A New Troodontid Theropod from the Late Cretaceous of Central China, and the Radiation of Asian Troodontids

Abstract

A new troodontid dinosaur, Xixiasaurus henanensis gen. et sp. nov., from the Upper Cretaceous Majiacun Formation of the Xixia Basin, Henan Province, is erected, based on a partial skull. It is characterized by bearing 22 maxillary teeth, a distinct opening on the lateral surface of the base of nasal process of the premaxilla, the rostral end of the upper jaw forming a tapered U−shape, and the mandibular symphyseal region slightly inflected medially. Xixiasaurus is most closely related to the Mongolian Byronosaurus among troodontids. Byronosaurus, Urbacodon, and Xixiasaurus may form a new clade, suggesting an endemic radiation of troodontids across Asia, including multiple taxa without dental serrations. The discovery of Xixiasaurus in the Xixia Basin may imply that the Xixiasaurus−bearing Majiacun Formation is Campanian in age.

Osteology of a New Late Cretaceous Troodontid Specimen from Ukhaa Tolgod, Ömnögovi Aimag, Mongolia

“…This is different from the condition in Anchiornis and Archaeopteryx in which the nasal process is straight (Pei et al, 2017). As in all described troodontids and some basal paravians, the internarial bar is dorsoventrally flattened as can be observed on the right premaxilla (Makovicky et al, 2003;Norell et al, 2009;Lü et al, 2010;Pei et al, 2017).…”

“…2, 3), as in the derived troodontids Byronosaurus, Saurornithoides, and Zanabazar (Makovicky et al, 2003;Norell et al, 2009), but different from basal troodontid taxa like Jinfengopteryx, Sinovenator, and Mei Xu and Norell, 2004;Ji et al, 2005) in which the posterior margin of the naris is above the antorbital fossa. A well-defined pro-maxillary fenestra is not visible laterally at the anterior end of the antorbital fossa, unlike Jin fengopteryx, Mei, Sinovenator, Sinornithoides, and Geminiraptor (Russell and Dong, 1993;Xu et al, 2002;Xu and Norell, 2004;Ji et al, 2005;Senter et al, 2010), but is the same as the condition in derived troodontids Byronosaurus, Xixiasaurus, Saurornithoides, and Zanabazar (Makovicky et al, 2003;Norell et al, 2009;Lü et al, 2010).…”

“…However, it lacks many primitive features that are present in the Jehol troodontids, such as an external naris that posteriorly overlaps the anterior margin of the antorbital fossa, a deep anterior ramus of the maxilla, a reduced lateral lamina of the maxilla, the absence of a subotic recess, and metatarsal II almost as transversely wide as metatarsal IV. These characters are plesiomorphic for all paravians, but are not reported in Late Cretaceous troodontids such as Almas ukhaa, Byronosaurus, Gobivenator, Saurornithoides, Zanabazar, and Troodon (Makovicky et al, 2003;Norell et al, 2009;Lü et al, 2010;Tsuihiji et al, 2014). A subarctometatarsalian pes is reported in the basal troodontid Sinovenator as an intermediate state between the normal pes of other paravians and the typical arctometatarsalian pes of derived troodontids .…”

“…The interfenestral bar is inset from the lateral surface of the maxilla, as in Sinovenator, Xixiasaurus, Saurornithoides, and Gobivenator Lü et al, 2010;Tsuihiji et al, 2014), but differs from that of Byronosaurus (Makovicky et al, 2003) in which it is confluent with the side wall of the maxilla. An interfenestral canal connects the maxillary and antorbital fenestrae medial to the interfenestral bar as in IGM 100/972, Byronosaurus and Zanabazar (Makovicky et al, 2003;Norell, 2009, Norell et al, 2009).…”

New Specimens ofAnchiornis huxleyi(Theropoda: Paraves) from the Late Jurassic of Northeastern China

“…The posterior end of the posterior ramus tapers in PKUP V1068, as in Archaeopteryx and dromaeosaurids (Mayr et al, 2007;Turner et al, 2012). This is different from troodontids, where the posterior ramus has nearly parallel dorsal and ventral edges (e.g., Xu et al, 2002;Makovicky et al, 2003;Norell et al, 2009;Lü et al, 2010).…”