An unusual elateroid lineage from mid-Cretaceous Burmese amber (Coleoptera: Elateroidea)

We here report a new elateroid, Anoeuma lawrencei Li, Kundrata and Cai gen. et sp. nov., from mid-Cretaceous Burmese amber. Though superficially similar to some soft-bodied archostematans, Anoeuma could be firmly placed in the polyphagan superfamily Elateroidea based on the hind wing venation. Detailed morphological comparisons between extant elateroids and the Cretaceous fossils suggest that the unique character combination does not fit with confidence into any existing soft-bodied elateroid group, although some characters indicate possible relationships between Anoeuma and Omalisinae. Our discovery of this new lineage further demonstrates the past diversity and morphological disparity of soft-bodied elateroids.


Discussion
Subordinal and superfamilial placement of Anoeuma gen. nov.. The here presented fossil shares a rather typical habitus of soft-bodied elateroids. However, there are some characters which can be confusing if one tries to classify this lineage within the Coleoptera. The most intriguing is the ventral prothoracic structure which might seem to be unusual for Polyphaga, including Elateroidea. In Polyphaga, the propleuron is internallised as a cryptopleuron, while in the other three beetle suborders the propleuron remains external 22 . In Anoeuma gen. nov., the sclerite between prosternum and hypomeron (Figs. 4B,5B) could be potentially interpreted as a propleuron. If we follow this scenario, the only reasonable placement for the fossil would be in Archostemata, probably close to an aberrant soft-bodied Micromalthus LeConte, 1878. The new fossil shares a series of characters with Micromalthidae, including e.g., the absence of dorsal head protuberances (Figs. 4F,5F), the shortened elytra (Fig. 3B), and the higher number of abdominal ventrites (Fig. 3F) 23 . However, many other When we look at the hind wing, the folding pattern present in the fossil may look untypical for the polyphagan beetles; however, its hind wing venation looks elateroid-like. The radial cell of Archostemata is bordered by the radial bar, R 3+4 , and two cross-veins, while in Elateroidea (and Polyphaga in general, and also Anoeuma gen. nov.) the radial cell is bordered by the radial bar and R 3+4 only, with no cross-veins (Figs. 2C,3C) 26,27 . Further, in Archostemata, the RP branches are relatively well developed, and RP and MP 1+2 are connected by two cross-veins, while in Elateroidea (and also in Anoeuma gen. nov.), the RP branches are usually completely lost, and RP fuses with MP 1+2 distally (Figs. 2C,3C). Therefore, we believe the newly discovered fossil should be classified in Elateroidea rather than in Archostemata. The sclerite between prosternum and hypomeron could be interpreted as a well-developed protrochantin (Figs. 4B,5B). Indeed, a similarly shaped protrochantin (i.e., triangular with a slender process attached to procoxa) can be found in some other soft-bodied elateroids (e.g., 15 ).
Position of Anoeuma gen. nov. within Elateroidea. The soft-bodiedness and neoteny originated multiple times within the Elateroidea 1,5,7 . The classification of soft-bodied elateroids based solely on morphology is precluded by the fact that many related groups are differently affected by neoteny, thus being not similar, but on the other hand, some lineages look superficially very similar although they are only distantly related 1 . The morphology-based phylogenetic analyses failed to provide us with the natural classification of the Elateroidea 13 or to determine the phylogenetic position of a recently described fossil softbodied lineage 11 . Thus, since we cannot reliably use the morphological characters to test the position of Anoeuma gen. nov. within the Elateroidea, and the DNA, as an another potential source of data, is not available for the Mesozoic fossil taxa 28,29 , we have to rely on the comparison of the newly discovered fossil with other soft-bodied elateroids.

Scientific Reports
| (2021) 11:21985 | https://doi.org/10.1038/s41598-021-01398-w www.nature.com/scientificreports/ Recently discovered monogeneric Iberobaeniidae share with Anoeuma gen. nov. the shape of head, hypognathous mouthparts, and short tarsomeres 1-4. However, Iberobaeniidae can be distingushed by 2-segmented labial palps, pronotum without lateral carinae, not apparently shortened elytra, and much more reduced hind wing venation, with e.g., radial cell vestigial) 15,47 . Representatives of the soft-bodied click-beetle subfamily Omalisinae share with Anoeuma gen. nov. the general appearance, the slightly serrate antennae, tiny and ring-like antennomeres 2 and 3, and at least some have also a relatively long prosternum (although differently shaped) and a more developed mesoventrite (e.g., Cimbrion Kazantsev 48 ). Anoeuma gen. nov. is superficially most similar to Paradrilus Kiesenwetter in the prolonged frontal part of the cranium with (fronto)laterally inserted antennae, hypognathous mouthparts, and the pronotum without well-defined posterior angles (other omalisine genera have better developed posterior angles) 49 . Nevertheless, the fossil genus differs from Omalisinae in having the apparently narrower frontoclypeal part and even more hypognathous mouthparts, subtriangular prosternum with extremely slender prosternal process (prosternum transverse and without prosternal process or clearly subrectangular in Omalisinae), reduced elytra (all known Omalisinae have elytra long, covering whole or most of abdomen), tarsomeres 2-4 short and distinctly shorter than last tarsomere (Omalisinae usually have elongate tasomeres, with only tarsomere 4 relatively shorter, and last tarsomere never distinctly longer), and the elongate terminal maxillary palpomere, which is much longer than preceding ones (terminal palpomere usually only slightly longer than preceding one and never so long in Omalisinae). Therefore, although Anoeuma gen. nov. is in some characters more similar to Omalisinae than to other elateroid groups, and some of the characters which distinguish them are highly plastic in some soft-bodied elateroid lineages (e.g., 50 ), we prefer to place the newly discovered fossil in Elateroidea incertae sedis. Discoveries of further taxa related to Anoeuma gen. nov. in future would probably help us to better understand the systematic placement of this taxon, which we hypothesise may either represent a separate evolutionary lineage on a family rank or is related to the soft-bodied omalisine click-beetles.

Conclusions
Anoeuma gen. nov. exhibits a mixture of characters known in several neotenic elateroid lineages, but on the other hand, it differs from them in other taxonomically important characters. Additionally, the morphology of ventral prothorax and the dense body punctation are somewhat unique among soft-bodied Elateroidea. In the current state of knowledge, we are unable to place Anoeuma gen. nov. into any existing family in full confidence. However, since we refrain here from erection of a new suprageneric rank for Anoeuma gen. nov., we tentatively classify it as Elateroidea incertae sedis. Neotenics are rarely found not only in the fossil record but also recently in the field, mostly due to their cryptic lives. The flight-capable males usually fly only reluctantly and spend most of the day hidden in the soil or other substrates, and larviform females do not move much at all 5,47,51 . Therefore, every report of a new neotenic lineage is extremely important for understanding the past diversity and morphological variability of soft-bodied elateroids. Since the males of Anoeuma gen. nov. exhibit a number of characters which are typical for the lineages where the females are strongly affected by the paedomorphic development 5,41 , we expect that the females, when/if discovered, will be flightless and larviform.

Materials and methods
Materials. The Burmese amber specimens studied here were derived from amber mines near Noije Bum Fossil imaging. Photographs under incident light were mainly taken with a Zeiss Discovery V20 stereo microscope. Widefield fluorescence images were captured with a Zeiss Axio Imager 2 light microscope combined with a fluorescence imaging system. Confocal images were obtained with a Zeiss LSM710 confocal laser scanning microscope, using the 488 nm Argon laser excitation line 52 . Images under incident light and widefield fluorescence were stacked in Helicon Focus 7.0.2 or Zerene Stacker 1.04. Confocal images were stacked in Helicon Focus 7.0.2. Images were further processed in Adobe Photoshop CC to enhance contrast.