Paraves

Paraves is a branch-based clade defined to include all non-avian dinosaurs which are more closely related to birds than to oviraptorosaurs. Paravians comprises two major subgroups: Avialae, including Jeholornis and modern birds, and the Deinonychosauria, which includes the dromaeosaurs and troodonts, which may or may not form a natural group.

Like other theropods, early paravians were bipedal, walking on their two hind legs. However, whereas most theropods walked with three toes contacting the ground, fossilized footprint tracks confirm that many basal paravians, including dromaeosaurs, troodonts, and some early avialans, held the second toe off the ground in a hyperextended position, with only the third and fourth toes bearing the weight of the animal. This is called functional didactyly. The enlarged second toe bore an unusually large, curved sickle-shaped claw (held off the ground or 'retracted' when walking). This claw was especially large and flattened from side to side in the large-bodied predatory eudromaeosaurs. In these early species, the first toe (hallux) was usually small and angled inward toward the center of the body, but only became fully reversed in more specialized members of the bird lineage. One species, Balaur bondoc, possessed a first toe which was highly modified in parallel with the second. Both the first and second toes on each foot of B. bondoc were held retracted and bore enlarged, sickle-shaped claws.

An increasingly asymmetric carpal joint, a trend that can be traced back to primitive coelurosaurs, allowed the forelimbs to elongate and an elaboration of their plumage, traits that made the evolution of wings possible.

The teeth of basal paravians were curved and serrated, but not bladelike except in some specialized species such as Dromaeosaurus albertensis. The serrations on the front edge of dromaeosaur and troodont teeth were very small and fine, while the back edge had serrations which were very large and hooked. Paravians generally have long, winged forelimbs, though these have become smaller in flightless birds and some extinct lineages such as the troodonts. The wings usually bore three large, flexible, clawed fingers in early forms. The fingers became fused and stiffened and the claws highly reduced or lost in some advanced lineages.

Most dromaeosaurs seem to have been predatory, though some smaller species (especially among the troodonts and avialans) are known to have been at least omnivorous; and it is possible that an omnivorous diet was the ancestral state for this group, with more strict carnivory evolving in some lineages.

The most extreme examples of miniaturization and progenesis (accelerated sexual maturation which emerge during a phase that would be considered juvenile in the predecessors, at which both growth and further anatomical development slows down or cease) are found in Paraves. The ancestors of Paraves first started to shrink in size in the Early Jurassic 200 million BC, and fossil evidence show that this theropod line evolved new adaptations four times faster than other groups of dinosaurs, and was shrinking 160 times faster than other dinosaur lineages were growing. Turner et al. (2007) suggested that extreme miniaturization was ancestral for the clade, whose common ancestor has been estimated to have been around 65 centimeters long and 600-700 grams in mass. In Eumaniraptora, Dromaeosauridae and Troodontidae went later through four independent events of gigantism, three times in dromaeosaurs and once in troodonts, while the body mass continued to decrease in many forms within Avialae. Fossils shows that all the earliest members of Paraves found to date started out as small, while Troodontidae and Dromaeosauridae gradually increased in size during the Cretaceous period.

The ancestral paravian is a hypothetical animal: the first common ancestor of birds, dromaeosaurs, and troodonts which was not also ancestral to oviraptorosaurs. Little can be said with certainty about this animal. But the work of Xu et al. (2003), (2005) and Hu et al. (2009) provide examples of basal and early paravians with four wings, adapted to an arboreal lifestyle who would only lose their hind wings when some adapted to a life on the ground and when avialans evolved powered flight. Newer research also indicates that gliding, flapping and parachuting was another ancestral trait of Paraves, while true powered flight only evolved once, in the lineage leading to modern birds.