A primitive representative of the Parabathynellidae (Bathynellacea, Syncarida) from the Yilgarn Craton of Western Australia

Billibathynella humphreysi gen. et sp. nov. is described from calcrete aquifers located in the Yilgarn Craton of north‐western Australia. This is the first parabathynellid known from the Australian Precambrian shields, which have never been inundated by the sea. A comparison of the primitive species so far known from Australia and other continents points to the new species as being the most primitive among the parabathynellids. It further suggests that the new genus has an affinity to Notobathynella Schminke, but differs in having a six‐segmented antenna and a large epipod of the male thoracopod VIII. An attempt to relate the primitiveness of the new species to the historiogeological characteristics of the region has led to the conclusion that the recent parabathynellids could have emerged from freshwater epigean ancestors. It is further assumed that the transition of their ancestors to groundwaters happened in Notogaea.


Introduction
Bathynellaceans are typical inhabitants of continental groundwaters, which occur worldwide. Since no mechanism is known by which the species could have been dispersed beyond the interstitial, they are particularly interesting subjects for historical zoogeography (Schminke 1974;Camacho and Coineau 1989;Camacho 2003). There are two families in this order, the Bathynellidae and the Parabathynellidae. Because of the greater richness in taxonomic characters, the species of the latter family are taxonomically better studied than those of the Bathynellidae. Currently, we know almost 120 parabathynellid species comprising 35 genera (cf. Schminke 1986;Coineau 1996;Camacho et al. 2000). Of these, five genera are known from Notogaea: Notobathynella Schminke, 1973, Chilibathynella Schminke, 1973, Atopobathynella Schminke, 1973, Hexabathynella, Schminke, 1972, and Kimberleybathynella Cho et al., 2005. Here we describe a new species, for which the sixth genus of the Australian continent is erected. This species is particularly interesting because of two following points of view. First, it displays primitiveness in all characters and is comparable to the stem species of the Parabathynellidae, as hypothesized by Schminke (1973). Second, the locality where the new species was discovered is the Yilgarn Craton of Western Australia, which has not been inundated by sea water since at least the Palaeozoic (Humphreys 1999). Hence the new species is expected to bring a novel aspect to the taxonomy as well as the historical zoogeography of the family Parabathynellidae.

Material and methods
The material was collected using a large hand net (350 mm mesh) after abseiling into the well. The well is concrete lined at the top but with bare calcrete near the floor, and about 6 m deep. The water was 0.2 m deep with the following characteristics: temperature 26.0uC; pH 7.49; specific conductance 2.61 mS cm 21 ; dissolved oxygen 6.37 mg l 21 . The well is reported by the land manager to have consistently yielded 30,000 gallons h 21 (136 kl h 21 ) of irrigation water, for which it is no longer used. Description of the typical calcrete habitat is provided by Humphreys (1999Humphreys ( , 2001. The samples were prepared and mounted in a mixture of glycerin-formalin. For drawing and investigation, a Nikon Eclipse E600 microscope with differential interference contrast equipment was used with oil immersion. The type materials of the new species herein described are permanent preparations deposited in the collection of the Western Australian Museum, Perth (WAM).

Diagnosis
Parabathynellidae. Body elongated and cylindrical. Antennule seven-segmented. Antenna seven-segmented. Labrum flat with numerous teeth on its free margin. Incisor process of mandible with more than five spines. Mandibular palp twice as long as wide. Maxilla foursegmented. Thoracopods I-VII with exopod of more than five segments (up to 10). Male thoracopod VIII almost rectangular, 1.5 times longer than width; protopod protruded at inner distal corner; epipod large, triangular, its distal part covering penial region of protopod; basipod without setae, inner margin of basipod drawn out into projection. Uropod with numerous spines on sympod; endopod with two distal spines, two plumose setae near base, three distal setae and a protrusion on disto-outer margin, inner spines variable in number; exopod with numerous setae and a basi-ventral seta. Anal operculum flat to slightly concave. Furcal rami elongated, with two large distal spines and numerous spines on inner margin.

Etymology
The generic name refers to one taxon and is named after Dr W. F. Humphreys, alias Bill (Perth, Australia) for his great contributions to the research on the Australian groundwater fauna. Gender feminine.

Description of male
Body length 6.30 mm (other males: 6.16, 5.45 mm), approximately 13 times as long as wide. Head as long as anterior three thoracic segments combined ( Figure 1). Antennule ( Figure 2A) seven-segmented. First segment with one seta on inner distal margin, with five simple dorsal setae, and with one dorsal, two lateral and one ventromedial plumose seta. Second segment with one group of four plumose setae and with six simple setae on inner distal margin. Third segment with two lateral setae and six setae on inner distal margin. Inner flagellum of third segment with three simple setae. Fourth segment with one stub seta and one plumose seta on dorsal margin, and with two stub setae and two plumose setae on outer distal apophysis. Fifth segment with four setae on inner margin, with two aesthetascs and one simple seta dorsally. Sixth segment with four setae on inner margin, with two aesthetascs and one seta dorsally, and with one aesthetasc. Seventh segment with three subterminal aesthetascs and four simple setae.
Labrum ( Figure 2C) flat with 28 teeth. Fourteen frontal teeth of more or less equal size flanked by small teeth on both sides. Inner surface with numerous combs of ctenidia and teats.
Pleotelson ( Figure 3B, C) with one seta near the base of furcal rami on both sides. Anal operculum flat.
Furcal rami four times as long as wide, with two large distal spines, and 21 spines on inner margin, and with two dorsal setae.

Description of female
The female differs from the male in thoracopod VIII. Body length 6.02 mm (other females: 5.94, 5.82, 5.80 mm).
The right and left thoracopods VIII ( Figure 2G) not fused, cone-shaped, as large as the endopod of the male thoracopod VIII, with two distal denticles.

Remarks
The new species is probably the most primitive member of the Parabathynellidae. The richness of the extremities in teeth, claws and setae, the high number of segments of thoracopodal exopod and finally the large body size are the indices (Table I). From the Australian continent two genera are known as being primitive: Chilibathynella and Notobathynella. According to Schminke (1973), Chilibathynella has a balloon-shaped male thoracopod VIII, five-segmented antenna, one-segmented exopod of thoracopods I-VII and thus differs from the new species significantly. Notobathynella, in contrast, is similar to the new species in many aspects. For example, the male thoracopod VIII of Notobathynella is very similar to that of the new genus in its rectangular form, in having the massive protopod with prominent penial region, in the basipod drawn out into projection, and in the triangular exopod. Although fewer in number, the exopods of thoracopods I-VII of Notobathynella display a relatively large number of segments (2-3-4-4-4-3-3). However, Notobathynella has a five-to six-segmented antenna and lacks the basi-ventral seta on the uropodal exopod. In addition, the epipod of the male thoracopod VIII never covers the penial region of the protopod and the basipod bears a seta. Hence, we propose for the new species a new genus Billibathynella gen. nov.
In terms of the primitiveness of the extremities, Allobathynella Morimoto and Miura, 1957, occurring in East Asia, is also comparable with the new species, B. humphreysi. According to Schminke (1973), this genus includes the most primitive members of the Parabathynellidae. However, even the basal representative of Allobathynella, A. gigantea pluto Morimoto, 1963, is at most only 3.30 mm long and has a six-segmented antenna, a 7 Setation on maxilla 5-10-17-11 3-4-10-7 4-5-10-8 3-6-11-7 3-4-11-5 3-6-21 2-3-10-2 Number of exopodal segments of thoracopods I-VII 7-9-10-10-10-9-9 4-6-6-7-7-7-6 2-3-4-4-4-3-3 1-1-1-1-1-1-1 2-3-3-3-3-3-3 3-4-5-5-5-5-5 3-4-5-5-5-4-4  , 1963;3, Notobathynella remota Schminke, 1973;4, Chilibathynella australiensis Schminke, 1973;5, Psalidobathynella stocki Schminke, 1979;6, Afrobathynella trimera Schminke, 1976;7, Batubathynella malaya (Sars, 1929). labrum with 14 teeth, a mandible with seven teeth on the incisor process and 12 spines in the spine row, an exopod formula of thoracopods I-VII of 4-5-6-6-6-6-6, 18 spines on the uropodal sympod, and six spines on the furcal rami. Except for the eight-segmented antennule (cf. Morimoto 1963), the primitiveness of A. gigantea thus does not go beyond that of B. humphreysi. The new species rather seems to resemble the hypothetical stem species of the Parabathynellidae, which Schminke (1973) has reconstructed based on the summation of primitive characters of the family known at that time. The differences of the stem species from B. humphreysi may exist only in the three-segmented mandibular palp, in the male and female thoracopods VIII with the form slightly different from the other thoracopods, the possession of two epipods on each of thoracopods I-VII, the presence of two-segmented pleopods on the first and second pleomeres, and the number and the position of the setae on the pleotelson. Billibathynella humphreysi has been collected from irrigation wells located in the calcrete aquifer in the arid zone of Australia, where typical karst features have been developed, with sinkholes with a large water body (Sanders 1974;Barnett and Commander 1985;Humphreys 2001). Since the sinkholes usually provide unconfined spaces, one could argue that the unusual body size of B. humphreysi has been developed secondarily in adaptation to the free water, as supposed by Noodt (1964) for Bathynella baicalensis Bazikalova, 1954 and Baicalobathynella magna Bazikalova, 1954, the largest and the most primitive representatives of the family Bathynellidae from the bottom of Lake Baikal. Accordingly, the marine interstitial ancestor of the Parabathynellidae could have invaded the continental groundwater via the coastal interstitial (thalassoid origin: Boutin and Coineau 1987;Coineau 1996;Camacho 2003). As in both species from Lake Baikal, however, B. humphreysi does not differ from other species of the Parabathynellidae in basic body plan, even lacking pleopods, and thus shows no sign of adaptation to the free water habitat. The habitat of the nominative form of Allobathynella gigantea Morimoto, 1959 also supports the primitiveness of B. humphreysi. A. gigantea is much smaller (2.35 mm) and equipped with fewer teeth, setae, or spines on the proper extremities and thus seems to be more suitably adapted to interstitial spaces than B. humphreysi. However, the former species is likely to favour free water, since it was found abundantly under stones on the bottom of a shallow dug well exposed to the faint sunlight (Morimoto 1959). In terms of the origin of the Parabathynellidae, on the other hand, it is worthy to mention that both B. humphreysi and A. gigantea occur in regions which have mostly not been submerged by the sea since the Palaeozoic (Morimoto 1959;Watts and Humphreys 2000). As Schminke (1972) remarked for the two bathynellids from Lake Baikal, it does not seem to be a simple coincidence that B. humphreysi, the most primitive form within the Parabathynellidae, occurs just in one of the oldest far inland areas of the world. This fact strongly suggests that the extant parabathynellids have been derived from a surface water limnic ancestor (limnocoid origin: Schminke 1973). It is also probable that the transition to groundwater happened in Notogaea. A way to test this hypothesis on the centres of origin of the family, however, would be an extensive investigation of the groundwater fauna of old cratons in far inland areas of the gondwanan fragments.