Microraptorians were a group of basal dromaeosaurids with slender proportions and long limbs. With the exception of Hesperonychus from the late Cretaceous 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 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 other dromaeosaurids, microraptorians were carnivores with relatively large, serrated teeth and an hyper-extendable second toe equipped with a curved claw.
Most microraptorians were small dinosaurs, with taxa like Microraptor and especially Zhongjianosaurus being among the smallestnon-avian dinosaurs known. However, some microraptorians, such as Tianyuraptor and Changyuraptor, were 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 since 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.
Feathers
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 able to use these wings to glide or take off from the ground.
Characteristic features
Microraptorines can be distinguished from other dromaeosaurids due to the following features:
Microraptoria is usually classified as a clade of dromaeosaurids. Senter and colleagues expressly coined the name without the subfamily suffix-inae to avoid perceived issues with erecting a traditional family-group taxon, should the group be found to lie outside Dromaeosauridae proper. Sereno offered a revised definition of the sub-group containing Microraptor to ensure that it would fall within Dromaeosauridae, and erected the subfamily Microraptorinae, attributing it to Senter et al., though this usage has only appeared on his online TaxonSearch database and has not been formally published. The cladogram below follows a 2012 analysis by paleontologists Phil Senter, James I. Kirkland, Donald D. DeBlieux, Scott Madsen and Natalie Toth.
