Microraptor

Credit: AMNH.

*Microraptor* is an extinct genus of paravian from China during the Early Cretaceous.

## History

The first *Microraptor* specimen is of a small individual with faint feather preservation, not well preserved and lack the skull. Mark Norell (2002) described BPM 1 3-13, which was not attributed to an existing species and was not named. Later the same year, Stephen Czerkas et al. named *Cryptovolans pauli*, referring two specimens to the species, the first having well-preserved feathers, from the Jiufotang Formation, Early Abian, Liaoning, China. The specimen is now part of the Paleontology Museum of Beipiao collection. The holotype is catalogued LPM 0200, the counterslab LPM 0201 and the paratype LPM 0159. The slab and counterslab represent BPM 1 3-13 and the paratype is a smaller individual. These make two individuals, both articulated but compressed and partially damaged.

Czerkas et al. (2002) erected the genus based on the primary feathers (which they believed was of a bird), a co-ossified sternum, a tail with 28-30 vertebrae and a third finger that bears a short phalanx III-3. Some of the feathers Czerkas attributed to the forewings were actually attached to the legs - the rest of the features are identical to *Microraptor*. However, BPM 1 3-13 has a longer tail with 24-26 vertebrae. Later studies and more descried specimens show that *Cryptovolans* shows no unique features, and all of the features are present to varying degrees. Phil Senter et al. (2004) reviewed the situation, concluding they all represent individual variation, *Cryptovolans* and *M. gui* being junior synonyms of *M. zhaoianus*. Other researchers, such as Alan Feduccia and Tom Holtz, have since supported this conclusion.

BMNHC PH881 shows several new characteristics of *Microraptor*: probable iridescent black plumage, a bifurcated tailfan and no nuchal crest. The tailfan was the same, except bearing two elongate narrow feathers at the center, and the nuchal crest observed in the holotype was determined to be taphonomic distortion. New specimens have been uncovered from the Shangheshou Bed, Jiufotang Formation in Liaoning, China. *Microraptor* is the most common non-avialan from the Jiufotang Formation, as 300+ undescribed specimens attributable to *Microraptor* or microraptorians were reported in 2010. However, many of these have been composited by private fossil owners.

Norell et al. (2002) described BPM 1 3-13 as having four wings on the limbs, with Czerkas mistaking the animal having feathers on it's arms on the illustrated cover of Feathered Dinosaurs and the Origin of Flight in the same year. Additionally, Czerkas concludes Norell had seen what he wanted to and criticized him for not concluding dromaeosaurs were birds - he was convinced Norell had been subject to an extreme case of pareidolia. Norell uses a definition for "bird" that would exclude BPM 1 3-13, but not as an Avialae. However, Czerkas conclusions would be disproven when a new specimen was published by Xu the following year. The first specimen was discovered in 2001 and four more were bought from private collectors by the Institute of Vertebrate Paleontology and Paleoanthropology (Xu's museum) between 2001 and 2003. Xu considered these specimens new species, based on proportional differences. Most of them had hindwings. He named the new species *M. gui*, which was questioned by Senter due to the differences correlating with size and likely age differences. Two more specimens of *M. zhaoianus* were described by Hwang et al. (2002), before *M. gui* was named. Further, Czerkas believed *Microraptor* had better flight capabilities than *Archaeopteryx*, citing the fused sternum and asymmetrical feathers as derived features. Czerkas also states the volant animal was a clear dromaeosaur, proposing Dromaeosauridae as a group of basal birds. Later, Czerkas (2002) argues that *Deinonychus* was secondary flightless. Currently, there is not enough evidence to prove if Dromaeosauridae descended from an ancestor with aerodynamic abilities. Xu et al. (2003) proposes basal dromaeosaurs were likely small, arboreal, gliding creatures, and Turner et al. (2007) suggested this ancestor was flightless and could not fly, by was ~65 centimeters long and ~600-700 grams in weight.

### Taxonomy

Because of unusual description circumstances, the name "*Microraptor*" was considered controversial. The first *Microraptor* was from the chimeric "Archaeoraptor", a forgery formed from Microraptor, *Yanornis* and an undescribed third species that was assembled in China and smuggled into the United States to put for sale. The forgery was revealed by Xu Xing in Beijing's Institute of Vertebrate Paleontology and Paleoanthropology, and Storrs L. Olson, the bird curator of the Nation Museum of Natural History of the Smithsonian, published the Microraptor elements. The material was published in an obscure journal under Archaeoraptor liaoningensis to remove the name from the paleornithological record by assigning the remains to the chimeric elements least likely to have an avian origin. However, Xu found the remainder of the specimen and described M. zhaoianus one year later. Since Archaeoraptor takes International Code of Zoological Nomenclature (ICZN) priority, Microraptor would typically be a junior synonym of Archaeoraptor. However, some doubt Olson met the requirements in his description, especially since he assigned the material as the lectotype before the genus was even erected. Since Xu's name was used more and Archaeoraptor is a chimera, the ICZN rendered Archaeoraptor a nomen vanum and Microraptor a nomen protectum.

### "Tetrapteryx" Hypothesis

The unique wings of *Microraptor* raised questions about flight evolution in birds, and if they ever went through a 'four-winged stage'. As early as 1915, William Beebe argued bird evolution may have gone through the four-winged stage, known as "tetrapteryx". Chatterjee and Templin noted it was possible. However, the unusually long hindwing feathers in some dinosaurs, *Archaeopteryx*, some modern birds (such as raptors) and dinosaurs with primary feathers on their feet (*Pedopenna*), they argued morphological and phylogenetic evidence indicates bird flight had shifted from four limbs to two limbs at some point, and modern birds had four-winged ancestors or ancestors with long feathers on their feet.

## Description

Adult *Microraptor* were among the smallest non-avian dinosaurs, estimated at 77 centimeters (2.53 feet) long and 1 kilogram (2.2 pounds) heavy. Holtz estimates it at 90 centimeters (3 feet) and Benson et al. (2012) estimates a maximum length of 1.2 meters (3.9 feet). Along with being so small, *Microraptor* is one of the first non-avialan dinosaurs discovered with feather impressions and wings, with *M. zhaoianus* having the most feather impressions. Among early birds and feathered dinosaurs, *Microraptor* also has flight feathers on all four limbs. Their bodies were coated in a thick coat of feathers and a tail with a diamond-shaped tailfan, which may have aided in flight stabilizing. Xu et al. (2003) compared the long plumes on *Microraptor*'s head to the Phillipine eagle. Light and dark bands on some specimens may indicate colour, with some having a definite iridescent black colouration. Unserrated and partially serrated teeth with constricted waists and elongate upper arm bones are shared with primitive avians and primitive Troodontidae. In fact, *Microraptor* is similar to Sinovenator, Hwang et al. (2002) noting in a *M. zhaoianus* description the two are both very primitive members of groups closely affiliated with one-another and close to the deinonychosaurian split.

Quanguo Li et al. (2021) announced the plumage colouration in *Microraptor* based on BMNHC PH881, an exceptional specimen that also highlights several unknown characteristics. Fossilized melanosomes were analyzed with scanning electron microscope techniques and comparisons to melanosome patterns in modern birds. The cells were shaped in a matter consistent with modern cells that produce a black, glossy pigment. These are rod-shaped and narrow, arranged in stacks, similar to starlings. This indicates an iridescent black colouration, the function of which is unknown. It has been suggested *Microraptor* used this to display like many modern birds do with similar plumage.

## Paleobiology

### Ground Movement

Analysis on *Microraptor* feet suggest it was an awkward walker. For example, the front wing feathers would have hindered because their wrists had limited movement and their feathers were extremely long. Corwin Sullivan et al. (2010) found that, with the wings folded as far as possible, the wings would have dragged across the ground. Additionally, the same would have happened if the wrists were in a neutral position, or in a forward striking motion. By keeping the wings fully elevated and the upper arm positioned backwards could the feathers avoided dragging. Because repeated dragging across terrain would have worn down the wing feathers, it is impossible for *Microraptor* to have grabbed or manipulated objects.

### Flight

*Microraptor* had four wings on each limb, resembling tandem wing aircraft. They had long pennaceous feathers on the arms and hands (10-20 centimeters (3.9-7.9 inches) long on the legs, 11-15 centimeters (4.3-5.9 inches) long on the feet). The feathers on the legs were true flight feathers, with asymmetrical vanes on feathers from the arms, legs and tail. Like modern birds, the primaries were anchored to the hand and secondaries to the arm. This is true for the hindwings, as well. The feather bases extend close to, or contact the bones like modern birds, creating strong anchors. This is not apparent in most fossils, however, as decayed soft tissue obstructs it under natural light.

Originally, *Microraptor* was thought a glider, likely being mostly arboreal because the hindwings would have hindered sprinting. However, some paleontologists suggest their feathers helped them "parachute" from trees, possibly to attack. This was considered a precursor to true flight or gliding. Chatterjee and Templin (2007) tested this: the wing surface was too narrow to parachute without injury from any height, but they did not deny *Microraptor* may have had the ability to parachute short distances between close branches. Wind tunnel experiments demonstrate a high-lift coefficient with high drag was the best strategy for *Microraptor* when gliding at low elevations. *Microraptor* did not need the modern wing considered effective for gliding. The idea *Microraptor* was an arboreal glider requires *Microraptor* to have climbed or lived in trees. However, it's limb proportions are more alike ground birds than climbing bird, and it lacks the skeletal adaptations required for climbing trees.

Stephen Czerkas argued (based on *Cryptovolans* specimens) that *Microraptor* was a good flier, and possibly better than *Archaeopteryx*. He cite a fused sternum, asymmetrical feathers and feathers at the shoulder girdle indicate powered flight. Today, most agree *Microraptor* has the anatomical characteristics of a less-advanced flying animal. Some studies propose the shoulder joint is too primitive to perform a full flap. Theropods have shoulders facing downwards and slightly backward, allowing the socket to limit the arm from travelling upwards; the opposite is true for birds. Studies suggest the shoulder did not allow for a high, upwards position near to the vertebral column (like birds) until the enantiornithes. However, others disagree: paraves' shoulder girdles curved in way do high the upstroke of the wing could be near-vertical. Since the shoulder could have been advanced, with the propatagium linking the wrist and shoulder and an alula, *Microraptor* may have achieved powered flight.

Some studies conclude *Microraptor*'s wings could generate enough lift for powered flight launch without a full vertical stroke. A 2016 study on incipient paravian flight calculates *Microraptor* was capable of wing-assisted incline running, alongside wing-assisted leaping and ground-based launching.

Czerkas and Gregory S. Paul et al. have argued *Microraptor* could fly and was a dromaeosaurid, proposing later and larger forms were secondarily flightless. Xu et al. suggested the Dromaeosauridae ancestors were likely small, arboreal gliders. However, primitive dromaeosaurs have short arms not preferable for gliding. Study on most paraves showed most were able to achieve the lift required for powered flight. These studies conclude *Microraptor* is a product of convergent evolution, as it is independent from bird ancestors. Despite being a non-flier, its feet were similar to raptorial birds such as hawks. This indicates that theropod dinosaurs carried out the same specialized tasks that some birds do, no matter how rare the trait(s) may be.

### Hindwing Posture

Sankar Chattejee (2005) suggested *Microraptor* had to have the fore and hindwings at different levels (like a biplane) to glide or fly, but it was also able to overlay (like a dragonfly). Chatterjee was able to calculate possible gliding methods based on the biplane model - finding it likely used a phugoid glide style. The feathers in the biplane wing structure could have controlled drag and trajectory, for example, with the hindwings helping the animal control gliding flight. Chatterjee also used algorithms to test if *Microraptor* was capable of powered flight, finding it met the requirement and could theoretically fly.

However, some paleontologists doubt the biplane model. Alexander et al. (2010) described a lightweight 3D physical model used in experiments. They configured the hindwings in different positions. They found the biplane model was not unreasonable, but was structurally deficient and needed heavy-headed weight distribution. This study suggests a laterally abducted structural and aerodynamic option. Broughham and Brusatte oppose this, saying their methods do not match the recent data, and that the study is insufficient. They criticize the anatomy of the model, mostly that the hip did not match Dromaeosauridae. This anatomy prevented the legs from spreading horizontally. Additionally, their model was crushed, which they argued was inaccurate to the actual anatomy. Alexander et al. (2010) responds to the criticisms noting that *Hesperonychus* shows hip sockets partially upward, possibly allowing more of a splay.

### Diet

In 2010, further preparation of the M. zhaoianus type fossil announced probable gut contents. In the gut were mammalian bones (a possible skull, a limb, a foot and fragmented vertebrae). The foot is similar to *Eomaia* and *Dinodelphys*. This mammal has a length of 80 millimeters (3.1 inches) and a mass of 20-25 grams (0.71-0.88 ounces). The unguals are less curved and indicates lesser climbing capabilities than the two similar mammals. In the National Academy of Sciences (December 6, 2011 issue), Jingmai O'Connor et al. described a *Microraptor* with bird fossils in it's body cavity: a partial wing and feet. They concluded the animal had engulphed a perched bird whole. Fish scales found in the abdomen of *Microraptor* were announced in 2013, showing that the dinosaur did not only hunt in an arboreal environment. They also found the first three teeth are inclined anterodorsally, which is indicative of piscivory. In 2019, *Indrasaurus* was described from the stomach of Microraptor, who had swallowed the item head-first like many carnivorous birds and lizards. The lizard lacked pitting and scars, which suggests the individual died shortly after consumption and digestion had not occurred. *Microraptor* has never preserved gastric pellets, like the close parave *Anchiornis* did. This proposes *Microraptor* passed indigestible matter into it's feces rather than coughing-up a pellet. The scleral ring indicates *Microraptor* was a nocturnal hunter - but the discovery of iridescence has challenged this idea, since no modern nocturnal birds have such a colouration. *Microraptor* was an opportunistic generalist carnivore.

### Molting

*Microraptor* likely experienced sequential molting, as evidenced by a patch of 3 strangely-shaped feathers among the plumage. It may have molted similar to songbirds, where they shed several feathers at a time rather than all at once. This patch appears by the forewing of *M. gui*, being noticed by Yosef Kiat et al. (2020). Not only does this prove it molted, but that it was so well-adapted towards flight that molting was a part of their lifestyle. Stephen Brusatte has commented on this, stating that *Microraptor* thus makes the most convincing cases of flight in non-avian dinosaurs.

## Classification

Phil Senter, James I. Kirkland, Donald D. DeBlieux, Scott Madsen and Natalie Toth (2012) conclude the following cladogram:

## Gallery

### Fossils

*M. gui* holotype IVPP V 13352.Beijing Museum of Naturak History PH1084.*M. zhaoianus* specimen.Specimen from the Shandong Tianyu Museum of Nature.A cast of *M. gui*.Beijing Museum of Naturak History PH1084.Beijing Museum of Naturak History PH1084.Beijing Museum of Naturak History PH1084.Beijing Museum of Naturak History PH1084.
## References

* https://en.wikipedia.org/wiki/Microraptor
* https://www.sciencenews.org/article/microraptor-dinosaur-fossil-molting-feathers-songbirds
* https://archive.org/details/g.s.paul2016theprincetonfieldguidetodinosaurs/page/n87/mode/2up
* https://www.nature.com/articles/s41467-022-35039-1