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A New Alvarezsaurid Dinosaur from the Nemegt Formation

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A New Alvarezsaurid Dinosaur from the Nemegt Formation www.nature.com/scientificreports OPEN A new alvarezsaurid dinosaur from the Nemegt Formation of Mongolia Sungjin Lee 1, Jin-Young Park1, Yuong-Nam Lee1*, Su-Hwan Kim1, Junchang Lü2,4, Rinchen Barsbold3 & Khishigjav Tsogtbaatar3 Alvarezsaurid diversity has been markedly increased by recent discoveries from China. However, the number of alvarezsaurid specimens in the Nemegt Formation of Mongolia remained low since the initial report on Mononykus olecranus in 1993. Here we report three new alvarezsaurid specimens from this formation, which were associated with each other and also with multiple oviraptorid skeletons in a small multi-species assemblage. Two of the alvarezsaurid specimens represent a new taxon, Nemegtonykus citus gen. et sp. nov., which is mainly distinguished from other alvarezsaurids by the frst sacral vertebra with a subtrapezoidal lamina, the second sacral centrum which is directly co-ossifed with ilium, the posterodorsally oriented postacetabular process of ilium, and partial co-ossifcation between metatarsals II and IV. The other specimen is very similar to M. olecranus in morphology and referred to cf. Mononykus sp. Our phylogenetic analysis recovered Nemegtonykus as a parvicursorine forming a polytomy with several other taxa from the Gobi Desert. The presence of three alvarezsaurid individuals in the same locality indicates that the abundance of alvarezsaurids have been greatly underestimated in the Nemegt dinosaur faunas. Alvarezsaurs are an extraordinary group of animals which are, in general, characterized by greatly shortened forelimbs with an enlarged medial-most manual digit (particularly parvicursorines) and contrastingly elongate distal hind limbs with a specialized arctometatarsal condition in derived forms1–14. Teir records span across four continents, but most taxa are from either Asia1,6,8,9,11,13–22 or South America3,12,23–26, with only fragmentary specimens from North America5,27 and Europe28. Despite their nearly global distribution, alvarezsaur fossils have been extremely rare compared to other contemporary dinosaurs. Te rarity and a lack of complete alvarezsaur specimens have led to many difculties in interpreting of their ecology or phylogenetic relationships. Teir somewhat peculiar morphology, which is strikingly similar to that of birds, has caused extensive debate regarding their place on the tree of life1,2,4–12,16,17,19,29–38. Early studies suggested an ornithomimid afnity for alva- rezsaurs was proposed35,36,39, but they are now generally thought to be basal maniraptorans6–12,40–44. Te origin of alvarezsaurs has also been much discussed. It was initially suggested that alvarezsaurs originated in South America3–5,27, but recent discoveries of basal alvarezsaurs in China support an Asian origin of this clade6,9,11. Te Nemegt Basin in the Gobi Desert, which holds three major Upper Cretaceous sedimentary formations (Nemegt, Baruungoyot, and Djadochta), is home to many alvarezsaurids, specifcally parvicursorines. However, there has been only one alvarezsaurid taxon, Mononykus olecranus, known from the Nemegt Formation1. In fact, alvarezsaurid specimens are relatively rare in the Nemegt Formation compared to the older Baruungoyot and Djadochta formations, each of which has yielded two alvarezsaurid taxa4,15–17,19. In 2008, an international team of the Korea-Mongolia International Dinosaur Expedition (KID) found a small assemblage (the surface area of the region containing the assemblage is less than 0.5 m2) of theropods comprising multiple individuals of ovi- raptorids and alvarezsaurids from the Nemegt Formation at Altan Uul III in the Gobi Desert, Mongolia (Figs 1 and S1). Te site is located on a gentle slope, and many of the fossils were buried inside the rock. Te oviraptorid specimens consist of the holotype of Gobiraptor minutus and at least two other larger undescribed individu- als mainly represented by a sacrum and partial hind limb elements45. Similarly, alvarezsaurid specimens in this assemblage include postcranial elements of three individuals which share peculiar hind limb morphology of alva- rezsaurids, such as the elongated distal hind limbs, greatly reduced fbula, and the unique arctometatarsal condi- tion. (Figs 2–7 and S2–4). All three alvarezsaurid specimens are described in this paper, including the holotype and referred specimens of a new taxon Nemegtonykus citus gen.et sp. nov. Te other specimen is distinguished from Nemegtonykus but more similar to M. olecranus, thus referred to cf. Mononykus sp. Multiple alvarezsaurid 1School of Earth and Environmental Sciences, Seoul National University, Seoul, 08826, Republic of Korea. 2Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China. 3Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar, 15160, Mongolia. 4Junchang Lü is deceased. *email: [email protected] SCIENTIFIC REPORTS | (2019) 9:15493 | https://doi.org/10.1038/s41598-019-52021-y 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Map showing the locality (marked with a red star) where the multi-species assemblage was discovered. Figure 2. Skeletal reconstruction of Nemegtonykus citus gen. et sp. nov. (MPC-D 100/203) with missing parts in grey. specimens in this assemblage indicate that alvarezsaurids were more abundant than the previous fossil record suggested during the Nemegt time. Results Systematic palaeontology. Dinosauria Owen, 184246 Teropoda Marsh, 188147 Maniraptora Gauthier, 198648 Alvarezsauria Bonaparte, 199123 Alvarezsauridae Bonaparte, 199123 Parvicursorinae Karhu and Rautian, 199615 Nemegtonykus citus, gen. et sp. nov. Holotype. MPC-D 100/203, partially disarticulated postcranial elements including six dorsal vertebrae, two sacral vertebrae, 21 caudal vertebrae, fve separated dorsal ribs, nearly complete lef scapulocoracoid, nearly com- plete lef ilium, partial right ilium, partial lef pubis, other partial pelvic elements, complete lef femur, com- plete lef tibiotarsus, partial right tibia, nearly complete lef fbula, lef tarsometatarsus consisting of distal tarsal co-ossifed with metatarsals II and IV, and isolated lef pedal phalanges III-1, IV-1, and IV-2 as well as possible II-1 and II-2. Many of the elements such as the sacrum, caudal vertebrae, and lef hind limb were naturally artic- ulated and close to each other. Te other elements were not articulated but very close to the articulated bones. Te exceptions are the dorsal ribs which were in contact with the pelvic elements of MPC-D 102/111 (Gobiraptor minutus holotype). Te ribs are assigned to MPC-D 100/203 rather than MPC-D 102/111 because of their mor- phology and size. Teir tuberculum and capitulum are arranged to ft the diapophysis and parapophysis which, in case of alvarezsaurids, are located on the same horizontal plane. Tey are also too small to be a part of MPC-D 102/111 but perfectly match the dorsal vertebrae of MPC-D 100/203. SCIENTIFIC REPORTS | (2019) 9:15493 | https://doi.org/10.1038/s41598-019-52021-y 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 3. Axial skeleton and pelvis of the holotype specimen of Nemegtonykus citus gen. et sp. nov. (MPC-D 100/203). (a,b) Photograph (a) and interpretative illustration (b) of the dorsal-sacral succession and pelvis in lef lateral view. (c,d) Photograph (c) and interpretative illustration (d) of the dorsal-sacral succession and pelvis in dorsal view. (e) Caudal vertebrae in dorsal view. Abbreviations: ant, antitrochanter; brs, brevis shelf; dv, dorsal vertebra(e); p, pubis; poa, postacetabular process; pra, preacetabular process; pup, pubic peduncle; sac, supracetabular crest; sl, sacral lamina; sv, sacral vertebra(e); tp, transverse process(es). Scale bars equal 1 cm. Referred specimen. MPC-D 100/207, possible pelvic element, nearly complete right femur, partial right tibia which is articulated with the fbula, partial lef astragalus with tarsometatarsus, distal ends of right metatar- sals II, III, and IV, and possible right pedal phalanges II-1 and IV-1. Te elements were disarticulated but located close to each other with matching sizes. Tey also exhibit the same light-coloured texture that makes them dis- tinguishable from other specimens. Locality and horizon. Altan Uul III49–53, Ömnögovi Province, Mongolia (Figs 1 and S1). Upper Cretaceous Nemegt Formation49–51,53–55. Etymology. Te generic name refers to the Nemegt Formation (the origin of the holotype specimen) and onyx (‘claw’ in Greek); specifc name means ‘swif’ in Latin, referring to the hypothesized cursorial lifestyle of this taxon. Diagnosis. An alvarezsaurid of moderate size distinguished from other alvarezsaurids by the following unique set of characters (autapomorphies are marked with an asterisk, see Supplementary Information for difer- ential diagnosis): completely co-ossifed frst and second sacral vertebrae; anteroposteriorly elongate and subtrap- ezoidal lamina formed by transverse process-sacral rib complex and postzygapophyses on the frst sacral vertebra, which is co-ossifed with preacetabular part of ilium* (similar structure present in Xixianykus); co-ossifcation between the second sacral centrum and preacetabular part of ilium without any contribution of sacral ribs*; partially co-ossifed scapulocoracoid*; greatly reduced pubic peduncle (similar to Qiupanykus); posterodorsally oriented postacetabular process of ilium*; distinct fossa on dorsal surface of ilium near antitrochanter (also pres- ent in Xixianykus); prominent wedge-shaped tubercle on the posterolateral
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