First Discovery Of Theropod Fossils From The Late Cretaceous Nenjiang Formation In The Songliao Basin

The Songliao Basin is one of the world’s largest Cretaceous terrestrial sedimentary basins.The well-preserved lacustrine deposits in the basin offer a precious chance to recover a nearly complete sequence of Cretaceous continental sedimentary records.However,theropod remains from the Songliao Basin are scarce,which makes us unclear about scientific questions such as the composition of the terrestrial vertebrates and their co-evolution with the environment in the Songliao Basin.The theropod fossils discovered recently from the Nenjiang Formation in the Songliao Basin are comprehensively evaluated and explored in this research.This study is of great significance for understanding the diversity and paleogeographic distribution of theropods in the Late Cretaceous of the Songliao Basin.Theropod teeth are explored utilizing detailed descriptions and qualitative comparisons.There are at least three different types of theropods in the fauna,respectively Troodontidae,Dromaeosauridae and Tyrannosauroidea.The troodontid teeth are extremely small,markedly labiolingually compressed,and exhibit large hook-like distal denticles.One well-preserved tooth of troodontid presents a distinct constriction between the crown and root.The dromaeosaurid teeth are small to medium,markedly labiolingually compressed,strongly curved distally,and exhibit chisel-like denticles.One tooth of dromaeosaurid remains can be further attributed to Dromaeosaurinae by bearing relatively large denticles that are subequal in size on both carinae.The tyrannosauroid teeth are large,robust,and with large chisel-like denticles on mesial carinae.The tibia,fibula and caudal vertebrate are poorly preserved.It is difficult to identify these theropod skeletons.A bioerosion trace can be observed in the tibia.Details of histological structures are described based on a thin section of the tibia.The bone wall thickness is low,the medullary cavity is large and the degree of vascularization is high.One line of arrested growth is visible in the cortex.There is no external fundamental system,and no inner circumferential lamellae.The histological section reveals that the individual was very young and was growing rapidly at the time of death.Compared with the theropods in the dinosaur fauna of similar ages in the adjacent area,the type of theropod assemblage of the Songliao Basin is quite similar to those of the Heilongjiang River Basin,the Gobi Desert,and the Jiaolai Basin,indicating that there could have been a faunal exchange between these localities in Late Cretaceous.It is the first discovery of theropod fossils from the Nenjiang Formation,the first discovery of the Troodontidae from the Songliao Basin,also the first discovery of the Dromaeosaurinae from China.This study extends the known geographic range of these clades of theropods and expands knowledge of the terrestrial ecosystem during the Late Cretaceous of the Songliao Basin.The materials also provide important clues for the discovery of more theropod fossils in this area.