Phylogenetic relationships of paravian theropods.

摘要

Coelurosauria is the most diverse clade of theropod dinosaurs, with much of that diversity present in Paraves---the clade of dinosaurs containing dromaeosaurids, troodontids, and birds (avialans). Paraves therefore has at least 150 million years of evolutionary history that continues to the present day. The clade represents the most diverse living tetrapod group (there are over 9,000 extant species of bird), and it is at the root of the paravian radiation, when dromaeosaurids, troodontids, and avialans where diverging from one another, that we find the morphology and soft tissue changes associated with the origin of avian flight. Within the first 15 million years of paravian evolutionary history the clade exhibited nearly four orders of magnitude difference in body size, a value that is similar to the wide body size disparity present today in carnivorans or varanoid squamates. In this aspect, the paravian clade serves as an important case study in characterizating the pattern, process, and dynamics of evolutionary size change. This last point is of particular interest because of the historical significance placed on the role body size reduction played in the origin of avian flight.;The present study reviews and revises the membership of the Dromaeosauridae and attempts to provide an apomorphy-based diagnosis for all known and valid taxa. Furthermore, this study examines four new paravian taxa, re-evaluates the enigmatic paravian Jinfengopteryx elegans, and incorporates new morphologically information from two undescribed troodontid species from the Late Cretaceous of Mongolia. The four new paravian taxa are shown to be members of the Dromaeosauridae. Two of the new species occupy derived positions within the Dromaeosauridae close to the well known Velociraptor mongoliensis . One taxon, the incompletely known Shanag ashile, occupies a basal though unresolved postion, but shows many affinities with Gondwanan and Asian dromaeosaurid taxa. Finally, the last new species Mahakala omnogovae is here depicted as the basal-most dromaeosaurid and as a result serves a crucial role in understanding the evolution and ancestral morphology of Dromaeosauridae.;The most detailed and comprehensive phylogenetic analysis of paravians to date is conducted to explore the phylogenetic history of these new dromaeosaurid taxa, as well as to examine the general pattern of paravian relationships within the broader context of Coelurosauria and increased emphasis on the role basal avialans have on understanding character optimizations at the base of Paraves. A large dataset of 472 characters and 99 taxa was constructed by merging two large datasets, one examining coelurosaur relationships broadly (based on the TWiG dataset) and the other examining avialan relationships specifically. This merged dataset was then supplemented with novel character analysis focusing on paravian taxa. This dataset is important in that it bridges a phylogenetic gap that had persisted between studies on birds and studies on all other coelurosaurs.;All the most parsimonious trees recovered in the cladistic analysis support the monophyly of Paraves, Troodontidae, Dromaeosauridae, and Deinonychosauria. A new clade of basal troodontids was discovered including the two undescribed Mongolian troodontids and Jinfengopteryx elegans. Recently proposed relationships within Dromaeosauridae are further supported, with a fundamental dichotomy present between a clade of Gondwanan/Asian dromaeosaurids and a clade of Laurasian dromaeosaurids. Avialan monophyly is strongly supported with Archaeopteryx, Sapeornis, Jeholornis, and Jixiangornis forming the successive sister taxa to the Confuciusornis node. This topology supports a more basal position for Sapeornis than previous phylogenetic analyses and indicates a progressive acquistion of a fully "avian" shoulder morphology.;Paravian body size evolution is examined empirically using body mass estimates obtained from femoral length measures. Ancestral node reconstructions are estimated on the phylogeny treating the body mass as a continuous morphological trait and optimizing it as such across the tree. This method shows that extreme miniaturization, previously thought unique to bird origins, occurred progressively earlier in theropod history and was followed by at least three episodes of substantial size increase within Deinoncyhosauria.