New Fossil Xyelidae (Hymenoptera: Symphyta) from the Mesozoic of Northeastern China.

One new genus and species, Leptoxyela eximia gen. et sp. nov., and one new species, Scleroxyela cephalota sp. nov., are described and illustrated based on two well-preserved compression fossils from the Lower Cretaceous Yixian Formation and the Middle Jurassic Jiulongshan Formation of China, respectively. Leptoxyela eximia gen. et sp. nov. is placed in the tribe Angaridyelini, 1966 and Scleroxyela cephalota sp. nov. in the tribe Xyeleciini Benson, 1945; while both tribes are in the subfamily Macroxyelinae Ashmead, 1898 of Xyelidae. A key to the genera of Angaridyelini is provided. In addition, we investigated various angles between Rs+M and 1-Rs for known fossil species of Macroxyelinae, and we found the angle and the length of 1-Rs are correlated; however, we could not see any correlation between the angles and the fossil ages even within a tribe. Furthermore, based on Sc2 connected to R before Rs, the angle between Rs+M and 1-Rs, and the length of the first flagellomere, we believe that the tribe Ceroxyelini may have only one genus of Ceroxyela, and suggest that Isoxyela and Sinoxyela should be transferred to Gigantoxyelini.

1. Introduction

Hymenoptera, one of four mega-diverse insect orders, comprises more than 155,000 described species [1,2] and plays a fundamental role in almost all terrestrial ecosystems [3,4]. Hymenoptera consists of the suborders of Apocrita and Symphyta [5]. Symphyta include about 9000 species in the world [6]. The most important diagnosis trait of Symphyta is the presence of broad waists, contrary to the Apocrita. As the most basal group of Symphyta, the small family Xyelidae has a special status in the evolutionary history of the Symphyta and Hymenoptera [7,8,9].

The earliest appearances of xyelids have been reported from the Middle or Upper Triassic of Kyrgyzstan [10], and the Upper Triassic of Australia [11,12], South Africa [13], Argentina [14] and Japan [15]. The family reached its maximal of diversity between the Middle Jurassic and the Early Cretaceous, and its past distribution was much broader than today [16]. There are four subfamilies in the Xyelidae: Archexyelinae Rasnitsyn, 1964; Macroxyelinae Ashmead, 1898; Madygellinae Rasnitsyn, 1969 and Xyelinae Newman, 1834 [17], with numerous fossil records. Hitherto, more than 85 species within 50 genera belonging to four subfamilies of Xyelidae have been reported in the Mesozoic and Cenozoic [18,19,20,21].

Macroxyelinae are a common subfamily of Xyelidae in the Mesozoic and can easily be differentiated from all other subfamilies because of the following combination of characters: the sclerotized apical bridge between C and R1 next to pterostigma, short and wide ovipositor and at least partially sclerotized pterostigma [10]. Macroxyelinae encompass five tribes: Angaridyelini Rasnitsyn, 1967; Ceroxyelini Rasnitsyn, 1969; Gigantoxyelini Rasnitsyn, 1969; Macroxyelini Ashmead, 1898 and Xyeleciini Benson, 1954 [22]. Up to date, 28 genera containing 44 fossil species have been reported in Macroxyelinae (Table 1).

Table 1

Known fossil species of Macroxyelinae.

Tribe	Taxon	Locality and Horizon	Angle between 1-Rs and Rs+M of Forewing	Reference
Xyeleciini	Bolboxyella bolboica	E. Siberia, K1	-	Rasnitsyn, 1990
	Microxyelecia brachycera	Kazakhstan; J3	108°	Rasnitsyn, 1969
	Proxyelia pankowskii	USA; P	145°	Jouault, Aase and Nel, 2021
	Scleroxyela cephalota sp. nov.	China; J2	128°	This paper
	Scleroxyela daohugouensis	China; J2	130°	Zheng et al., 2021
	Xyelecia xiejiaheensis	China; N	130°	Hong, 1983
	Xyelites lingyuanensis	China; K1	125°	Zhang and Zhang, 2000
	Xyelites trigeminus	E. Siberia, K1	115°	Rasnitsyn, 1966
	Uroxyela sicicauda	E. Siberia, K1	118°	Rasnitsyn, 1966
Angaridyelini	Angaridyela chengdeensis	China; K1	143°	Ren, 1995
	Angaridyela endemica	China; K1	154°	Zhang and Zhang, 2000
	Angaridyela exculpta	China; K1	139°	Zhang and Zhang, 2000
	Angaridyela minor	E. Siberia, K1	124°	Rasnitsyn, 1966
	Angaridyela pallipes	E. Siberia, K1	152°	Rasnitsyn, 1966
	Angridyela robusta	China; K1	132°	Zhang and Zhang, 2000
	Angridyela suspecta	China; K1	125°	Zhang and Zhang, 2000
	Angaridyela vitimica	E. Siberia, K1	125°	Rasnitsyn, 1966
	Baissoxyela tarsalis	E. Siberia, K1	×	Rasnitsyn, 1969
	Ceratoxyela decorosa	China; K1	145°	Zhang and Zhang, 2000
	Leptoxyela eximia gen. et sp. nov.	China; K1	130°	This paper
	Lethoxyela excurva	China; K1	140°	Zhang and Zhang, 2000
	Lethoxyela vulgata	China; K1	137°	Zhang and Zhang, 2000
	Liaoxyela antique	China; K1	150°	Zhang and Zhang, 2000
	Nigrimonticola longicornis	Kazakhstan; J3	113°	Rasnitsyn, 1966
	Ophthalmoxyela brachyua	Kazakhstan; J3	×	Rasnitsyn, 1966
Ceroxyelini	Ceroxyela dolichocera	E. Siberia, K1	120°	Rasnitsyn, 1966
To be determined	Sinoxyela viriosa	China; K1	155°	Zhang and Zhang, 2000
	Isoxyela rudis	China; K1	157°	Zhang and Zhang, 2000
Macroxyelini	Anthoxyela anthophaga	Russia; K1	150°	Rasnitsyn, 1982
	Anthoxyela baissensis	Russia; K1	170°	Rasnitsyn, 1977
	Anthoxyela orientalis	China; K1	167°	Gao and Ren, 2008
	Anthoxyela turgensis	Russia; K1	165°	Rasnitsyn, 1990
	Brachyoxyela brevinodia	China; K1	159°	Gao, Zhao and Ren, 2011
	Brachyoxyela gracilenta	China; K1	158°	Gao, Zhao and Ren, 2011
	Chionoxyela nivea	E. Siberia, K2	-	Rasnitsyn, 1993
	Megaxyela petrefacta	USA; N	155°	Brues, 1908
	Megaxyela yaoshanica	China; N	155°	Zhang, 1989
	Paleoxyela nearctica	USA; P	160°	Jouault, Aase and Nel, 2021
Gigantoxyelini	Abrotoxyela curva	China; J2	180°	Zheng et al., 2020
	Abrotoxyela lepida	China; J2	165°	Gao, Ren and Shih, 2009
	Abrotoxyela multiciliata	China; J2	172°	Gao, Ren and Shih, 2009
	Chaetoxyela hirsuta	E. Siberia, K1	145°	Rasnitsyn, 1966
	Gigantoxyela quadrifurcata	E. Siberia, K1	158°	Rasnitsyn, 1966
	Heteroxyela ignota	China; K1	180°	Zhang and Zhang, 2000
	Magnaxyela rara	China; J2	170°	Zheng et al., 2020
	Shartexyela mongolica	Mongolia; J3	165°	Rasnitsyn, 2008

Note. The systematic position of fossil Symphyta in the table was based on the description in original papers, and it does not reflect certain different opinions. J—Jurassic, K—Cretaceous, P—Paleogene, N—Neogene, J2—Middle Jurassic, J3—Late Jurassic, K1—Early Cretaceous.

In this report, we describe a new genus and species as well as a new species of a previously erected genus Scleroxyela, which enhance our knowledge of the Xyelidae in the Mesozoic of northeastern China.

2. Materials and Methods

The holotype specimen of Leptoxyela eximia gen. et sp. nov. (Figure 1) was collected from the Lower Cretaceous, Yixian Formation; Huangbanjigou, Chaomidian Village, Shangyuan Township, Beipiao City, Liaoning Province, China. The holotype specimen of Scleroxyela cephalota sp. nov. (Figure 2) was collected from the latest Middle Jurassic, Jiulongshan Formation; Daohugou Village, Shantou Township, Ningcheng City, Inner Mongolia, China. All specimens are housed at the fossil collection of the Key Lab of Insect Evolution and Environmental Changes, at the College of Life Sciences and Academy for Multidisciplinary Studies, Capital Normal University (CNUB; Dong Ren, curator), in Beijing, China.

Figure 1

Photographs of Leptoxyela eximia gen. et sp. nov., holotype (specimen CNU-HYM-LB-2021002) female. (A,B), latero-ventral view as preserved. (C), line drawing of latero-ventral view. (D), line drawing of forewing. (E), line drawing of antenna. (F), ovipositor under alcohol. (G), line drawing of ovipositor. (H), middle leg under alcohol. (I), middle leg under alcohol, showing leg tibial spurs (white wireframe). (J), middle leg under alcohol, showing planter lobes (white wireframe). Scale bars: 1 mm in (A–D); 0.5 mm in (E–J). Abbreviations: cx1—coxa1; cx3—coxa3; fla1–fla21—flagellomeres I-XXI; nt—notaulus; pe—pedicel; pl2—mesopleuron; pl3—metapleuron; pn—pronotum; sc—scape; scl—mesoscutellum; smm—medial mesoscutal suture; t8–t9—tergum 8–9; v1—first valvula; v2—second valvula (vI + v2—ovipositor); v3—third valvula (ovipositor sheath); vr1—first valvifer; vr2—second valvifer.

Figure 2

Photographs of Scleroxyela cephalota sp. nov., holotype (specimen CNU-HYM-NN-2021004) male. (A), dorsal view as preserved. (B), ventral view as preserved. (C), line drawing of dorsal view with forewings and hind wings. (D), line drawing of forewing. (E), line drawing of hind wing. (F), Photo of body under alcohol. (G), Photo of genitalia under alcohol. (H), line drawing of genitalia. (I). Photo of head under alcohol. Scale bars: 1 mm in (A–E); 0.25 mm in (F–I). Abbreviations: t7–t9, tergum 7–9.

The photographs were taken with a Nikon SMZ 25 and an attached Nikon DS-Ri 2 digital camera system. Line drawings were prepared using Adobe Illustrator CC and Adobe Photoshop CC. Statistical analyses were performed using R v.4.0.3 (R Core Team, Auckland, New Zealand) The wing venation nomenclature used in this paper follows Huber and Sharkey (1993) [23]. Antennal thread means flagellar segments behind the enlarged first flagellar segment. For wing venation, 1-Rs and 2-Rs refers to the 1st and 2nd segments of Rs; 1-M means the 1st segment of M and 1-Cu, 2-Cu and 3-Cu mean 1st, 2nd and 3rd segments of Cu.

3. Results

Systematic palaeontology.

Order: Hymenoptera, Linnaeus, 1758.

Suborder: Symphyta, Gerstaecker, 1867.

Superfamily: Xyeloidea, Newman, 1834

Family: Xyelidae, Newman, 1834

Subfamily: Macroxyelinae, Ashmead, 1898

Tribe: Angaridyelini, Rasnitsyn, 1966

Genus: Leptoxyela, Dai, Rasnitsyn, and Wang gen. nov.

Type species: Leptoxyela eximia, Dai, Rasnitsyn, and Wang sp. nov.

Etymology: the generic name is a combination of Greek “Lepto-”, meaning thin and referring to the long and thin terminal part of flagellum, and the generic name Xyela.

Diagnosis: Antenna with thread, longer than first flagellomere. Pterostigma sclerotized basally; 1-Rs slightly longer than half 1-M, shorter than half 2-Rs; Sc1 connected to C beyond level of Rs and Sc2 connected to R just before origin of Rs; Rs1 closer to pterostigma than to 1-Rs2 along wing margin; 1m-cu apparently longer than 2-Cu; ovipositor sheaths short and moderately wide.

Species included: type only.

Remarks: Leptoxyela gen. nov. is attributed to Macroxyelinae, based on the sclerotisation connecting C and R before pterostigma; short and moderately wide ovipositor sheaths, and further to Angaridyelini based on pterostigma sclerotized only basally. Within Angaridyelini, the new genus is similar to Nigrimonticola in having long antennal thread; Sc1 connected to C beyond level of Rs and Sc2 connected to R just before origin of Rs. It can be differentiated from Nigrimonticola by its longer 1m-cu (see Key below for details).

Leptoxyela eximia Dai, Rasnitsyn, and Wang gen. et sp. nov.

Material: CNU-HYM-LB-2021002, housed at the College of Life Sciences and Academy for Multidisciplinary Studies, Capital Normal University.

Etymology: the specific name is from Latin “eximius”, meaning exceptional and referring to the well-preserved fossil specimen.

Locality and horizon: Lower Cretaceous, Yixian Formation; Huangbanjigou, Chaomidian Village, Shangyuan Township, Beipiao City, Liaoning Province, China.

Diagnosis: as for the genus.

Description (Figure 1): Female sawfly in latero-ventral view with incomplete head and wings. Body length 10 mm (excluding antennae), forewing (as preserved) 8 mm long and antenna 5.7 mm long. Head and thorax brown, legs pale, antenna and abdomen of coloration intermediate between that of the head and that of the legs, legs and antenna with some darker spots.

Head distorted. Antenna very long, scape ca. 5× as long as wide, almost half as long as first flagellomere, pedicel somewhat longer than wide, first flagellomere very thin, as long as cell 1mcu, antennal thread about 1.7× as long as first flagellomere, 1.1× as long as segments 1–3 combined, with 20 segments visible, each three times as long as wide.

Thorax distorted, with no important characters available.

Forewing with pterostigma sclerotized basally; costal area broad, Sc closer to R than to C, sub-parallel to R, with two branches, Sc1 connected to C beyond level of Rs and Sc2 connected to R just before origin of Rs; R only slightly curved; 1-Rs slightly longer than half the length of 1-M, at most half the length of 2-Rs; 1r-rs slightly longer than 2r-rs, inclined toward wing base, 3-Rs arched posteriorly; Rs1 closer to Rs2 than to pterostigma along wing margin; 1m-cu almost as long as 3-Cu and about 1.2× as long as 2-Cu; 1-Cu curved in middle. Cell 1rm long and narrow, significantly longer than 1r; cell 1r about 1.6 times as long as wide and longer than cell 2r; cell 2r trapezoidal; length ratio of cells 1r:2r = 5:3; cell 1mcu nearly as long as cell 1r; cell 2mcu longer and wider than 1mcu, about 2.2× as long as wide and 1.1× as long as cell 2rm; 2rm as long as cell 3rm; cell 1cua and cell 1a incomplete. Hind wing only fragmentarily preserved.

Foreleg and middle leg partly preserved, but structure of each part of hind leg well-preserved; foreleg with coxa thin as preserved; mesopleuron and metapleuron partly preserved; hind leg with coxa elongate, about 2.1× as long as wide; trochanter small and trapezoidal, trochantellus clearly visible; femur thick and slightly wider medially, about 3.3× as long as wide; hind tibia long, about 1.4× as long as femur, and with long and sharp terminal spur; tarsus incomplete (many segments of tarsi preserved).

Abdomen with nine segments visible, ovipositor sheath broad and short; first valvifers roughly triangular; second valvifers large, longer than first valvifers; ovipositor well preserved, first valvula as wide as second valvula.

Tribe: Xyeleciini, Benson, 1945

Genus: Scleroxyela, Zheng, Hu, D. Chen, J. Chen, Zhang, and Rasnitsyn, 2021

Type species: Scleroxyela daohugouensis, Zheng, Hu, D. Chen, J. Chen, Zhang, and Rasnitsyn, 2021.

Species included: type species and Scleroxyela cephalota, Dai, Rasnitsyn, Shih, and Wang sp. nov.

Scleroxyela cephalota, Dai, Rasnitsyn, Shih, and Wang sp. nov.

Material: CNU-HYM-NN-2021004, housed at the College of Life Sciences and Academy for Multidisciplinary Studies, Capital Normal University.

Etymology: the specific name is from Greek “kephalon”, meaning head and referring to the large head.

Locality and horizon@ latest Middle Jurassic (late Callovian), Jiulongshan Formation; Daohugou Village, Shantou Township, Ningcheng County, Inner Mongolia, China.

Description (Figure 2): Male (body length 5.47 mm) sawfly in ventral view with nearly complete forewings. Body and antenna dark, legs pale with some darker spots. Head (width 1.45 mm, length 0.9 mm) massive, eye large, about a quarter of head width. Antenna incomplete, with first flagellomere slightly longer than thread.

Thorax and head nearly equal in width. Medial mesoscutal line short, not extending behind notauli; notauli V-like; mesoscutellum wider than prescutum, about 1.4× as wide as mesoscutum; cenchri slightly shorter than wide; metascutum triangular-shaped; metapostnotum slender and remaining part of the thorax not discernable.

Forewing (6.42 mm long) with pterostigma completely sclerotised, costal area obviously widened proximad base of Rs; Sc with two branches, posterior branch of Sc (Sc2) short, subvertical to R, shorter than 1-Rs and connected to R well before origin of Rs; anterior branch (Sc1) long, reaching C beyond origin of Rs; R distinctly bent before origin of Rs and greatly thickened before pterostigma; 1-Rs half the length of 1-M; 2r-rs ca. 1.5× as long as 1r-rs; 3-Rs arched posteriorly; 1m-cu half the length of 3-CuA, roughly equal to 1cu-a; 3r-m about 1.3× as long as 2r-m; M+Cu slightly curved in its basal third; CuP straight, extended to the top of cell 2a. Cell 1r about twice as long as 2r and 2.6× as long as wide; cell 2r trapezoidal, length ratio of cells 1r:2r = 2:1; cell 1cua narrow and long, distinctly broader at M base, about 7.5 times as long as wide, and twice as long as cell 1mcu; cell 2mcu long and narrow, longer than 2rm and about 3.6 times as long as wide; cell 1a 1.75 times as long as 2a.

Hind wing with 1-Rs vanishing short (1r-m meeting Rs close to Rs base); limit between 1-M and 1r-m obscure; M+Cu nearly straight.

Foreleg and middle leg incomplete, hind leg with coxa elongate, about 1.5× as long as wide; trochanter small and trapezoidal; femur oval, slightly wider medially, about 4.6× as long as wide; hind tibia long, about 1.1× as long as femur; tarsi not preserved.

Abdomen with nine segments visible, genitalia visible, hypopygium large, diamond-shaped; gonocoxa wide, subtriangular (narrowed basally), apparently wider than gonostylus; gonostylus long, with distinct gonomacula.

Remarks. Scleroxyela cephalota sp. nov. can be assigned to the subfamily Macroxyelinae based on the pterostigma slerotised and with sclerotisation connecting C and R near pterostigma, to the tribe Xyeleciini based on Sc2 connected to R before Rs and 1-Rs at most half the length of 1-M, and to the genus Scleroxyela based on pterostigma completely sclerotized and Sc1 reaching C distinctly behind Rs. Within the genus, the new species differs from S. daohugouensis in having the head wider, pterostigma wider than 1r-rs length, Sc2 connected to R more distant from Rs base, and forewing shorter (6.42 mm long vs. 11.1 mm in S. daohugouensis). These differences are unlikely to be sexually dependent.

4. Discussion

We describe one new genus and two new fossil species, from Huangbanjigou and Daohugou of China, respectively. Both Daohugou and Huangbanjigou are important insect deposits containing abundant, well-preserved insect fossils [24]. The Daohugou area, an important locality for the Middle Jurassic insects, located in northeast of China, is assumed to be located in a humid and warm–temperate climate area by the paleoenvironment reconstruction analysis. The Huangbanjigou area, as an important section of the Yixian Formation, also has a large number of rare animal, insect and plant fossils [25]. However, different experts have different views on the age of the Yixian Formation. Three opinions have been offered: Late Jurassic [26], transition from Late Jurassic to Early Cretaceous [27,28], and Early Cretaceous [29,30,31]. Nowadays, the academic community generally supports the results of radioactive geochronological dating method, considering the Yixian Formation as the Early Cretaceous, Barremian–Aptian age [32,33]. The position of biota of the Yixian Formation within the Jehol Biota, which it belongs to, is discussed by Rasnitsyn (2020) [34]. Up to date, there are 5 species of Macroxyelinae reported from Daohugou, and 14 species from Huangbanjigou [35].

The angle between Rs+M and 1-Rs is usually obtuse in Macroxyelinae, but the angle varies among the tribes. The formation of angle between Rs+M and 1-Rs is discussed (red in Figure 3) using a series of Macroxyelinae fossils. Our observations, including the angle of all fossil species of the subfamily Macroxyelinae, are summarized in Table 1. The angle varies from 108° to 145° in the tribe Xyeleciini, from 113° to 154° in the tribe Angaridyelini, 120° in the tribe of Ceroxyelini (only Ceroxyela Rasnitsyn, 1966), from 150° to 170° in the tribe of Macroxyelini, and from 145° to 180° in the tribe Gigantoxyelini.

Figure 3

Angle of Rs+M and the section of Rs in the subfamily Macroxyelinae. (A), Uroxyela sicicauda, Rasnitsyn, 1966 (Xyeleciini). (B), Ceroxyela dolichocera, Rasnitsyn, 1966 (Ceroxyelini). (C), Leptoxyela eximia, gen. et sp. nov (Angaridyelini). (D), Megaxyela yaoshanica, Zhang, 1989 (Macroxyelini). (E), Sinoxyela viriosa, Zhang and Zhang, 2000 (tribe to be determined). (F), Abrotoxyela multiciliata, Gao, Ren and Shih, 2009 (Gigantoxyelini).

We try to compare and investigate the angle changes between Rs+M and 1-Rs for the tribes in the subfamily Macroxyelinae. We found that there are two trends in the degree of the angle allowing the creation of two informal groups of Macroxyeline tribes (Angaridyelini + Ceroxyelini + Xyeleciini, angle = 108–155°) and (Gigantoxyelini + Macroxyelini, angle = 145–180°). Note that some overlappings of the morphospaces of some tribes are recorded (Figure 4). Interestingly, we found that the first group is also characterized by a short 1-Rs, unlike the second one with generally longer 1-Rs, again with some overlapping. However, we cannot see any correlation between the angle and the fossil age, which is also an interesting observation. Additionally, these fossils show a transition from an obtuse angle toward a perfectly linear alignment of Rs+M and 1-Rs. The angles between Rs+M and 1-Rs became broadened from the tribes Xyeleciini, Angaridyelini and Ceroxyelini to the tribe Macroxyelini, and then to the tribe Gigantoxyelini, while nearly reaching a linear alignment in some species of Macroxyelinae.

Figure 4

Different types of antennae in the subfamily Macroxyelinae. (A), Isoxyela rudis Zhang and Zhang, 2000. (B), Gigantoxyela quadrifurcata Rasnitsyn, 1966. (C), Scleroxyela daohugouensis Zheng et al., 2021. (D), Leptoxyela eximia gen. et sp. nov. (E), Ceratoxyela decorosa Zhang and Zhang, 2000.

The tribe Ceroxyelini was established by Rasnitsyn (1969) initially consisting of only one genus of Ceroxyela. Subsequently, Zhang and Zhang (2000) reported two genera, Sinoxyela and Isoxyela, which were also placed in tribe Ceroxyelini based on their pterostigma of the forewing being sclerotized and basally membranous. In this report, we believe that Isoxyela and Sinoxyela cannot be attributed to Ceroxyelini primarily based on these different characters: Sc2 meeting R before Rs for Isoxyela and Sinoxyela; the pterostigmal desclerotisation is less extensive than in Ceroxyela; the angle between Rs+M and 1-Rs is 120° in Ceroxyela, strongly different from those of Sinoxyela (155°) and Isoxyela (157°); and the length of 1-Rs and the first flagellomere of Isoxyela and Sinoxyela are obviously different from those of Ceroxyela.

The first flagellomere, an important diagnosis trait in Xyelidae [36], is also diversified among the subfamily Macroxyelinae (Figure 5): first flagellomere varies from being three times longer than thread (Isoxyela and Gigantoxyela), to as long as it (Scleroxyela), to 0.6 times shorter than thread (Leptoxyela), and up to 0.4 times so (Ceroxyela). Based on the above study on the veins and antennae, we believe that the tribe Ceroxyelini may only consist of one genus of Ceroxyela, and that the genera Isoxyela and Sinoxyela should not be included in Ceroxyelini. We would suggest to transfer these two genera to Gigantoxyelini tentatively, based on the sclerotized pterostigma and basal Sc2 until more information is available. We also hope that there will be better phylogenetic results in the future to reveal the relationship within Ceroxyelini.

Figure 5

The changes of angle between 1-Rs and Rs+M of forewing for known fossil species of five tribes in Macroxyelinae and the relationship between the angle and fossil age. Angaridyelini in black; Ceroxyelini in yellow; Gigantoxyelini in purple; Macroxyelini in blue; Xyeleciini in red. J2—Middle Jurassic; J3—Late Jurassic; K1—Early Cretaceous; Eo—Eocene; M—Miocene.

5. Conclusions

As the most basal group of Symphyta in Hymenoptera, the family Xyelidae has a special status in the evolutionary history of the Symphyta [7,8,9]. The earliest appearances of xyelids have been reported from the Middle or Upper Triassic of Kyrgyzstan [10], and the Upper Triassic of Australia [11,12], South Africa [13], Argentina [14] and Japan [15]. Xyelidae reached its maximum diversity between the Middle Jurassic and the Early Cretaceous, and its past distribution was much broader than today [6]. Macroxyelinae is a common subfamily of Xyelidae in the Mesozoic [10]. Macroxyelinae encompasses five tribes: Angaridyelini Rasnitsyn, 1967; Ceroxyelini Rasnitsyn, 1969; Gigantoxyelini Rasnitsyn, 1969; Macroxyelini Ashmead, 1898 and Xyeleciini Benson, 1954 [22]. Until now, 28 genera containing 44 fossil species have been reported in Macroxyelinae

We described Leptoxyela eximia gen. et sp. nov. and Scleroxyela cephalota sp. nov., based on two well-preserved compression fossils from the Lower Cretaceous Yixian Formation and the Middle Jurassic Jiulongshan Formation of China, respectively. Leptoxyela eximia gen. et sp. nov. is placed in the tribe Angaridyelini and S. cephalota sp. nov. in the tribe Xyeleciini; while both tribes are in Macroxyelinae of Xyelidae. After investigating various angles between Rs+M and 1-Rs for known fossil species of Macroxyelinae, we reported that the angle and the length of 1-Rs are correlated, however, we could not see any correlation between the angles and the fossil ages even within a tribe. In addition, we suggested that Isoxyela and Sinoxyela should be transferred to Gigantoxyelini and the tribe Ceroxyelini may have only one genus of Ceroxyela.