Study Of A New Specimen Of Sinovenator (Theropoda:Troodontidae) From The Early Cretaceous Of Western Liaoning,China

Troodontidae is a group of dinosaurs that have been found as a close relative of Avialae,and its study is a hot-spot in the studies of dinosaurs.As the most basal member of troodontids,Sinovenator plays a key role in the study of the origin and early evolution of this family.Here,this study reports a new specimen of Sinovenator(PMOL-AD00102),discovered from the lowest part of the Yixian Formation at the Lujiatun locality of western Liaoning,China.By using high-resolution computed tomographic(CT)technology and three dimensional visualization to reveal the cranial anatomy of PMOL-AD00102,this study not only enriches our knowledge in the osteology of Sinovenator,but also firstly describes the paleoneurology and the morphological information of inner ear of Sinovenator.First,the autapomorphies of Sinovenator are revised with added four diagnostic features: well-developed medial shelf on the jugal;lateral groove on the pterygoid flange of the ectopterygoid;slender bar in the parasphenoid recess;lateral surface of the anterior cervical vertebrae bearing two pneumatic foramina.Second,this study confirms three important characters of Sinovenator,such as a prominent otosphenoidal crest,an incipient subotic recess and a basisphenoid recess.These findings indicate that the braincase of Sinovenator is not as primitive as previously suggested,although it still shows an intermediate state between derived troodontids and non-troodontid paravians by having an initial stage of the subotic recess and the otosphenoidal crest.Third,some novel and notable morphologies of troodontids are revealed.For example,the quadratojugal wraps the lateral and the posterior surfaces of the quadrate in troodontids for quadrate-quadratojugal articulation;the presence of the epipterygoid in Sinovenator,as firstly reported in Troodontidae,implies that the loss of the epipterygoid is likely a derived character that is present in later diverging taxa of the family and supports the hypothesis that the loss of the epipterygoid is possibly homoplastic in derived troodontids and avialans;by comparison with other dinosaurian taxa,the morphology of the footplate varies in different lineages;the slender atlantal ribs of PMOL-AD00102 support the hypothesis that the atlantal rib has an evolutionary trend to reduce the size along the theropod lineage.Fourth,the study of the paleoneurology of Sinovenator firstly shows the structures of the endocast of the Early Cretaceous troodontids.Apart from well-developed olfactory bulb,the endocast of Sinovenator is similar to that of Archaeopteryx,bearing enlarged cerebrum and cerebellum,developed optic lobe and distally expanded flocculus.Additionally,the well-developed olfactory bulb indicates that Sinovenator has a discriminating sense of smell.Lastly,the reconstructed inner ear of Sinovenator by the CT-scan images firstly shows the structures of troodontid inner ear.And also,based on empirical formula,this study concludes the hearing of Sinovenator is similar to that of Archaeopteryx and better than that of other dinosaurs having a hearing of low frequencies,such as Allosaurus,Psittacosaurus and Brachiosaurus.