The first Late Triassic Chinese triadophlebiomorphan (Insecta: Odonatoptera): biogeographic implications

The clade Triadophlebiomorpha represents a morphological ‘link’ between the Paleozoic griffenflies (Meganisoptera) and the modern taxa. Nevertheless they are relatively poorly known in the body structures and paleobiogeography. The Triassic dragonfly is extremely rare in China with only one previously recorded. A new family, Sinotriadophlebiidae Zheng, Nel et Zhang fam. nov., for the genus and species Sinotriadophlebia lini Zheng, Nel et Zhang gen. et sp. nov., is described from the Upper Triassic Baijiantan Formation of Xinjiang, northwestern China. It is the second Chinese Triassic odonatopteran and the second largest Mesozoic representative of this superorder in China. The discovery provides new information for the clade Triadophlebiomorpha during the Late Triassic and expands its distribution and diversity in Asia. The find reflects a close relationship between the two Triassic entomofaunas from Kyrgyzstan and the Junggar Basin, and provides a Carnian age constraint on the lowermost part of the Baijiantan Formation.

In China, Mesozoic dragonflies have been largely found in Middle Jurassic to Lower Cretaceous strata but are little known from the Triassic although a fragmentary "protodonate" (Zygophlebiidae) has been recently described from the Middle-Upper Triassic Tongchuan Formation of Shaanxi Province, China 1 . In the Junggar Basin of Xinjiang, northwestern China, a few insect fossils have been previously unearthed from Triassic strata 2, 3 . Ten insect orders have been recently recorded from the Lower Jurassic of the Tuziakeneigou outcrop in Karamay City including a damsel-dragonfly Dorsettia sinica Zheng et al., 2016, which reflects a weak influence of the end-Triassic extinction on damsel-dragonflies or a quick dispersal of damsel-dragonflies during the earliest Jurassic 4 . In addition, four orders of insect fossils have been unearthed from the Upper Triassic Baijiantan Formation of the Shendigou outcrop in Karamay City, somewhat enhancing the distribution and diversity of the insect fauna during the Late Triassic. The clade Discoidalia is divided into two subclades: the Stigmoptera (including the modern Odonata) and Triadophlebiomorpha (Zygophlebiida + Triadophlebiida) 5 . The clade Triadotypomorpha is more widespread than the clade Triadophlebiomorpha as the former occurs in Kyrgyzstan, western Europe (France, Germany and Spain) and Australia [5][6][7] . All members of the clade Triadophlebiida are known from the Madygen Formation of southwestern Kyrgyzstan 8 .
Here we describe a new family Sinotriadophlebiidae attributable to the clade Triadophlebiida from the Upper Triassic Baijiantan Formation of the Shendigou outcrop, Karamay City, Xinjiang, northwestern China (Fig. 1A,B, 45° 44′ N, 85° 0′ E). The Baijiantan Formation unconformably underlies the Lower Jurassic Badaowan Formation and conformably overlies the Middle-Upper Triassic Karamay Formation in the Shendigou outcrop (Fig. 1C). It consists of grey-white or grey-yellow mudstone and silty mudstone interbedded with sideritic bands, yielding

Discussion
Sinotriadophlebia gen. nov. has a highly developed N with a kink in ScP, obvious nodal furrow, aligned Cr and Sn, and a typical odonatoid DC basally open but distally delimited by a characteristically oblique vein MAb between MA and MP. These characters are apomorphies of the Discoidalia Bechly, 1996. It should be noted that Bechly 10 assigned the two subclades Triadophlebiomorpha Pritykina, 1981 andTriadotypomorpha Bechly, 1996 to the clade Triadophlebioptera Bechly, 1996, considering that Triadotypomorpha also has a typical odonatoid DC closed distally by MAb. However, MAb is in fact absent from Triadotypus guillaumei Grauvogel and Laurentiaux, 1952, the type genus of the Triadotypidae Grauvogel and Laurentiaux, 1952 within Triadotypomorpha 11 . In addition, Bechly 10 put the Piroutetiidae Nel, 1989 in the Triadotypomorpha, a position rejected by Nel et al. 5 who included this family in the Triadophlebiomorpha, and here we restore it in its original position. A further difference between Sinotriadophlebia and Triadotypus is that MP is simple in the latter but it has numerous posterior branches in the former.
Sinoriadophlebia has the following synapomorphies of the Triadophlebiomorpha: petiole very long and of unique shape (not formed by a fusion of CuA + CuP + AA with the anal margin as in other odonates with petiolated wings); MP distinctly curved distal of its origin at the distal angle of DC; and discoidal vein MAb very long and oblique. It also shares a characteristic of this clade in the presence of numerous posterior branches of MP, unlike in the Triadotypomorpha. Within the Triadophlebiomorpha, the affinity of the new genus with the Zygophlebioidea Pritykina, 1981 (sensu Bechly, 1996) can be excluded because Sinoriadophlebia does not share the peculiar pattern of branching of IR2 and IR1 on RP2 in Zygophlebioidea 8,10 . Sinotriadophlebia also has the following synapomorphies of the Triadophlebiida Bechly, 1996(Mitophlebiidae Pritykina, 1981+ Triadophlebioidea Pritykina, 1981: CuA + CuP + AA fused with MP and Sn very oblique. The Mitophlebiidae can be excluded from consideration by Sn not being concavely curved in Sinotriadophlebia. The Triadophlebioidea comprises Paurophlebiidae Bechly, 1996 and Triadophlebiidae Pritykina, 1981. The absence of the fusion of ScP with RA from the base to N in Sinotriadophlebia excludes affinity with Paurophlebiidae and Triadophlebiidae can also be excluded by the following differences: RP3/4 unforked in Sinotriadophlebia but forked in Triadophlebiidae; In conclusion, Sinotriadophlebia can be attributed to neither the Mitophlebiidae nor the Triadophlebioidea (Fig. 4). A new family is proposed based on the following combined differences: large size with wing length reaching up to 100 mm (within the Triadophlebiida, all species are less than 80 mm except Neritophlebia longa Pritykina, 1981 being 120-125 mm in length); distal part of ScP crossing N very obliquely with crossveins beneath it; CuA with numerous parallel posterior branches (a character also present in Triadotypus, but more developed than in Sinotriadophlebia); RP and MAa fused into a long basal stem (a character also present in Triadotypus, but not in all other Triadophlebiida); Sn extremely oblique and very elongate, with several crossveins beneath it (a character shared with the Neritophlebiinae).
Sinotriadophlebia lini is the second largest Mesozoic odonatopteran from China, being slightly smaller than the damsel-dragonfly Hsiufua chaoi (107.6 mm wing for forewing length) from the Middle-Upper Jurassic of Inner Mongolia 12 . In addition, the Triassic Iverya averyi Béthoux & Beattie, 2010 is currently considered within Triadotypidae, showing combined characters of Triadotypidae and Triadophlebiomorpha. It shares the characters present in the Triadophlebiomorpha but not in Triadotypus: oblique MAb, well separated RP and MAa, and long petiole. However, it also shares with Triadotypus a CuA with numerous posterior branches and a simple MP, unlike in Triadophlebiomorpha 7 . The discovery of much better preserved specimens is necessary to determine its exact position. Numerous fossils including insects, kazacharthrans, fishes, plants and sporopollen have been unearthed from the Baijiantan Formation of the Shendigou outcrop in Xinjiang 13,14 . The sporopollen assemblage in the Baijiantan Formation is mostly inherited from that of the upper part of the Karamay Formation 13 and the megaspore assemblage resembles that of the Upper Triassic Haojiagou Formation in the southern Junggar Basin 15 , all indicating a Late Triassic age. The Baijiantan Formation is found in the northwestern Junggar Basin and can be correlated with the Huangshanjie-Haojiagou formations in the southern and eastern basin (Fig. 5). The Huangshanjie and Haojiagou formations are considered to be Carnian 16 and Norian-Rhaetian in age 17,18 , separately. Therefore, the age of the Baijiantan Formation is probably Carnian-Rhaetian.
The clade Triadophlebiida were previously only recorded from the Madygen Formation of southwestern Kyrgyzstan 8 . The age of the Madygen Formation is considered to be Ladinian-Carnian [19][20][21] . The discovery of Sinotriadophlebia therefore points to a Carnian age for the lowermost part of the Baijiantan Formation. The relatively short distance between the Madygen fossil site and northwestern Xinjiang suggests a close relationship between the two insect faunas.

Methods
The specimen was examined dry under a Nikon SMZ1000 stereomicroscope. Photographs were taken using a Canon 5D digital camera and the line drawings were prepared from photographs using image-editing software (CorelDraw X7 and Adobe Photoshop CS6). The specimen is housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS).
The higher classification of Triadophlebiida, as well as family and generic characters followed in the present work, is based on the phylogenetic system proposed by Bechly 25 and modified by Nel et al. 5 .