Synchrotronia idinineteena Soriano and Pollock, 2014

Soriano, Carmen, Pollock, Darren, Néraudeau, Didier, Nel, Andre & Tafforeau, Paul, 2014, First fossil record of polypore fungus beetles from Lower Cretaceous amber of France, Acta Palaeontologica Polonica 59 (4), pp. 941-946 : 942-945

Synchrotronia idinineteena Soriano and Pollock sp. nov.

Fig. 1 .

Etymology: After beamline ID19, in which the specimen was detected and imaged.

Holotype: Specimen IGR.ARC-330.1, figured in Tafforeau et al. (2006), but no description was provided.

Type locality: Archingeay-Les Nouillers, Charente-Maritime, southwest France.

Type horizon: Uppermost Albian, Lower Cretaceous ( Néraudeau et al. 2002).

Diagnosis.—As for genus.

Description

Body.—Length 3.2 mm, width 1.4 mm, antennal length 1.3 mm, elytra length 2.2 mm. Body subovate, elongate, dorsally convex.

Head.—About two times narrower than base of pronotum. Labrum visible, about two times as wide as long. Three-segmented maxillary palpi: first palpomere two times as long as second, and nearly three times shorter than third, which is securiform and twice as broad as the two others. Eyes laterally inserted, very close to antennae, shallowly emarginate anteriorly. Antennae widely separated, subserrate, antennomeres 4–10 distinctly triangular, widest distally, without club. Antennomeres 1.3–2 times as long as wide, and 1.8–3 times wider distally than proximally, except for last one which is quite elliptical (two times as long as wide). First antennomere twice as long as wide and with a constriction at its basal third. Second and third antennomeres the shortest (scarcely half the length of first antennomere) and the narrowest, about 1.3 times as long as wide. Fourth and sixth as long as first one and slightly broader, 1.4 times as long as wide. Fifth antennomere slightly shorter and narrower than fourth and sixth however with similar proportion length/width. Antennomere 7–10 with same length as fourth, fifth, and sixth, while they are slightly wider, with proportion comparable to those of second and third antennomeres. Neck-like constriction not distinct, visible in sagittal cut ( Fig. 2 ), but with a posterior carina meeting anterior margin of pronotum.

Prothorax.—1.7 times wider than long, anteriorly rounded, widest posteriorly, broader than elytra at shoulders, with a lateral carina in its posterior 3/4; posterior margin bisinuate, posterolatral angles acute, but without groove; prosternal process long (3/4 of prosternum) and narrowed apically; sutures not visible; procoxal cavities seem to be closed and separated from each other. Mesothorax slightly shorter than prothorax; mesocoxal cavities rounded, a little longer than wide, separated by an elongate concavity for insertion of prosternal process; mid coxal cavities closed laterally by sterna. Metathorax parallel-sided, as long as pro- and mesothorax combined and approximately 1.8 times wider than long; metepimeron about two times as wide as long; metepisterna elongate and anteriorly narrowed; discrimen dividing metaventrite along half of its length from its posterior edge.

Prothoracic coxae.—very large and protruding; metacoxae transverse (about three times as wide as mesocoxae), extending laterally to elytra. Trochanters obliquely attached to femora, so that femora abut coxa. Meso- and metafemora almost as long as maximal length of metathorax; profemora slightly shorter; all femora three times as long as wide, quite flattened, slightly broadened from base to apical third, then narrowing more abruptly. Tibiae about twice as narrow as femora in widest part; protibiae curved in distal fourth, with a strong apical hook; meso- and metatibiae conical with apex two times as wide as base; two tibial spurs on each meso- and metatibia, approximately as long as widest part of tibia. Tarsal formula 5-5-4; last tarsomere with two short claws. Prothoracic tarsi with tarsomere 1 about four times shorter than prothoracic tibia, cylindrical with apex slightly bulging, tarsomeres 2, 3 and 4 each 0.5 times as long as tarsomere 1, tarsomeres 2 and 4 quite triangular; tarsomere 2 longer than wide and tarsomere 4 slightly wider than long; tarsomere 3 wider than long and bilobed; tarsomere 5 conical with widest part at apex. Mesothoracic and metathoracic tarsomeres cylindrical with apex slightly bulging, except for bilobed mesothoracic tarsomere 4 and metathoracic tarsomere 3; mid tarsomere 1 approximately 1.8 times as short as mesotibia; tarsomeres 2, 3 and 5 about two times shorter than tarsomere 1. Metathoracic tarsomere 1 almost two times as short as metatibia; tarsomere 2 two times as short as tarsomere 1; tarsomere 4 two times as short as tarsomere 2.

Abdomen.—Approximately as long as meta- and mesothorax together, with apex slightly rounded and five ventrites; tergites deformed because the abdomen is completely desiccated under the elytra.

Elytra.—Without striations or insertion of setae, probably smooth, approximately three times as long as wide, parallel-sided from their anterior margin for 2/3 of their length, then gradually narrowed; epipleurae visible on ventral face whole length of elytra, each approximately nine times as long as wide, strongly narrowing posteriorly and slightly curved; maximal epipleural width approximately equal to maximal width of metathoracic femora. Elytra covering most of pygidium. Metathoracic wings present, macropterous, hidden under elytra but visible in sagittal cut ( Fig. 2 ).

Discussion.— Tetratomidae is an example of a relatively obscure group whose classification has become better understood through increased study of the larvae (e.g., Nikitsky 1998); historically, its constituents (especially the Eustrophinae ) were placed in Melandryidae , but now have separate family status. The five subfamilies comprise 13 genera and approximately 140 species. As far as is known, all species are associated with decaying trees and/or tree-rotting fungi Young and Pollock 2002; Lawrence and Leschen 2010; Pollock 2012), like the new fossil species S. idinineteena ( Adl et al. 2011) . Adults are active nocturnally and can be seen crawling on dead trees and fungal fruiting bodies.

The species of Eustrophinae are all rather uniform in shape: subovate to oval, with convex dorsal surface. The Palaearctic fauna presently comprises four genera in two tribes: Holostrophini ( Pseudoholostrophus Nikitsky, 1983 , Holostrophus Horn, 1888 ) and Eustrophini ( Eustrophus Illiger, 1802 and Synstrophus Seidlitz, 1898 ) ( Nikitsky 1998; Young and Pollock 2002). The Holostrophini have an Asiamerican distribution, with three Nearctic species and about 20 from Russian Far East to the Oriental region. The Palaearctic Eustrophini are known from the Holarctic and Oriental regions, with only a single species– Eustrophus dermestoides (Fabricius, 1792) –known from western Europe. This species is widespread (ranging from Spain to Turkey) ( Nikitsky 2008) and is the only one presently sympatric with the type locality of Synchrotronia . However, as discussed below, the holotype of S. idinineteena has greater affinity to the Holostrophini than to the Eustrophini .

The 5-5-4 tarsal formula exhibited by the type specimen place this new genus/species within Tenebrionoidea (= Heteromera auct.); other salient anatomical features place this taxon in Tetratomidae (sensu Nikitsky 1998) , like the simple narrow tarsi of all legs, procoxae separated by a prosternal process combined with the simple metacoxa and the relatively short spurs of the metatibiae, and more specifically within the subfamily Eustrophinae . Two characters that are important in separating the two tribes within Eustrophinae are (i) furrowing (or lack) on meso- and metatibiae, and (ii) size and separation of eyes. Other characters, such as presence or absence of laminae on the metendosternite, are not ordinarily visible on amber inclusions. According to Nikitsky (1998), members of Holostrophini are characterized as having no tibial furrows and also smaller, more widely separated eyes. These latter characters are also present on the holotype of S. idinineteena ; other features that lend support for a placement among the Eustrophinae (Holostrophini) include the subovate body shape, which is distinctly convex dorsally, the rather large prosternal process, and the bisinuate posterior pronotal margin.

Three features that are often considered diagnostic for this family are noteworthy, in relation to the single examined specimen of S. idinineteena : antennomere shape, basal pronotal impressions, and upper elytral surface. The antennae of S. idinineteena are unique among eustrophines, in that they are not clubbed, and that antennomeres 4–10 are distinctly triangular, and very similar in structure to each other. Adults of extant species possess antennae that range from slightly to distinctly clubbed ( Nikitsky 1998; Pollock 2008, 2012); no phylogenetic analysis has been done for the Eustrophinae , or the Tetratomidae , but perhaps the clubbed antenna represent an apomorphic state.

According to Nikitsky (1998) a key character for members of Eustrophinae is the presence of short, paired longitudinal impressions along the posterior margin of the pronotum; these vary from being only slightly impressed, to being distinctly incised. The holotype of S. idinineteena seems not to have these impressions, however.

The fact that the holotype of S. idinineteena seems to be glabrous dorsally could be an artifact of the preservation process. However, it seems obvious that the elytra are not distinctly striate. Within Eustrophinae , members of Holostrophini also share this non-striate condition, whereas most members of Eustrophini have distinctly impressed elytral striae ( Pollock 2008).

In summary, although based on only a single specimen, the holotype of S. idinineteena is sufficiently complete, and exhibits enough diagnostic features to warrant its placement within Tetratomidae : Eustrophinae : Holostrophini . The combination of features definitely indicates a new (and extinct) species, which cannot be placed into any described genus. Based on overall structural similarity, it seems closest to Pseudoholostrophus Nikitsky, 1983 . It seems probable that the feeding preferences of the Cretaceous Synchrotronia are very similar to those of extant species of this tribe, i.e., various wood-rotting fungi.

This new finding represents the first record of the family Tetratomidae in the Cretaceous. Table 1 lists the other species of superfamily Tenebrionoidea described from this period. Nevertheless, the systematic position of many of these fossil taxa remains doubtful, due to the lack of anatomical characters. Future implementation of Synchrotron X-Ray tomography will surely aid to reconstruct the anatomy and more precise systematic placement of new insect taxa preserved in Cretaceous amber.

Figures

Fig. 1. 3D reconstruction of the holotype of polypore fungus beetle Synchrotronia idinineteena Soriano and Pollock gen. et sp. nov., University of Rennes I, Archingeay-Les Nouillers, Albian. Specimen tomographied in beamline ID19 of the ESRF, at 30 keV and 5.06 μ voxel size. In ventral (A), lateral (B, C), dorsal (D), frontal (E), and posterior (F) views.

Fig. 2. Sagittal cut of the 3D volume of the holotype of polypore fungus beetle Synchrotronia idinineteena Soriano and Pollock gen. et sp. nov., University of Rennes I, Archingeay-Les Nouillers, Albian. Arrows mark the wings preserved under the collapsed notum.