Elmidae, Curtis, 1830

KIREJTSHUK, ALEXANDER G., PATEL, RAMAN, RANA, RAJENDRA SINGH, PROKIN, ALEXANDER, NEL, ANDRÉ & JÄCH, MANFRED A., 2023, Discovery of the oldest known elmid larva (Coleoptera, Elmidae) from the Lower Eocene of Rajasthan (India, Palana Formation), Palaeoentomology 6 (3), pp. 235-241 : 237-239

Elmidae genus and species indetermined

( Fig. 3 )

Material. Specimen GU / R /B/G 3008 (a nearly complete larva), stored at Palaeontology laboratory, Department of Geology, H.N.B. Garhwal University, Srinagar Garhwal, Uttarakhand, India

Type locality and horizon. Lower Eocene , Palana Formation , Gurha opencast lignite mine, Bikaner district, Rajasthan, India .

General remarks. The characters observed in this larva are all diagnostic of Elmidae ( Fig. 3 ): Head with clustered stemmata on each side; frontal suture V-shaped, epicranial suture short; antenna shorter than head, with three antennomeres; labrum transverse and short; abdomen long, nine-segmented, segment IX long, with short ventral operculum in apical position, tuft of retractile tracheal anal gills present.

Description. Thorax and abdomen laterally compressed, but head dorsoventrally compressed; legs only partly preserved. Body ca. 5.70 mm long, elongate. Thorax and abdomen equally high. Thorax and abdominal segments I–V apparently somewhat expanded and subflattened, probably during fossilization, abdominal segments VI–IX apparently not deformed. Thorax and abdomen with traces of stigmata. Thoracic and abdominal integument rather sclerotized, almost as sclerotized as epicranium, apparently all segments finely and densely granulate, microsetae not visible. Posterior edges of abdominal segments I–VIII apparently finely serrate (serration partly traceable on upper parts of sides of abdominal segments III, IV and VIII). Prothorax ca. 1.8 times as long as mesothorax and 1.6 times as long as metathorax.

Epicranium turned about 90° around body axis, apparently widest at middle (but dorsoventrally compressed, particularly posteriorly, and appearing widest at base), apparently somewhat narrowing basally along retracted part of head, subtruncate at frons, mandibles subtriangular and short, heavily sclerotized and visible through epicranial cuticle, ocular area slightly traceable but heavily sclerotized. Median part of epicranium destroyed along epicranial sutures and partly missing. Right antenna moderately long, visible between anterior edge of frons and ocular area.

Prothorax larger (longer and apparently wider) than meso- and metathorax, dorsal surface (pronotum) longer than ventral one, with outlined, comparatively wide pro- and mesocoxae, and also two moderately thick metafemora (besides, one profemur and other sclerites of legs apparently partly traceable, as well as some remnants of mouthparts (apparently detached maxilla) or proleg sclerites.

Abdominal segments I–VIII approximately subequal in length and also to meso- and metathorax; segment IX about as long as segments VI–VIII together, and nearly twice as long as prothorax, in distal third subconical and sharply acuminate at apex; segment IX with several gill filaments protruding from the ventral operculum.

Figures

FIGURE 3. Fossil larva, Eocene of Rajasthan, GU/R/B/G 3008. A, Photograph of entire body. B, Line drawing of body. C, Photograph of head and thorax, enlarged. D, Line drawing of head and thorax. Scale bars = 1 mm (A, B), 0.5 mm (C, D).