Serrathroscinus, a new genus of Cephalobyrrhinae from mid-Cretaceous Kachin amber (Coleoptera: Limnichidae)

ABSTRACT

A new genus and species of Limnichidae, Serrathroscinus majaechi gen. et sp. nov., is described from mid-Cretaceous Kachin amber of Myanmar. While sharing a typical appearance with other members of Cephalobyrrhinae, Serrathroscinus is distinctive within the subfamily due to its fully and unambiguously serrate antennae.

KEYWORDS:
Burmese amber; Mesozoic; beetle; fossil; taxonomy

INTRODUCTION

The beetle family Limnichidae, as currently recognized (though not monophyletic), encompasses four subfamilies: Hyphalinae, Thaumastodinae, Cephalobyrrhinae and Limnichinae (Hernando and Ribera 2016, Cai et al. 2022). Among these, Cephalobyrrhinae is a small group that includes five extant genera: Cephalobyrrhus Pic, Erichia Reitter, Parathroscinus Wooldridge, Throscinus LeConte, and Ercus García and Jiménez-Ramos (Jäch and Pütz 2001, García and Jiménez-Ramos 2019). Notably, the genus Ersachus Erichson, while traditionally placed within Limnichinae, exhibits certain morphological similarities to Cephalobyrrhinae, particularly in the absence of excavations on the ventral surface for leg reception, suggesting a potential taxonomic relationship that warrants further investigation (Pütz et al. 2004, Arce-Pérez et al. 2014).

Fossil limnichids have been described from Eocene Baltic amber and mid-Cretaceous Kachin amber (Pütz et al. 2004, Hernando et al. 2018, Yu et al. 2018, Kirejtshuk and Prokin 2022, Li et al. 2022). Among these discoveries, Erichia cretacea Yu et al. and Anomocephalobus liuhaoi Li et al., both recovered from Kachin amber, have been assigned to Cephalobyrrhinae (Yu et al. 2018, Li et al. 2022). Palaeoersachus bicarinatus Pütz et al. from Baltic amber, while having some cephalobyrrhine-like traits, has been classified as Limnichidae incertae sedis (Pütz et al. 2004). In this study, we describe a new fossil representative of Cephalobyrrhinae from Kachin amber, contributing to the growing understanding of this subfamily’s paleodiversity.

MATERIAL AND METHODS

The Kachin amber specimen studied herein (Figs 1-18) originated from amber mines near Noije Bum (26°20’N, 96°36’E), Hukawng Valley, Kachin State, northern Myanmar. The amber specimen is deposited in the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences, Nanjing, China. The amber piece was trimmed with a saw mounted on a handheld rotary tool, ground with emery paper of different grit sizes, and finally polished with polishing powder.

Figures 1-2
General habitus of Serrathroscinus majaechi gen. et sp. nov., holotype, NIGP207074, under incident light: (1) dorsal view; (2) ventral view. Scale bars: 1 mm.

Photographs under incident light were taken with a Zeiss Discovery V20 stereo microscope. Confocal images were obtained with a Zeiss LSM710 confocal laser scanning microscope, using the 488 nm (Argon) laser excitation line (Fu et al. 2021). Images were stacked with Helicon Focus 7.0.2 and Adobe Photoshop CC, and were further processed in Adobe Photoshop CC to adjust brightness and contrast. Microtomographic data were obtained with a Zeiss Xradia 520 Versa 3D X-ray microscope at the micro-CT laboratory of NIGP and analyzed in VGStudio MAX 3.0. Scanning parameters were as follows: isotropic voxel size, 4.5026 μm; power, 3 W; acceleration voltage, 40 kV; exposure time, 2 s; projections, 2401.

TAXONOMY

Coleoptera Linnaeus, 1758

Limnichidae Erichson, 1846

Cephalobyrrhinae Champion, 1925

Serrathroscinus gen. nov.

https://zoobank.org/7E4B974A-EDD3-47A7-B72F-375C4293CBE5

Type species. Serrathroscinus majaechi sp. nov.

Diagnosis. Body elongate-oval. Ventral surface without grooves for reception of legs (Fig. 16). Antennae unambiguously serrate, with antennomeres 3-11 distinctly asymmetrical (Figs 1, 2). Anteclypeus present (Fig. 18). Pronotal disc with simple surface; posterior pronotal angles unproduced; posterior pronotal margin crenulate (Fig. 10). Prosternal process smooth and parallel-sided (Fig. 4). Elytral surface without striae; basal elytral margin smooth (Figs 10, 11). Abdominal ventrite 5 apically simply rounded (Fig. 8).

Etymology. The generic name is derived from the Latin serra (saw), referring to the fully serrate antennae, and Throscinus, a genus in Cephalobyrrhinae. The name is masculine in gender.

Serrathroscinus majaechi sp. nov.

Figs 1-18

https://zoobank.org/64C57E7A-E1A5-4113-9C00-9EFBD4D44DF1

Diagnosis. As for the genus.

Material examined. Holotype, NIGP207074, male.

Locality and horizon. Amber mine located near Noije Bum Village, Tanai Township, Myitkyina District, Kachin State, Myanmar; unnamed horizon, mid-Cretaceous, Upper Albian to Lower Cenomanian.

Figures 3-11
Details of Serrathroscinus majaechi gen. et sp. nov., holotype, NIGP207074, under confocal microscopy: (3), head, ventral view; (4) prothorax, ventral view; (5) mesothorax, ventral view; (6) metacoxae, ventral view; (7) fore leg; (8) aedeagus, ventral view; (9) head, dorsal view; (10, 11) elytral base, dorsal view. (el) Elytron, (md) mandible, (mtc) metacoxa, (pn) pronotum, (ps) prosternum, (pts) protarsus, (sc) scutellum. Scale bars: 200 μm.

Figures 12-18
X-ray microtomographic reconstruction of Serrathroscinus majaechi gen. et sp. nov., holotype, NIGP207074. Appendages removed in (16-18): (12) dorsal view; (13, 16) ventral view; (14, 17) lateral view; (15, 18) anterior view. Scale bar: 1 mm.

Description. Body elongate oval, about 3.6 mm long, 1.6 mm wide. Dorsal surface with fine decumbent hairs and interspersed stouter erect hairs.

Head hypognathous, largely exposed. Frontoclypeal suture present. Eyes large, strongly convex. Antennae 11-segmented; antennomeres 3-11 triangular, distinctly asymmetrical. Anteclypeus present. Mandible bidentate at apex. Apical maxillary palp fusiform.

Pronotal disc about 1.8× as wide as long, widest posteriorly, with sides slightly curved and converging anteriorly; surface simple; lateral pronotal carinae complete; posterior pronotal margin medially protruding backward and smooth, laterally straight and crenulate; posterior pronotal angles nearly right, not produced. Hypomera without ridges and associated grooves for reception of fore legs. Prosternum in front of procoxae transverse, as long as prosternal process; prosternal process parallel-sided, broad, wider than 1/3 maximum width of prosternum, apically rounded, fitting well into broad mesoventral cavity. Procoxal cavities broadly open externally. Protrochantin exposed.

Scutellar shield triangular. Elytra elongate, about 1.4× as long as combined width, slightly broader than pronotum; basal margin smooth; surface without any striae or grooves. Hind wings present. Mesocoxal cavities widely separated, partly closed laterally by mesepimera. Mesometaventral junction externally as straight line. Metaventrite with discrimen and katepisternal suture; crural impressions absent. Metacoxae narrowly separated, transverse; plates relatively well developed.

Legs slender. Tibiae without stout spines; tibial spurs present, possibly 1-1-1. Tarsi 5-5-5, relatively elongate. Pretarsal claws simple.

Abdomen with five ventrites; ventrites 1-3 appearing more solidly connected. Ventrite 1 without crural impressions. Ventrite 5 apically simply rounded, without notches. Aedeagus of trilobate type; penis and parameres slender, apically acute; penis almost as long as parameres.

Etymology. The species is named after the coleopterist Dr. Manfred A. Jäch.

DISCUSSION

Serrathroscinus could be confidently placed in subfamily Cephalobyrrhinae, as it has an elongate-oval body (often wider in Limnichinae), simply transverse metacoxae (small and ovoid in Hyphalinae, oblique in Thaumastodinae), and 5-segmented protarsi (4-segmented in Hyphalinae and Thaumastodinae), and lacks the excavations on ventral surface for the reception of legs (present in Limnichinae) (Hinton 1939, Satô 1966, Britton 1971, Wooldridge 1975).

Serrathroscinus has the typical appearance of Cephalobyrrhinae, particularly resembling Throscinus and Parathroscinus (Wooldridge 1984). However, Serrathroscinus is distinctive among Cephalobyrrhinae due to its fully and unambiguously serrate antennae. While the distal flagellomeres of some cephalobyrrhines might be more or less asymmetrically expanded apically, their basal flagellomeres are essentially cylindrical (e.g., Yoshitomi 2019: fig. 2G). Among the previously reported Cephalobyrrhinae, Anomocephalobus Li et al. might have the most distinctly serrate antennae (Li et al. 2022). Even so, antennomere 3 of Anomocephalobus is cylindrical (at most very weakly asymmetrical). However, in Serrathroscinus, all the flagellomeres (antennomeres 3-11) are distinctly asymmetrical.

The following characters further distinguish Serrathroscinus from other (potential) members of Cephalobyrrhinae. Serrathroscinus probably has a membranous anteclypeus, whereas according to the published line drawings, Throscinus and Parathroscinus appear to have no anteclypeus (Wooldridge 1984: figs 5, 6; García and Jiménez-Ramos 2019). Serrathroscinus has a smooth and parallel-sided (or even slightly dilated near apex) prosternal process, whereas the prosternal process of Ersachus and Ercus is somewhat triangular (with posteriorly converging sides) (Arce-Pérez et al. 2014, García and Jiménez-Ramos 2019), and the prosternal process of Palaeoersachus Pütz et al. is medially carinate (Pütz et al. 2004). Serrathroscinus has a pronotal disc with simple surface and unproduced posterior corners, whereas the posterior pronotal corners of Erichia are strongly produced, the pronotal disc of Erichia and Cephalobyrrhus has impressions near base (Pütz 1991, Jäch and Pütz 2001), and the disc of Palaeoersachus has paired longitudinal carinae at base (Pütz et al. 2004). The basal margin of elytra is generally described as serrate or crenulate for Cephalobyrrhinae (Pütz 1998). Although we cannot confirm this for every genus, a crenulate basal elytral margin is at least present in Cephalobyrrhus, Anomocephalobus and Erichia cretacea (Pütz 1998, Li et al. 2022: figs 9, 21). By contrast, the basal elytral margin is (almost) smooth in Serrathroscinus. The abdominal ventrite 5 is notched at apex in Parathroscinus, Ercus and Erichia cretacea (Wooldridge 1984, García and Jiménez-Ramos 2019, Li et al. 2022: fig. 20), whereas it is simply rounded in Serrathroscinus.

ACKNOWLEDGEMENTS

Carles Hernando and Hiroyuki Yoshitomi provided valuable insights into the morphology of the new fossil. Su-Ping Wu assisted with micro-CT reconstruction, Yan Fang with confocal microscopy, and Jing-Jing Tang with widefield microscopy. We also thank two anonymous reviewers for their constructive comments on the manuscript.

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ADDITIONAL NOTES

• ZooBank register
  https://zoobank.org/FA7AE5B0-E654-4E6B-9CB8-757ADC605183
• Data Availability
  The original micro-CT and confocal data are available in the Zenodo repository. https://doi.org/10.5281/zenodo.15725047
• Funding
  Financial support was provided by the National Key Research and Development Program of China (2024YFF0807601). Y-DL is supported by a scholarship granted by the China Scholarship Council (202108320010) and the Bob Savage Memorial Fund of the University of Bristol.
• How to cite this article
  Li Y-D, Huang D-Y, Cai C-Y (2025) Serrathroscinus, a new genus of Cephalobyrrhinae from mid-Cretaceous Kachin amber (Coleoptera: Limnichidae) Zoologia 42: e25029. https://doi.org/10.1590/S1984-4689.v42.e25029
• Published by
  Sociedade Brasileira de Zoologia at Scientific Electronic Library Online - https://www.scielo.br/zool

Edited by

• Editorial responsibility
  Marcel Gustavo Hermes

Data availability

The original micro-CT and confocal data are available in the Zenodo repository. https://doi.org/10.5281/zenodo.15725047

Publication Dates

• Publication in this collection
  28 Nov 2025
• Date of issue
  2025

History

• Received
  24 Apr 2025
• Accepted
  03 July 2025