Xing Xu

ABSTRACT

Deinonychosaurs are generally regarded to include two groups: dromaeosaurs and troodontids, and their fossil records are mainly restricted to the Paleolaurasian Cretaceous. Dromaeosaurs are a variety of small to medium-sized, agile animal, distinguishing from other coelurosaurians mainly in a number of unique features on feet and tail; troodontids are small-sized animals, characterized with very long and slender hindlimbs, and the largest relative brain size of all dinosaurs. Recent systematic works often support a monophyletic Deinonychosauria; however, some studies disagree, suggesting that either the Dromaeosauridae or the Troodontidae is the sistergroup to the Aves. Nevertheless, the consensus is that the three groups are more closely related to each other than to other coelurosaurian groups.

For the past few years some relatively complete deinonychosaurian specimens were collected from the Lower Cretaceous Yixian Formation of western Liaoning, China, some of which even preserve the soft tissue. The osteology and integument of deinonychosaurs from the Jehol Group of western Liaoning Province (China) are described in detail and the discussions on the coelurosaurian evolution, in particular the origin of birds and related issues are made.

The holotype (IVPP V12811) of Sinornithosaurus millenii was collected from the lower part of the Yixian Formation, including most of the cranial and postcranial elements. The diagnostic features for the species include deep excavation on the posteroventral margin of the premaxilla, a diastema between the premaxillary and maxillary teeth, a a number ofmany pits and ridges on the anterolateral surface of the antorbital fossa, the posterolateral process of the parietal beinglong and sharply posteriorly directed, thecolumnar-like margin of thepterygoid process of the quadrate, the bifurcated posterior margin of the dentary, a distinctive groove posterior to the anterior carina on the medial surface of the premaxillary tooth crowns, absence of ventrolateral processes on intercentrum of the atlas, T-shaped centrum of the first caudal in ventral view, transverse process on anterior caudals with rounded, and strongly anteroposteriorly expanded distal end, presence of two fossae separated by an oblique ridge on the lateral side of the anterior chevrons near the articular end, and pubis strongly curved psoteriorly. 96 osteological characters were analyzed, 21 of which support a close relationship between the Dromaeosauridae and the Aves, 9 of which suggest that dromaeosaurs are more closely related to troodontids, and 25 of which indicate that dromaeosaurs, troodontids, and birds are closely related to each other.

The holotype (IVPP V12330) and referred specimen (IVPP V13475) of Microraptor zhanianus were collected from the Jiufotang Formation of western Liaoning, China. The animal is characterized by distal articulation of penultimate phalanges of manual digits II and III small and skewed ventrally, short penultimate phalanges of manual digits II and III, penultimate phalanx of manual digit III extremely slender, presence of a pendant accessory crest at the base of the lesser trochanter on the femur, proximal pedal phalanges of the digit III and IV much more robust than the corresponding intermediate and penultimate phalanges.

The holotype (IVPP V13474) of Graciliraptor lujiatunensis was collected from the lowest part of Yixian Formation of western Liaoning, China. The diagnostic characters for the new taxon include extremely long and slender middle caudals, extremely slender tibia, proximal tibia shaft rectangular in cross section, astragalar medial condyle much larger than lateral one, metatarsal II much more robust than the other metatarsals and long and slender pedal phalanx III-1. The most conspicuous feature of G. lujiatunensis is that it might represent the most slender skeleton among non-avian theropods found to date.

In general, the Liaoning dromaesaurs are small in size and have a skull with an Archaeopteryx-like profile, a relatively small number of caudal vertebrae, a derived should girdle similar to Archaeopteryx, a proportionately long forelimb, a pelvic girdle similar to basal birds, and a pes showing some arboreal features. The Liaoning dromaeosaurids display a diverse morphology of integumentary structures, including plesiomorphic single and natal-down-like filaments, and derived pennaceous feathers along the hindlimb. These discoveries provide the strongest evidence to date that feathers evolved before the origin of birds and flight. In particular, the morphology and distribution of the feathers on Liaoning dromaesaurids, among other features, strongly suggest that dromaeosaurids might have been able to parachute or even glide, representing a group of transitional animals between the cursorial non-avian theropods and volant birds. 

Sinovenator changae is the most basal troodontid found to date. The holotype (IVPP V12615) and the referred specimen (IVPP V12583) of S. changae were collected from the lowest part of the Yixian Fomation of western Liaoning, China. The diagnostic features include straight and vertical anterior margin of antorbital fenestra, frontal with a vertical lamina anteriorly, surangular T-shaped in cross section, elongate manual phalanx I-1 as long as metacarpal II, and the presence of a prominent lateral cnemial crest that is continuous with the fibular crest. S. changae displays a number of features that are not found in more derived troodontids, but do occur in dromaeosaurs and avialans, such as a laterally directed glenoid fossa and opisthopuby.

On one hand, Sinornithosaurus millenii, Microraptor zhanianus and Graciliraptor lujiatunensis display some troodontid-like features, such as the subarctometatarsalian pes, providing more evidence for the hypothesis that Dromaeosauridae-Troodontidae is a monophyletic clade; on the other hand, they and Sinovenator changae have more bird-like features than the more derived members of each clade, respectively, suggesting a closer relationships to birds.

Based on observations on specimens and information from references, a 260-character list is made. Some of the character states are discussed in detail and characters with continuous state are analyzed using statistic method. A matrix composed of 260 characters and 46 taxa is constructed and analyzed cladistically using the NONA (ver 2.0) software package and formatting and character exploration was performed in WinClada. The strengths and weaknesses of particular relationships have been investigated using a variety of approaches. In order to evaluate the role of certain taxon in reconstructing coelurosaurian phylogeny, the data-matrix has been run with the particular taxon excluded. The significance of certain taxon in understanding the evolutionary process can be evaluated by its effect on the tree topology and the revelant statistics. A significant taxon either has a profound effect on tree topology and/or causes a big change in the revelant statistics that reflect a significant change of character evolution.

For the first time, this study evaluates the affect of combining information from different parts of the body in reconstructing coelurosaurian evolution. By data partitioning, this study evaluates and compares the affect of the cranial and postcranial information in recovering the cladgram and based on the resulted cladgram, the possible evolutionary model is reconstructed. Regarding the interrelationships of the major coelurosaurian groups, the analysis based on the postcranial information produces a result more congruent with the results of the complete analysisthan does the analysis based only on cranial information.

This analysis suggests that the Tyranosauridae lies outside the Coelurosauria, the Ornithomimosauria represents the most basal major coelurosauiran group, and the Maniraptora is mainly composed of two lineages, an Alvarezsauridae-Therizinosauroidea-Oviraptorosauria lineage and a Paraves lineage.  This result is more concordant with the current coelurosaurian fossil record because it suggests the major maniraptoran groups originated at times later than previously thought (earlier than Late Jurassic). The current analysis also supports a monophyletic Deinonychosauria containing the Dromaeosauridae and Troodontidae, but this relationship is weakly supported. 

A discussion of coelurosaurian evolution is presented based on results of this cladistic analysis. The coelurosaurian evolution is far beyond the origin of birds and characterized by complicated process. In particular, the body size plays a key role in the morphological changes: on one hand, it is closely related to the evolution of some key characters including flight; on the other hand, it brings much more homoplasies.

Arboreal features are also observed in some non-avian maniraptorans. A new approach is proposed to infer the arboreality in some non-avian maniraptorans and birds.  The quantitative study suggests there is an evolutionary trend toward arboreality within the Coelurosauria and the basal paraves might be arboreal animals.  Some more derived paraves improved their arboreal adaptations later.  This ecological transition is concordant with the independent phylogenetic analysis and the information from preserved integumentary structures

This study proposes the concept of the relative modification degree, using the relative contribution to the synapomorphies to quantitatively assess the structural modification. The quantitative study shows that at the bases of Coelurosauria, Maniraptora, and Paraves the relative modification degree is greater than at other nodes along the evolutionary history of the coelurosaurian dinosaurs. An examination of character distributions along the maniraptoran lineage reveals that major structural modifications toward avians were acquired in the early stages of maniraptoran evolution.

Key words: western Liaoning; Jehol Group; Deinonychosauria; Coelurosaurian evolution.