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

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.

Comparative analysis of vestibular ecomorphology in birds

“…; Azuma et al. ). Nevertheless, some flying birds have similar vestibular sizes (relative to body mass) to those of flightless birds (Fig.…”

“…; Azuma et al. ). Based on this evidence, the series of acquisition of bird‐like features of the labyrinth is broadly understood (e.g.…”

“…; Yi & Norell, ; Azuma et al. ), or of general patterns of labyrinth evolution and their relationships with ecology across tetrapods as a whole. As well as having significance for functional biology, these relationships have importance for the field of palaeobiology, because they can be used to constrain hypotheses of the ecologies of extinct species (Spoor et al.…”

The distribution of dental features in non-avian theropod dinosaurs: Taxonomic potential, degree of homoplasy, and major evolutionary trends

“…While basalmost coelurosaurs such as Bicentenaria, Zuolong and basal tyrannosauroids have denticulated mesial and distal carinae along the whole dentition, a mesial carina is absent in the mesial and distal dentitions of most compsognathids, and the distal carina is unserrated in the mesial teeth of these taxa. A mesial carina is, nevertheless, present in some mesial and lateral teeth of the basalmost neocoelurosaurs Ornitholestes and Fukuivenator, but mesial denticles are absent and the carina is restricted to the apical portion of the crowns (Azuma et al, 2016; C.H. personal obs.).…”

“…personal obs.). The basal neocoelurosaur Fukuivenator is characterized by a dentition in which all teeth appear to be fully unserrated (Azuma et al, 2016), an apomorphic condition among neocoelurosaurs given the presence of serrations in compsognathids, Ornitholestes, basal alvarezsauroids, therizinosaurs and oviraptorosaurs (see below). The systematic position of Fukuivenator is still unresolved in Neocoelurosauria and this taxon might be a basal paravian closely related to basal dromaeosaurs such as Halsz-karaptor due to the presence of D-shaped mesial teeth and unserrated crowns.…”