A new sawfly of Megalodontesidae, Jibaissodes peichenae sp. nov., is described from the Lower Cretaceous Yixian Formation of Northeastern China. It is established mainly based on the pectinate antenna comprising 42 flagellomeres and the proximal 28 bearing apical rami, which gradually shorten in length toward the apex of the flagellum. The pterostigma of the forewing is infuscated apically and on the hind wing, vein 1-Rs is nearly equal to 1r-m and slightly shorter than 1-M. The first tergum is widely excised posteriorly and roundly protruding laterally alike in Megalodontes. This find supports that pectinate antennae in extant sawflies of Megalodontesidae originated at least during or before the Early Cretaceous.

ramified antennae, Symphyta, taxonomy, Yixian Formation

Megalodontesidae is a small extant family with only one extant genus comprising 42 species and 12 fossil genera with 23 species (Taeger et al. 2018). Megalodontesidae comprises four subfamilies: three extinct subfamilies, Archoxyelydinae Wang, Rasnitsyn & Ren, 2013, Decorisiricinae Wang, Rasnitsyn, Shih, Sharkey & Ren, 2015, and Praesiricinae Rasnitsyn, 1968 and one extant subfamily, Megalodontesinae Konow, 1897. The sole extant genus, Megalodontes Latreille, 1803, which is distributed in temperate regions of the Palaearctic (Benson 1968; Goulet 1993; Blank et al. 2001; Taeger et al. 2010), and an extinct genus, Jibaissodes Ren, Lu, Guo & Ji, 1995, are characterized by having saw-like or comb-like ramified antennae.

Among extant symphytan insects, ramified antennae are present in various taxa, i.e., in Diprionidae, Pergidae, Tenthredinidae, and Megalodontesidae (Gao et al. 2016). Fossil records of ‘Symphyta’ with ramified antennae are very rare. Up to date, only two species with ramified antennae have been reported: Jibaissodes bellus Gao, Shih Labandeira, Santiago-Blay, Yao & Ren, 2016 with simply ramified antennae from the Lower Cretaceous Yixian Formation, and Atefia rasnitsyni Krogmann, Engel, Bechly & Nel, 2012 with the biflabellate antennae from the Lower Cretaceous Crato Formation, assigned to the superfamily Tenthredinoidea s. str. Latreille, 1802. The occurrence of biflabellate antennae in A. rasnitsyni has been suggested to indicate the antiquity of insect usage of long-range female attractants (Krogmann et al. 2013).

In this paper, we describe Jibaissodes peichenae sp. nov. based on a well-preserved specimen from the mid-Lower Cretaceous Yixian Formation of northeastern China. The new species with distinctly pectinate antennae contributes additional important morphological characters of the family Megalodontesidae.

For the paper we examined the holotypes of Jibaissodes peichenae sp. nov. (specimen no. CNU-HYM-LB2018033, part and counterpart) and Jibaissodes bellus Gao, Shih, Labandeira, Santiago-Blay, Yao & Ren, 2016 (specimen no. CNU-HYM-LB2011009, part and counterpart), which are housed in the Key Laboratory of Insect Evolution and Environmental Changes at the Capital Normal University, Beijing, China (CNUB; Dong Ren, curator). Both were collected from the mid-Lower Cretaceous Yixian Formation of Liaoning Province of China. The holotype of Jibaissodes giganteus Ren, Lu, Guo & Ji, 1995 (specimen no. BL92105, part, housed in the Geological Museum of China, Beijing, China, Jun Yao, curator) was also examined and redrawn for this paper. It was collected from the Lower Cretaceous Lushangfen Formation of Beijing of China.

The specimens were examined and photographed, either dry or wetted with 95% ethanol, by using a Nikon SMZ25 with an attached camera system. Line drawings were prepared using Adobe Illustrator CC and Adobe Photoshop CC graphics software. The wing venation nomenclature used in the paper was modified after Rasnitsyn (1969, 1980).

Jibaissodes peichenae sp. nov. is assigned to Megalodontesidae by two features typical for Megalodontesidae: undivided first tergum and absent Sc (Benson 1968; Goulet 1993; Wang et al. 2016). In addition, J. peichenae can be attributed to Megalodontesinae, by the pectinate antennae as a derived character state. Other fossil Megalodontesidae, i.e., Decorisiricinae, Archexyelinae, and Praesiricinae, have a synantennomere 3 (Blank et al. 2013) like the ancestral Xyelidae and a number of fossil taxa, which represents an ancestral character of Hymenoptera (e.g., Rasnitsyn 1996). The semicircular distal excision and the lateral round protuberances of the first abdominal tergum is a putative apomorphy of Jibaissodes + Megalodontes, since protrusions are obviously absent in Decorisiricinae, Archexyelinae, and Praesiricinae. On the forewing, M+Cu is straight, while it is curved in Archexyelinae and Praesiricinae.

All fossil megalodontesids share as a plesiomorphy of a curved vein 2A on the forewing (Wang et al. 2015, 2016). The almost straight 2A of the extant Megalodontes is an apomorphy of this taxon (Taeger 2002; Taeger et al. 2014). Therefore, we treat Jibaissodes and Megalodontes as separate groups. Although J. peichenae sp. nov. shares two features with Megalodontes (pectinate antennae and laterally protruding, medio-anteriorly dark first tergum; Taeger 2002), it is impossible to evaluate the coloration of head, pronotum, and mesonotum of J. peichenae as preserved on fossil with certainty. We need more fossil specimens to address whether Jibaissodes has close affinities with Megalodontes.

Since weak sexual dimorphism has been observed within several groups of Megalodontes having antennae with relatively long rami in males and slightly short rami in females (Taeger 2002), and thereby, we cannot rule out the possibility that these two species, J. peichenae with long ramified antennae and J. bellus with short ramified antennae, might be different sexes of the same species. However, J. peichenae differs from J. bellus also in the following characters: pterostigma of forewing infuscated apically and, particularly, the excised posterior edge of first tergum, which is more similar to Megalodontes (Taeger 2002; Taeger et al. 2014). Given that the holotype of J. peichenae lacks the tip of the abdomen, it is impossible to properly ascertain whether the two should be considered as the same species. Therefore, we treat them as separate for now, pending future finds of additional fossil specimens to confirm the relationship between these two species.

We are very grateful to Dr Xiaodan Lin and Mr Xiangbo Guo (College of Life Sciences, Capital Normal University) for their help in this research. We thank the Editorial Board of ZooKeys, and in particular, Dr Stephan M. Blank. We express our gratitude to Dr Lars Vilhelmsen, Dr Andreas Taeger, and Dr Meicai Wei for their critical but valuable reviews of the manuscript. We also thank the curator of the type specimen of Jibaissodes giganteus, Dr Jun Yao. This project was supported by grants to D.R. from the National Natural Science Foundation of China (grant nos. 31730087 and 41688103). T.P.G. was supported by the Program for Changjiang Scholars and Innovative Research Team in University (IRT-17R75), Project of High-level Teachers in Beijing Municipal Universities (grant no. IDHT20180518) and the Support Project of High-level Teachers in Beijing Municipal Universities in the Period of 13^th Five-year Plan (CIT&TCD201704090). M.W. was supported by Fundamental Research Funds of CAF (No. CAFYBB2019QB004) and the National Natural Science Foundation of China (grant no. 31702038). The authors declare that there are no competing interests in the manuscript.