A review of theropod dinosaurs from the Late Jurassic to mid-Cretaceous of Southeast Asia

Abstract

Several non-avian theropod dinosaurs, as well as some Mesozoic birds, have been reported from Southeast Asia. The fossils are dominantly found in northeastern Thailand, however, one bizarre theropod has been described from Laos, one theropod has been reported from Malaysia, and some avian and non-avian theropods have been recently reported from Myanmar. The temporal distribution of Southeast Asian theropods ranges from the Late Jurassic to the mid-Cretaceous. All non-avian theropod faunas from Southeast Asia consist of non-maniraptoran tetanurans. They show similarity to Chinese plus Japanese theropods during the Early Cretaceous in broad systematic terms. During this time, megaraptorans can be found only in Japan, Australia, Brazil, and possibly Thailand, whereas tyrannosauroids can be found in China, Europe, possibly Brazil and Australia. Spinosaurids, carcharodontosaurians, and some coelurosaurs such as ornithomimosaurs were almost cosmopolitan. Metriacanthosaurids, on the other hand, were endemic to Europe and Asia including China and Thailand during the Middle to Late Jurassic.

Résumé

Plusieurs dinosaures théropodes non aviens ainsi que des oiseaux mésozoïques ont été signalés en Asie du Sud-Est. On les trouve principalement dans le nord-est de la Thaïlande, cependant un théropode étrange a été décrit au Laos, un autre en Malaisie et des théropodes aviens et non aviens ont récemment été signalés au Myanmar. La distribution temporelle des théropodes d’Asie du Sud-Est va du Jurassique supérieur au Crétacé moyen. Toutes les faunes de théropodes non aviens d’Asie du Sud-Est sont constituées de tétanuriens non maniraptoriens. Ils présentent de manière générale des similitudes avec les théropodes chinois et japonais du début du Crétacé. Durant cette période les mégaraptoriens ne sont présents qu’au Japon, en Australie, au Brésil voire peut-être en Thaïlande, tandis que les tyrannosauroïdes se trouvent en Chine, en Europe, et probablement au Brésil et en Australie. Les spinosauridés, les carcharodontosauriens et certains coelurosaures tels que les ornithomimosaures sont eux presque cosmopolites. Les métriacanthosauridés sont par contre connus du Jurassique moyen au Jurassique supérieur, endémiques de l’Europe et de l’Asie, et plus spécifiquement de la Chine et de la Thaïlande.

Introduction

The most abundant and diverse non-avian theropod fossils found in Southeast Asia are from Thailand. Most of them are from the Sao Khua Formation, but also can be found in the Khok Kruat and Phu Kradung formations. Some of these finds were studied almost twenty years ago, whereas others have been discovered recently and need careful study. In this paper, an overview of theropod dinosaurs during the Mesozoic in Southeast Asia is provided, with a specific emphasis on new specimens and information on theropods found in Thailand.

The discovery of non-avian theropod dinosaurs from Thailand started in the late 1980s. Until now, eleven non-avian theropods and a Mesozoic bird have been reported. The theropods pertain to various groups and date from the Late Jurassic to the Early Cretaceous of northeastern Thailand (ca. 160–113 million years ago). There are two metriacanthosaurids (= sinraptorids) from the Phu Kradung Formation, which may be Late Jurassic or Early Cretaceous in age (prior to 125 million years ago). The finds consist of a lower leg (left tibia) of a metriacanthosaurid and cranial elements of another metriacanthosaurid (Buffetaut and Suteethorn, 2007, Chanthasit and Suteethorn, 2013). Seven theropods plus a bird from the Early Cretaceous Sao Khua Formation (ca. 130–125 million years ago) include a compsognathid (Buffetaut and Ingavat, 1984); several teeth of a spinosaurid (Siamosaurus suteethorni, Buffetaut and Ingavat, 1986); the postcranial skeleton of an ostrich-mimic dinosaur (Kinnareemimus khonkaenensis, Buffetaut et al., 2009); a maxilla of a carcharodontosaurid (Buffetaut and Suteethorn, 2012); the putative basal tyrannosauroid Siamotyrannus isanensis (Buffetaut et al., 1996), for which new studies suggest that it may not be a tyrannosauroid but an allosauroid (e.g. Rauhut, 2003a, Carrano et al., 2012) or that it could be a basal coelurosaur (Samathi, 2013, Samathi and Chanthasit, 2017); a Mesozoic bird (Buffetaut et al., 2005a); and two newly reported mid-sized, probable megaraptoran coelurosaurs, which look similar to Fukuiraptor from Japan (Samathi et al., 2019). Two theropods from the Early Cretaceous Khok Kruat Formation (ca. 125–113 million years ago) include an undescribed partial postcranial skeleton of a spinosaurid (Buffetaut et al., 2005b, Milner et al., 2007) and an undescribed partial skull and postcranial skeleton of a carcharodontosaurian (Azuma et al., 2011).

Other theropods from Southeast Asia have been recovered from Laos, Malaysia, and Myanmar. These include one spinosaurid from Laos named Ichthyovenator laosensis from the “Grès supérieurs” Formation reported in 2012 (Allain et al., 2012, Allain, 2014) and two spinosaurid teeth from the late Early Cretaceous of the Malay Peninsula (Sone et al., 2015). One coelurosaur and several enantiornithine birds preserved in amber were also recently reported from the mid-Cretaceous of Myanmar (Xing et al., 2016a, Xing et al., 2016b, Xing et al., 2017, Xing et al., 2018, Xing et al., 2019). Theropods from the Xinlong Formation of southern China, which belongs to the same paleobiogeographic province as Southeast Asia, includes the carcharodontosaur Datanglong guangxiensis (Mo et al., 2014b) and several teeth of carcharodontosaurids and spinosaurids (Buffetaut et al., 2008, Mo et al., 2014a, Mo et al., 2016).

Here, we review theropod dinosaurs found on the Southeast Asian mainland, as well as southern China, focusing mainly on Thailand from where most of the theropod faunas have been reported. We discuss their affinities based on recent information and recent finds, as well as comment on their evolution and paleobiogeography. Our review also focuses on the geologic age of, in particular, the finds from northeastern Thailand (Khorat Plateau). There, a circa 3–4 km thick sequence of continental redbeds (Morley et al., 2011), the Khorat Group, that has been divided into several formations, yields the dinosaur fossils, including those of theropods. However, as in other parts of the world, the continental redbed sequences of the Khorat Plateau have been notoriously difficult to date, leading to sometimes strongly differing age assignments of the fossils in the older and more recent literature.