Yun, Mongolian Geoscientist 50 (2020) 2-10 https://doi.org/10.5564/mgs.v50i0.1325 Mongolian Geoscientist Original article A Carcharodontosaurid tooth from the Hasandong Formation (Lower Cretaceous) of South Korea Chan-gyu Yun1,2* 1Vertebrate Paleontological Institute of Incheon, Incheon 21974, Republic of Korea 2Biological Sciences, Inha University, Incheon 22212, Republic of Korea *Corresponding author: [email protected] ARTICLE INFO ABSTRACT Article history: A large tooth of theropod dinosaur that was recovered from the Hasandong Received 04 April, 2020 Formation (Lower Cretaceous; Aptian-Albian) in Daedo island, Hadong Couty, South Gyeongsang Province of South Korea is redescribed. Although the tooth was Accepted 12 May, 2020 misidentified as a "Prodeinodon"-like megalosaurid theropod at the first time, detailed comparisons with known theropod dentition anatomy strongly indicate that this tooth belongs to an Acrocanthosaurus-like basal carcharodontosaurid theropod. This referral is supported by its combination of large size, ovoid-shaped cervix outline, mesial carina that does not reach the cervix, labially displaced distal carina and large number of denticles. This tooth is different from other carcharodontosaurid teeth from the same formation in several anatomical aspects (e.g., smaller overall size, presence of transverse lines adjacent to the distal carina, presence of interdenticular sulci in distal carina, denticle densities, crown basal ratio), indicating that carcharodontosaurid diversity in the Early Cretaceous of Korea could have been higher, although these differences may represent positional or individual variations. The presence of Acrocanthosaurus-like theropod teeth (e.g., "Prodeinodon", "Wakinosaurus") from early Cretaceous deposits (Valanginian-Cenomanian) of South Korea, Japan, Mongolia and China indicates that North American Acrocanthosaurus atokensis possibly represents a form that immigrated from the Asia. Keywords: Theropoda, Carcharodontosauridae, Daedo Island, Hadong County INTRODUCTION et al., 2019). The abundance of theropod teeth in Isolated theropod teeth comprise an important the fossil record is mainly due to the facts that role in understanding paleoenvironment, theropod dinosaurs were polyphyodont animals, paleoecology, paleodiversity and anatomical and their teeth covered with enamel represent information on various theropod clades in one of the hardest skeletal elements in the whole Mesozoic terrestrial ecosystems as they skeleton (e.g., Hendrickx et al., 2019). Thus, represent one of the most common fossils in acceptable taxonomic identities of isolated terrestrial Mesozoic formations (e.g., Hendrickx theropod teeth could be very important in © The Author(s). 2020 Open access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and source, provide a link to the Creative Commons license, and indicate if changes were made. 2 Yun, Mongolian Geoscientist 50 (2020) 2-10 dinosaur biogeography and evolution, especially considered as megalosaurids. Later, Yun et al. for the sediments that lack articulated skeletons (2007) redescribed KS 7001 briefly, and they or isolated, well preserved bony elements. followed the classification made by Lim et al. Despite the abundance of ichnofossils such as (2002) that KS 7001 is a megalosaurid. eggs or footprints, most dinosaur body fossils However, it is now known that laterally that have been reported from the Korean compressed crown morphology is not unique for Peninsula represent isolated, incomplete or the megalosaurid teeth (Hendrickx et al., fragmentary materials that are not yet widely 2015a), and the identifications of "Prodeinodon" reported to the international paleontological teeth as megalosaurid theropods are highly community (Lee, 2003; Choi and Lee, 2017). doubtful (e.g., Averianov and Skutschas, 2009). Among these fossils, isolated theropod teeth are Based on these factors, some authors have currently only reported from the Hasandong referred KS 7001 as an Indeterminate Formation (Aptian-Albian) that is distributed in Theropoda with unavailable further taxonomic the southeastern part of the Korean Peninsula resolution (Carrano et al., 2012; Choi and Lee, (Choi and Lee, 2017), and previous taxonomic 2017). However, it has been recently shown that identifications on most of these fossils were although most theropod teeth may not be made through preliminary descriptions at the diagnositic to the species level, they possess times when phylogenetically informative several important anatomical characters that can variations in theropod teeth were poorly be used for assigning them to at least several described. Nevertheless, they represent certain taxonomic levels (e.g., family-level important record for understanding theropod clade; Hendrickx et al., 2019). Here, the author diversity and evolution during the early redescribes KS 7001 to correctly identify its Cretaceous of the Northeastern Asia. taxonomic identity, and discusses about its In 2002, Lim et al. (2002) reported a large implications on dinosaur biogeography and theropod tooth (KS 7001) from the Hasandong evolution. Formation that is exposed in the Daedo island, Hadong County of South Gyeongsang Province, Institutional Abbreviations – AMNH, and they assigned the tooth as Megalosauridae American Museum of Natural History; IVPP, (Fig. 1). The key features that Lim et al. (2002) Institute of Vertebrate Paleontology and identified the tooth as a megalosaurid is the Paleoanthropology; KIGAM VP, Korea Institute strongly laterally compressed morphology of the of Geoscience and Mineral Resources; KS, crown, and its similarities with "Prodeinodon" Kyeongnam Science High School Museum. teeth from China and Mongolia that had been Anatomical Abbreviations – AL, apical length; Fig. 1. Remaining photographs of KS 7001, modified from Yun et al. (2007). Large theropod tooth in lingual (A), labial (B), mesial (C), distal (D), and distal (E), and basal (E) views. Scale bar = 20 mm. 3 Yun, Mongolian Geoscientist 50 (2020) 2-10 CA, crown andgle; CBL, crown base length; other known Northeastern Asian theropod teeth CBR, crown base ratio; CBW, crown base were made through an extensive review on the width; CH, crown height; CHR, crown height literature. ratio; CWR, crown width ratio; DA, distoapical denticle density; DAVG, average distal denticle SYSTEMATIC PALEONTOLOGY density; DC, distocentral denticle density; DSDI, ; MA, mesioapical denticle density; Allosauroidea Marsh, 1878 MAVG, average mesial denticle density; MB, Carcharodontosauridae Stromer, 1931 mesiobasal denticle density; MC, mesiocentral Carcharodontosauridae indet. denticle density; MCL, mid-crown length; MCW, mid-crown width; MDDD, minimum TAXONOMIC REFERRAL distal denticle density; MMDD, minimum KS 7001 is identified as a mesial tooth of basal mesial denticle density. carcharodontosaurid theropod, based on the combination of large size, large number of GEOLOGICAL BACKGROUND denticles, ovoid outline of base, nearly straight The Hasandong Formation is the middle unit of distal margin, mesial carina that does not reaches the Sindong Group which is approximately the cervix, and distal carina that is strongly 1200 thick on average, and it is the lowermost displaced labially (Hendrickx et al., 2015a, group of the Gyeongsang Supergroup (Lee, 2019; Soto et al., 2020). 2008). The formation is composed of alluvial KS 7001 is a large theropod tooth crown, as its and fluvial sediments, and shows alternating preserved height is over 60 mm. Although size channel and interchannel sediments (Lee, 2008). must be treated with caution when identifying The rocks comprising Hasandong Formation theropod tooth as this feature varies include reddish and grey sandstone, reddish to allometrically (Hendrickx et al., 2019), this greenish grey sandy mudstone, and dark grey feature has been demonstrated to be useful to shale (Lee, 2008). SHRIMP and LA-MC- differentiate the teeth of various theropod taxa ICPMS U-Pb analyses provide an age between (Hendrickx and Mateus, 2014; Hendrickx et al., 118.0 ± 2.6 Ma (Upper Aptian) for the 2019). According to Hendrickx et al. (2019), Hasandong Formation (Lee et al., 2018), and tooth crowns of more than 60 mm occur only in molluscan faunas indicate the age about Aptian Allosauroidea (Allosaurus, Sinraptor, to Albian (Yang, 1982). carcharodontosaurids including Numerous vertebrate fossils have been reported Acrocanthosaurus, Carcharodontosaurus, from the formation, including fish, turtles, Mapusaurus, Giganotosaurus), Ceratosaurus, pterosaurs, and dinosaurs (Choi and Lee, 2017). large megalosaurids (Afrovenator, Megalosaurus, Torvosaurus), spinosaurids, and MATERIALS AND METHODS derived tyrannosauroids. KS 7001 can be easily Unfortunately, the original fossil of KS 7001 is differentiated from spinosaurids by its lack of currently lost (C.S. Yun, pers. comm. 2020). coniodont condition, absence of fluted external Therefore, a remaining high-quality cast made surface, and much larger denticle sizes (e.g., of gypsum is mainly studied (Fig. 2). The Hendrickx et al., 2019; Yun, 2019). KS 7001 original descriptions made by Lim et al. (2002) differs from Ceratosaurus by its much elongate and Yun et al. (2007) are partly referenced for morphology (higher CHR) and lack of flutes in the parts (e.g.,