Microraptor

***Microraptor*** (meaning "small thief") is a genus of small, four-winged paravian dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. They date from the early Cretaceous Jiufotang Formation (Aptian stage), 125 to 120 million years ago. Three species have been named (*M. zhaoianus*, *M. gui*, and *M. hanqingi*), though further study has suggested that all of them represent variation in a single species, which is properly called *M. zhaoianus*. ***Cryptovolans***, initially described as another four-winged dinosaur, is usually considered to be a synonym of *Microraptor*.

Like *Archaeopteryx*, well-preserved fossils of *Microraptor* provide important evidence about the evolutionary relationship between birds and earlier dinosaurs. *Microraptor* had long pennaceous feathers that formed aerodynamic surfaces on the arms and tail but also on the legs. This led paleontologist Xu Xing in 2003 to describe the first specimen to preserve this feature as a "four-winged dinosaur" and to speculate that it may have glided using all four limbs for lift. Subsequent studies have suggested that *Microraptor* was capable of powered flight as well.

*Microraptor* was among the most abundant non-avialan dinosaurs in its ecosystem, and the genus is represented by more fossils than any other dromaeosaurid, with possibly over 300 fossil specimens represented across various museum collections.

## Description

With adult specimens estimated up to 77 centimetres long (2.53 ft) and with a weight estimated up to 1 kilogram (2.2 lb), *Microraptor* was among the smallest-known non-avian dinosaurs. Holtz estimated it at 90 centimetres (3.0 ft). An estimate by Benson *et al.* in 2012 was that *Microraptor* had a maximum length of 1.2 m (3.9 ft). Aside from their extremely small size, *Microraptor* were among the first non-avialan dinosaurs discovered with the impressions of feathers and wings. Seven specimens of *M. zhaoianus* have been described in detail, from which most feather impressions are known. Unusual even among early birds and feathered dinosaurs, *Microraptor* is one of the few known bird precursors to sport long flight feathers on the legs as well as the wings. Their bodies had a thick covering of feathers, with a diamond-shaped fan on the end of the tail (possibly for added stability during flight). Xu *et al.* (2003) compared the longer plumes on *Microraptor*'s head to those of the Philippine eagle. Bands of dark and light present on some specimens may indicate color patterns present in life, though at least some individuals almost certainly possessed an iridescent black coloration. Several anatomical features found in *Microraptor*, such as a combination of unserrated and partially serrated teeth with constricted 'waists', and unusually long upper arm bones, are shared with both primitive avians and primitive troodontids. *Microraptor* is particularly similar to the basal troodontid *Sinovenator*; in their 2002 description of two *M. zhaoianus* specimens, Hwang *et al.* note that this is not particularly surprising, given that both *Microraptor* and *Sinovenator* are very primitive members of two closely related groups, and both are close to the deinonychosaurian split between dromaeosaurids and troodontids.

### Coloration

In March 2012, Quanguo Li *et al.* determined the plumage coloration of *Microraptor* based on the new specimen BMNHC PH881, which also showed several other features previously unknown in *Microraptor*. By analyzing the fossilized melanosomes (pigment cells) in the fossil with scanning electron microscope techniques, the researchers compared their arrangements to those of modern birds. In *Microraptor*, these cells were shaped in a manner consistent with black, glossy coloration in modern birds. These rod-shaped, narrow melanosomes were arranged in stacked layers, much like those of a modern starling, and indicated iridescence in the plumage of *Microraptor*. Though the researchers state that the true function of the iridescence is yet unknown, it has been suggested that the tiny dromaeosaur was using its glossy coat as a form of communication or sexual display, much as in modern iridescent birds.