some representatives of the three remaining subfamilies: Microsilphinae, Omaliinae, and Proteininae. In particuliar, Sinanthobium differs from the monogeneric Microsilphinae by lacking a pronounced three-antennomere club and having a broader pronotum and shorter elytra. Sinanthobium is distinguishable from Proteininae mainly in having a well-developed pronotal hypomeron with a subtriangularly produced process, since in extant proteinine beetles the postcoxal sclerite of prothorax has a spiracle and is significantly separated from the pronotal hypomeron. It is noteworthy that the modern Proteininae (except Silphotelini) have the mesothoracic spiracle surrounded by a sclerotised peritreme, which is triangular in shape and thus superficially similar to an omaliine postcoxal process. This proteinine structure appears to be a secondary sclerotisation of the intersegmental membrane, not homologous with the postcoxal process of the pronotal hypomeron of that of Omaliinae. With only two southern hemisphere genera Alloproteinus Steel and Silphotelus Broun (Steel Reference Steel1966), the most basal tribe Silphotelini of Proteininae has a hypomeral projection similar to the new genus and various Omaliinae (Newton and Thayer Reference Newton and Thayer1995). Since ocelli were not visible in the new genus, the new genus resembles some members of the proteinine tribe. It can be easily separated from Silphotelus by relatively short elytra (not covering the whole abdomen as the latter) and by the five metatarsomeres (not four metatarsomeres). The new genus superficially resembles Alloproteinus in habitus and body size, but it differs from the latter by lacking a small notch on each side behind antennal insertions and having a broader body shape, and antennomeres 8–10 quadrate (not distinctly transverse as in Alloproteinus). Therefore, it is the best solution to place the new genus in the Recent subfamily Omaliinae, even though one of the most critical features, head with a pair of ocelli, is not preserved in the mainly ventral-sided tiny specimen.
Within the modern subfamily Omaliinae, Sinanthobium can be tentatively assigned to the presently Holarctic and Oriental tribe Anthophagini as supported by the combination of: antennomeres 8–10 only quadrate, not transverse; and metatarsomeres 1–4 together longer than metatarsomere 5. Notably, the omaliine tribe Anthophagini is something of a taxonomic dumping-ground and is unlikely to be monophyletic (Newton et al. Reference Newton, Thayer, Ashe and Chandler2000). Thus, the character states mentioned above are likely to be plesiomorphic. Even though the meaning and definitiveness of placement in Anthophagini are weak, it is still a practical solution to place this fossil in the modern group rather than leaving it as incertae sedis within Omaliinae. In general habitus, Sinanthobium superficially resembles several extant genera of Anthophagini, including Anthobium Leach (Coleoptera: Staphylinidae) and Olophrum Erichson (Coleoptera: Staphylinidae). They share relatively broad body shape, transverse pronotum, and non-striate elytra. However, Sinanthobium can be separated from Anthobium by its small body size, quadrate antennomeres 8–10 (not longer than wide as in modern Anthobium) and shorter elytra with nearly four abdominal segments exposed; from Olophrum by it much smaller body size, more transverse pronotum and sharply pointed postcoxal process of the pronotal hypomeron.
Among the eight described Mesozoic omaliine genera, the new genus is separable from all genera except Morsum Ryvkin (Coleoptera: Staphylinidae) by its oval-shaped body with transverse pronotum and gradually tapered abdomen. The mid-Jurassic genus Morsum, bearing a very broad abdomen, is unfortunately poorly preserved. The head, pronotum, and legs are not preserved. However, judging from the relatively short elytra and first four sub-parallel abdominal segments of Morsum, it is better to regard them as two different genera. Lin (Reference Lin1976) described the first fossil rove beetle of China, Protostaphylinus mirus Lin (Coleoptera: Staphylinidae), from the Middle Jurassic Haifanggou Formation near Haifanggou Village, Beipiao, Liaoning Province, China. However, its systematic position within extant staphylinid subfamilies has been uncertain. It displays the most similarities to the late-Jurassic omaliine genera Globoides and Porrhodromus in general habitus, and shares several characters with modern Omaliinae, including its overall shape and size, rounded posterolateral margin of elytron, abdominal segments with only a pair of paratergites and abdominal sternite II somewhat developed. We here suggest that the genus Protostaphylinus probably belongs to Omaliinae, making it the first described Chinese fossil record for this subfamily.
To date, one rove beetle subfamily Glypholomatinae (Cai et al. Reference Cai, Huang, Thayer and Newton2012) and six other subfamilies, including Apateticinae, Olisthaerinae, Tachyporinae, and Trigonurinae (personal observation), have been known from the Middle Jurassic Daohugou biota. Thus, the present find of a new omaliine beetle from the Daohugou beds provides evidence that this subfamily has a long evolutionary history, and originated comparatively early, more than 165 million years ago. In addition, it indicates that at least some of omaliines have changed very little from the Jurassic to the present.
Acknowledgements
They are deeply grateful to Dr. Margaret Thayer (Field Museum of Natural History, Chicago, Illinois, United States of America) and one anonymous reviewer for constructive suggestions. Financial support was provided by the National Basic Research Program of China (2012CB821903), Outstanding Youth Foundation of Jiangsu Province (Grant No. BK2012049), the National Natural Science Foundation of China (91114201 and J1210006), Chinese Academy of Sciences (KZCX2-YW-QN104).
