Yprezethinus, PARKER, 2022

PARKER, JOSEPH, 2022, Transitional morphology and Afrotropical affinity of a bythinoplectine rove beetle from the early Eocene of India (Coleoptera: Staphylinidae: Pselaphinae), Palaeoentomology 5 (5), pp. 452-460 : 453-456

Yprezethinus gen. nov.

urn:lsid:zoobank.org:act:2DEC8AE3-3EBA-4791-A08E-AFDAD243E43A

Type species. Yprezethinus grimaldii sp. nov. here designated.

Etymology. The generic name is a combination of “Ypresian” and Zethinus Raffray , a Recent bythinoplectine genus to which the new taxon is phylogenetically close.

Diagnosis. The new genus, Yprezethinus , and its single species, Y. grimaldii , can be distinguished from all other Pselaphinae by the following combination of characters: (1) Head laterally excavate, with fossae extending from clypeus to anterior margin of eye; capable of holding retracted maxillary palpi; (2) antenna 11-segmented with enlarged ovular club formed from tightly appressed antennomeres 10 and 11; (3) maxillary palpomere 3 approximately twice as long as wide, ~2/3 palpomere 2 length, bearing narrow, elongate, apically globose tubercle; (4) maxillary palpomere 4 shorter than 3, rounded-conical,bearing narrow, apically globose tubercle (similar to that of palpomere 3), as well as prominent apical pseudosegment; (5) procoxae and metacoxae contiguous; (6) tarsi seemingly 2-segmented, with short first visible segment (probable composite of tarsomeres 1 and 2) and long second visible segment (probable tarsomere 3); and (7) all tarsi with single claws.

Description. Body length: ~ 0.8 mm. Body form relatively flattened and elongate (“euplectite body form” sensu Parker, 2016a) ( Fig. 1A ).

Head

Head transverse, with prominent frontal rostrum extending from clypeus to anterior margins of eyes (rostrum challenging to visualize in the holotype Tad-130 but discernable with specimen in dorsal view). Lateral margins of head deeply excavate, with large dorsal fossae that extend either side of frontal rostrum to anterior margins of eyes ( Fig. 1B ). Antennae mounted on underside of frontal rostrum. Antennae composed of 11 antennomeres; antennal clubs formed from tightly appressed antennomeres 10 and 11, separation of these segments observable as a narrow groove encircling the club ( Fig. 1C ; “ sillon annulaire ” sensu Coulon, 1989). Maxillary palpi 4-segmented with conspicuous additional 5th apical pseudosegment ( Fig. 1B , APs). Maxillary palpomere 3 enlarged, longer than palpomere 4. Both palpomeres 3 and 4 bearing narrow, elongate, apically globose tubercles on dorsal faces ( Fig. 1B, D ). Maxillary palpi capable of retracting into lateral excavations of head. Head venter with median gular sulcus.

Thorax

Prothorax approximately as wide as long; subequal in width to head. Prothoracic morphology largely occluded. Pterothorax ~1.7× longer and ~1.3× wider than prothorax. Dorsal and ventral fovea are mostly indiscernible in Tad-130, but mesoventrite bearing possible lateral mesocoxal fovea (LMCF) following the foveal system of Pselaphinae from Chandler (2001). Apparent carina extending from mesocoxal cavity to lateral margin of mesoventrite.

Abdomen

Abdomen approximately equal in width to pterothorax and ~1.3× pterothorax length. Dorsal view of abdomen occluded, but five tergites (IV–VIII) visible. Tergites IV–VI subequal in length and width; VII ~ 1.2× longer than VI, narrowing posteriad; VIII ~ 0.5× shorter than VII, narrowing to abdominal apex. Distinct paratergites present on at least tergites V–VII (those on IV likely present but occluded). Six sternites clearly visible (III– VIII) ( Fig. 1A ); entire apical margin of sternite III visible between and laterally to metacoxae.

Legs

Procoxae and mesocoxae projecting ventrally, contiguous at midline. Metacoxae relatively flat and broadly conical, contiguous at midline, projecting posteriorly ( Fig. 1E ). Trochanters of all legs short such that apex of coxa sits adjacent to base of femur. Tarsi with two visible tarsomeres; first visible tarsomeres very short ( Fig. 1F ).

Elytra and flight wings

Taxonomic characters of the elytra are mostly occluded in Tad-130, but putative humeral carina present. Flight wings present.

Remarks. Yprezethinus belongs to the tribe Bythinoplectini based on its possession of the following combination of characters: (1) head with prominent frontal rostrum; (2) margins of head anterior to eyes deeply excavate, capable of accommodating the retracted maxillary palpi ( Fig. 1B ); (3) complex maxillary palpi, with palpomere 3 subequal to or longer than 4 (3 longer than 4 in Yprezethinus ) ( Fig. 1B ); (4) contiguous metacoxae ( Fig. 1E ), a plesiomorphic state in Pselaphinae , seen in the supertribes Faronitae and Euplectitae (the latter including Bythinoplectini ); (5) tarsi composed of 2 visible segments ( Fig. 1F ; Coulon 1989) argued that the first visible tarsomere is actually composed of the dorsum of morphological tarsomere 1 and the venter of morphological tarsomere 2); (6) tarsi with single claws ( Fig. 1F ). Within Bythinoplectini , Yprezethinus is placed within the subtribe Bythinoplectina on account of the lateral margins of the head being completely excavate, from clypeus (below the antennal insertions of the frontal rostrum), extending posteriorly to the eyes ( Coulon, 1989; Chandler, 2001).

To my knowledge, Yprezethinus is the first described fossil representative of Bythinoplectini . According to Coulon’s (1989) generic revision of Bythinoplectini , Yprezethinus belongs within informal “Section V” of this tribe, a group of eight genera distributed in the Neotropical and Afrotropical ecozones that all possess maxillary palpomeres 3 and 4 with narrow, elongate, apically globose tubercles—a putative synapomorphy ( Fig. 1B, D ). Within Section V, Yprezethinus appears to be extremely close to a putatively monophyletic group of similar genera centered on Zethinus Raffray (herein the “ Zethinus -group”). For comparison to extant Zethinus - group character states, confocal images of the cephalic morphology of two species of Zethinus are shown in Figure 2 . Zethinus illustrates clearly the laterally excavate head of the subtribe Bythinoplectina , in which the antennae are mounted on a frontal rostrum, below which the maxillary palpi can be fully retracted into large fossae ( Fig. 2A–C ). Zethinus also exemplifies the characteristic Section V maxillary palp morphology, with palpomeres 3 and 4 bearing elongate tubercles ( Fig. 2D ). Genera within the Zethinus -group are distinguished from the remaining Section V Bythinoplectini genera by the morphology of the antennal club, which is spherical or ovoid and composed of the apical-most two antennomeres that are tightly appressed hemispheres ( Fig. 2A ; note that in Zethinus species, the apical two antennomeres are 7 and 8). Yprezethinus possesses similar antennal club morphology ( Fig. 1C ). Yprezethinus is therefore placed into crowngroup Bythinoplectini , and the Zethinus -group of Section V specifically.

Importantly, Zethinus -group genera possess a maximum of nine antennomeres ( Dichozethinus Jeannel has eight antennomeres; Zethinus Raffray and Pseudozethinus Coulon both have nine). Yprezethinus is therefore distinguished from these other genera by its possession of 11 antennomeres—the plesiomorphic condition in Pselaphinae ( Chandler, 2001; Parker, 2016a). Hence, I hypothesize that although Yprezethinus sits within the bythinoplectine crown, it is a stem-lineage of the Zethinus -group, which has not undergone any of the antennomere fusions or losses characteristic of crowngroup members of this putative clade. If Section V bythinoplectines as a whole form a monophyletic group, then the genus Hendecameros Comellini may represent an early-branching lineage: this taxon possesses the plesiomorphic character states of 11-segmented antennae and distinct (non-appressed) antennomeres 10 and 11 ( Coulon, 1989). I speculate that Yprezethinus may capture a transitional form between a Hendecameros- like ancestor and Recent Zethinus -group genera, in which antennomeres 10 and 11 show the derived state of appression, but no reduction in antennomere number has yet occurred.

Figures

FIGURE 1. Yprezethinus grimaldii Holotype AMNH Tad-130. A, Habitus with visible abdominal sternites labelled. B, Head and pronotum with segments of maxillary palpomeres numbered (MP2–MP4). Bythinoplectina-type lateral excavation, eye, apical pseudosegment of maxillary palp (APs) and mandible (Md) indicated. C, Right antenna with antennomeres numbered. Note the club formed from tightly appressed, hemisphere-shaped antennomeres 10 and 11. D, Close-up view of apically globose tubercles of maxillary palpomeres 3 and 4. E, Proximal leg articulation, showing conically projecting metacoxa (MC), short metatrochanter (MT) and femur (F). F, Right metatarsus (F) with bythinoplectine-type enlarged third tarsomere spanning distance between arrowheads, and minute first visible tarsomere.

FIGURE 2. Cephalic morphology of Zethinus-group Bythinoplectini. A, B, Zethinus leleupi males (Democratic Republic of Congo: Mabali). C, D, Zethinus sp. nov. male (Democratic Republic of Congo: Kinshasa). A, Dorsal head and cephalic appendages of Zethinus showing antennae mounted under the frontal rostrum, either side of which are large excavations that extend posteriorly to the anterior margins of the eyes. Tightly appressed antennomeres 7 and 8 together form an ovoid antennal club; arrowheads indicate the boundary between these antennomeres—the “sillon annulaire” sensu Coulon, 1989. MP: left maxillary palp. B, Frontal view of Zethinus head showing retracted maxillary palpi inside excavations lateral to frontal rostrum. C, Head of Zethinus in lateral view with maxillary palp retracted into excavation. D, Maxillary palp of Zethinus showing diagnostic Bythinoplectini Section V morphology with narrow, elongate, apically globose tubercles on palpomeres III and IV. Prominent apical pseudosegment (APs) is indicated.