A new non-averostran neotheropod specimen from the Early Jurassic of Southwestern China and its implications for early theropod evolution

Abstract Averostra (= Ceratosauria + Tetanurae) is a crucial clade that includes nearly all theropods from the Toarcian (Early Jurassic) to the Late Cretaceous and ultimately gave rise to birds. Although recent discoveries have improved our understanding of the transition from non-averostran neotheropods to averostrans, many key fossils remain fragmentary, limiting further insight into this evolutionary sequence. Here, we describe a new neotheropod specimen from the Lower Jurassic Lufeng Formation of Yunnan, China. The specimen includes a postcranial skeleton, lacking cervical and dorsal vertebrae, with osteohistological data suggesting that it represents an immature individual. It is distinguished from other neotheropods, particularly referred specimens of Sinosaurus (e.g. KMV8701 and LDM-L10), by a unique combination of characters. However, owing to the absence of overlapping material with the sub-coeval Shuangbaisaurus anlongbaoensis, we refrain from naming a new species. Multiple phylogenetic analyses consistently recover the specimen as the sister taxon to Averostra. It retains many plesiomorphic non-averostran traits but also exhibits features approaching those of averostrans, illustrating the gradual morphological transition towards Averostra. Nonetheless, the presence of several tetanuran-like characters further underscores the complexity of early theropod evolution and suggests that some features traditionally regarded as diagnostic of Tetanurae originated before the emergence of Averostra.