Microraptor facts for kids

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Microraptor
Temporal range: Lower Cretaceous
Close-up of a cast
Scientific classification
Kingdom: Animalia
Class: Sauropsida
Subclass: Archosauria
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
(unranked): Coelurosauria
Family: Dromaeosauridae
Genus: Microraptor
Microraptor fossil1
A cast of the type specimen of Microraptor gui at the American Museum of Natural History in New York. The feathers are on both front and rear legs.
Microraptor gui holotype
Same fossil with arrows

Microraptor was a small feathered dinosaur related to Velociraptor. It had flight feathers, and could glide and probably fly. About two dozen well-preserved fossil specimens have been found in Liaoning, China. They are from the Lower Cretaceous Jiufotang Formation, 125 million years old.

Adult specimens are 42–83 centimeters (1.4–2.7 ft) long, so Microraptor was one of the smallest dinosaurs.

Description

Microraptor scale
Wingspan & body size compared with human. Scale bar: 1 m (3.3 ft).

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

Microraptor Restoration
Restoration with colouration based on fossilized melanosomes

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.

Paleobiology

Wings and flight

Microraptor had four wings, one on each of its forelimbs and hindlimbs, somewhat resembling one possible arrangement of the quartet of flight surfaces on a tandem wing aircraft of today. It had long pennaceous feathers on arms and hands 10–20 cm long (3.9–7.9 in) with legs and feet 11–15 cm long (4.3–5.9 in). The long feathers on the legs of Microraptor were true flight feathers as seen in modern birds, with asymmetrical vanes on the arm, leg, and tail feathers. As in modern bird wings, Microraptor had both primary (anchored to the hand) and secondary (anchored to the arm) flight feathers. This standard wing pattern was mirrored on the hindlegs, with flight feathers anchored to the upper foot bones as well as the upper and lower leg. Though not apparent in most fossils under natural light, due to obstruction from decayed soft tissue, the feather bases extended close to or in contact with the bones, as in modern birds, providing strong anchor points.

It was originally thought that Microraptor was a glider, and probably lived mainly in trees, because the hindwings anchored to the feet of Microraptor would have hindered their ability to run on the ground. Some paleontologists have suggested that feathered dinosaurs used their wings to parachute from trees, possibly to attack or ambush prey on the ground, as a precursor to gliding or true flight. In their 2007 study, Chatterjee and Templin tested this hypothesis as well, and found that the combined wing surface of Microraptor was too narrow to successfully parachute to the ground without injury from any significant height. However, the authors did leave open the possibility that Microraptor could have parachuted short distances, as between closely spaced tree branches. Wind tunnel experiments have demonstrated that sustaining a high-lift coefficient at the expense of high drag was likely the most efficient strategy for Microraptor when gliding between low elevations. Microraptor did not require a sophisticated, ‘modern’ wing morphology to be an effective glider. However, the idea that Microraptor was an arboreal glider relies on it to have regularly climbed or even lived in trees, when study of its anatomy have shown that its limb proportions fall in line with modern ground birds rather than climbers, and its skeleton shows none of the expected adaptations in animals specialized for climbing trees.

Microraptor gui holotype under UV light
M. gui holotype under two different UV light filters, revealing extent of preserved feathers and soft tissue

Describing specimens originally referenced as a distinctive species (Cryptovolans pauli), paleontologist Stephen Czerkas argued Microraptor may have been a powered flier, and indeed possibly a better flyer than Archaeopteryx. He noted that the Microraptor's fused sternum, asymmetrical feathers, and features of the shoulder girdle indicated that it could fly under its own power, rather than merely gliding. Today, most scientists agree that Microraptor had the anatomical features expected of a flying animal, though it would have been a less advanced form of flight compared to birds. For example, some studies suggest the shoulder joint was too primitive to allow a full flapping flight stroke. In the ancestral anatomy of theropod dinosaurs, the shoulder socket faced downward and slightly backward, making it impossible for the animals to raise their arms vertically, a prerequisite for the flapping flight stroke in birds. Studies of maniraptoran anatomy have suggested that the shoulder socket did not shift into the bird-like position of a high, upward orientation close to the vertebral column until relatively advanced avialans like the enantiornithes appeared. However, other scientists have argued that the shoulder girdle in some paravian theropods, including Microraptor, is curved in such a way that the shoulder joint could only have been positioned high on the back, allowing for a nearly vertical upstroke of the wing. This possibly advanced shoulder anatomy, combined with the presence of a propatagium linking the wrist to the shoulder (which fills the space in front of the flexed wing and may support the wing against drag in modern birds) and an alula, much like a "thumb-like" form of leading edge slot, may indicate that Microraptor was capable of true, powered flight.

Other studies have demonstrated that the wings of Microraptor were large enough to generate the lift necessary for powered launching into flight even without a fully vertical flight stroke. A 2016 study of incipient flight ability in paravians demonstrated that Microraptor was capable of wing-assisted incline running, as well as wing-assisted leaping and even ground-based launching.

Stephen Czerkas, Gregory S. Paul, and others have argued that the fact Microraptor could fly and yet is also very clearly a dromaeosaurid suggests that the Dromaeosauridae, including later and larger species such as Deinonychus, were secondarily flightless. The work of Xu and colleagues also suggested that the ancestors of dromaeosaurids were probably small, arboreal, and capable of gliding, although later discoveries of more primitive dromaeosaurids with short forelimbs unsuitable for gliding have cast doubt on this view. Work done on the question of flight ability in other paravians, however, showed that most of them probably would not have been able to achieve enough lift for powered flight, given their limited flight strokes and relatively smaller wings. These studies concluded that Microraptor probably evolved flight and its associated features (fused sternum, alula, etc.) independently of the ancestors of birds. However, further research on Archaeopteryx suggested that it too was capable of powered flight, showing basal paravians to be more volant than previously assumed.

Hindwing posture

Microraptor models
Wind tunnel experiments with different wing configurations

Sankar Chatterjee suggested in 2005 that, in order for Microraptor to glide or fly, the forewings and hindwings must have been on different levels (as on a biplane) and not overlaid (as on a dragonfly), and that the latter posture would have been anatomically impossible. Using this biplane model, Chatterjee was able to calculate possible methods of gliding, and determined that Microraptor most likely employed a phugoid style of gliding: launching itself from a perch, the animal would have swooped downward in a deep U-shaped curve and then lifted again to land on another tree. The feathers not directly employed in the biplane wing structure, like those on the tibia and the tail, could have been used to control drag and alter the flight path, trajectory, etc. The orientation of the hindwings would also have helped the animal control its gliding flight. Chatterjee also used computer algorithms that test animal flight capacity to test whether or not Microraptor was capable of true, powered flight, as opposed to or in addition to passive gliding. The resulting data showed that Microraptor did have the requirements to sustain level powered flight, so it is theoretically possible that the animal flew as opposed to gliding.

Some paleontologists have doubted the biplane hypothesis, and have proposed other configurations. A 2010 study by Alexander et al. described the construction of a lightweight three-dimensional physical model used to perform glide tests. Using several hindleg configurations for the model, they found that the biplane model, while not unreasonable, was structurally deficient and needed a heavy-headed weight distribution for stable gliding, which they deemed unlikely. The study indicated that a laterally abducted hindwing structure represented the most biologically and aerodynamically consistent configuration for Microraptor. A further analysis by Brougham and Brusatte, however, concluded that Alexander's model reconstruction was not consistent with all of the available data on Microraptor and argued that the study was insufficient for determining a likely flight pattern for Microraptor. Brougham and Brusatte criticized the anatomy of the model used by Alexander and his team, noting that the hip anatomy was not consistent with other dromaeosaurs. In most dromaeosaurids, features of the hip bone prevent the legs from splaying horizontally; instead, they are locked in a vertical position below the body. Alexander's team used a specimen of Microraptor which was crushed flat to make their model, which Brougham and Brusatte argued did not reflect its actual anatomy. Later in 2010, Alexander's team responded to these criticisms, noting that the related dromaeosaur Hesperonychus, which is known from complete hip bones preserved in three dimensions, also shows hip sockets directed partially upward, possibly allowing the legs to splay more than in other dromaeosaurs.

Ground movement

Microraptor by durbed
Restoration of two individuals by the ground

Due to the extent of the hindwings onto most of the animal's foot, many scientists have suggested that Microraptor would have been awkward during normal ground movement or running. The front wing feathers would also have hindered Microraptor when on the ground, due to the limited range of motion in the wrist and the extreme length of the wing feathers. A 2010 study by Corwin Sullivan and colleagues showed that, even with the wing folded as far as possible, the feathers would still have dragged along the ground if the arms were held in a neutral position, or extended forward as in a predatory strike. Only by keeping the wings elevated, or the upper arm extended fully backward, could Microraptor have avoided damaging the wing feathers. Therefore, it may have been anatomically impossible for Microraptor to have used its clawed forelimbs in capturing prey or manipulating objects.

Feeding

Microraptor Skeletons by Qilong
Skeletal restorations of various specimens

In 2010 researchers announced that further preparation of the type fossil of M. zhaoianus revealed preserved probable gut contents. These consisted of mammalian bones, including possible skull, limb, and vertebral fragments and also a whole foot. The foot skeleton is similar to those of Eomaia and Sinodelphys. It corresponds to an animal with an estimated snout to vent length of 80 mm (3.1 in) and a mass of 20–25 g (0.71–0.88 oz). The unguals of the foot are less curved than in Eomaia or Sinodelphys, indicating that the mammal could climb but less effectively than in the two latter genera. Other than mammals, Microraptor ate lizards, like Xianglong.

In the December 6, 2011 issue of Proceedings of the National Academy of Sciences, Jingmai O'Connor and coauthors described a Microraptor specimen containing bird bones in its abdomen, specifically a partial wing and feet. Their position indicate that the dinosaur swallowed a tree-perching bird whole.

In 2013 researchers announced that they had found fish scales in the abdominal cavity of a Microraptor specimen. This new finding corrects the previous position that Microraptor hunted only in an arboreal environment. They also argued that the specimen showed a probable adaptation to a fish-eating diet: the good preservation of the mandible shows that the first three teeth were inclined anterodorsally, a character often associated with piscivory. Microraptor was an opportunistic feeder, hunting the most common prey in both arboreal and aquatic habitats.

Based on the size of the scleral ring of the eye, it has been suggested Microraptor hunted at night. However, the discovery of iridescent plumage in Microraptor has cast doubt on this conclusion, as no modern birds that have iridescent plumage are known to be nocturnal (though, contrary to media claims about this discovery, the study authors themselves haven’t ruled out nocturnality yet, as ‘’Microraptor’’ is not similar in diet and behaviour to any modern iridescent birds)

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