Pycnomerus rimatara, Porch, Nick & Smith, Tessa R., 2017

Porch, Nick & Smith, Tessa R., 2017, New Pycnomerus Erichson (Coleoptera: Zopheridae: Pycnomerini) from Rimatara, French Polynesia, Zootaxa 4237 (1), pp. 154-166 : 157-161

Pycnomerus rimatara sp. nov.

(Fig.2,3,4)

Type material. Holotype: French Polynesia, Austral archipelago, Rimatara , Maunutu Swamp, C 1T2 2008 excavation, 500 ml sediment sample, 100–105 cm depth—articulated male head and prothorax, dorsally and ventrally complete except for appendages. BPBM, Honolulu.

Paratypes (8): French Polynesia, Austral archipelago, Rimatara , Maunutu Swamp C 1T2 2008 excavation, 500 ml sediment sample, 50–55 cm depth, 1 prothorax, 1 pronotum only; ditto, except 60–65 cm depth, 1 prothorax ; ditto, except 70–75 cm depth, 1 prothorax ; ditto, except 100–105 cm depth, 1 prothorax ; ditto, except 110–115 cm depth, articulated male head and prothorax; French Polynesia, Austral archipelago, Rimatara , Maunutu Swamp, MATO 2008 core, 330–335 cm depth, 1 prothorax ; ditto, except 430–435 cm depth, 1 prothorax. All but two specimens ( MATO 430–435 cm and C1T2 100–105 cm) in BPBM, otherwise in Porch.

Diagnosis. This species is distinguished from the known co-occurring species, P. prebblei n. sp., described herein, by differing physical dimensions, shape and microsculpture. P. rimatara has a flattened pronotal disk with bilinear arrangement of large elongate punctures either side of the middle of the disk, in contrast to P. prebblei , which has a convex pronotal disk with evenly distributed, evenly sized punctures. The posterior angles of the pronotum are obtuse in P. rimatara and rounded in P. prebblei and the sides of the pronotum distinctly anteriorly divergent in the former species and subparallel in P. prebblei . This latter character is evident in the ratio of median width relative to maximum width of the pronotum: P. rimatara <0.97, P. prebblei >0.97. The anterior pronotal angles in P. rimatara are not distinctively produced, whereas those of P. prebblei are obtusely produced to almost the level of the anterior margin of the medial part of the pronotum.

Description. Colour: Dark piceous-black.

Prothorax approximately as wide as long, greatest width at 1/4–1/5 of the prothorax length. Lateral margins straight, weakly converging behind widest point until rounded close to base, weakly-moderately crenulate and with narrow marginal sulcus adjacent to narrow, raised, border. Anterior angles weakly rounded and slightly produced behind middle. Anterior margin straight to very weakly bisinuate and unbordered except at anterior angles where sulcus of lateral margin continues around the anterior angle sinuously, a row of small punctures medially. Posterior angles obtuse. Posterior margin weakly lobed, rounded to very obtuse angulate, completely margined by sulcus, a continuation of lateral sulcus. Pair of deep fovae in basal sulcus, either side of middle, extending just to base of pronotum and merging with punctures which crenulate it. Basal border narrow, punctate, appearing thicker adjacent to basal fovae.

Pronotal disk moderately flat over medial third, a little declivous to anterior margin, and curving to anterior and posterior lateral margins in anterior and posterior thirds, respectively. Disk indistinctly impressed either side of middle and bearing two diffuse fields of oval punctures, either side of middle, that are separated by their own, to several times their own, length. Punctures in basal third diverging and laterally oriented, becoming convergent and medially oriented in middle third, becoming smaller in size towards anterior margin. Punctures mainly parallel but convergent in anterior third. Microsculpture of pronotal disk weak, pronotal disk between puncture appearing shiny but with a reduced longitudinally elongate mesh which gives an impression of fingerprints.

Prosternum densely punctate with punctures usually separated by about their length or less, closer on middle in front coxae and on middle and apex of prosternal process where they begin to coalesce. Punctures smaller in middle of prosternum and on prosternal process, larger on sides of prosternum. Anterior of prosternum with moderately well-developed margin that extends across much of prosternum, composed of many small punctures and a shallow punctured sulcus; with a well-developed fringe of golden setae remaining on some specimens. Apex of prosternal process truncate to slightly emarginate. Hypomera punctate.

Head (based on holotype and a paratype still articulated with prothorax) widened in front of eyes until widest 1/3 eye length in front of eyes. Frontal margins very weakly bisinuate, narrowed (at approx. 50o angle) to moderately wide anterior margin which is weakly bisinuate with greatest development on right side. Longitudinally elongate depressions of deep confluent punctures adjacent to each eye, from just before middle of eye to just before hind margin of eye, deeper rounded to elongate (varies across specimens) depressions on each side of frons at termination of weakly indicated frontoclypeal suture. Frontoclypeal suture extends from margin of clypeus to fovae, absent across middle of frons between fovae. Head shining with punctures separated by their diameter to several times their diameter. Eye coarsely facetted, about 7–8 ommatidia in length along lateral margin, dorsally and distinctly longer ventrally (estimated 10–11 ommatidia). Males with setose pit on submentum.

Physical dimensions and Ratios

Prothorax measurements based on 7 specimens. Measurements of Holotype in parentheses. Pronotum length: 0.72–0.95 mm (0.87 mm). Pronotum width: 0.75–0.96 mm (0.85 mm). Anterior width of pronotum: 0.60–0.77 mm (0.71 mm). Medial width of pronotum: 0.71–0.93 mm (0.82 mm). Basal width of pronotum: 0.60–0.78 mm (0.64 mm). Lateral length of pronotum: 0.57–0.78 mm (0.74 mm). Pronotum length/width ratio: 0.96–1.04 (1.02).

Ventral Prothorax: Prosternum length: 0.50–0.67 mm (0.62 mm). Length prosternum in front of coxae: 0.30– 0.42 mm (0.36 mm). Anterior foramen: 0.49–0.61 mm (0.57 mm). Basal foramen: 0.47–0.64 mm (0.56 mm). Prosternal process intercoxal width: 0.17–0.22 mm (0.19 mm). Maximum prosternal process width: 0.25–0.34 mm (0.29 mm). Ratio of Prosternal length/Length in front of coxae: 1.58–1.70 (1.70).

Head measurements based on 5 specimens including 2 types (articulated heads and prothoraces listed above) and 3 additional specimens considered to belong to this species noted below in ‘additional material’. Some measurements not possible for all specimens; noted as ‘NA’ for holotype measurement if unavailable. Measurements of Holotype in parentheses. Clypeus width: 0.51–60 mm (0.56 mm). Interocular width (dorsal): 0.41–0.48 mm (0.45 mm). Temporal width: 0.53–0.58 mm (NA). Eye length (dorsal): 0.13–0.16 mm (0.15 mm). Eye length (ventral): 0.19–0.20 mm (0.2 mm). Ratio Eye length dorsally/Eye length ventrally: 1.09–1.14 (1.10). Ratio interocular width dorsally/interocular width ventrally: 1.08–1.14 (1.10).

Distribution. Known only from Rimatara and expected to be a Rimatara endemic. Likely extinct: material known from other Austral Archipelago islands (Tubuai, Raivavae and Rapa) and nearby Cook Islands (Mangaia and Atiu) represent different taxa.

Etymology. Named for its occurrence and apparent endemism on the island of Rimatara , French Polynesia.

Associated (Non-type) material. Although currently impossible to prove unreservedly that different sclerites are conspecific with the type material (unless articulated), we consider it likely that the abundant elytral type from the Rimatara sediments matches the heads and pronota of P. rimatara . For Rimatara , given only one species is abundant, we are sufficiently confident to consider the sclerites conspecific but because we are not able to be 100% certain, we must exclude the elytra from the type series. Similarly, although the holotype is an articulated head and pronotum we do not include isolated heads in the type series due to the possibility that they may represent more than one species.

Other non-type material (sclerites other than prothorax or articulated head and prothorax): French Polynesia, Austral archipelago, Rimatara , Maunutu Swamp C 1T2 2008 excavation, 500 ml sediment sample, 60–65 cm depth, 1 head; ditto, except 70-75 cm depth, 1 left elytron fragment (not illustrated) ; ditto, except 80–85 cm depth, 1 complete left elytron ; ditto, except 90–95 cm depth, 1 complete right elytron ; ditto, except 100–105 cm depth, 1 largely complete head, 1 pronotum, 2 complete left elytra, 2 complete right elytra ; ditto, except 120–125 cm, 1 complete right elytron, 1 complete left elytron, 1 largely complete left elytron; French Polynesia, Austral archipelago, Rimatara , Maunutu Swamp, MATO 2008 core, 315–320 cm depth, 1 complete right elytron ; ditto, except 355–360 cm depth, 1 complete right elytron ; ditto, except 445–450 cm depth, 1 head. Representative sclerites in BPBM and Porch.

Description of elytra considered to belong to P. rimatara : subparallel, widest about middle, weakly sinuate and narrowing to rounded-subangular humerus, marked with one-several blunt teeth, to apex. Elytral intervals 1-6 mostly wider than striae, becoming narrower from interval 6. Intervals flat, widened at elytral base and apex where they are up to twice as wide as striae. Intervals apparently lacking microsculpture, shiny, and with a single row of very widely spaced punctures, mostly>4-5 times the length of the transversely elongate punctures. Striae with distinctive 'oxhide ingot' punctures separated by about, to slightly more than, their length; an oxhide ingot is a bronze age metal ingot (usually copper or tin) with a characteristic shape that is very similar to the distinctive puncture shape of some Pycnomerus elytra ( Pulak, 1998; see Figure 1 A for an example). Elytron length (1.91–2.45 mm, Av. 2.15 mm). Elytron width (0.49–0.58 mm, Av. 0.54 mm).

Figures

FIGURE 1. Measurements. Not all measurements for a sclerite or set of sclerites were always measured reflecting the variable nature of the subfossil material (an articulated head and pronotum may obscure, for example, the temporal width of the head) and preservation. A. An example of an oxhide ingot with the shape of the distinctive punctures found on many Pycnomerus elytra—see text for further details.