A bizarre theropod from the Early Cretaceous of Japan highlighting mosaic evolution among coelurosaurians.

Kazunori Miyata,Yoichi Azuma,Masateru Shibata,Takuya Imai,Xing Xu,Soichiro Kawabe

doi:10.1038/srep20478

Abstract

Our understanding of coelurosaurian evolution, particularly of bird origins, has been greatly improved, mainly due to numerous recently discovered fossils worldwide. Nearly all these discoveries are referable to the previously known coelurosaurian subgroups. Here, we report a new theropod, Fukuivenator paradoxus, gen. et sp. nov., based on a nearly complete specimen from the Lower Cretaceous Kitadani Formation of the Tetori Group, Fukui, Japan. While Fukuivenator possesses a large number of morphological features unknown in any other theropod, it has a combination of primitive and derived features seen in different theropod subgroups, notably dromaeosaurid dinosaurs. Computed-tomography data indicate that Fukuivenator possesses inner ears whose morphology is intermediate between those of birds and non-avian dinosaurs. Our phylogenetic analysis recovers Fukuivenator as a basally branching maniraptoran theropod, yet is unable to refer it to any known coelurosaurian subgroups. The discovery of Fukuivenator considerably increases the morphological disparity of coelurosaurian dinosaurs and highlights the high levels of homoplasy in coelurosaurian evolution.

Highlights

There has recently been a great increase in our understanding of coelurosaurian evolution, bird origins in particular, through comparative studies about modern and fossil birds and non-avian dinosaurs[1,2]
The Kitadani Dinosaur Quarry of the Lower Cretaceous Tetori Group in Fukui, Japan (Fig. 1) represents one of the richest Cretaceous terrestrial localities in Japan, having produced numerous specimens of plants, bivalves, gastropods, fish, small mammals, turtles, crocodyliforms, avialan and non-avialan dinosaur eggshells[9], various pterosaur, avialan, and non-avialan dinosaur ichnofossils[10], and several dinosaur taxa including the allosauroid Fukuiraptor kitadaniensis[11,12], titanosauriform Fukuititan nipponensis[13], iguanodontian Fukuisaurus tetoriensis[14], and hadrosauroid Koshisaurus katsuyama[15]. The excavation in this quarry in the summer of 2007 resulted in the discovery of a specimen preserving the majority of a small-sized theropod skeleton, which represents the most complete non-avialan dinosaur specimen recovered in Japan
The quarry is locally referred to the Lower Cretaceous Kitadani Formation (Akaiwa Subgroup, Tetori Group)

Summary

Introduction

There has recently been a great increase in our understanding of coelurosaurian evolution, bird origins in particular, through comparative studies about modern and fossil birds and non-avian dinosaurs[1,2]. The Kitadani Dinosaur Quarry of the Lower Cretaceous Tetori Group in Fukui, Japan (Fig. 1) represents one of the richest Cretaceous terrestrial localities in Japan, having produced numerous specimens of plants, bivalves, gastropods, fish, small mammals, turtles, crocodyliforms, avialan and non-avialan dinosaur eggshells[9], various pterosaur, avialan, and non-avialan dinosaur ichnofossils[10], and several dinosaur taxa including the allosauroid Fukuiraptor kitadaniensis[11,12], titanosauriform Fukuititan nipponensis[13], iguanodontian Fukuisaurus tetoriensis[14], and hadrosauroid Koshisaurus katsuyama[15] The excavation in this quarry in the summer of 2007 resulted in the discovery of a specimen preserving the majority of a small-sized theropod skeleton, which represents the most complete non-avialan dinosaur specimen recovered in Japan. We describe the specimen and discuss its implications for understanding coelurosaurian evolution

Methods

Results

Conclusion