All specimens were measured electronically using a Leica M420 microscope equipped with a Syncroscopy AutoMontage Photomicroscopy system (SYNCROSCOPY, Synoptics Ltd.). Measurements for various body parts, given in mm, are encoded as follows:

LH length of head, measured along midline from anterior margin of labrum to the virtual line connecting posterior supraorbital setae;

WH width of head, at level of anterior supraorbital setae;

WPm maximal width across pronotum;

WPa width across anterior angles of pronotum;

WPp width across posterior angles of pronotum;

LP length of pronotum from base to apex along midline;

WE width of elytra, at level of 4^th umbilicate setae;

LE length of the elytra, from apex of scutellum to apex of left elytron;

SBL standardized body length, a sum of LH, LP, and LE.

In addition, nine ratios between these measurements were calculated: WH/WPm, WPm/WE, WPa/WPp, WPm/WPp, WPm/LP, WE/SBL, WE/LE, LE/SBL, and LP/LE. All values are given as mean ± standard deviation.

Digital photographs of the dorsal habitus of new species were taken with the AutoMontage system using a Leica M420 microscope. Line drawings of selected body parts were made using a camera lucida on an Olympus BX 50 microscope. Scanning electron micrographs were made with coating on an ESEM FEI Quanta 200.

Dissections were made using standard techniques. Genitalia were dissected from the abdomens of specimens previously softened in boiling water for 20–30 minutes. Contents of the abdomen were cleared using boiling 10% KOH for 2–3 minutes to remove internal tissues, and then washed in hot water before examination. After examination, genitalia were mounted on plastic transparent boards in dimethylhydantoin formaldehyde resin (DMHF, Steedman 1958) and pinned beneath the specimen. In some species, investigation of body parts was undertaken as follows. The whole specimen was cleared, using boiling 10% KOH for ~5 minutes, then washed and dissected. Disassembled body parts from one specimen were placed on a plastic transparent board, properly oriented, mounted in DMHF and pinned together with the specimen labels.

The author had no opportunity to investigate the type material of the New Zealand anillines. The concept of Pelodiaetus used in the paper is based on the material identified by Moore during his work on New Zealand fauna of Anillina (Moore 1980).

Criteria for recognizing new species were the following (Sokolov et al. 2004): two or more similar forms that are sympatric were considered separate species if they differed in genitalic morphology and at least one external character; allopatric forms that were similar in external morphology were considered separate species if they differed in general form of the median lobe or the armature of the internal sac; allopatric forms were considered conspecific if they show intergradation of external characters and/or intergradation of the shape and armature of the median lobe. Therefore, morphological recognition of species was based on gross external characters, including forebody proportions, and form and structure of the elytra, and fine external characters, such as chaetotaxy of different body parts, structure of the labio-maxillar complex, shape of ventral sclerites, and patterns of forebody microsculpture. Genitalic characters included the form of the median lobe, the armature of the internal sac observed retracted inside the median lobe, the shape of spermatheca, and the structure of ovipositor sclerites.

The terms used in the paper and the scheme of descriptions follow that of the author’s previous publications on New Zealand Anillini (Sokolov 2015, 2016). Numbering of the umbilicate pores of elytra follows that of Erwin (1974) and Giachino and Vailati (2011).

The range of Pelodiaetus species stretches across three regions of the South Island of New Zealand (Fig. 8) from Canterbury in the north through Otago and down to Southland and Stewart Island in the south. The members of the genus inhabit almost all the territory of the coastal lowlands, and in Otago penetrate deeply inland along the Clutha River valley. This rather large territory harbors only two species, one of which is endemic to the Christchurch area and the other of which occupies the rest of the genus range. Morphologically, the two species are almost identical and can be distinguished from each other only after dissection and genitalia investigation, suggesting that diversification between species took place relatively recently.

Geologically, the range of the genus is associated with old terranes composed of Paleozoic to Early Cretaceous volcanics or sediments (Mortimer 2004, Michaux 2009). The range of P. sulcatipennis occupies areas on older island arc-derived and oceanic terranes, namely, the Brook Street, the Murihuku, the Dun Mountain-Maitai, and part of the younger Rakaia Terrane (Wandres and Bradshaw 2005, Liebherr et al. 2011, Cooper and Ireland 2013). The range of the presumably more recently derived P. nunni is associated with the younger Rakaia Terrane only (Wandres and Bradshaw 2005, Liebherr et al. 2011, Cooper and Ireland 2013).

Search of concordant taxon-area relationships reveals that non-montane representatives of another anilline endogean taxon, the genus Zeanillus, demonstrate almost identical distribution to the members of the genus Pelodiaetus (Fig. 10). Indeed, the range of Pelodiaetus in its main contours repeats the range of the monotypic subgenus Brounanillus Sokolov (Fig. 10A, star), but, in comparison to the range of the latter, demonstrates evident features of northward and westward expansions. Interestingly, northward expansion of the range of Pelodiaetus corresponds to the range of the nominotypical subgenus Zeanillus (Fig. 10A, heart), a group of closely related species inhabiting coastal foothills of southern and central Canterbury. Similar to the members of Pelodiaetus, in the Christchurch area the members of Zeanillus s. str. are represented by their own endemic species Z. phyllobius (Broun). The similarity in general distribution and particular species ranges between the members of Pelodiaetus and Zeanillus may point to their at least partly shared evolutionary history in the South Island.

Figure 10. 

Map of southern half of South Island, New Zealand, showing ranges of subterranean non-montane Anillina. A – Zeanillus (combined ranges of subgenera Zeanillus s. str. (heart) and Brounanillus (star), white dots represent approximate boundary between taxon ranges); B – Pelodiaetus. Elevation scale bars are given in meters.

In the history of Zealandia (Mortimer et al. 2017), the remnant of which is New Zealand (Heads 2016), there was a critical time point for the local biota known as the Oligocene “drowning” (Haq et al. 1987, Stöckler et al. 2002). Depending on the scenario of maximal marine transgression, i.e., on how much the land of Zealandia was above sea level at that time, there exist two concepts of the shaping of modern New Zealand biota, called the “dispersal” and “vicariance” models (Heads 2016). One group of scientists assumes that complete submergence of subaerial lands with subsequent elimination of terrestrial biota took place and believes that all modern lineages arrived in New Zealand after the Oligocene (dispersal model) (Waters and Craw 2006, Landis et al. 2008, Wallis and Trewick 2009). Another group supports the idea that during maximum marine transgression, Zealandia was only reduced to a certain number of low-lying islands on which the Gondwanan-aged relict taxa survived drowning (vicariance model) (Lee et al. 2001, Knapp et al. 2007, Giribet and Boyer 2010, Conran et al. 2014). The second scenario implies that reduction in size of subaerial lands caused corresponding reduction in species diversity, and that these islands remained isolated for tens or hundreds of thousands of years, thus providing enhanced opportunities for geographic speciation (Cooper and Cooper 1995).

Contemporary distribution of the members of the genus Pelodiaetus and its relatives seems to fit better the vicariance model, because it is likely that their ancestors persisted on Zealandia after it’s rifting off and drifting away from Australia. The distribution of putative overseas relatives of Pelodiaetus serves as an indirect support for this assumption. As mentioned under the genus description, the members of Pelodiaetus are very similar to the members of the primarily Australian genus Illaphanus that comprises 29 species of small to medium-large (0.86–1.74 mm) beetles. Besides the Australian mainland, representatives of this genus are known from Tasmania and from the Lord Howe and Norfolk Islands (Giachino 2005). All Illaphanus species have grooved elytra, no denticle in front of the posterolateral angles of the pronotum, and, with some exceptions, are lacking the mental tooth in the labial complex. The state of the latter character in most cases distinguishes the members of Illaphanus from New Zealand representatives of Pelodiaetus, which are characterized by the blunt mental tooth. However, the state of the mental tooth varies within Illaphanus, and some species of Illaphanus with the developed mental tooth are almost indistinguishable from the members of Pelodiaetus. Notably, a small Illaphanus with the mental tooth, Ill. norfolkensis Giachino, occurs on Norfolk Island (Giachino 2005), which is famous for its endemic fauna of considerably older age than the island itself (Heads 2011). It has been hypothesized that endemic taxa surviving today on the island evolved on other former islands existing in the vicinity on the Norfolk Ridge, the strip of continental crust that extends from New Zealand to New Caledonia. During the breakup of East Gondwana, the Norfolk Ridge together with the Lord Howe Rise and the rest of Zealandia drifted away from Australia (Heads 2016). Hence, the occurrence of Illaphanus species on Norfolk and Lord Howe Islands supports the idea that Pelodiaetus-like forms might have persisted on the territory that split from East Gondwana as the Zealandia continent ca. 84 Ma ago (Mortimer et al. 2019).

One more piece of evidence of the persistence of the representatives of the Pelodiaetus lineage in Zealandia is the geographical distribution of the members of Pelodiaetodes, presumably the sister-taxon to Pelodiaetus (Sokolov 2015). Externally, besides their large size, the members of Pelodiaetodes can be distinguished from the members of Pelodiaetus only by the well-developed denticle anterior to the posterolateral angles of the pronotum. This character is not known among the members of Illaphanus and other genera with grooved elytra and can be considered as an autapomorphy of Pelodiaetodes. The genus comprises two morphologically distinct subgenera; its modern distribution is disjunct and limited by the Northland Allochthon (Bradshaw 2004) on the North Island (nominotypical subgenus, four species) and one locality in the Oamaru region on the South Island (subgenus Monosetodes Sokolov, one species). Given that the lands of the Northland Allochthon were completely submerged during the late Oligocene and became subaerial only around the late- to mid-Miocene after subduction of the Pacific Plate under the Australian one (Bradshaw 2004), the later arrival of the members of Pelodiaetodes to the North Island can be assumed. The Oamaru region remained submerged from the Late Cretaceous to the early Oligocene, but some lands of this region were likely exposed during the Oligocene “drowning” from the late Oligocene to the early Miocene (Thompson et al. 2014), which is supportive of the idea of the late arrival of the members of Pelodiaetodes to the Oamaru region as well. The absence of Pelodiaetodes-like forms in Australia but their presence as two morphologically distinct taxa in New Zealand suggests that the genus Pelodiaetodes arose after the East Gondwana breakup, but before New Zealand acquired its modern contour, i.e., somewhere on the Zealandia continent. Apparently, the members of Pelodiaetodes managed to survive different epochs of Zealandia inundations including the Oligocene “drowning,” and finally reach the places of their modern distribution.

Additionally, it is worth mentioning a strange ecological pattern of co-occurrences of the members of Pelodiaetus with other anillines. In the examined material, I came across four cases of syntopic co-occurrences of New Zealand anillines. Three of these cases were related to Pelodiaetus species: namely, Pelodiaetus nunni co-occurred with Z. (Zeanillus) phyllobius (Broun) at Ahuriri (Christchurch area, Canterbury), P. sulcatipennis with Z. (Zeanillus) nunni Sokolov at Woodside Glen (Dunedin area, Central Otago), P. sulcatipennis with Z. (Brounanillus) pallidus (Broun) at Picnic Gully (Taieri Mouth, South Otago), and one case related to its sister-taxon, where Z. (Nunnanillus) pellucidus Sokolov co-occurred with Pelodiaetodes (Monosetodes) nunni Sokolov at Glen Warren Reserve (Oamaru, North Otago). In these examples, the co-occurring species were represented by two contrasting morphotypes, a small (first place in the listing above) and a large one. In all cases but one, a small species of the Pelodiaetus lineage co-occurs with a large one from the unrelated clade, i.e., a small Pelodiaetus lives together with a large Zeanillus. However, in the fourth case, on the contrary, a small Zeanillus lives with a large Pelodiaetodes. Generally, miniaturization is one of the ways of speciation in the Anillini allowing species to occur syntopically and to exploit resources more effectively (Sokolov 2013, Andújar et al. 2017). Often these species are closely related and probably formed as a result of sympatric speciation, as was demonstrated for North American Anillinus lescheni Sokolov and Carlton and A. stephani Sokolov and Carlton (Sokolov et al. 2004), for some European Typhlocharis Dieck (Pérez-Gonzáles and Zaballos 2013), and for Mexican Zapotecanillus oaxacanus Sokolov and Z. nanus Sokolov (Sokolov 2013). In the case described in this paper, the co-occurrence of small and large species belonging to unrelated taxa demonstrates evolutionary adaptations of representatives of different genera to living together in one community, and also suggests intermixture of different faunas, reflecting the complicated evolutionary history of endogean biota on the Zealandia continent. Apparently, the evolution of the local anilline community was accompanied by the subsequent extinction and replacement of one taxon with another, either because of dramatic historical events or as a result of species competition, or as a combination of both.

Comparison of species composition in Australia and New Zealand helps to explain the origin of the South Island anilline faunas. In Australia, as many as 29 (85.3%) of 34 known species of Anillini belong to the genus Illaphanus, the sister-taxon to the Pelodiaetus lineage. The same dominance of representatives of the Pelodiaetus lineage can be seen on the North Island, where four species (80.0%) of five recorded for the island belong to Pelodiaetodes. At the same time, on the South Island, only three species (20.0%) of 15 anillines belong to the Pelodiaetus lineage. The significant decline in the number of species of the Pelodiaetus lineage on the South Island suggests that the Anillini faunas of the two islands may have different origins. The presence of species with Australian roots on the North Island is in concord with well-documented southeastern Australia-New Zealand connections (Heads 2014, 2016). By contrast, on the South Island, the fauna of Anillini has been shaped by the representatives of Zeanillus, whose ancestors supposedly inhabited a territory of the Campbell Plateau (Sokolov 2016). The geological data show that the Campbell Plateau together with Marie Byrd Land formed the West Antarctic margin, rather distant from the Australia/Pacific Plate margin of East Gondwana (Siddoway 2008, Heads 2014). Thus, the anilline community of the South Island could have resulted from an intermixture of ancient endogean faunas from different parts of East Gondwana and perhaps was dramatically modified by the Oligocene “drowning.” The possibility that different parts of East Gondwana could harbor different terrestrial faunas seems plausible. Several recent investigations have demonstrated examples of diversification of ancient terrestrial groups that had taken place prior to major geological events that split ancient supercontinents (San Mauro et al. 2005, Murienne et al. 2014, Heads 2014, Buckley et al. 2015).