Polycytella triassica Tillyard, 1922

Béthoux, Olivier & Anderson, John M., 2021, The Polycytellidae viewed as Gondwanan Glosselytrodea, Palaeoentomology 4 (6), pp. 550-558 : 554-555

Polycytella triassica Tillyard, 1922

( Fig. 1 )

Holotype. Specimen GSQ81a [part, preserving a wing in dorsal aspect, but mostly as impression (i. e., negative view) of ventral side, partly with vein section as positive elevation, in particular along RA], housed at the Queensland Museum ( Brisbane , Australia); and specimen NHMUK In.33279 (counterpart of GSQ81a), housed at the Natural History Museum (London, United-Kingdom; not investigated) ( Rix, 2021).

Diagnosis. Forewing: length about 8.2 mm, aspect ratio 3.3; in the 3 rd quarter of wing length, cells between RA and CuA post as wide as long; occurrence of an intercalary vein between CuA post and CuP (originating before the end of the latter; but see below).

Type locality and horizon. Denmark Hill, Ipswich, Queensland, Australia; Blackstone Formation; Carnian (possibly lowermost Norian).

Redescription. A right forewing, with most of the margins and apex missing; preserved length 7.2 mm (about 8.2 mm if complete), width 2.2 mm; area between anterior wing margin and ScP broad, with at least 4 rows of small cells; in this area, occurrence of a conspicuous ambient vein parallel to the anterior wing margin; just distal to the end of ScP, area between anterior wing margin and RA broad (0.6 mm wide opposite the end of ScP); MP+CuA ant rectilinear in its basal half, then slightly deflected posteriorly; area between RA and MP+CuA ant broadening from wing base to the ca. 3/5 of wing length, then tapering, with 4 veins at its broadest; area between MP+CuA ant and CuA post broadening from the wing base to the ca. 3/5 of wing length, then tapering, with 4 veins at its broadest; CuA post simple, very slightly bent opposite the end of CuP; occurrence of an intercalary vein between CuA post and CuP, originating about 1.6 mm before the end of the latter; claval furrow concave, rectilinear, closely bordered by two convex vein-like elements, the anterior one herein regarded as CuP, the posterior one as AA1 or a derivative of cross-venation (thereafter ‘AA1’); distally, claval furrow and both of these elements barely distinguishable from a single vein, ending 4.7 mm distal to wing base; two well-defined veins (AA?) converging towards the end of CuP, with 4 rows of cells between AA1 and the first of these veins; the second of these veins located in a depression.

Discussion. The polarity of the specimen GSQ81a is not evident at first glance. The fact that cross-veins are preserved as depressions points towards a negative imprint, while the claval furrow preserved as a depression points towards a positive imprint. One possibility, then, is that the specimen is a negative imprint of a ventral side. This interpretation is corroborated by portions of RA appearing as convex tubular elements (in particular opposite the end of ScP), representing a positive imprint of the dorsal side of a then right forewing. In turn, if this is a right forewing, negatively imprinted cross-veins can only be accounted for by a negative imprint of a ventral side.

Whether an intercalary vein occurs between CuA post and CuP (as represented on Fig. 1 ), or whether this vein is the genuine CuA post instead, is not evident to assess, essentially because the posterior part of the marginal band is poorly preserved. Considering that, on one hand, CuA post is a conspicuously convex vein in Polycytella rasnitsyni , and that, on the other, the vein of concern lies in a depression, while the vein anterior to it (‘CuA post ’ on Fig. 1 ) is elevated, we considered it unlikely that it could be CuA post. The development of an intercalary vein in this area is not unlikely, given that similar structures occur in the area between the anterior wing margin and ScP, and in the area anterior to RA.

Figures

FIGURE 1. Polycytella triassica Tillyard, 1922, holotype specimen (GSQ81a), right forewing (dorsal aspect, but mostly as impression of ventral side). A, Interpretative drawing. B, Photograph (G. Thompson, Queensland Museum, Brisbane, Australia; ‘cf’, claval furrow).