A Short-Armed Troodontid Dinosaur from the Upper Cretaceous of Inner Mongolia and Its Implications for Troodontid Evolution

Xing Xu,Qingwei Tan,Dong Xiao,Corwin Sullivan,Fenglu Han,Andrew Allen Farke

doi:10.1371/journal.pone.0022916

Abstract

BackgroundThe Troodontidae represents one of the most bird-like theropod groups and plays an important role in our understanding of avian origins. Although troodontids have been known for over 150 years, few known derived troodontid specimens preserve significant portions of both the forelimb and the hindlimb.Methodology/Principal FindingsHere, we report a new troodontid taxon, Linhevenator tani gen. et sp. nov., based on a partial, semi-articulated skeleton recovered from the Upper Cretaceous Wulansuhai Formation of Wulatehouqi, Inner Mongolia, China. L. tani has an unusual combination of primitive and derived character states, though our phylogenetic analysis places it in a derived clade within the Troodontidae. As a derived taxon, L. tani has a dromaeosaurid-like pedal digit II, and this species also possesses a humerus that is proportionally much shorter and more robust than those of most other troodontids.Conclusion/SignificanceThe combination of features present in Linhevenator indicates a complex pattern of character evolution within the Troodontidae. In particular, the discovery of Linhevenator suggests that derived troodontids have independently evolved a highly specialized pedal digit II and have significantly shortened the forelimb over the course of their evolution.

Highlights

Troodontids are a group of small theropods characterized by long legs and enlarged braincases [1]
Six troodontid species are known from the Upper Cretaceous of Mongolia and China, including Byronosaurus jaffei [17], Borogovia gracilicrus [18], Saurornithoides mongoliensis [19], Zanabazar junior [20,21], Tochisaurus nemegtensis [22], and Xixiasaurus henanensis [23]
L. tani can be referred to the Troodontidae based on several troodontid synapomorphies: nasal and lacrimal form a lateral shelf on the snout [25,46]; lateral flange overhangs descending process of lacrimal [1,46]; anterior process of lacrimal significantly elongated to form entire dorsal border of antorbital fenestra [1,21]; metatarsus highly asymmetrical, with short and slender metatarsal II and stout metatarsal IV

Summary

Introduction

Troodontids are a group of small theropods characterized by long legs and enlarged braincases [1]. Phylogenetic analyses have varied to some extent in their placement of the Troodontidae, the majority of recent analyses place them alongside dromaeosaurids in Deinonychosauria, which in turn emerges as the sister clade to the Avialae [2,3,4,5,6,7,8]. This places troodontids in a pivotal phylogenetic position with respect to the study of avian origins. Troodontids have been known for over 150 years, few known derived troodontid specimens preserve significant portions of both the forelimb and the hindlimb

Methods

Results

Conclusion