Ovipositor and mouthparts in a fossil insect support a novel ecological role for early orthopterans in 300 million years old forests

A high portion of the earliest known insect fauna is composed of the so-called ‘lobeattid insects’, whose systematic affinities and role as foliage feeders remain debated. We investigated hundreds of samples of a new lobeattid species from the Xiaheyan locality using a combination of photographic techniques, including reflectance transforming imaging, geometric morphometrics, and biomechanics to document its morphology, and infer its phylogenetic position and ecological role. Ctenoptilus frequens sp. nov. possessed a sword-shaped ovipositor with valves interlocked by two ball-and-socket mechanisms, lacked jumping hind-legs, and certain wing venation features. This combination of characters unambiguously supports lobeattids as stem relatives of all living Orthoptera (crickets, grasshoppers, katydids). Given the herein presented and other remains, it follows that this group experienced an early diversification and, additionally, occurred in high individual numbers. The ovipositor shape indicates that ground was the preferred substrate for eggs. Visible mouthparts made it possible to assess the efficiency of the mandibular food uptake system in comparison to a wide array of extant species. The new species was likely omnivorous which explains the paucity of external damage on contemporaneous plant foliage.


Introduction
The earliest known insect fauna in the Pennsylvanian, ca. 307 million years ago, was composed by species displaying mixtures of inherited (plesiomorphic) and derived (apomorphic) conditions, such as the griffenflies (stem relatives of dragon-and damselflies), but also by highly specialized groups, such as the gracile and sap-feeding megasecopterans, belonging to the extinct taxon Rostropalaeoptera. A prominent portion of this fauna were the so-called 'lobeattid insects'. They have been recovered from all major Pennsylvanian outcrops, where some species can abound Béthoux and Nel, 2005a). Indeed, at the Xiaheyan locality, China, for which quantitative data are available, they collectively account for more than half of all insect occurrences (Trümper et al., 2020). Additionally, another extinct group, the Cnemidolestodea, composed of derived relatives of lobeattid insects, was likewise ubiquitously distributed during the Pennsylvanian until the onset of the Permian (Béthoux, 2005b).
The phylogenetic affinities of lobeattid insects are debated. They have been regarded as stem relatives of either Orthoptera (crickets, grasshoppers, katydids;Béthoux and Nel, 2002;Béthoux and Nel, 2005a) or of several other lineages within the diverse Polyneoptera (Aristov, 2014;Rasnitsyn, 2007). A core point of the debate is the presumed wing venation ground pattern of insects, which, however, will remain elusive until Mississipian or even earlier fossil wings are discovered. Ecological preferences of lobeattid insects are also poorly known. Traditionally, they have been regarded as foliage feeders (Labandeira, 1998) but, given their abundance, this is in contrast to the paucity of documented external foliage damage during that time.
The Xiaheyan locality is unique in several respects (Trümper et al., 2020), including the amount of insect material it contains. Over the past decade, a collection of several thousand specimens was unearthed, allowing for highly detailed analyses of, for example, ovipositor and mouthparts morphology of extinct insect lineages (Pecharová et al., 2015b). These character systems are investigated herein in a new lobeattid species, based on hundreds of remains, using reflectance transforming imaging (RTI) together with more traditional approaches. Dietary preferences were inferred using a comparative morphometric and biomechanical analysis of gnathal edge shape based on an extensive dataset of extant polyneopteran species, with a focus on Orthoptera. Together, this investigation provides information regarding the phylogenetic affinities of loebattid insects and on their preferred mode of egg laying and dietary niche.

Mandibular mechanical advantage
The head and mouthpart morphology could be investigated in more detail in six specimens (see Appendix 1) while we could study the mechanical advantage (MA; see Section 1.5 of Appendix 1) of their mandibles in four of the six (viz. . The MA is defined as the inlever to outlever ratio and thus indicates the percentage of force transmitted to the food item (i.e. the effectivity of the lever system). Therefore, the MA allows for a size-independent comparison of the relative efficiencies of force transmission to the food item. Low MA values usually indicate quick biting with low force transmission typical for predators, while high MA values indicate comparatively slow biting with higher force transmission typical for non-predatory species.
Calculation of the MA along the entire gnathal edge revealed characteristic MA curve progressions for the studied taxa (Appendix 1, Section 2.3, and Appendix 1- figure 9). Compared to the studied fossils, extant Dermaptera, Embioptera, and Phasmatodea showed comparatively high MAs with an almost linear curve progression towards more distal parts of the mandibular incisivi whereas Plecoptera, Zoraptera, and Grylloblattodea were located at the lower end of the MA range with a gently exponential decrease towards the distal incisivi. The analysed extant Orthoptera occupy a comparatively wide functional space, with lineages at the higher and lower ends of the MA range. The composite fossil mandible representation (CFMR) of Ct. frequens (see Materials and methods) is located in the centre of the observed range of MAs for Orthoptera ( Figure 4).
A polynomial function of the fifth order resulted in the best relative fit on the MA curves according to the Akaike information criterion (AIC) value (-661.3, see Materials and methods). The five common coefficients were subjected to a principal component analysis (PCA, Figure 4E), and, because phylogenetic signal was detected (K = 1.03316; p = 0.0001), also analysed using a phylogenetic principal component analysis (pPCA) (Appendix 1, Section 2.3, and Appendix 1- figure 10). The first four principal components (PCs) accounted for 96.8 % (PCA)/96 % (pPCA) of the variation in MA (Appendix 1-table 2).
In both PCAs, PC1 mainly codes for the vertical position of the MA curve, that is, the effectivity of the force transmission along the whole toothrow, while PC2 mainly codes for the curvature, that is, whether there is an almost linear or a gently exponential decrease in the effectivity of force transmission. Due to the narrow distribution of species along PC3, it was not possible to associate a clear biomechanical pattern to this PC.
The CFMR of Ct. frequens is located at the centre of the first three PCs ( Figure 4E). Omnivorous Orthoptera and all herbivore taxa, with the exception of Apotrechus, are located along the width of PC1, while there is a tendency for the carnivorous taxa within the sampling to be spread along PC2.

Phylogenetic implications
Our analysis of material of Ct. frequens provides unequivocal evidence that olis2 occurs in this species. Therefore, the new species was an orthopteran. The ovipositor configuration in Ct. elongatus furthermore conforms that observed in extant cave crickets (Raphidophoridae) in which olis2 occurs in addition to olis1 and interlocks gs9 and gp9 (Figure 3A-C; Appendix 1, Section 2.2). Indeed, this structure is present in ensiferan ('sword-bearing') Orthoptera possessing a developed ovipositor and is absent in caeliferan ('chisel-bearing') Orthoptera (Cappe de Baillon, 1920;Cappe de Baillon, 1922;Kluge, 2016;and see below). It follows that the new species is either more closely related to Ensifera than to Caelifera (owing to the possession of olis2), or it is a stem-orthopteran and olis2 was secondarily lost in Caelifera.
Further evidence for the phylogenetic placement of Ct. frequens is based on the lack of jumprelated specializations in the hind-leg. Such specializations define the taxon Saltatoria within Orthoptera, and therefore Ct. frequens can be confidently excluded from crown-Orthoptera. This conclusion is furthermore corroborated by wing vein characteristics: Ct. frequens lacked a forked CuPa vein before its fusion with the CuA vein. Such a forked CuPa vein is typical for Panorthoptera, which includes crown-Orthoptera and their nearest stem relatives (Béthoux and Nel, 2002). Given this evidence, based on the configuration of several body parts, Ct. frequens, and its various Pennsylvanian relatives collectively referred to as 'lobeattid insects' are stem relatives of Orthoptera ( Figure 3C). The absence of olis2 in Caelifera therefore is the consequence of a secondary loss.  figure 7I); (C-D) Specimen CNU-NX1-764, (C) color-coded interpretative drawing, and (D) photograph (composite). (E) Principal component analysis of the mandibular mechanical advantage. Color-coding: (A-D) red, lacina (la); salmon, cardinal and stipital sclerites (ca and st, respectively); dark blue-purple, mandible (md); yellow, tentorium, including anterior tentorial arm (ata), posterior tentorial arm (pta), and corpotentorium (ct). Other indications: co, coronal cleavage line; fc, frontal cleavage line.
Even though it is unclear how far posteriorly olis2 extends in Ct. frequens, the asserted phylogenetic placement of this species provides new insights on the evolution of ovipositor interlocking mechanisms in Orthoptera ( Figure 3). The one in Ct. frequens is best comparable to the one of Rhaphidophoridae, the main difference concerning the rachis ('ball' as in 'ball-and-socket'), which is limited to a short protrusion in these insects, while the aulax ('socket' as in 'ball-and-socket') extends further posteriorly. In addition, gs9 extends more ventrally, concealing gp8 for some distance. Compared to Gryllacrididae the only notable difference in Ct. frequens is the ventral extension of gs9 in the former. In Anostostomatidae, the ventral margin of gs9 enters a socket in gp8, regarded as composing the premises of a third olistheter (olis3). The most parsimonious hypothesis is that this new structure ultimately replaces olis2 in Tettigoniidae and thereby allows a coupling of gs9 with gp8.
Grylloidea (true crickets) and Ct. frequens are separated by more severe morphological differences. A gp9 is not present in all Grylloidea and, if present, it occurs at the ovipositor base and is reduced compared to, for example, Rhaphidophoridae. Gs9 and gp8 are connected by an olistheter and we suggest that it might represent a variant of olis2, assuming a hypothetical case (shaded scheme in Figure 3C) in which olis2 interlocks gs9, gp9, and gp8 altogether. The reduction of gp9 would then mean that only olis2 connects gs9 and gp8. The alternative is a convergent acquisition of an olis3, as in Tettigoniidae.
Unlike other orthopterans displaying a well-developed external ovipositor, Caelifera use valves for digging a tunnel to accommodate their entire abdomen and, additionally, dig egg pods (Fedorov, 1927;Stauffer and Whitman, 1997;Uvarov, 1966). The shoving operation to move forward is accomplished by powerful, rhythmic, dorso-ventral openings and closings of two sets of valves (Thompson, 1986), gs9 and gp8+ gp9, the two latter ones being interlocked via olis1. Even though gp9 is often reduced, it plays an important role in the closing of the ovipositor via muscles attached to it (Thompson, 1986). Obviously, an olistheter interlocking gs9 and gp8 (i.e. olis2) would impede such movements. Given the ovipositor configuration and phylogenetic placement of Ct. frequens, it follows that the olis2 was lost in Caelifera, a likely consequence of their highly derived oviposition technique.
The evolutionary scenario resulting from our findings in Ct. frequens addresses a long-standing debate on the respective position of the two main lineages of Orthoptera, Ensifera and Caelifera. On the basis of early, fossil Saltatoria/Orthoptera displaying elongate ovipositors, palaeontologists already assumed that caeliferans derived from ensiferans (Sharov, 1968). However, the placement of the corresponding fossils remained contentious, leaving it possible that both, Ensifera and Caelifera, derived from an earlier, unspecialized assemblage (Ander, 1939). The discovery of an elongate ovipositor in the stem-orthopteran Ct. frequens provides a definitive demonstration that caeliferans derived from ensiferans. Because rock-crawlers can also be understood as possessing an elongate ovipositor, which would render the term 'Ensifera' ambiguous, it is proposed to coin a new taxon name, Neoclavifera, to encompass species bearing an olis2, that is, all extant orthopterans and their stem relatives as currently known ( Figure 3C; Appendix 1, Section 2.1.1).
Another important input on the early evolution of orthopterans regards the abundance of lobeattids. Indeed, these insects are emerging as the main component of the Pennsylvanian insect fauna. They have been reported in high numbers from all major Pennsylvanian deposits Béthoux and Nel, 2005a; and Appendix 1, Section 2.1), such as Miamia bronsoni at Mazon Creek . At Xiaheyan, they collectively account for more than half of all insect occurrences (Trümper et al., 2020). Besides a high abundance, lobeattids and other stemorthopterans compose a species-rich group at Xiaheyan, where they represent about a third of all insect species currently known to occur at this locality (Appendix 1, Section 3, taxon Archaeorthoptera). Orthoptera, which represent the bulk of extant polyneopteran insect diversity, therefore must have diversified early during their evolution.

Ovipositor shape and use
Extant Orthoptera resort to a wide diversity of substrates where to lay eggs, including ground, decaying leaves or wood, and stems or leaves of living plants (Cappe de Baillon, 1920;Cappe de Baillon, 1922;Ingrisch and Rentz, 2009;Rentz, 1991). This operation aims at ensuring a degree of moisture conditions suitable for eggs to fully develop, and providing protection, for example against predation. Ground is the preferred substrate of the majority of Orthoptera, including Caelifera (Agarwala, 1952;Stauffer and Whitman, 1997;Uvarov, 1966; and see above). Within this group, the epiphytic and endophytic habits of several, inner lineages represent derived conditions (Braker, 1989;Ramme, 1926). This habit translates into finely serrated ovipositor valves, including gs9.
As for 'ensiferan' Orthoptera, they generally possess a pointed and elongate ovipositor used to insert eggs in various substrates. In Grylloidea (including true crickets), females insert eggs in the ground using a needle-like ovipositor, or deposit them in subterranean chambers or burrows adults may inhabit, in which case the ovipositor is usually reduced (Cappe de Baillon, 1922;Loher and Dambach, 1989;Otte and Alexander, 1983). However, within Grylloidea, three groups, the Trigonidiinae (sword-tail crickets), the Aphonoidini, and the Oecanthinae (tree crickets), evolved oviposition in plants. In the former, which lay eggs in soft plant material, gs9 displays serration in its distal third, along its dorsal edge (Kim, 2013;Otte and Perez-Gelabert, 2009). In contrast, both Aphonoidini and Oecanthinae lay eggs in more robust plant material, translating into apices of gs9 provided with strongly sclerotized sets of teeth and hooks (Loher and Dambach, 1989). In Oecanthinae, in which oviposition functioning was studied in most detail, the alternate back and forth movements of gp8 induce apices of gs9 to alternately approximate and diverge (Dambach and Igelmund, 1983), and therefore act as a shoving tool.
The Rhaphidophoridae commonly lay eggs into the ground, or, alternatively, into rotten leaves or wood (Hubbell, 1936). In the latter case, the ovipositor is often curved. Interestingly, Ceuthophilus spp. use the ovipositor tip, somewhat truncated, to rake ground surface above oviposition holes (Hubbell and Norton, 1978), presumably to hide them. Anostostomatidae lay eggs in the ground or on walls of subterranean chambers (Monteith and Field, 2001;Stringer, 2001). These preferences also apply to both Gryllacrididae (Hale and Rentz, 2001;Morton and Rentz, 1983) and Stenopelmatidae (Davis, 1927; not represented in Figure 3C), in which the ovipositor, if well developed, is long, narrow, and rectilinear to curved (Cadena-Castañeda, 2019;Ingrisch, 2018).
Although most Tettigoniidae (katydids) lay eggs in the ground, a variety of plant tissues, including galls, are also targeted by members of this very diverse family (Cappe de Baillon, 1920;Gwynne, 2001;Rentz, 2010). As above, shape and serration relate, to a large extent, to the preferred substrate. A needle-shaped ovipositor generally indicates preference for ground, a sickle-shaped one for plant tissues. Curved ovipositors indicate preference for decaying wood, and more strongly falcate ones, which are usually also laterally flattened (as opposed to sub-cylindrical), preference for either bark crevices or leaf tissues. Katydids laying eggs in hollow grass stems or leaf sheaths possess straight to slightly falcate, flattened, and unarmed ovipositors. Marked serration on the dorsal side of the ovipositor indicates preference for plant tissues.
Given the relation of ovipositor shape and substrate in extant species, Ct. frequens, with its needleshaped ovipositor including ventrally oriented teeth, likely oviposited in the ground ( Figure 5). It is therefore unlikely that Pennsylvanian stem-orthopterans were responsible for endophytic oviposition traces documented for this epoch (Béthoux et al., 2004;Laaß and Hauschke, 2019). More likely candidates for these endophytic egg laying are the extinct Rostropalaeoptera (Béthoux et al., 2004;Pecharová et al., 2015a).

Dietary preferences
Unlike in an extant tropical forest, a limited proportion of Pennsylvanian plant foliage experienced external damage, in particular generalized feeding types such as margin and hole feeding. Although such damages were reported from multiple localities, they are so rare that their occurrence was considered worth being reported (Correia et al., 2020;Iannuzzi and Labandeira, 2008;Laaß and Hauschke, 2019;Scott and Taylor, 1983). Quantitative data from Pennsylvanian localities indicate that generalized external damages were indeed rare, and concentrated on pteridosperms ('seed ferns'; Donovan and Lucas, 2021;Xu et al., 2018). Such damages have been traditionally assigned to Orthoptera and their purported stem relatives (Labandeira, 1998). Indeed, investigation of mouthparts morphology in a subset of these insects suggested that, at least for the representatives belonging to the Panorthoptera/Saltatoria ( Figure 3C), these insects were herbivores (Labandeira, 2019). However, there is an inconsistency between the paucity of damage on Pennsylvanian plant foliage on the one hand, and the abundance of lobeattid insects on the other. If these insects were all external foliage feeders, evidence of such damage would be more prevalent.
Given the reconstruction of the mandibular gnathal edge and its position in PC space in relation to other Orthoptera and Polyneoptera ( Figure 4E; Appendix 1, Section 2.3), Ct. frequens was likely an omnivore species -not a solely herbivorous or carnivorous one. The new species is the second most common insect species at Xiaheyan, where it occurs in all fossiliferous layers at a rate of ca. 10 %. This implies that a significant portion of Pennsylvanian neopteran insects were opportunistic, omnivorous species, which reconciles the paucity of foliage damage with the abundance of stem-Orthoptera.

Fossil material
The studied specimens are housed at the Key Laboratory of Insect Evolution and Environmental Changes, College of Life Sciences, Capital Normal University, Beijing, China (CNU). All specimens were collected from the locality near Xiaheyan village, where insect carcasses deposited in an interdeltaic bay (Trümper et al., 2020).
The adopted morphological terminology is detailed in Appendix 1, Section 1.1. Documentation methodology is detailed in Appendix 1, Section 1.2.1. General habitus was investigated based on a selection of 23 specimens (including the holotype; Appendix 1, Section 2.1.2). Ovipositor morphology was investigated based on four specimens (Appendix 1, Section 1.2.2). Head and mouthparts morphology was investigated based on six specimens (Appendix, 1 Section 1.2.3).
To ensure an exhaustive documentation of ovipositor, head and mouthparts morphology, we also computed RTI files for details of several specimens. RTI files are interactive photographs in the sense that light orientation can be modified at will. The approach, originally developed in the field of archaeology (see Earl et al., 2010 and references therein), has also been applied to a variety of sub-planar fossil items (Béthoux et al., 2016;Hammer et al., 2002;Jäger et al., 2018;Klug et al., 2019;among others).
We computed RTI files based on sets of photographs obtained using a custom-made light dome as described elsewhere (Béthoux et al., 2016), driving a Canon EOS 5D Mark III digital camera coupled to a Canon MP-E 65 mm macro lens. Sets of photographs were optimized for focus using Adobe Photoshop CC 2015.5. RTI computing was then performed using the RTIbuilder software (Cultural Heritage Imaging, San Francisco, CA) using the HSH fitter (a black reflecting hemisphere placed next to the area of interested provided reference). Several snapshots were extracted using the RTIviewer software (Cultural Heritage Imaging, San Francisco, CA), including those in 'normals visualization' mode, which provides a color-coded image according to the direction of the normal at each pixel (i.e. the direction of the vector perpendicular to the tangent at each pixel; see Figure 2C and F). This allows to quantify subtle height differences in fossilized structures.

Comparative analyses
The phylogeny adopted for comparative analyses is based on the most comprehensive account to date (Song et al., 2020), which is largely consistent with previous analyses (Song et al., 2015;Zhou et al., 2017), except for the position of the Rhaphidophoridae, either regarded as sister group of the remaining Tettigoniidea or of a subset of it. The same applies to the Schizodactylidae (splay-footed crickets), which lack a developed ovipositor.
Fossil ovipositor morphology was compared to original material of extant species and to literature data (Appendix 1, Sections 1.3.1, 2.2). Multiple interpretations of the fossil ovipositor morphology were considered. Among these, the favoured interpretation is the only one consistent with observations made on all specimens.
The MA of the mandibles, that is, the inlever to outlever ratio, indicates the effectivity of force transmission from the muscles to the food item (Appendix 1-figure 1). Apart from force transmission, the MA can also indicate the dietary niche and feeding habits (Blanke, 2019;Sakamoto, 2010;Westneat, 2004). The MA was extracted from 43 extant polyneopteran species (Appendix 1figure 9) including 31 orthopterans and one CFMR of the newly described fossil species (Appendix 1, Sections 1.3.2, 1.4, Appendix 1-table 1). The CFMR was derived from a Procrustes superimposition (R package 'geomorph' v.3.0.5; Adams et al., 2013) of four fossil specimens which showed low levels of overall distortion and a mandible orientation suitable for extraction of individual MAs (Appendix 1-figure 9). For comparison of species and inference of the dietary niche, a PCA and, due to the detection of significant phylogenetic signal, a pPCA (R package 'phytools' v.0.6-44; Revell, 2012) were performed (for results of the pPCA, see Appendix 1-figure 9, Appendix 1- Appendix 1 Appendix 1-figure 1. Workflow for the extraction of the mandibular mechanical advantage based on 3D models.  Figure 2D), (B) drawing and (C) photograph (light-mirrored).
Appendix 1- figure 9. Outlines of the mandibular gnathal edges for all studied taxa.
Appendix 1- figure 11. Results of the principal component (PC) analysis of the mandibular mechanical advantage for the first two PCs together with results for the first two PCs after phylogenetic signal correction. Large dots, distribution of species in PC space uncorrected for phylogenetic signal; small dots, distribution of species in PC space corrected for phylogenetic signal. Although phylogenetic signal was significant, differences do not affect the relative position of the sampled species to each other in PC space.

Documentation of fossil material 1.2.1 General aspects
Handmade draft drawings were produced using a LEICA MZ12.5 dissecting microscope equipped with the aid of a drawing tube (Leica, Wetzlar, Germany). Photographs were taken using Canon EOS 550D or 5D Mark III digital cameras (Canon, Tokyo, Japan), coupled to a Canon 50 mm macro lens, a 100 mm macro lens, or a Canon MP-E 65 mm macro lens, all equipped with polarizing filters. Each specimen was photographed under dry condition and covered with a thin film of ethanol. When available, both imprints were photographed. These photographs were optimized using Adobe Photoshop CC 2015.5 (Adobe Systems, San Jose, CA) and assembled, together with handmade drawings, into a single, multi-layered document. Reproduced photographs referred to as 'composites' are a combination of photographs of a dry specimen and the same under ethanol.
In addition to traditional photographs, we computed RTI files for details of several specimens (see main document). The corpus of data was used to produce illustrations using Adobe Illustrator CS6 (Adobe Systems, San Jose, CA). Multi-layered documents (photographs only) and RTI files are Appendix 1-table 2 Continued provided in the associated Dryad dataset (Chen et al., 2021). Investigated specimens are listed in the Section 2.1.2.
Measurements were based on complete specimens illustrated herein and are provided in the following format: minimum/average/maximum.

Head and mouthparts morphology
The head and mouthpart morphology was investigated based on six specimens. Four of them (viz. CNU-NX1-326, −747, -754, -764) were investigated for the MA (see Section 1.4) of their mandibles. The specimens CNU-NX1-749, and -756 were excluded from the analysis because their mandibles were preserved with a slight rotation in the frontal plane; this impeding an accurate measurement of the MA (see below).

Documentation of extant material 1.3.1 Ovipositor morphology
We complemented the available literature on the morphology of female terminalia which form the ovipositor in polyneopteran lineages and in Orthoptera in particular (Ander, 1956;Bradler, 2009;Cappe de Baillon, 1920;Cappe de Baillon, 1922;Klass et al., 2003;Kluge, 2016;Walker, 1919; and see Klass, 2008 and references therein) by preparation of material belonging to various extant species (see Section 2.2). External habitus was photographed under various angles. Terminalia, together with the ultimate abdominal segments, were then cut off and mounted in a polyester resin. Three to four sections were made at various levels and hand-polished. Direct observation and photographs (same equipment as above) were used to document them.

Mandible morphology
To allow for inferences about the potential feeding ecology of the fossils, the MA was studied on a phylogenetically diverse sample of extant species including several lineages of polyneopteran insects. Twenty-nine recent taxa of Polyneoptera (Appendix 1-table 1) were investigated using micro-computed tomography (µCT) carried out at several synchrotron facilities: Beamline BW2 and IBL P05 of the outstation of the Helmholtz Zentrum Geesthacht at the Deutsches Elektronen Synchrotron (DESY), the beamline TOMCAT at the Paul Scherrer Institute (PSI), the TOPO-TOMO beamline of the Karlsruhe Institute of Technology (KIT), and beamline BL47XU of the Super Photon Ring 8GeV (SPring-8).

Analysis of the mandibular MA 1.4.1 Introduction
The MA is a straightforward biomechanical metric which was first introduced for vertebrates (Westneat, 1995;Westneat, 2004) and was used since in studies on vertebrate and arthropod jaw mechanics (Blanke et al., 2017;Cooper and Westneat, 2009;Cox and Baverstock, 2015;Dumont et al., 2014;Fabre et al., 2017;Fujiwara and Kawai, 2016;Habegger et al., 2011;Olsen and Gremillet, 2017;Sakamoto, 2010;Senawi et al., 2015;Weihmann et al., 2015). The MA is defined as the inlever to outlever ratio. For dicondylic insect mandibles, the inlever is the distance between the application of the input force and the joint axis, while the outlever arm is the distance from the biting point to the joint axis (Appendix 1-figure 1).
The MA thus indicates the percentage of force transmitted to the food item (i.e. the effectivity of the lever system). Although more detailed investigations concerning muscular insertion angles, muscle volumes, spatial arrangements, and muscle characteristics would be needed to quantify the absolute forces applied to a given food item, the MA is a useful mechanical performance index: It allows a size-independent comparison of the relative efficiencies within the mandibular lever system and it can be readily measured in a wide array of dried museum specimens as well as freshly collected ones. Here, we used it to assess the efficiency of the mandibular lever system of insect fossils.
Automatic segmentations of the mandibles were performed using the software ITK-snap (Yushkevich et al., 2006) after which STL files were imported into the software Blender (http:// www. blender. org) for further processing (Appendix 1-figure 1). The gnathal edge was defined sensu Richter et al., 2002 as the area from the pars molaris (proximal to the mouth opening) to the pars incisivus (distalmost tooth). Since the homology of subparts of the gnathal area is debated (Fleck, 2011;Richter et al., 2002;Staniczek, 2000), the gnathal outline, as seen when orienting the mandible in line with the rotation axis (Appendix 1-figure 1), was scaled as a percentage of tooth row length. For this, ~800 points for each specimen were wrapped against the gnathal outline in Blender and the distance between each point orthogonal to the mandibular rotation axis (=outlever) was measured. Similarly, one point was placed at the insertion point of M. craniomandibularis internus on the mandible and the distance between this point orthogonal to the rotation axis was measured (i.e. inlever). MA measurements were carried out on the segmentations of the left mandible for each specimen. All measurements and calculations were carried out in the R software environment (v. 1.1.383) using custom scripting. Separate MAs for each studied fossil were computed and combined to a CFMR using a Procrustes superimposition as implemented in geomorph v.3.0.5 in order to account for uncertainties in MA extraction due to potential distortion artefacts. From this superimposition, the mean MA shape was extracted and used together with the MAs of recent species for the further analysis steps. Polynomial functions of the 1st-20th order were fitted against all MA profiles. The AIC was used to determine the polynomial function with the best relative fit whose coefficients were then used for further analysis.

Phylogenetic signal
Phylogenetic signal was assessed using the most recent comprehensive phylogenetic estimate as a basis (Song et al., 2020). The phylogeny was pruned in order to contain only the taxa analysed here. The fossils were fitted into the phylogenetic estimate based on inference derived from wing venation and leg and ovipositor morphology (see main text).
Phylogenetic signal was assessed using the K statistic as implemented in geomorph v.3.0.5 (Adams et al., 2013) with 10,000 random permutations. This test statistic was found to be the most efficient approach to test for phylogenetic signal (Pavoine and Ricotta, 2013). Since significant phylogenetic signal was detected, a PCA as well as pPCA as implemented in the phytools package v.0.6-44 (Revell, 2012) were carried out in order to compare the analysed specimens in MA shape space.

Systematic palaeontology
In this section the systematics at the family-group level and below conforms to the ICZN to ensure that the new species name is valid under this Code, while that above the family-group, left ungoverned by the corresponding code, conforms to the principles of cladotypic nomenclature (Béthoux, 2007b;Béthoux, 2007a), itself compliant with the PhyloCode (Cantino and Queiroz, 2020). Specifically, a cladotypic definition corresponds to an apomorphy-based definition using two species as internal specifiers (each being anchored to a specimen designated as type). There are minor discrepancies between cladotypic nomenclature practice on the one hand and recommendations of the PhyloCode on the other. Notably, the first author to have associated the selected defining character state and a taxon is to be acknowledged under the former procedure.

Definition
Species that evolved from the hypothetical ancestral species in which the character state 'in female ovipositor, occurrence of a locking mechanism composed of a rhachis on gonostylus IX and of an aulax on gonapophysis IX' (also called 'secondary olistheter'; as opposed to 'in female ovipositor, absence of a locking mechanism composed of a rhachis on gonostylus IX and of an aulax on gonapophysis IX'), as exhibited by linderi Dufour, 1861 (currently assigned to Dolichopoda Bolívar, 1880) and artinii Griffini, 1913(currently assigned to Homogryllacris Liu, 2007, was acquired.

Reference phylogeny
The monophyly of Orthoptera, which includes all extant species sharing the defining character state of Neoclavifera, is beyond doubt. Wipfler et al., 2019, andSong et al., 2020, compose two recent accounts on the topic. It follows that the acquisition of the defining character state in the cladotypic species/specifiers, attested by Cappe de Baillon, 1920, very probably occurred once (Kluge, 2016). It is considered lost in Caelifera.

Qualifying clauses
Several qualifying clauses are explicit when using a cladotypic definition, but they need to be specified for a PhyloCode usage. The name Neoclavifera shall be considered as invalid as that of a taxon if it occurs that (i) the defining character state was acquired by the cladotypes/specifiers convergently, (ii) the defining character state is a plesiomorphy, (iii) the cladotypes/specifiers belong to a single species, and/or (iv) the defining character state does not occur in the specifiers (unless it is secondarily lost). There is no known evidence that one of these clauses might be challenged in our case.

Discussion
At the first glance, the name Chopard, 1920, appears as a suitable name. However, it is an explicit reference to the sword-like shape of the ovipositor valves in the corresponding insects, which composes a pre-occupation under cladotypic nomenclature (conversely, the taxon name Caelifera Ander, 1936, is an explicit reference to chisel-like shape of the valves). In other words, the name etymologically refers to a character state different from that used to define the new taxon, which makes it unavailable for the aimed purpose. The same applies to the taxon name Dolichocera Bei- Bienko, 1964 ('long horned';and, conversely, 'Brachycera Bei-Bienko, 1964' for 'short horned'), favoured by Kluge, 2016. Moreover, current classificatory schemes customarily regard Ensifera and Caelifera as sister groups, while our results predict that Caelifera is to be nested within Ensifera. Prolonged ambiguity on the conversion of 'Ensifera' as a defined taxon is then to be expected, not mentioning the fact that Ensifera Lesson, 1843 is a genus name for sword-billed hummingbirds, and Ensifera ensifera (Boissonneau, 1839) its type species. Given this situation, and the absence of a name composing a direct reference to the occurrence of olis2, we propose to coin a new one. Based on our literature survey, Kluge, 2016, is the first author to have discriminated a taxon on the basis of the defining character state only. This author stated that an olis2 is the autapomorphy of 'Dolichocera', but the name being a direct reference to another character state (see above), it follows that a new one is needed, hence Neoclavifera.
The meaning of the terms 'rhachis' and 'aulax' is critical to the proposed definition. Modest elevation and groove likely made the transition from adjoined smooth surfaces to ones bearing a proper rhachis and aulax. It is therefore necessary to define rhachis and aulax, as follows: a rhachis is a projection whose base is narrower than its projected part at its widest (best assessed in crosssection), and an aulax is its counterpart.
As defined, and based on species currently known, the composition of the taxa Archaeorthoptera and Neoclavifera overlap. We hypothesize that the defining character state of Archaeorthoptera was acquired in a hypothetical ancestral species distinct from the one of Neoclavifera, but the order of acquisition of their respective defining character states remains unknown.

Etymology
Based on 'frequens' ('frequent' in Latin), referring to the abundance of the species at Xiaheyan.

Differential diagnosis
Compared to Ct. elongatus (Brongniart, 1893), it is most closely related species (Appendix 1, Section 2.1), smaller size (deduced from forewing length) and prothorax longer than wide (as opposed to quadrangular).

General description
Body length (excluding antennae, including ovipositor) about 42-52 mm (based on female individuals only). Head: prognathous, head capsule heart-shaped in dorsal view; md with strongly sclerotized and prominent incisivi and a well-sclerotized molar area; la with a strong apical tooth and a smaller sub-apical one; mp well-developed, with five observed segments; tentorium composed of well-developed ata, ct, and pta, dorsal arms not visible; co located in the midline along the dorsal side of the head capsule, then branching into two diverging fc; ant long, filiform. Thorax: prothorax longer than wide, longer than head; boundary between mesothorax and metathorax not visible. Wings: ScP reaching RA distal to the two-thirds of wing length; RA with few or no anterior veinlets; RA and RP strong, parallel for a long distance; RA-RP area narrow in its basal half; at the wing base, R and M + CuA distinct; MA and MP simple for a long distance, with similar numbers of terminal branches, usually 1-3, rarely more than 4; CuA diverging from M + CuA and fusing with CuPa; CuA+ CuPa posteriorly pectinate. Forewing: length 31.5/36.1/41.2 mm, largest width 6.9/8.3/10.7 mm, membranous; ScP with anterior veinlets; RA-RP fork slightly distal to the point of divergence of M and CuA (from M + CuA); RP branched distally, near the second third of wing length, usually with 11-17 branches reaching apex, and occasionally 1-2 veinlets reaching RA; first split of M + CuA (into M and CuA) near the first fourth of wing length; between the origin of CuA (from M + CuA) and the first fork of RP, M very weak; first fork of M near wing mid-length; MA distinct from RP, connected to it by a short cross-vein, or occasionally fused with it for a short distance; median furrow located along M and then MP; CuA+ CuPa with most of its main branches further branched, with a total of 16-26 terminal branches; in basal part, CuA+ CuPa emitting strong posterior veinlets, vanishing before they reach the claval furrow; CuPb concave, weak and simple; AA1 with 3-4 branches; AA2 with about 10 branches; cross-veins mostly not reticulated, except along the apical and postero-apical section of the wing margin, and in the ScP/ScP+ RA RP area (where they are particularly strong); longitudinal pigmented areas located (i) along R, (ii) along CuA, and then the main stem of CuA+ CuPa, and (iii) along the posterior wing margin, distal to the endings of the first branches of CuA+ CuPa; these three areas merge distally; additional pigmented area along AA1. Hind wing: as in forewing, except for the following: slightly shorter than forewing; RA-RP fork opposite the point of divergence of M and CuA (from M + CuA); RP usually with 11-16 branches reaching apex; M forked at the first quarter of the wing; M with 5-8 branches reaching posterior wing margin; CuA+ CuPa with 5-8 branches; pigmented area forming an arc covering the apex, beginning along RA and ending close to the end of CuPb; plicatum well developed, with plica prima anterior reaching the posterior wing margin opposite the end of ScP (on RA). Legs: Fore-leg femur 4.9-6.3 mm long, 1.0-1.3 mm wide, tibia 5.2-6.3 mm long; mid-leg femur 5.2-6.4 mm long, tibiae 5.9-7.3 mm long; hind-leg femur 7.5-11.5 mm long, tibia 9.8-12.0 mm long; spines, probably in two rows, present along the ventral side of tibia of all legs, concentrated near the apex (fore-leg, at least 12 spines; mid-leg, at least 8 spines; hindleg, at least 15 spines); tarsus 5-segmented, second, third, and fourth segments shorter, terminal tarsal segment with paired claws and arolium (deduced from well-preserved fore-legs). Abdomen: abdomen about 17-23 mm long (based on female individuals only); female with a prominent sword-like ovipositor (see more detailed interpretation below and specimen descriptions).

Specimens description
Holotype, CNU-NX1-326 ( Figure 1) Positive and negative imprints of an almost complete female individual, viewed dorsally, very well preserved, with head, thorax, leg remains (including wellexposed fore-legs) and  (5), paired claws and arolium visible; mid-and hind-legs incomplete and/or not well exposed. Legs: spines well exposed on foreleg tibiae and distal part of a mid-leg tibia. Abdomen: bent (probably a consequence of decay), about 17 mm long, ovipositor viewed laterally, possibly slightly obliquely; bases of gp8 strongly sclerotized, well visible.
CNU-NX1-749 (Figure 2A-C, and Appendix 1-figure 7A-E) Positive and negative imprints of an almost complete female individual, wings incomplete and overlapping, body about 45 mm long. Head: about 6.4 mm long, 3.5 mm wide. Thorax: prothorax about 5.6 mm long, 3.7 mm wide. Legs: fore-leg femur 4.9 mm long, 1.2 mm broad, tibia 5.8 mm long, 0.8 mm broad, tarsus about 3.8 mm long; mid-leg femur 5.9 mm long, 1.0 mm broad, tibiae 7.3 mm long, 0.8 mm broad, tarsus about 4.9 mm long; hind-leg femur 6.1 mm long, 1.1 mm broad, tibia 10.1 mm long, 0.7 mm broad; spines visible, or even well-exposed, on each exposed tibiae. Abdomen: about 17 mm long (excluding ovipositor); sword-like ovipositor viewed laterally, about 8.4 mm long; antero-basal apophyses of gs9, gp9, and gp8 distinct, well delineated; near the ovipositor base, dorsal and ventral edges of gs9 and gp8, and ventral edge of gp9 well delineated; dorsal edge of gp9 visible in the distal half of the ovipositor; olis1 and olis2 visible near the ovipositor base, strongly sclerotized; olis1 located along the ventral edge of gp9 and dorsal edge of gp8; olis2 located close to (or along) the ventral edge of gs9, and laterally on gp9; olis1 and olis2 converging; ventral edge of gp8 with teeth more prominent and densely distributed near the apex.
CNU-NX1-742 ( Figure 2C-F, Appendix 1-figure 8A-C) Positive and negative imprints of an almost complete female individual, partly disarticulated, left forewing missing; body about 52 mm long. Head: detached from the rest of the body, mouthparts not discernible. Thorax: prothorax about 7.0 mm long, 3.6 mm width. Wings: a forewing and two hind wings visible, poorly preserved. Legs: fore-leg femur 5.7 mm long, 1.1 mm broad, tibia 6.2 mm long, 0.9 mm broad; spines well exposed on one hind-leg tibia, some visible on one foreleg tibia. Abdomen: strongly bent, segments not discernible; ovipositor very well preserved, detached from the rest of the abdomen, about 9.5 mm long; antero-basal apophyses of gs9, gp9, and gp8 distinct, well delineated; near the ovipositor base, dorsal and ventral edges of gs9 and gp8, and ventral edge of gp9 well delineated; dorsal edge of gp9 visible at the extreme base and in the distal half of the ovipositor; olis1 and olis2 visible near the ovipositor base, strongly sclerotized; olis1 located along the ventral edge of gp9 and dorsal edge of gp8; olis2 located close to (or along) the ventral edge of gs9, and laterally on gp9; olis1 and olis2 converging; ventral edge of gp8 with teeth more prominent and densely distributed near the apex.
CNU-NX1-754 ( Figure 4A and B, Appendix 1-figure 7I-K) Positive and negative imprints of an almost complete individual, well-preserved, wings overlapping, incomplete and partly creased, end of abdomen missing. Head: about 6.8 mm long 4.5 mm wide; md with strongly sclerotized and prominent incisivi and a well-sclerotized molar area; terminal teeth of la visible; ca distinguishable; co located in the midline along the dorsal side of the head capsule, then branching into two diverging fc. Thorax: prothorax about 5.9 mm long, 4.4 mm wide. Legs: fore-leg femora 5.3 mm long and 1.1 mm broad, tibiae 5.2 mm long and 0.7 mm broad, tarsus about 4.0 mm long; mid-leg femur 5.4 mm long and 1.1 mm broad, tibia 5.9 mm long and 0.8 mm broad, tarsus about 4.5 mm long; fore-and mid-leg tarsi well preserved, five-segmented with paired claws and arolium; second, third, and fourth segments shorter, ventral process (projecting forward) of third and fourth segments visible; hind-leg femora 7.5 mm long; end of hind-leg tibiae missing, 7.1/5.9 mm long, 0.7 mm broad; spines well exposed on one of the forelegs tibiae. Abdomen: about 14 mm as preserved, segments not discernible.
CNU-NX1-764 ( Figure 4C and D) Positive and negative imprints of an almost complete, isolated head, posterior part possibly overlapping with prothorax; mouthparts well preserved; md in occlusion, 2.1 mm long, 1.1 mm wide at their base, provided with strongly sclerotized and prominent incisivi and a well-sclerotized molar area; distal part of la visible, provided with a strong apical teeth and a smaller sub-apical one; tentorium composed of well-developed ata, ct, and pta, dorsal arms not visible; ct 1.2 mm long and 0.3 mm wide.
CNU-NX1-752 (Appendix 1- figure 2A and B) Positive and negative imprints of a partly incomplete individual, head and prothorax well exposed, a single fore-leg preserved, wings partly spread, right hind wing creased, most of abdomen missing. Thorax: prothorax about 7.0 mm long, 4.0 mm wide. right hind-leg, femur 8.3 mm long, tibia 11.1 mm long, tarsus about 6.6 mm long, with five tarsal segments, claws, and arolium visible; spines visible on one of the hind-leg tibiae.

CNU-NX1-750 (Appendix 1-figure 3C and D)
Positive and negative imprints of an almost complete individual, forewings overlapping hind wings, complete set of legs, abdomen poorly preserved and incomplete. Thorax: prothorax about 5.9 mm long, 4.2 mm wide. CNU-NX1-747 (Appendix 1-figure 4A-D) Positive and negative imprints of an almost complete individual, left forewing and right hind-leg missing. Head: about 6.0 mm long, 4.4 mm wide; md about 1.7/2.0 mm long, 1.4 mm wide at their base; apical tip of la visible, ct 0.9 mm long 0.2 mm wide; compound eye oval; circumocular ridge well developed. Thorax: prothorax about 5.5 mm long, 3.3 mm wide. Right forewing: preserved length about 29 mm, best width 8.2 mm; RP simple for 12.3 mm, with 10 branches, two of them reaching ScP + RA; MA and MP with two branches each; CuA+ CuPa with 19 terminal branches visible. Hind wings: plicatum folded, with numerous anal veins, not clearly discernible. Left hind wing: length 30.1 mm, best width 7.8 mm; RP simple for 9.6 mm, with 11 branches reaching wing apex and a single veinlet reaching ScP + RA; MA and MP simple for a long distance, each with three branches; CuA+ CuPa posteriorly pectinate, with six terminal branches. Right hind wing: overlapping with right forewing, only partly discernible; RP simple for 8.3 mm, with nine branches preserved. Legs: right legs poorly preserved and/or incomplete; left fore-leg femur 6.3 mm long and 1.0 mm wide, tibia 5.2 mm long and 0.6 mm wide; mid-leg femur 5.0 mm long and 1.1 mm wide, tibia 6.8 mm long and 0.6 mm wide; hind-leg femur 8.6 mm and 1.0 mm wide, tibia 9.8 mm long and 0.6 mm wide; spines well exposed on both foreleg tibiae, and the preserved mid-leg and hind-leg tibiae.

Grylloblatta chandleri
Our observations corroborate previous accounts (Walker, 1919;Walker, 1943), in particular regarding the occurrence of a long olis1 connecting gp9 and gp8. Its rh is slightly dejected externally. We also noticed the occurrence of an olistheter interlocking left and right gp9 along their dorsal margins. A specimen we observed had an egg engaged in the ovipositor. Due to the large diameter of the egg olis1 unlocked, as well as the dorsal gp9-gp9 olistheter. It can then be assumed that olistheters are comparatively labile structures in the species. In resting position (i.e. without engaged egg), when viewed externally, the ventral part of gp9 is not concealed by gp9.
Most of the area of gp9 concealed by gs9 is not as strongly sclerotized as its ventral part, except for the very base and its dorsal, ventral, and apical margins. (Linnaeus, 1764) (schematized under 'Caelifera' in Figure 3C)

Anacridium aegyptium
Our observations corroborate previous accounts on other caeliferan species reporting the occurrence of an olis1 connecting gp9 and gp8 along the entire ventral edge of the former (Kluge, 2016; Thompson, 1986). Unlike reported by Ander, 1956, we found no evidence of an olistheter interlocking the 'inner' (i.e. gp9) and 'posterior' (i.e. gs9) valves (i.e. olis2). The gp8 and Ander, 1956, 'lateral basivalvular sclerite' are extensively fused: they share the same lumen, and the dorsal and ventral fusion points are conspicuous in cross-section, owing to a clear invagination, coupled to a substantial and well-delimited thickening, of their shared wall.

2.2.3
Ceuthophilus sp. (Figure 3A and B; schematized under 'Rhaphidophoridae' in Figure 3C) We concur with previous accounts reporting that olis2 occurs in this lineage and in other Rhaphidophoridae (Cappe de Baillon, 1920;Gurney, 1936;Kluge, 2016). Unlike other orthopterans, the rh of olis2 is a short projection directed posteriorly, while its al covers a broader range (as it is, the antero-ventral half of gp9). Viewed laterally, the al of olis2 is slightly convex. This configuration possibly provides some degree of rotational freedom to gs9 vs. gp9 and gp8 (interlocked by olis1, which extends more posteriorly than olis2, including its rh), using the rh of olis2 as a slightly movable axis. This supposed ability would allow gp8 postero-ventral teeth to be exposed (instead of concealed by gs9) and then used by the insect to appreciate the adequacy of substrate for oviposition. The gp8 is only partially concealed by gs9. (Linnaeus, 1758,) (schematized under 'Tettigoniidae' in Figure 3C)

Tettigonia viridissima
The observed configuration of the ovipositor valves conforms that described by Cappe de Baillon, 1920. Unlike assumed by Kluge, 2016; among others we argue that the olistheter interlocking gs9 and gp8 (thereafter olis3) is not homologous with olis2. Firstly, a protrusion from gs9 and directed towards gp9 (viz., the characteristic features of olis2) occurs at various levels along the ovipositor. It is clearly distinct from another well-delimited olistheter (viz. olis3). Secondly, as stated by Kluge, 2016, the Anostostomatidae possibly represent an 'intermediate' stage is which a well-delimited olis2 co-occurs with the premises of an olis3, in the shape of a projection of the ventral margin of gs9 into gp8. If two olistheters occur (in addition to olis1), they cannot be homologous. It follows that there is an olis3 besides olis2.

Analysis of the mandibular MA
Progression of MA curves for the studied taxa are represented in Appendix 1- figure 9. Results of the PCA are summarized in Appendix 1-table 2 and represented in Appendix 1-figure 10, including the pPCA. Animated versions of the PCA represented in Figure 3E are provided in the associated Dryad dataset (Chen et al., 2021).