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Genus † Archaeomalthus gen. nov. Type species. Archaeomalthus synoriacos Yan et al., sp. nov., by monotypy. Diagnosis Antennal sockets visible from above; propleuron exposed, narrow, not reaching anterior pronotal margin; procoxae slightly widened, almost contiguous medially; metanotum with metascutum delimited by very distinct longitudinal bulges (alacrista) (Figures 1 (b) and 2(i,k,l)); anterior intercoxal process of metaventrite strongly developed, completely separating mesocoxae. Abdomen with seven visible sternites. Derivation of name Generic name from Greek ‘archaios’ – ‘ancient’ and Micromalthus LeConte, 1878, type genus of family Micromalthidae. Description Body elongated, slender, probably slightly flattened, with distinct shoulder region, slightly rounded laterally posterior to prothorax. Total length ca. 1.18 mm, maximum width at posterior metathoracic region ca. 0.4 mm. Cuticular surface largely smooth on all body regions, without recognizable scales, punctures or tubercles. Head large, as long as pronotum, rectangular, slightly narrower than anterior pronotal margin; with widely separated well-developed compound eyes, inner margins of eyes are finely bordered; ocelli not recognizable, probably absent. Labrum not articulated, apparently fused with clypeus. Concave transverse line present on anterior head capsule, possibly indicating clypeofrontal strengthening ridge. Antennae inserted dorsally but still widely separated, articulatory area close to anteromesal edge of eyes. Mandibles sickle-shaped, with distal halves curved inwards. Maxillae not recognizable. Proximal elements of labium not visible as separate structures, submentum apparently fused with gula and ventral wall of head capsule (Figure 2 (a,b)). Temporal region posterior to eyes very long, more than half as long as the length of head; narrowed neck region absent. Dorsal protuberances missing. Gular plate present, large. Pronotum almost quadrangular, as long as wide, with distinct lateral carina; anterior and posterior angles distinct, triangular; anterior pronotal margin at the same level as the anterior prosternal edge. Prosternal intercoxal process not visible, apparently missing; wide membranous area present posterior to procoxae. Large spiracle recognizable in prothoracic-mesothoracic intersegmental region. Elytral bases distinctly wider than maximum pronotal width; pronoto-elytral angle, i.e. humeral bulges, very distinct, rounded. The shape of elytra roughly quadrangular, with truncated apices. Mesal base enclosed by the semioval scutellar shield. Window punctures and other patterns of surface sculpture absent, degree of sclerotization uniform. Metanotum with distinct longitudinal median groove delimited with strongly developed alacrista; additional longitudinal bulges visible laterally. Mesocoxae widely separated, more or less rounded, nearly triangular, not transverse. Mesocoxal cavities laterally closed by mesepimeron and metanepisternum. Metaventrite large, distinctly narrowed anteriorly, with large triangular, apically rounded anterior intercoxal process; separating mesocoxae completely. Transverse suture of metaventrite not recognizable, probably absent. Metatrochantin not exposed. Metacoxae short, transverse, reaching the lateral edge of ventrite, without recognizable metacoxal plates. Abdomen of parabolic shape, evenly narrowing posteriorly, with rounded apex, as long as meso- and metaventrite combined; at least basal sternites separated by externally visible intersegmental membranes. Four distal abdominal tergites not covered by the elytra. Species included Monotypic. Occurrence Babiy Kamen`locality, Tom`river, Kuznetsk Basin, S. Siberian, in the upper part of the Maltsevo Formation. Remarks Generally, size alone does not justify a placement of a beetle in a specific supraspecific taxon. However, after taking into consideration all the characters mentioned in the Diagonisis and Description, a length of only slightly more than 1 mm should be seen as an additional argument for keeping Archaeomalthus in Micromalthidae. All other archostematan beetles with the exception of the strongly flattened Crowsoniella relicta (Pace 1975) are much larger. The degree of cuticular sclerotization can rarely be adequately estimated on fossil imprints. Therefore, it cannot be used as a solid diagnostic character, even though some surface features such as tubercles, large punctures, etc., can be used for species identification and also in a phylogenetic context (e.g. Beutel et al. 2008; Hörnschemeyer 2009). In the case of Archaeomalthus, the visibility of metathoracic fine details (including metapostnotum) on mostly ventral imprint and elytra (Figure 2 (i,k)), preserved only as faint lines, suggests that Archaeomalthus was overall weakly sclerotized. Elytra of most fossilized beetles, in contrast, are hard structures overlapping the body and usually retaining many structural details (Ponomarenko 1969). Beetle fossils previously known from Babiy Kamen`locality, Petrodromeus Ponomarenko et Volkov, 2013 and Permunda Ponomarenko et Volkov 2013 (Adephaga; Trachypachidae) both have finely preserved elytra with well-defined longitudinal striae (Ponomarenko and Volkov 2013). The abdomen of Archaeomalthus is distinctly shorter than the head and pronotum combined; however, this character is variable in the extant Micromalthus due to varying degrees of extension of the intersegmental membranes. Thus it is not used in the generic diagnosis. The abdominal segments are often more contracted in males than in females, possibly due to different physiological conditions (e.g. development stage of eggs). The length of the abdomen, especially of females, varies in M. debilis Hörnschemeyer et al. 2010. |
