Microraptoria (formerly known as Microraptorinae) is a clade of basal dromaeosaurid theropod dinosaurs. The first microraptorians appeared 125 million years ago in China. Many are known for long feathers on their legs and may have been semi-arboreal gliders. Most microraptorians were relatively small; adult specimens of Microraptor range between 77–90 centimetres long (2.53–2.95 ft) and weigh up to 1 kilogram (2.2 lb), making them some of the smallest known dinosaurs. However, the largest members of this clade were as large as 1.2 meters (4 ft) in length and weighed upwards of 4 kg (8.8 lb).
Microraptorians were a group of basal dromaeosaurids with slender proportions and long limbs. With the exception of Hesperonychus from the late Cretaceous (Campanian Stage) of North America, all microraptorians have been found in the Yixian and Jifuotang Formations of Liaoning county, China, hence why microraptorians are sometimes referred to as "Liaoning dromaeosaurs". These formations (collectively known as the Jehol group) have been dated to the early Cretaceous and at that time would have been part of a temperate wetland ecosystem threatened by frequent volcanic eruptions. Like all other dromaeosaurids, microraptorians were carnivores with large, serrated teeth and a hyper-extendable second toe equipped with a curved claw.
Most microraptorians were small dinosaurs, with taxa like Microraptor and especially Zhongjianosaurus being among the smallest non-avian dinosaurs known to science. However, some microraptorians, such as Tianyuraptor and Changyuraptor, were significantly larger and similar to other dromaeosaurids in size. Many microraptorians also had long and robust arms and legs, in contrast to the stockier eudromaeosaurs, although long arms are not universal to the group, as the basal microraptorian Tianyuraptor had unusually short arms by dromaeosaurid standards. Considering this, it is likely that the small size and long wings of some microraptorians are an example of convergent evolution with other small paravians and early birds such as Anchiornis and Archaeopteryx.
The fossilization conditions of the Jehol group are very accommodating to the preservation of soft structures in fossils, and as a result many microraptorians have been preserved with a covering of feathers. Not only have long, advanced feathers been preserved on the arms and tails of many specimens, but a few species even have long feathers on their legs. This condition has also been seen in other paravians such as Anchiornis, and has caused these kinds of dinosaurs to be labelled as "four-winged dinosaurs". The largest known "four-winged" dinosaur, Changyuraptor,is a microraptorian. It is possible that some microraptorians such as Microraptor were semi-arboreal and used these wings as gliding mechanisms. Analysis of the hip joins of Hesperonychus also show that they were capable of splaying their legs further apart than ground-hunting dromaeosaurids, further cementing the theory that these animals were capable of gliding flight.
Below is a list of the known genera of microraptorians: