Description of a new species of Eucinetidae (Coleoptera, Scirtoidea) from Cretaceous Burmese amber

Abstract Eucinetus parvussp. nov. is described from late Cretaceous Burmese amber, representing the second record of Eucinetidae from the Burmese amber and the first species of the family with simple, not piercing, mouthparts. A comparison between Mesocinetidae and Eucinetidae is provided.


Introduction
The family Eucinetidae Lacordaire, 1857 is a relatively small group in the polyphagan Coleoptera, consisting of 10 extant genera and fewer than 60 species distributed worldwide (Leschen 2016;Lawrence 2019). The genera of living Eucinetidae, Nycteus Latreille, 1829, Bisaya Reitter, 1884, Euscaphurus Casey, 1885, Jentozkus Vít, 1977, Tohlezkus Vít, 1977, Eucilodes Vít, 1985, Eucinetella Nikitsky, 1996, Proeuzkus Vít, 2000, and Noteucinetus Bullians & Leschen, 2004, with exception of Eucinetus Germar, 1818, are mostly very limited in their geographic distribution and contain only a few species. Because of the peculiar shape of the metacoxae, these beetles are usually called "plate-thigh beetles" (Guéorguiev 2004). The living species can usually be found under bark or are extracted from leaf litter, and they have been found in a relationship with myxomycete or basidiomycete fungi on which adult beetles and larvae probably feed (Leschen 2016). Historically, Eucinetidae has been included in the superfamily Dascilloidea (Crowson 1955), but Crowson (1960) created Eucinetoidea for Eucinetidae, Scirtidae, and Clambidae based on both adult and larval characters; Lawrence and Newton (1982) accepted this systematic arrangement, while Lawrence and Newton (1995) later claimed the priority of Scirtoidea, which also included Decliniidae Nikitsky, Lawrence, Kirejtshuk & Gratshev, 1994(Nikitsky et al. 1994. Molecular studies (McKenna et al. 2015;Zhang et al. 2018) also supported the close relationships of these families, together with Derodontidae, and they indicated that Eucinetidae may be close to the basal group in Polyphaga. Within the family, species of Bisaya, Jentozkus, Tohlezkus, Eucilodes, and Proeuzkus have subsuctorial mouthparts, the function of which remains unclear until now.
The fossil record of Eucinetidae is sparse. The fossil genus Mesocinetus Ponomarenko, 1986, which was described from the early Cretaceous of the Gurban-Eren Formation in western Mongolia (Ponomarenko 1986), was subsequently placed in its own family, Mesocinetidae (Kirejtshuk and Ponomarenko 2010). The family Mesocinetidae was thought to be related to Scirtidae and Eucinetidae, and included four other late-Jurassic genera (Manoelodes Kirejtshuk & Ponomarenko, 2010, Manopsis Kirejtshuk & Ponomarenko, 2010, Parashartegus Kirejtshuk & Ponomarenko, 2010, and Shartegus Kirejtshuk & Ponomarenko, 2010. Two species of the genus Huaxiacinectus Hong, 1995, which were described from early Cretaceous of the Huachi-Huanhe Formation in China (Hong 1995), were attributed to Eucinetidae, although the true attributions of these species remain unknown and need study. Jałoszynski (2019) recently described a eucinetid specimen from Burmese amber with highly modified piercing mouthparts as Cretohlezkus alleni Jałoszynski, 2019. The oldest species of the genus Eucinetus was in Bembridge Marls from the late Eocene of the Isle of Wight (Kirejtshuk et al. 2019).
Here, a new fossil species of the extant genus Eucinetus (Eucinetidae) with simple mouthparts from Burmese amber is presented. This new species demonstrates the ancient origin of this lineage of beetles.

Materials and methods
The specimen included in this study is embedded in Burmese amber from the Hukawng Valley of northern Myanmar (Cruickshank and Ko 2003;Dong et al. 2015: fig. 1). The age of this amber is generally considered to be near the Albian/Cenomanian (98.79 ± 0.62 Ma) (Shi et al. 2012). The amber specimen is deposited in the Museum of Biology, Sun Yat-sen University, China (SYSBM).
For preparation, the amber material was polished with emery papers of various grits and polished with polishing powder. Images were taken using a Nikon DS-Ri2 camera mounted on a Nikon SMZ25 microscope; layers were captured and aligned using NIS-Elements software and processed in Photoshop CC. The line drawings were prepared in Adobe Illustrator CC and figures were compiled in Photoshop CC. The length of the beetle specimen was measured from the anterior margin of head to the apex of elytra; the width is the maximum width of the elytra. Morphological terminology of Eucinetidae follow Leschen (2016 Diagnosis. The new species can be distinguished from all the extant species of Eucinetus by the combination of the following characters: relatively small and narrower body; slenderer mesepimeron and matanepisternum; relatively short antenna with the scape shorter than pedicel. It can also be easily separated from Eucinetus nikolaevae by much smaller body (1.9 mm long compared to 2.8 mm in E. nikolaevae), slender body-shape, and sub-rectangular labrum.
Head relatively small, subtriangular; posterior margin nearly truncate. Eyes moderately large, protruding, and finely facetted. Temple behind eyes moderately long; posterior angles orthogonal. Antennae closely inserted in front of eyes in antennal fossae. Antenna (Fig. 1D) 11-segmented, short, and not extending beyond posterior margin of pronotum; scape relatively small and longer than wide; pedicel dilated and distinctly wider than adjacent segments; antennomere 3 nearly as long as following antennomeres; antennomeres 3-10 increasingly transverse toward apex; terminal antennomere larger with rounded apical margin. Frontoclypeal suture absent; labrum long, wide, and subrectangular, with nearly straight apical margin. Maxillary palp (Fig. 1D) 4-segmented; first segment very short; palpomeres 2 and 3 subequal and cylindrical; terminal segment longest, not wider than penultimate one, and fusiform anteriorly, with sharp apex. Labium with simple prementum; labial palp 3-segmented; terminal palpomere largest and fusiform, with apex elongate and sharp.
Pronotum transverse, widest posteriorly; anterior margin broadly rounded, lateral margins gradually widened posteriorly, and posterior margins sinuate; disc with distinct microsculpture, uniformly covered by dense, short setae. Prosternum highly reduced with very narrow area in front of procoxae; prosternal process narrow. Notosternal suture present. Procoxae strongly transverse and projecting, nearly contiguous, and protrochantins exposed; procoxal cavities externally widely opened.
Elytra elongate, about 2.3 times as long as wide, lateral margins gradually narrowed posteriorly, apex sharp; dorsal surface with dense, distinct, transverse microsculpture and covered by dense, uniform setae; epipleuron not extending to apex and relatively narrow at base. Mesoventrite short, with deep, longitudinal, middle depression fitting fore femur. Mesanepisternum large and subrectangular; mesepimeron large and subtrapezoid. Mesocoxal cavities (Figs 1C, 2) large and subovate; laterally widely open to mesepimeron; mesocoxae moderately separated and not projecting; mesotrochantins concealed. Metaventrite short, transverse, and not narrowed towards lateral margins; metanepisternum subtriangular and elongated. Metacoxae contiguous, with large metacoxal plates covering hind femora and most of abdominal ventrite 1; metacoxal plates with anterior margin only slightly oblique, lateral margins curved. Tarsal formula 5-5-5. Abdomen with five visible ventrites, terminal ventrite subtriangular. Fore leg with small, indistinct trochanter; femur elongate and slightly curved; tibia short and flattened, gradually widened toward apex; apical spurs highly reduced; tarsi 5-segmented, with basal four tarsomeres short and almost in same length; claws small. Mid leg with small trochanter and enlarged femur; tibia flattened and broadened apically, with pair of apical spurs unequal in length; apex with fringe of small spines; tarsus with first tarsomere longest, nearly the same length as following two segments combined, tarsomeres 2-5 gradually shortened, tarsomeres 2-4 with fringe of spines apically. Hind leg (Fig. 1E) with femur elongate and slightly dilated, mostly concealed by metacoxal plate; tibia longer than femur, flattened and widened apically, and apex with pair of short unequal apical spurs and fringe of spines; first tarsomere longest and almost same length as following two segments combined; tarsomeres 2-4 gradually shortened; last tarsomere with pair of small claws, nearly same length as penultimate one.

Discussion
The placement of the new species in Eucinetidae is based on the combination of hypognathous head, fusiform body shape, 5-5-5 tarsal formula, fringes of spines on apex of tibiae and tarsomeres, and huge metacoxal plates. Unlike Cretohlezkus, which was described from Burmese amber (Jałoszynski 2019), the new species has a simple prementum and a relatively long metaventrite and metepimeron, which are similar to those in some species of Eucinetus and Nycteus. Antennomere 3 in Nycteus is distinctly shorter than the following segment, different than in the new species. The characters discussed above suggest the placement of the new fossil taxon in the genus Eucinetus. However, the diagnosis of this genus is very broad and requires further research. The occurrence of Eucinetidae in Burmese amber with both simple and piercing mouthparts suggests that the family habits were already very diverse 99 Ma ago and this diversity has been retained to the present.
Eucinetidae is closely related to the fossil family Mesocinetidae, which is distinguished mostly by the large metacoxal plates, very short metaventrite, and widened metanepisternum (Vít 1995(Vít : fig. 10, 1997(Vít : figs 20, 21, 2000 fig. 3). Mesocinetidae differs from Eucinetidae in having small metacoxal plates, a relatively long metaventrite, and a subrectangular metepimeron (Kirejtshuk and Ponomarenko 2010). How-ever, the differences between these families need further research, as intermediate taxa are found in Eucinetidae. The extant genus Eucilodes is similar to Mesocinetidae in sharing some of those characters (Vít 1985: figs 5, 21), but it has piercing mouthparts absent from known taxa of Mesocinetidae. Similarly, some species of Eucinetus and Nycteus have relatively long metaventrites and metanepisterna (Vít 1977(Vít : figs 1, 3, 1979(Vít : figs 9, 10, 1985 fig. 7), while in other species of the same genera these structures are much shorter (Vít 1990: fig. 28). The genera of Eucinetidae with subsuctorial mouthparts was found to be monophyletic in Jałoszynski's (2019) morphological phylogenetic analysis, but more comprehensive samples and molecular data are required to verify this hypothesis.