A New Titanopteran from Southwestern Korean Peninsula: The Triassic Age of the Nampo Group and its Tectonic Implications

Titanopterans are well-known as giant predatory insects in the Triassic, but not only their rare occurrences have been limited to Central Asia and Australia, but also their phylogenetic anity remains unresolved. The age of the nonmarine sequences of the Nampo Group at the southwestern Korean Peninsula is unclear, and the tectonic anity of the surrounding area is contentions. Here we report a new titanopteran Magnatitan jongheoni gen. et sp. nov. the Amisan Formation, Nampo Group, which marks the rst discovery of the titanopteran fossil from outside Central Asia and Australia, presenting a possible circum-Tethys Ocean distribution, at least, during the Late Triassic. The new fossil shows a clearly divided CuPb, which will help understand the evolution of titanopterans in the future. Moreover, the occurrence of a titanopteran nally conrms the Late Triassic age of the Nampo Group. In China, similar Late Triassic non-marine sequences are widespread in the Cathaysia Block, in which various geological features similar to those in the southwestern Korean Peninsula, such as a Paleozoic magmatism and an eclogite facies with Neoproterozoic protoliths, have been recently documented as in the southeastern Korean Peninsula. Such similarities may suggest a close tectonic anity between the northeastern Cathaysia Block and the southwestern Korean Peninsula.


Introduction
Titanoptera is an iconic insect group of the Triassic. The prominently large size (a wingspan of up to 400 mm is documented by Gigatitan vulgaris 1 ), and the raptorial forelegs equipped with stout spines, make them the most impressive predatorial insects of the Triassic 1,2 . Both sexes seemed to have had 'sound apparatus' in forewings, which may have produced deep and resonant acoustic signals in Triassic forests 2 (but see ref [3]). Despite the fame, however, their phylogenetic a nity remains unstable. Although titanopterans are often considered closely-related to orthopterans, no consensus has been made on their phylogenetic position. Titanoptera was treated as a separate order closely related to Orthoptera 1,4 . It was argued that titanopterans originated from the Permian orthopteran Tcholmanvissiidae 5 , rendering this family paraphyletic 1 . Subsequently, it was suggested the Carboniferous protorthopteran family Geraridae 6 as a sister group of Titanoptera 7 . In a reinvestigation of the wing veins Order Titanoptera was derived from the Tchomanvissidae 8 . Recently, however, the close relationship between the Tchomanvissidae and Titanoptera was questioned 9 (but see ref [10]). The incomplete understanding of its a nity is mainly due to the small number of fossils with limited paleogeographic occurrences; to date, all titanopteran fossils have been known only from Central Asia and Australia 1,11−13 .
The complicated suite of geologic features in the Korean Peninsula has led to a variety of interpretations on the tectonic processes which have formed them. Especially regarding the collision of the Sino-Korean (North China) Craton and South China Craton at around the Permo-Triassic, several disparate hypotheses have been put forward for the tectonic a nity and the movement of the Korean Peninsula [14][15][16][17][18][19][20] .
Disagreements derives not only from the way of interpreting the various geological data, but also from not-well constrained ages of several sedimentary basins which developed under the tectonic framework Seongjuri formations are dominated by conglomerate and sandstone facies, while the fossiliferous Amisan and the Baegunsa formations are comprised of coal-bearing shale and sandstone, with minor amount of conglomerates in the lower part of the Baegunsa Formation 38 . The Nampo Group is unconformably overlain by the Early to Middle Jurassic Oseosan Volcanic Complex, which consists of light gray to gray tuff and lapilli tuff, overlain by thick tuffaceous sedimentary rocks of gray to dark gray shale, sandstone, and conglomerate 27 . The basement rocks of the Chungnam basin is largely of the Neoproterozoic and Paleozoic metamorphic rocks (the Wolhyeonri complex) in the west of the Oseosan area, while the Paleoproterozoic granite gneiss is prevalent to the east of the Oseosan area 33 (Fig. 1). Various hypotheses have been proposed for the development of the Chungnam Basin, under the view that the Chungnam Basin is part of the Daedong Supergroup, which include development of half-grabens or pull-apart basin from the late stage of the Songrim orogeny 39 , deposition in syn-orogenic (Songrim orogeny) piggyback basins 16 , and syntectonic deposition during the early stages of the Daebo orogeny 30 . Subsequent researchers 21 considered the "subbasins" of the Chungnam Basin as piggyback basins formed during the Daebo orogeny 21 . Most recently, a basin development model of post-collisional extensional deformation at the Late Triassic was proposed 26,28 , which is based on the assumption that an eastward extension of the Sulu collisional belt runs through this area 17,19 .
The Amisan formation is known to be up to 1,000 m thick, and consists of the lower sandstone units, the lower shale unit, the middle sandstone units, the middle shale unit, and the upper sandstone unit 38 .
Diverse fossils have been documented from the Amisan Formation, including plants [40][41][42][43][44][45] , bivalves 46 , conchostracans 25 , insects 47,48 , and shes 49 . Of them, plant and conchostracan fossils have been used for age dating for the Amisan Formation. The Late Triassic age was suggested, based on the plant fossil assemblages 45 , but most of the genera in the subsequent paleobotanical researches appeared to have ranges up to Lower Jurassic or even up to Cretaceous 24,41,43,44,50−52 . The conchostracan species of the formation were also considered to be Triassic in age, based on poorly-preserved specimens 25 , but the Mesozoic occurrence of the bivalve genus Margaritifera, the only known bivalve genus from the Amisan Formation, was then known to be restricted to post-Triassic 46 . In addition, the Jurassic radiometric ages of the Nampo Group 29,30 have questioned the Triassic age from the paleontological evidence, calling for more convincing evidence 25 .

Material And Methods
A single forewing with well-preserved venations is discovered from the middle shale unit of the Amisan formation exposed in the Myeongcheon section (Fig. 1). The sample was photographed submerged in water, with a Canon EOS 60D using a Canon EF 100 mm f/2.8 USM macro lens. High-angle lighting was used for the maximal re ective image of the specimen. Images were cropped and enhanced by means of contrast and brightness in Adobe Photoshope CS6. For the relief-enhanced images, raw images for polynomial texture mapping were acquired by a self-crafted system which permits lighting from 50 different directions and the same Canon EOS60 camera setting mentioned above. The specimen was white-coated with magnesium oxide for this process. In order to enhance surface details of the fossil, 50 images were taken and converted into a PTM format le which was then run in RTI Viewer software which is freely downloadable at http://culturalheritageimaging.org/What_We_Offer/Downloads/.
The specimen was also gold-coated using a Cressington 108 Auto sputter coater with 10 mA for 80 s. A eld-emission electron probe microanalyzer (JEOL JXA-8530F) and a low vacuum eld emission scanning electron microscope (JEOL JSM-7200F-LV) at the Korea Polar Research Institute were used to acquire X-ray elemental maps and surface analysis. X-ray elemental maps for C, N, O, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, and Ba were obtained using ve wavelength dispersive spectrometry (WDS) and nineteen energy dispersive spectrometry (EDS), with an acceleration voltage of 20 kV, beam current of 200 nA, beam size of 13 µm, dwell time of 10 ms, and step size of 13 µm. Raw data of X-ray elemental maps were imported and processed for brightness and contrast by ImageJ 53 . Qualitative point data was generated from the gold-coated specimen using an accelerating voltage of 5 kV and probe current of 10 nA by EDS equipped at the low vacuum eld emission scanning electron microscope.
General wing venation nomenclature follows ref [54], which was also veri ed by a subsequent research 55 .
Interpretation of titanopteran speci c wing venation pattern follows 8

Etymology
The generic name is derived from the Latin 'Magna,' referring to its large size, and the general name of the group 'titan'. The species name was designated in honor of the collector of the specimen, Prof. Jongheon Kim (Kongju National University) who has made signi cant paleontological contributions for the Nampo Group over the last thirty years.

Holotype
GNUE112001, an almost complete left forewing. The specimen is housed in the Gongju National University of Education.

Type locality and horizon
The middle shale unit of the Amisan Formation, Upper Triassic, Myeongcheon Section, Seongju-myeon, Boryeong City, Chungcheongnam-do, Republic of Korea. .

Diagnosis
ScP strong; RA and RP are divided near to distal half of the wing, CuPb divided into CuPbα and CuPbβ.
ScP with many branches and dichotomously rami ed at distal part; RA extends one branch at distal part; division of MA and MP close to fusion of CuA and CuPaα; MP rami ed into two branches near to reseparated CuPaα from CuA. area between CUA to posterior wing margin widened.

Description
Forewing large with pointed apex, anterior margin anteriorly bowed, posterior margin posteriorly bowed, preserved length 57.3 mm, width 19.0 mm; distal part of ScA preserved, weak; area between ScA to anterior wing margin narrow; ScP long and strong, anteriorly pectinate with many branches and dichotomously rami ed at distal part; R concave; RA and RP are divided near the distal half of the wing; RA extends one branch weakly at distal part; separating M and CuA preserved, nearby M divided into MA and MP; M gently twisted, divided into MA1 and MA2 at more distal than dividing RA and RP; free part of basal M short; MP concave, dichotomously rami ed between dividing R and dividing MA; area between MA and MP wide with dense and sturdy cross-veins; CuA seperated from M, fused with CuPaα°, free part of basal CuA short; CuA + CuPaα° long, gently concave; branch of CuPaα° + CuPaα* long, posteriorly pectinated with CuPaα* and CuPaα°; CuPaβ seperated from CuPaα• near the widest area between MA and MP; fused CuPaα• + CuPaβ + CuPb long; CuPb separated from CuPaα• + CuPaβ, divided into CuPbα and CuPbβ, basal of CuPb short; fused CuPaα•+CuPaβ longer than separated free CuPaβ; preserved AA1 and AA2 long without any branch; cross-veins dense, long, and simple, but organized networks present near the margin.

Titanopteran and the Late Triassic paleoecology of the Nampo Group
Compared to the large number of plant fossils (more than twenty species reported), only a small number of invertebrate fossils have been documented from the Nampo Group: i.e., a single species of bivalve 46 , four species of conchostracans 25 , and two species of insects 47,48 . The occurrence of the titanopteran fossil from the Amisan Formation, therefore, adds a new component to the Late Triassic ecology of the Nampo Group. Given its large size, Magnatitan must have played an important role in the food web as daunting predators on insects, invertebrates, and possibly small tetraopds, such as amphibians (Fig. 3).

Familial assignment and presence of divided CuPb
The two most important synapomorphies of titanopterans are CuPaα•+CuPaβ and CuPb having the same point of origin 8  It was argued that the divided CuPb is present in some members of Archaeorthoptera, Panorthoptera and Titanoptera 9 , considering this feature in those groups as results of convergent evolution, questioning the close relationship between the Titanoptera and the orthpoteran Family Tcholmanvissidae. In contrast, a subsequent research reinterpreted the wing vein homology of those groups and proposed the divided CuPb was not present in titanopterans, reinstating the close relationship of the Titanoptera and the Tcholmanvissidae of the Archaeorthoptera 10 . One of the signi cant features of Magnatitan jongheoni gen. et sp. nov. is the presence of divided CuPb, while su cing the criteria for titanopterans. The new fossil in this study, therefore, will provide a new data for future discussion on the homology of orthopteran wings.

Mineralogy of the fossil
In recent paleontological researches using WDS analyses on fossiliferous shale, various body structures, such as brains in Cambrian stem-arthropod fossils 57 , and wing venations and a subgenetal plate in Cretaceous elcanid orthopteran fossils 58 , were highlighted in the carbon elemental maps by different carbon concentrations, indicating that the fossils were preserved as thin carbonaceous lm on dark shale. The mineralogy of the new titanopteran fossil from the Amisan Formation in this study, however, seems different from those in the previous studies. Carbon distribution on the fossil surface is scarce (Fig. 4C), and there is less silicon (Fig. 4J), compared to the matrix. Instead, enriched on the fossil surface are aluminum and potassium ( Fig. 4K and L).
Along the thick wing veins, dendrite-like overgrowth textures are present (Fig. 4A). Qualitative point analysis by EDS equipped at the low vacuum eld emission scanning electron microscope, however, has shown that the total EDS spectra of the wing membrane area, the wing veins, and the dendrite-like textures are comparable to each other. Similar results have been acquired from a XRD microstructural analysis applied to the wing and the matrix rocks. In the elemental maps, a slight amount of magnesium and oxygen is present along the thick veins ( Fig. 4E and I), while traces of titanium are recognized along the dendrite-like overgrowth (Fig. 4F). There are prominent absence of iron, calcium, and potassium along the dendrite-like overgrowth (Fig. 4D, G. and L). At present, however, the mineralogy of the fossil is not clearly identi ed, requiring further research.

First documentation of titanopteran in East Asia
Previously, the occurrences of titanopterans have been restricted to Central Asia including the Triassic Madygen Formation of Kyrgyzstan, the Permian of Tyulgan District of Russia, the Triassic near the Lake Wivenhoe, Queensland, Austrailia, and the Middle Triassic of New South Wales, Australia. A recent report documented a fragmentary forewing of titanopteran from the Carboniferous of France 3 , but further investigations would merit, given the much earlier age and the incomplete preservation. In the Triassic paleogeography, Central Asia was located to the north of the Tethys Ocean, while Australia was located to the south of the Ocean, and the two areas were connected by the regions spreading along the western margin of the Tethys Ocean, such as Europe, northern Africa, Arabia, and India 59 . This implies that titanopterans may have also inhabited in those regions along the western margin of the Tethys Ocean (Fig. 5). By the Late Triassic, the East Asian plates, including the Sino-Korean, South China, and Indochina cratons were accreted to form a large continental crust, located to the east of the Tethys Ocean 59 (Fig. 5). Accordingly, the new discovery of titanopteran fossil from Korea may suggest a circum-Tethys Ocean distribution of titanopterans, at least in the Late Triassic. The possible widespread distribution of titanopterans implies a critical role that this giant insect predators may have played in the Triassic terrestrial ecology. Future investigations on the Triassic nonmarine deposits in the circum-Tethys Ocean regions may lead to new discoveries of titanopterans.

Triassic age of the Nampo Group and development of the Chungnam basin
Titanopterans are known as representative predatorial insects of the Triassic 2 , although an occurrence from the Upper Permian has been documented 56 . The occurrence of a titanopteran in this study, therefore, evinces the Triassic age for the Amisan Formation. Previously, the Family Paratitanidae to which the new genus Magnatitan belongs, composed of two genera, the Lower to Middle Triassic Paratitan 1 and the Middle to Upper Triassic Minititan 56 . The worldwide "coal accumulation gap" interval is known from the Lower Triassic to the Middle Triassic 60-62 , whose cause is still debated (see ref [62]). The frequent occurrences of coal-bearing layers in the Amisan Formation and the Baegunsa Formation, therefore, constrains the age of these formations as post-Middle Triassic. This result con rms the Late Triassic age for the Amisan Formation proposed from the previous paleontological researches 25,45 . In this regard, the occurrences of the plant fossil genera Lobatannularia and Sphenophyllum from the Baegunsa Formation, which overlies the Amisan and the Jogyeri formations, may suggest the Upper Triassic spanning up to the Baegunsa Formation. It was even regarded the two genera as relic components from the Late Paleozoic Cathaysian ora of East Asia 24,25 . If so, the uppermost unit of the Nampo Group, the Seongjuri Formation remains to be the only unit with a possibility of stretching up into the Lower Jurassic. The youngest detrital zircon data from the Seongjuri Formation shows a range from 197 to 175 Ma 26  At present, it is inferred that the Chungnam Basin was formed under an extensional regime during the Late Triassic. Being closely situated to the Okchen Belt, the Chungnam Basin was subsequently affected by the Daebo orogeny, resulting in the NE-SW trending thrusts in the area (Fig. 1), which involved shortening of both the sedimentary rocks and the basement 28

. The distribution of the Oseosan Volcanic
Complex is elongated in NE-SW trend, and is bordered by a NE-SW trending thrust (the Jangsan Fault) to the southeast (Fig. 1). Because NE-SW trending folds and thrusts are typical features formed by the Jurassic Daebo Orogeny in the Okcheon Belt, we interpret that the Oseosan Volcanic Complex was deposited in a piggyback basin or a small-scale foreland basin formed under the in uence of the Daebo orogeny. The age of the Oseosan Volcanic Complex (178 to 172 Ma) 27 also corresponds well with the time range of the Daebo orogeny. The small-scale NE-SW trending imbricated sheets within the Oseosan Volcanic Complex (see ref [28]) may have also formed during the subsequent phase of the orogeny. Although the deposits of the basin are now distributed in three currently-isolated areas (the Ocheon, Oseosan, and Seongju areas) (Fig. 1), given the same stratigraphic sequences occurring in all areas, the Chungnam Basin must have developed as a single basin.

Tectonic a nity of the southwestern Korean Peninsula
In  Fig. 6), presenting similar depositional environments to the Nampo Group. Given the northeast-trending border of the northeastern Cathaysia Block (Fig. 6), this similarity may suggest a close tectonic a nity between the northeastern Cathaysia Block and the southwestern Korean Peninsula.
The link between the northeastern Cathaysia Block and the southwest Korean Peninsula can be corroborated by the recent petrological discoveries, which show remarkable similarities between the two regions. For the last couple of decades, various geological features, mostly unique in the Korean Peninsula, have been documented in the Wolhyeonri Complex, which unconformably underlies the Nampo Group in the Ocheon and Oseosan areas of (Fig. 1). The Wolhyeonri Complex includes serpentinized ultra ma c rocks, migmatitic gneiss, and amphibolites with garnet granulite relics, retrograded from an eclogite 70 . The ma c intrusive body with eclogite relics and marbles occur as a lensshaped exposure within granitic gneiss 17 . The eclogites in this area show a metamorphic age of ~ 230 Ma (Triassic), with ~ 880 Ma (Neoproterozoic) of the protolith age 17,70 , and have been treated as evidence of an extension of the Permo-collisional belt into this area 17,18 . More recently, the early-middle Paleozoic orogenic signals were reported from the Wolhyeonri Complex [71][72][73] , which was interpreted as evidence of a continental arc-magmatism 73 , or of subduction and the following collisional process of a microcontinent 71 .
The Precambrian rocks and their subsequent alteration within the Cathaysia Block are well represented in the Badu Complex and the Chencai Complex 74,75 , located in the northeastern Cathaysia Block (Fig. 6). In these areas, garnet-bearing amphibolites frequently occur with ma c rocks [74][75][76] . Recently, retrograded eclogite and ma c granulite from the Badu Complex was documented 74 , whose metamorphic age is 234 Ma (Triassic), with 997 Ma (Neoproterozoic) for the protolith age of the ma c granulite, which are comparable to the metamorphic and the protolith ages of the eclogites in the Wolhyeonri Complex. The Chencai Complex presents various high-grade metamorphic rocks including migmatitic gneiss and marble, and various ma c intrusive rocks, which mainly shows the Neoproterozoic and early to middle Paleozoic orogenic signals [77][78][79][80] . The northeastern Cathaysia Block was further affected by the Triassic orogeny, which may have been attributed to the collisions of South China with North China and Indochina 81 . Notably, the early-middle Paleozoic event in South China is known as the Kwangsian (Wuyi-Yunkai) orogeny, which formed a wide compressive regime in the Cathaysia Block and the southern part of the Yangtze Block 82,83 . This early-middle Paleozoic Kwangsian orogeny is closely comparable to the early-middle Paleozoic orogenic signals from the Wolhyeonri Complex, as recently suggested 84 . Interestingly, the late Triassic Wuzao Formation, a non-marine sequence with minor amount of coal measures, unconformably overlies the Chencai Group in the Chencai Complex 85 (Fig. 6), assuming a similar geological appearance to the Nampo Group, which unconformably overlies the metamorphic and the ma c intrusive rocks in the Wolhyeonri Complex (Fig. 1).
To sum up, the common occurrence of Triassic eclogites with Neoproterozoic protolith age, similar lithologic components, and the similar early-middle Paleozoic orogenic signals, point toward the tectonic linkage between the northeastern Cathaysia Block and the southwestern Korean Peninsula (Fig. 6). Taken together with the common occurrences of the non-marine Late Triassic sequences, we propose that the southwestern Korean Peninsula represents the eastward extension of the Cathaysia Block. This tectonic linkage offers a new insight for interpretation of debated geological features in the Korean Peninsula. The early-middle Paleozoic orogenic signals in the Wolhyeonri Complex can be viewed in connection with the Kwangsian orogeny in the Cathaysia Block, whose nature is still debated 86

Declarations
Competing Interest The authors declare that they have no known competing interests or personal relationships that could have appeared to in uence the work reported in this paper.