Mirolydidae, a new family of Jurassic pamphilioid sawfly (Hymenoptera) highlighting mosaic evolution of lower Hymenoptera

We describe Pamphilioidea: Mirolydidae Wang, Rasnitsyn et Ren, fam. n., containing Mirolyda hirta Wang, Rasnitsyn et Ren, gen. et sp. n., from the late Middle Jurassic Jiulongshan Formation of Daohugou, Inner Mongolia, China. The new taxon is characterized by unique forewing venation with the presence of forewing SC, 1-RS almost as long as 1-M, M + Cu straight, 2r-rs strongly reclival, and antenna with homonomous flagellum, revealing new and important details in antennal evolutionary transformations. Thus, M. hirta with a combination of primitive and more derived characters highlights its transitional state in the Pamphilioidea and complex mosaic evolution within Pamphilioidea in the late Middle Jurassic. The body of this species is densely covered with thin and long setae, suggesting its possible habit of visiting gymnosperm reproductive organs for pollen feeding and/or pollination during the late Middle Jurassic, much earlier than the appearance of angiosperm flowers.

Furthermore, a critical comparison between fossil and extant pamphilioids is lacking, mainly because known fossils are poorly preserved., Herein we describe Mirolydidae Wang, Rasnitsyn et Ren, fam. n., including one genus and its type species, with distinct characters including antenna, wing venation and legs. The new mirolydid species is based on two specimens collected from the latest Middle Jurassic Jiulongshan Formation at Daohugou Village, Ningcheng County, Inner Mongolia, China. This unique fossil taxon can not be classified into any existing families because it has a striking combination of the unique plesiomorphies and a number of hardly questionable apomorphies in the forewing venation, and special antenna. In our view, this makes it necessary to erect a new family. The discovery of this unique and unusual pamphilioid in the mid Mesozoic is of considerable significance to enhance our understanding of the morphological evolution within the Pamphilioidea or even within the basal Hymenoptera; furthermore, the new data also necessitate a re-evaluation of the evolution of antennal morphology in Pamphilioidea. Etymology. The generic name is a combination of the Latin "Mira-" meaning strange and unique, and Lyda, a junior synonym of Pamphilius Latreille, 1802, often used as a suffix for generic names in Pamphilioidea. Gender feminine.

Mirolyda hirta
Etymology. The specific name is derived from the Latin word "hirtus", meaning hairy, referring to the thin and long setae covering the entire body.

Diagnosis. As for genus.
Description. Holotype CNU-HYM-NN2012103 (p/c): Male: Part of head, antenna, thorax, abdomen and leg entirely or predominantly dark, pterostigma infuscated; otherwise color pale; whole body densely covered with thin and long setae (those on head sides being up to three times as long as the length of basal flagellomeres); wings with sparse and short setae (Fig. 1c,f- Head. Head (Fig. 1f) nearly rounded, about 1.2 times as wide as long; fore margin of clypeus curved and protruding medially; mouth separated from foramen magnum by a bridge; seven or eight basal flagellomeres connected tightly (in contrast to following ones), representing the partially dissociated composite 1st flagellar segment (Fig. 1c,d).
Thorax. Prothoracic structure poorly preserved; except that pronotum seems to be very short; mesonotum not distinct; mesopostnotum with round fovea centrally; cenchri long, reaching beyond metanotum midlength; metascutellum rounded triangular, comparatively small; further backward dorsal structures obscure. Mesopseudosternum triangular, well distant from the fore margin of mesoventropleuron.
Wings. Forewing (Fig. 1e) with pterostigma lanceolate, weak and very narrow; the posterior branch of SC subvertical, slightly shorter than 1-RS; R almost straight before RS base and then distinctly bent at RS base; vein 1-RS proclival and relatively long, nearly equal to 1-M in length; M + Cu straight; RS between 1r-rs and 2r-rs arched strongly towards wing posterior margin; 2r-rs inclined basally; 2r-m separated from 2r-rs by 0.43 times of its own length, located distal to middle of cell 2mcu; 3r-m inclined towards wing apex, separated from apex of cell 3r by 0.83 times of its length, and 1.54 times as long as 2r-m; 1m-cu 0.34 times as long as 3-Cu; 2m-cu at middle of cell 3rm. Cell 2r rather small and widened strongly, almost 0.76 times and 0.5 times as long as cells 1r and 3r, respectively; cell 1mcu 1.89 times as long as wide, and as long as cell 2rm; cell 2rm almost 0.85 times as long as 3rm, and 0.66 times as long as and 0.64 times as wide as, cell 2mcu. Hind wing (Fig. 1d, in red) with SC absent; cell r tapering apically; crossvein 1r-m aligned with 1-M; 1-RS rather long, about 0.59 times as long as 1-M & 1r-m 3r-m separated from apex of cell r by almost its own length; crossvein m-cu slightly longer than 3r-m, joining 2-M distal to midlength of cell rm, separated from 3r-m by a distance equivalent to nearly its length; crossvein cu-a proximal to midlength of cell mcu.
Paratype CNU-HYM-NN2012171 (p/c). Sex unknown. Integument generally brown to dark brown, with scattered setae on whole body, especially, thin and long setae on head, mesothorax and the first abdominal segment (Fig. 2a,b,e and g).
Head (Fig. 2a,e) almost 1.3 times as wide as long; antennal toruli separated as wide as the torular diameter, separated from compound eyes by almost 1.5 times torular diameter; scape 1.85 times as long as maximum wide; pedicel rectangular, 0.86 times as long as maximum wide; antenna long ( Fig. 2f), with at least 45 articles preserved; the composite first flagellomere thick and straight, almost 5.1 times as long as wide, with fusion traces of seven inflexibly connected basal flagellomeres as indicated by faint lines when observed under alcohol (Fig. 2c); remaining flagellomeres subquadratic, flexibly connected, tapering distally to form flagellar thread; ocelli small, positioned at posterior part of the head; separated from each other by about their own diameter, separated from the posterior margin of antennal toruli by more than diameter.
Pronotum short, 0.06 times as long as wide; mesoscutum with medial line and notauli strongly impressed; mesoprescutum small, nearly 0.25 times as long as mesonotum; mesoscutellum separated from the mesoprescutum by almost its own length; mesopseudosternum triangular, almost 0.5 times as long as wide, and well distant from the fore margin of mesoventropleuron; metanotum with cenchri small, almost 0.8 times as long as wide, and almost half length of the metanotum; metascutellum rounded quadrate and as long as cenchri. Legs (Fig. 2b) with femora fusiform, hind femur about 5.3 times as long as wide; tibiae comparatively narrow and long, with two preapical spurs preserved, one located almost at the midlength of tibia but the other one distad (Fig. 2b,d); hind tibiae about 0.6 times as wide as hind femora.
Abdomen with five segments preserved; the first one divided medially, covered with long and slender pubescence (Fig. 2g), scattered from the base of first tergum to its periphery.
Forewing ( Fig. 2f) with SC two branched, anterior branch of SC meeting C almost at origin of 1-RS, posterior branch vertical, 0.42 times as long as 1-RS; SC + R equal to 1-RS in length; 1-RS with anterior end slightly proximal to its posterior end, and almost as long as 1-M; M + Cu straight; 1cu-a placed proximad middle of cell 1mcu. Cell 1r long, equal to cell 1mcu in length; the latter about 1.85 times as long as wide. Venation of hind wing same as that in holotype. Phylogenetic analysis. Analysis of the morphological data matrix using NONA yielded only one most parsimonious tree, presented in Fig. 3, with the following characteristics: tree length 26, consistency index (CI) 76 and retention index (RI) 76. The major conclusions of our phylogenetic analysis are as follows: Tenthredinoidea, firstly separated from Xyelioidea, is the sister group to the big branch formed by taxa of superfamily Pamphilioidea. Our target taxon Mirolyda gen. n. is a sister group to the clade (Pamphiliidae + (Xyelydidae + Megalodontesidae)), supported by the following combination of characters: (i) separated oral and mandibular foramina (character 1, state 1) supporting position of the new genus within the superfamily Pamphilioidea; (ii) forewing with 1-RS not shorter than 1-M (character 8, state 0), supporting its basal position within Pamphilioidea, and (iii) three unambiguous characters indicative of its sister and not ancestral position in respect to the remaining Pamphilioidea: homonomous antenna (character 3, state 0), forewing with 1-RS not shorter than 1-M (character 8, state 0), and forewing with 2r-rs reclival (character 11, state 1), forewing with R distinctly with an angle at RS base (character 7, state 1). Two families of Pamphilioidea, Pamphiliidae and Megalodontesidae, are both recovered as monophyletic with moderate support and with multiple unambiguous morphological characters.  18 . In contrast, Rasnitsyn presented evidence of the order Palaeomanteida (= Miomoptera) to be a better candidate for the ancestry 2 . Irrespective of their different opinions, wings of Parasialidae and Palaeomanteida (particularly Palaeomanteidae) could be both taken as a rough model of the hymenopteran ancestor, which shows that SC has several fore branches and costal area is very wide, occupying up to 1/4 of wing width. The wide costal area (1/4~1/3 of wing width) is common in other Holometabola, e.g., some permotrichopterans and a few mecopterans. The new species of M. hirta has a wide costal area almost 1/3 of its wing width, which probably represents the ancestral character state for the Hymenoptera. In summary, unique and exceptional wing structures indicate that M. hirta has a combination of primitive and more derived characters highlighting its transitional state in the Pamphilioidea. Such mosaic evolution within Pamphilioidea in the latest Middle Jurassic indicates that the evolutionary process of sawflies is far more complex than we previously thought 19 .

On the modes of homonomous antenna in Pamphilioidea. Given that the antennal diversity of lower
Hymenoptera, ranging from the xyelid-like with the composite, thick and long first flagellomere (Fig. 4a) occurring in Xyelidae, Xyelotomidae, and Xyelydidae, etc., to the homonomous antenna in many siricids, tenthredinids, etc, it is widely accepted that the xyelid-like antenna is the plesiomorphic character in Hymenoptera (Table 1). On account of previously accumulated observations of antenna 20,21 , Wang et al. hypothesized several modes of antennal transformations during the evolution of the order 22 . One of the modes is exemplified by Archoxyelyda mirabilis Wang, Rasnitsyn & Ren (Megalodontesidae) that exhibits an extraordinary antenna form with the first nine flagellomeres thick, and tightly connected to each other, in contrast to the following 10-12 segments which are thinner and flexibly connected (Fig. 4d). The first set of thicker and consolidated flagellomeres are interpreted as a possible result of incomplete separation of primary components of the composite first flagellomere in Xyelidae (Fig. 4a).
The  thin, like the one shown in Fig. 4e. Irrespective of how it evolved, we should hypothesize independent acquisition of similar antennal structure by these two genera, because Mirolyda with its long 1-RS represents a putatively sister group to all remaining Pamphilioidea, whilst Archoxyelyda is rooted deeply within the superfamily 4 , and to hypothesize homology of the antennal structure of the two genera in antennal morphology would result in inferring massive homoplasy in remaining Pamphilioidea.
Traces of the former composite nature of the first flagellomere are known to occur in many living Pamphiliidae 23 . In particular, Caenolyda Konow and Acantholyda Costa have proceeded the same way of segmentation of the composite first flagellar segment into its component primary segments independently and so form a recent and partial parallel to Mirolyda and Archoxyelyda. The case of Caenolyda and Acantholyda is qualified as recent because of their terminal position in the cladogram and missing fossil record 4 . It is unlikely to be homologous with the process in Mirolyda and Archoxyelyda because the antennae of the latter genera suggest simultaneous segmentation of the composite segment into all primary ones at once, whilst Caenolyda and Acantholyda demonstrate a rather gradual separation of primary segment one by one starting from the distalmost one.
As a result, it is possible to conclude that our new data highlight existence of at least four modes of restoring homonomous antenna in Pamphilioidea: (i) direct reduction of both length and width of first flagellar segment (Pamphilius Latreille, Neurotoma Konow, Megalodontes); (ii) segmentation of the first segment at once into all primary ones when being wide (Archoxyelyda) (Fig. 4d); (iii) same as ii, except that segmentation starts after first segment became as thin as the following segments (Mirolyda) (Fig. 4c); (iv) same as ii, but segmentation proceeds by gradual detaching of apical primary segments one by one instead of their simultaneous segmentation (Fig. 4e).
On the hairy body of M. hirta gen. et sp. n. One of the most interesting characters in M. hirta is its pubescent (hairy) body with long and thin setae, which are present but much less in the Xyelidae 24 , e.g. Chaetoxyela Rasnitsyn. However, most extant bees often have specialized branched or feathery body setae (hairs) that help them in the collection of pollens 25 . Diverse pubescences often serve different functions, like in bumblebees, thick hairs for insulation, branched hairs for collecting pollen and colored hairs for signals (either warning, or attracting, or both). Moreover, the shape and structure of hairs also differ, like in bees, there are a few compound hairs such as spirally twisted hairs, spatulate hairs, etc 25 . Besides hymenopterans, the Bombyliidae, the so called "bee flies", one of the largest families in Diptera, are also covered by hairs on their body, and have long, slender legs 26 . The adults of bee flies feed on nectar and pollen, and are believed to be important pollinators of many plants 27 . The new species of M. hirta is covered by irregularly scattered setae on the wing membrane as well (Figs 1I and 2e), including a few on the veins, extremely long hairs near the wing base, and on the head, mesothorax and the first abdominal tergum (Figs. 1f and 2g). Besides, the surface structures (setae, small bristles, etc.) on wings have been considered as having close relationships with flight ability in Hymenoptera and also in other related orders [28][29][30] . Based on aforementioned, it is possible that these hairs of M. hirta might have served important and valuable functions such as collecting pollens, providing insulation, sensing the input signals, etc. However, evidence and direct proof are pending.

Materials and Methods
Examined taxa and terminologyh. The two type specimens, both with part and counterpart, were collected from a finely laminated tuff in the latest Middle Jurassic Jiulongshan Formation at Daohugou Village, Ningcheng County, Inner Mongolia, China. The Daohugou locality is now considered to be one of the most important insect Lagerstätten 31,32 . Because of new calibrations for the Jurassic System, this deposit should be now considered as latest Middle Jurassic (late Callovian) in age 9 , approximately 165 Mya -164 Mya.
The specimens studied in this paper were examined and then photographed, either dry or wetted with 95% ethanol, with a Leica DFC450 digital camera attached to a Leica M250 C dissecting microscope (Leica, Wetzlar, Germany). The line drawings were prepared using Adobe Illustrator CS2 and Adobe Photoshop CS5 software. The wing venation nomenclature used in this article is modified after Rasnitsyn

Phylogenetic analysis.
A phylogenetic analysis was conducted to elucidate the position of our new family within Pamphilioidea and to clarify familial relationships. In this study, we used the Xyelioidea and Tenthredinoidea as outgroups and nine taxa of superfamily Pamphilioidea as ingroups to carry out the phylogenetic analysis. Eighteen characters were identified and scored for all taxa. The character selection was partly based on the characters used by Ronquist et al. 4 , Vilhelmsen et al. 6 , and Wang et al. 10 in their phylogenetic analysis of the Pamphilioidea and Symphyta. A complete list of the taxa (Additional file 1: ) and the character state matrix (Additional file 2: Table S2) used in the phylogenetic analysis are provided. The phylogenetic analysis was carried out in NONA 33 in conjunction with WinClada 34 . Tree searches were performed using an heuristic search method (options: set to hold 10 000 trees, 1000 replications, 100 starting tree replication, multiple TBR + TBR search strategy). Character codings were set up by using Nexus Data Editor 0.5.0 35 with all characters unordered and of equal weight.