A new sawfly of Megalodontesidae (Insecta, Hymenoptera, Pamphilioidea) with pectinate antennae from the Early Cretaceous of China

Abstract A new sawfly of Megalodontesidae, Jibaissodes peichenaesp. 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.

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.

Material and methods
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 (1969Rasnitsyn ( , 1980.
Diagnosis. Antenna pectinate, with 42 flagellomeres, flagellum longer than head width, proximal 28 flagellomeres with apical rami, rami gradually shortening in length toward apex of flagellum; scape almost 3 times as long as first flagellomere. Anterior margin of pronotum round, with weakly concave posterodorsal margin. Forewing with pterostigma infuscated apically; vein 1cu-a strongly curved. Hind wing vein 1-Rs nearly equal to 1r-m, slightly shorter than 1-M.  (Figs 1A, B, 3A). Fossil incomplete: distal abdominal segments missing, part of the legs invisible. Middle and surrounding regions of compound eyes and part of posterior head pale, remainder of head dark. Thorax and legs entirely or predominantly dark. First tergum except hind margin and fifth tergum laterally dark. Body about 14.9 mm (but distal abdominal segments missing), antenna 11.4 mm long; forewing about 13.6 mm in length, maximum width 6.0 mm; hind wing about 8.5 mm in length, maximum width 3.8 mm.

Description. Body
Head. Large, about as wide as thorax. Head ( Fig. 2A) 5.3 mm wide and 4.6 mm long, almost circular. Compound eyes 1.8 mm long, 1.1 mm wide; right mandible large, bent and sickle-shaped; antenna (Fig. 2D, E) with 42 flagellomeres, basal 28 flagellomeres with rami extending from apicolateral angle, apical 14 flagellomeres without distinct rami, longest ramus about 0.8 mm in length, longest ramus slightly longer than total length of following three flagellomeres; rami gradually shortening in length toward apex of flagellum, first flagellomere much shorter than scape, 0.3 mm wide, 0.5 mm long; scape 1.4 mm long, maximum 0.4 mm in width; pedicel 0.8 mm long, maximum 0.5 mm wide. Thorax. Maximum width 4.9 mm; pronotum short, apex round, with weakly concave posterodorsal margin. Mesoscutum large, with medial line and notauli strongly impressed, tapering to acutely rounded base; mesoscutum without line to mesoscutellum; mesoscutellum tapering to acutely sharp apex; metascutum with cenchri present and small. Metatibia (Fig. 3B, C) with two preapical (near distal 0.7 of length) and two apical spurs; metabasitarsus long but shorter than remaining tarsomeres combined; metafemur, metatibia, and metabasitarsus lengths 3.4 mm, 4.6 mm, and 1.1 mm, respectively.
Gender. Unknown. Remarks. The species can be assigned to Jibaissodes according to the following diagnostic characters: forewing vein 1r-rs present, shorter than 2r-rs, cell 2rm longer than 3rm and cell 1mcu small. Jibaissodes was described from a poorly preserved fossil specimen with a left and a right forewing, a right hind wing, and parts of head and thorax (Ren et al. 1995). Jibaissodes peichenae sp. nov. is distinctly differentiated from the type species of J. giganteus by the following characters: forewing with pterostigma not completely infuscated; forewing vein 3r-m distinctly longer than 2r-m (about 1.7 times as long as 2r-m); 1cu-a distinctly curved; and hind wing vein 1-Rs nearly equal to 1r-m. Furthermore, J. peichenae is distinguished from J. bellus by the following characters: antenna pectinate, rami short (vs rami long in J. bellus); forewing with pterostigma infuscated apically (vs infuscated medially and apically in J. bellus).
Distribution. Huangbanjigou, near Chaomidian Village, in Shangyuan County, adjacent to Beipiao City, in Liaoning Province of China. Collected from the mid-Lower Cretaceous Yixian Formation, dated as latest Barremian to earliest Aptian, 125 Ma (Ren et al. 1995;Wang et al. 2015).
Etymology. The species epithet is dedicated to Miss Peichen Yao, the daughter of Dr Jun Yao, the specimen donator.

Discussion
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(Wang et al. , 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 an-tennae, 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.