Introduction

The Saldidae is a small family of insects belonging to Heteroptera. About 335 extant species have been described in this cosmopolitan family (Schuh and Polhemus 2009). Most saldids are littoral, inhabiting lake shores, beaches and stream banks and they are predaceous, feeding on small insects and decaying animal materials (Brooks and Kelton 1967).

Cobben (1959) proposed a classification of Saldidae, and divided Saldidae into three subfamilies: Aepophilinae, Chiloxanthinae and Saldinae. Schuh and Polhemus (1980) later considered the Aepophilinae to be of family rank based on their cladistic and phenetic analysis of the infraorder Leptopodomorpha. At present, Saldidae is divided into two subfamilies, Chiloxanthinae and Saldinae (Schuh and Slater 1995). The phylogenetic analyses concerning relationships within Saldidae (Polhemus 1977, Schuh and Polhemus 2009) present valuable information and conclusions.

To date, 6 incontrovertible fossil species in 3 generahave been reported: Oligosaldina Statz & Wagner, 1950 with three species, Oligosaldina rottensis, Oligosaldina rhenana and Oligosaldina aquatilis, found from Upper Oligocene deposits in Germany; Propentacora froeschneri (= Oreokora froeschneri) found in Miocene Latah Formation in USA (Lewis 1969); Salda exigua Germar & Berendt, 1856 found in Eocene Baltic amber, and Salda littoralis found in Recent Late Glacial clay (Jessen 1923).

However, 2 genera assigned to this group previously are not saldids. Leptosalda chiapensis (Cobben, 1971) from Mexico amber was assigned to the subfamily Leptosaldinae within Saldidae first, but was later transferred to Leptopodidae by Schuh and Polhemus (1980). Popov (1973) erected a subfamily Saldoniinae in Saldidae with one genus Saldonia and one species Saldonia rasnitsyni Popov, 1973, but later (Popov 1985) transferred the genus to Archegocimicidae, synonymized Saldoniinae under Archegocimicidae, and added two more species Saldonia sibirica Popov, 1985 and Saldonia maculata Popov, 1985, all from the Lower or Middle Jurassic of Transbaikalia, Russia. Archegocimicidae is similar to Saldidae, and it was assigned to the infraorder Leptopodomorpha (Popov 1985, 1989, Popov et al. 1994). Polhemus (1977) thought Saldonia probably should be classified into Dipsocoridae based on its wing venation. Cobben (1987) didn’t consider this genus as a member of the infraorder Leptopodomorpha, but he didn’t give detailed explanation.

In this paper, we described a new fossil shore bug, Brevrimatus pulchalifer gen. et sp. n., from the Yixian Formation, Baitugou, Nanyingpan Village, Sanbeigou Town, Duolun County, Inner Mongolia, China. Xing et al. (2005) and Zhang et al. (2004), respectively, based on isotope data and abundant statistical analysis of fossils data came to the consistent opinion that the age of the Yixian Formation is Early Cretaceous. And this opinion has been accepted widely (Swisher et al. 1999, Lu 2000, Zhou et al. 2003, Fürsich et al. 2007). Here we consider the age of the Yixian Formation as the Early Cretaceous (about 125 Ma).

Material and methods

Our fossil specimen is deposited in the Key Laboratory of Insect Evolution and Environmental Changes, Capital Normal University, Beijing, China. It was examined with the LEICA MZ 12.5 dissecting microscope. The specimens were examined without alcohol and under alcohol. Photos were taken by a Nikon Digital Camera DXM1200C. Line drawings were made with Photoshop graphic software. Morphological terminology used here follows that of Schuh and Slater (1995).

The body length was measured from the apex of head to the apex of abdomen; body width, at the maximal width of body; pronotum length, along the midline; pronotum width, across the broadest part at its posterior angles; wing length, from the basal to the apex of anterior margin; wing width, at the maximal width of the wing. All measurements are in millimeters (mm).

Systematic paleontology

Order Hemiptera Linnaeus, 1758

Suborder Heteroptera Latreille, 1810

Infraorder Leptopodomorpha Popov, 1971

Family Saldidae Amyot & Serville, 1843

Subfamily Chiloxanthinae Cobben, 1959

Discussion

The Leptopodomorpha consists of four extant families (Saldidae, Aepophilidae, Leptopodidae, Omaniidae) and three extinct families (Archegocimicidae, Mesolygaeidae, Palaeoleptidae). Popov et al. (1994) synonymized Mesolygaeidae to Archegocimicidae. But herein we think it is better to treat them as two separated families, because of their distinct difference in forewing. We compared our fossil with all the families in Leptopodomorpha. The body sizes of aepophilids and omaniids are less than 2mm, while the new species reaches to 8mm, much larger than aepophilids and omaniids. In Leptopodidae, rostrum at most reaches to the base of the fore coxae, while rostrum of the new species reaches to the base of the hind coxae. Besides that, anterior margin of pronotum is distinctly narrower than head in Leptopodidae, but anterior margin of pronotum of the new species is almost as wide as head. All the extinct families from Mesozoic are contemporaneous with the new fossil species. But they are different in some characters. Nine cells present in Archegocimicidae (Handlirsch 1906–1908), and the arrangement of the cells (Popov 1985) are totally different from the new species. Fore wing of Palaeoleptidae is nearly completely coriaceous except for small membrane (Poinar and Buckley 2009), which is different from the new species with large membrane. And wing venation consists of eight cells in Palaeoleptidae, which differs from the new species with five cells. The pronotum of Mesolygaeidae is divided into two parts (Zhang 1991), but in the new species no groove present on pronotum. The structure of end of abdomen is also different between the new species and mesolygaeids. So we classified our fossil into Saldidae based on the combined characters: compound eyes large and reniform, rostrum long, posterior margin of pronotum indented, hemelytra with costal fracture, medial fracture well developed and membrane with five cells.

Phylogenetic analysis

The new genus possesses some typical Chiloxanthinae characters, such as costal fracture very long, female sternum VII truncate with mesal concavity and base of ovipositor exposed. On the other hand, it possesses short medial fracture as Saldinae. Therefore, we carried out phylogenetic analyses to determine the placement of our new genus.

For the phylogenetic analyses, we selected three extant genera from Chiloxanthinae, five extant genera from Saldinae, our new fossil genus, and an unambiguous fossil species Oligosaldina aquatilis as in-group. Following previous studies (Polhemus 1977, Schuh and Polhemus 1980, 2009), we chose representatives from the family Leptopodidae (Patapius thaiensis Cobben, 1968) and Aepophilidae (Aepophilus bonnairei Signoret, 1879) as our out-group taxa. The 12 taxa that we chose for these phylogenetic analyses are listed in Table 1. We carried out phylogenetic analyses respectively with the fossil taxon Oligosaldina aquatilis andwithout this fossil taxon.

Most character information of the extent taxa was extracted from literatures (Cobben 1959, 1969, Drake 1961, Cobben and Polhemus 1966, Polhemus and Evans 1969, Polhemus 1972, 1977, 1991, Cobben 1980, King and Fordy 1984, Chen and Zheng 1987, Vinokurov 2005, 2009, Schuh and Polhemus 2009). The descriptions for the 17 characters and character states are listed in the Appendix. All characters were treated as unordered and weighted equally. A maximum parsimony analysis of the character matrix (Table 2) edited by NDE (Nexus Data Editor) version 0.5.0 (Page 2001), was performed on NONA (Goloboff 1998), using the Multiple TBR+TBR search strategy, options set to hold 10000 trees, 1000 replications with 100 starting tree replication. The unambiguous characters were mapped by WinClada (Nixon 2000).

Phylogenetic results

For the phylogenetic analyses excluding fossil species Oligosaldina aquatilis, we got two equally most parsimonious trees (Fig. 3A, B), with the following main characteristics: tree length = 28, consistency index (CI) = 82, retention index (RI) = 87. The strict consensus tree is shown in Figure 3C. Phylogenetic resultsindicate Saldidae is a monophyletic group, which is supported by four synapomorphies: posterior pronotal margin indented distinctly (Character 4:1); eversible glands present posterolaterally between sterna VI and VII (Character 9:1); eggs with aeropyles (Character 15:1); larval organ present (Character 16:1). Some synapomorphic characters, such as apicolateral sclerotized structures of penis present (Character 13:1) and filum gonopori coiled one to four times, like a watch-spring (Character 14:1) supported the monophyly of the subfamily Saldinae. Chiloxanthinae with our fossil species included is a monophyletic group, which is supported by four synapomorphies: five well defined cells in membrane (Character 6:1); medial fracture long (Character 8:2); female subgenital plate truncate with concavity along the midline (Character 11:2); base of ovipositor exposed (Character 12:1). In summary, phylogenetic results suggest our new fossil genus is in Chiloxanthinae and short medial fracture was treated as a reversal character.

For the phylogenetic analysis including fossil species Oligosaldina aquatilis, we got one most parsimonious tree (Fig. 3D), tree length = 28, CI = 82, RI = 88. The monophyly of Saldidae is supported by four synapomorphies (Character 4:1, 9:1, 15:1 and 16:1) as the results above. In this phylogenetic result, besides Character 13:1 and Character 14:1, short costal fracture of hemelytra (Character 7:1) supports the monophyly of the subfamily Saldinae. Five well defined cells in membrane (Character 6:1) indicate that our new genus should be in the branch of Chiloxanthinae. Therefore, both of the phylogenetic analyses suggest our fossil species should be classified into Chiloxanthinae.

Comparison with Chiloxanthinae indicates the new fossil species differs from other extant chiloxanthines in its short medial fracture. Besides this character, the boundary between corium and membrane is not clear in Enalosalda, which is different from Brevrimatus gen. n. with clear boundary. Paralosalda has four membrane cells, which is different from Brevrimatus gen. n. with five cells. Embolar modification of female is well developed in Pentacora, but in Brevrimatus gen. n. the embolar region is slightly thickened. Sublateral cell of membrane is shortest in Chiloxanthus, which differs from Brevrimatus gen. n. with the lateralmost cell is shortest. We further compared it with other fossil Saldidae. The arrangement of the cells of Brevrimatus gen. n. is similar to that of Oligosaldina, but lateralmost cell of membrane is distinctly smaller than that of Oligosaldina. Long costal fracture is present on Brevrimatus gen. n., but absent on Oligosaldina. A deep furrow is present in the pronotum of Oligosaldina, while it is absent in our new genus. Propentacora contains five closed cells in the wing membrane, but the corial veinof Propentacora appears to continue between the third and fourth membrane cells, which is different from the new genus. Brevrimatus gen. n. is distinctly different from Salda, which can be seen in phylogenetic result. Comparing our fossils with the fossil species Salda exigua, we can separate them in the following characters: Brevrimatus gen. n. possesses five closed cells in the forewing membrane, while Saldonia exigua has three closed cells and rostrum of Brevrimatus gen. n. reaches to the base of hind coxae, while in Saldonia exigua, rostrum just reaches to the fore coxae. Therefore Brevrimatus gen. n. is different from all other fossil genera. In geological age, all of the previously recorded fossil saldids are from Cenozoic. So far, Brevrimatus gen. n. found in the Lower Cretaceous sedimentary stratum is the oldest saldid.