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Description
I'm late for a bandwagon that I never planned on jumping on (namely Wyatt-Andrews-Art 's Dinos Through the Decades challenge), but sometimes an idea slips into your head and leaves you with no choice but to realize it.

Understandably, most entries for Dinos Through the Decades have focused on extinct animals that have been known to science for many decades or even centuries. However, there are some more recently discovered species for which our understanding has changed dramatically as well, and one of my favorite examples of this is Microraptor.

The original 2000 description of the type species, M. zhaoianus, is often overlooked these days, at least from a paleoartistic perspective. This is probably because the type specimen (IVPP V12330) is not particularly impressive, especially by Jehol standards, being a partial skeleton with poorly-preserved feather traces around the front and hind limbs. (In fact, I've often come across the misconception that the iconic IVPP V13352 specimen, which would not be described until a few years later, is the holotype of Microraptor instead.) Even so, Microraptor had a few claims to fame upon its description: the fact that it preserved any feathers at all was still noteworthy at a time when feathers had only been recently confirmed in non-avialan dinosaurs, it was the smallest non-avialan dinosaur known then (only about 40 cm long, though later specimens would show that it could grow substantially larger), and the tail of the type specimen had been part of the notorious "Archaeoraptor" chimera. Based on its small size and features of its toes and claws, its describers suggested that it may have been arboreal. Paleoartistic depictions of Microraptor from this time are relatively hard to come by, but generally showed it as a small, fuzzy theropod that climbed about in trees, though without specific adaptations for moving through the air.

That would change in 2003, when new specimens of Microraptor with more completely preserved plumage were described. These included the more famous IVPP V13352 specimen, which was described as the type specimen of a second species of Microraptor, M. gui (now commonly considered the same species as M. zhaoianus). These specimens revealed the presence of large, complex feathers on both the front and hindlimbs as well as the tail, very similar to the flight feathers of modern birds. It was suggested that the wing-like surfaces on all four limbs allowed Microraptor to glide, and it was reconstructed doing so by splaying its limbs to its sides. The supposed gliding adaptations, along with the argument that the long hindlimb feathers would have impeded locomotion on the ground, were used as further evidence for the idea that Microraptor primarily lived in trees. The image of Microraptor gliding from tree to tree with four splayed wings became a very popular one.

However, it was not long before some researchers pointed out that the ability to splay the hindlimbs in the manner originally reconstructed would be very unusual for a dinosaur, and there was little to indicate that Microraptor was much different from other dinosaurs in that regard. Several competing hypotheses were proposed for how the hindlimb feathers may have functioned with the legs held beneath the body in a more typical dinosaurian posture, with a "biplane-like" model receiving much press. Some of these potential gliding postures were notably shown being experimentally tested on an episode of Nova, "The Four-Winged Dinosaur", which aired in 2008.

Microraptor got another makeover in 2012, when it was inferred to have had largely glossy, iridescent feathers across its body based on analyzing the melanosomes preserved in the feathers of a new specimen (BMNHC PH881). This specimen additionally preserved a pair of especially elongate feathers at the tip of the tail, and the authors of the study reinterpreted the "feather mohawk" that had been shown on the head of previous depictions as a taphonomic artifact.

It was also around this time that several publications came out disputing the interpretation of Microraptor as a primarily arboreal animal, with some researchers even arguing for a mostly terrestrial lifestyle instead. One study comparing its limb proportions and overall anatomy to extant animals in detail found that it had more in common with terrestrial mammals and birds than with those that habitually climb. At the very least, the discovery of fish remains preserved as gut contents as well as analyses of gliding performance suggesting that Microraptor was a decent but not particularly specialized glider seemed to indicate that it was not a "dinosaurian flying squirrel" that was strictly tied to the trees.

Our knowledge about the life appearance of Microraptor hasn't changed much since then, but further studies have shed light on its possible lifestyle and life history. Perhaps most notably, a growing number of analyses have found evidence that Microraptor could have taken off under its own power; in other words, it was likely capable of powered flight instead of having been limited to gliding. This was not a total surprise; it had already been previously observed that Microraptor was as similar or more similar to flying birds in just about every flight-related part of its anatomy than was Archaeopteryx, for which powered flight abilities have also been debated but nonetheless considered. However, the flight apparatus of Microraptor still had notable differences from that of extant flying birds, so how well it could fly once it got into the air requires more research.

Some more findings about Microraptor that have been published in the last few years include additional gut contents revealing that it also fed on lizards, possible evidence of molting implying that it replaced its wing feathers in a specific sequence (similar to many flying birds today), and analyses of its hindlimb proportions suggesting that it was likely a faster runner than most other theropods of its size, contrary to previous assumptions that it would have been clumsy on the ground.

To be fair, there are a number of well-respected paleontologists who still consider an arboreal lifestyle for Microraptor plausible. However, I personally find the evidence that it was not an arboreal specialist to be fairly convincing, and I suspect that the pendulum will swing further in that direction in the future. I certainly wouldn't go as far as to claim that Microraptor never ventured into trees, but I do think that essentially all of the arguments for the idea that it spent most of its time living and feeding in trees are at least questionable.

The possible grasping function of its foot, for example, could equally be interpreted as a predatory adaptation , and its first toe is still unreversed and much smaller than in most birds that regularly perch in trees (even primarily terrestrial species that mostly use trees as roosts, like galliforms). It has also been found that ecology is very difficult to infer from claw curvature alone in extant animals, with large amounts of overlap in claw shape among different functional categories.

How the hindlimb feathers might have affected walking and running requires study, as their orientation and the potential muscular mechanisms controlling them in Microraptor remain uncertain. However, it can be said that domestic pigeon breeds with large hindlimb feathers are still quite capable of moving on the ground, especially individuals in which the longer feathers on the toes have worn away (which appears to be more similar to the condition seen in the preserved feathering of Microraptor). Furthermore, even if the hindlimb feathers were a hindrance in locomotion, an arboreal lifestyle would be unlikely to solve the problem, as tree climbing promotes crouched postures that lower the body's center of gravity, which, if anything, would bring the hindlimb feathers into closer and more frequent contact with the substrate. Ironically, the simplest way to keep the hindlimb feathers clear would have probably been to stand with the metatarsals raised high off the substrate, a stance more typical of terrestrial birds than arboreal ones.

Lastly, if Microraptor was capable of powered flight, that would remove climbing as a requirement for it to have gotten airborne.

A non-exhaustive selection of papers that have been published on Microraptor is listed below:

Anatomy

Pei, R., Q. Li, Q. Meng, K.-Q. Gao, and M.A. Norell. 2014. A new specimen of Microraptor (Theropoda, Dromaeosauridae) from the Lower Cretaceous of western Liaoning, China. American Museum Novitates 3821: 1–28. doi: 10.1206/3821.1

Xu, X., Z. Zhou, and X. Wang. 2000. The smallest known non-avian theropod dinosaur. Nature 408: 705–708. doi: 10.1038/35047056

Xu, X., Z. Zhou, X. Wang, X. Kuang, F. Zhang, and X. Du. 2003. Four-winged dinosaurs from China. Nature 421: 335–340. doi: 10.1038/nature01342

Plumage

Hone, D.W.E., H. Tischlinger, X. Xu, and F. Zhang. 2010. The extent of the preserved feathers on the four-winged dinosaur Microraptor gui under ultraviolet light. PLoS ONE 5: e9223. doi: 10.1371/journal.pone.0009223

Kiat, Y., A. Balaban, N. Sapir, J.K. O'Connor, M. Wang, and X. Xu. 2020. Sequential molt in a feathered dinosaur and implications for early paravian ecology and locomotion. Current Biology 30: 3633–3638. doi: 10.1016/j.cub.2020.06.046

Li, Q., K.-Q. Gao, Q. Meng, J.A. Clarke, M.D. Shawkey, L. D'Alba, R. Pei, M. Ellison, M.A. Norell, and J. Vinther. 2012. Reconstruction of Microraptor and the evolution of iridescent plumage. Science 335: 1215–1219. doi: 10.1126/science.1213780

Aerial locomotion

Chatterjee, S. and R.J. Templin. 2007. Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui. PNAS 104: 1576–1580. doi: 10.1073/pnas.0609975104

Dececchi, T.A., H.C.E. Larsson, and M.B. Habib. 2016. The wings before the bird: an evaluation of flapping-based locomotory hypotheses in bird antecedents. PeerJ 4: e2159. doi: 10.7717/peerj.2159

Dececchi, T.A., H.C.E. Larsson, M. Pittman, and M.B. Habib. 2020. High flyer or high fashion? A comparison of flight potential among small-bodied paravians. Bulletin of the American Museum of Natural History 440: 295–320.

Dyke, G., R. de Kat, C. Palmer, J. van der Kindere, D. Naish, and B. Ganapathisubramani. 2013. Aerodynamic performance of the feathered dinosaur Microraptor and the evolution of feathered flight. Nature Communications 4: 2489. doi: 10.1038/ncomms3489

Evangelista, D., G. Cardona, E. Guenther-Gleason, T. Huynh, A. Kwong, D. Marks, N. Ray, A. Tisbe, K. Tse, and M. Koehl. 2014. Aerodynamic characteristics of a feathered dinosaur measured using physical models. Effects of form on static stability and control effectiveness. PLoS ONE 9: e85203. doi: 10.1371/journal.pone.0085203

Pei, R., M. Pittman, P.A. Goloboff, T.A. Dececchi, M.B. Habib, T.G. Kaye, H.C.E. Larsson, M.A. Norell, S.L. Brusatte, and X. Xu. 2020. Potential for powered flight neared by most close avialan relatives, but few crossed its thresholds. Current Biology 30: 4033–4046. doi: 10.1016/j.cub.2020.06.105

Non-aerial locomotion

Dececchi, T.A. and H.C.E. Larsson. 2011. Assessing arboreal adaptations of bird antecedents: testing the ecological setting of the origin of the avian flight stroke. PLoS ONE 6: e22292. doi: 10.1371/journal.pone.0022292

Dececchi, T.A., A.M. Mloszewska, T.R. Holtz, Jr., M.B. Habib, and H.C.E. Larsson. 2020. The fast and the frugal: divergent locomotory strategies drive limb lengthening in theropod dinosaurs. PLoS ONE 15: e0223698. doi: 10.1371/journal.pone.0223698

Persons, W.S. IV and P.J. Currie. 2016. An approach to scoring cursorial limb proportions in carnivorous dinosaurs and an attempt to account for allometry. Scientific Reports 6: 19828. doi: 10.1038/srep19828

Diet

O'Connor, J., Z. Zhou, and X. Xu. 2011. Additional specimen of Microraptor provides unique evidence of dinosaurs preying on birds. PNAS 108: 19662–19665. doi: 10.1073/pnas.1117727108

O'Connor, J., X. Zheng, L. Dong, X. Wang, Y. Wang, X. Zhang, and Z. Zhou. 2019. Microraptor with ingested lizard suggests non-specialized digestive function. Current Biology 29: 2423–2429. doi: 10.1016/j.cub.2019.06.020

Xing, L., W.S. Persons IV, P.R. Bell, X. Xu, J. Zhang, T. Miyashita, F. Wang, and P.J. Currie. 2013. Piscivory in the feathered dinosaur Microraptor. Evolution 67: 2441–2445. doi: 10.1111/evo.12119

Description
I'm late for a bandwagon that I never planned on jumping on (namely Wyatt-Andrews-Art 's Dinos Through the Decades challenge), but sometimes an idea slips into your head and leaves you with no choice but to realize it.

Understandably, most entries for Dinos Through the Decades have focused on extinct animals that have been known to science for many decades or even centuries. However, there are some more recently discovered species for which our understanding has changed dramatically as well, and one of my favorite examples of this is Microraptor.

The original 2000 description of the type species, M. zhaoianus, is often overlooked these days, at least from a paleoartistic perspective. This is probably because the type specimen (IVPP V12330) is not particularly impressive, especially by Jehol standards, being a partial skeleton with poorly-preserved feather traces around the front and hind limbs. (In fact, I've often come across the misconception that the iconic IVPP V13352 specimen, which would not be described until a few years later, is the holotype of Microraptor instead.) Even so, Microraptor had a few claims to fame upon its description: the fact that it preserved any feathers at all was still noteworthy at a time when feathers had only been recently confirmed in non-avialan dinosaurs, it was the smallest non-avialan dinosaur known then (only about 40 cm long, though later specimens would show that it could grow substantially larger), and the tail of the type specimen had been part of the notorious "Archaeoraptor" chimera. Based on its small size and features of its toes and claws, its describers suggested that it may have been arboreal. Paleoartistic depictions of Microraptor from this time are relatively hard to come by, but generally showed it as a small, fuzzy theropod that climbed about in trees, though without specific adaptations for moving through the air.

That would change in 2003, when new specimens of Microraptor with more completely preserved plumage were described. These included the more famous IVPP V13352 specimen, which was described as the type specimen of a second species of Microraptor, M. gui (now commonly considered the same species as M. zhaoianus). These specimens revealed the presence of large, complex feathers on both the front and hindlimbs as well as the tail, very similar to the flight feathers of modern birds. It was suggested that the wing-like surfaces on all four limbs allowed Microraptor to glide, and it was reconstructed doing so by splaying its limbs to its sides. The supposed gliding adaptations, along with the argument that the long hindlimb feathers would have impeded locomotion on the ground, were used as further evidence for the idea that Microraptor primarily lived in trees. The image of Microraptor gliding from tree to tree with four splayed wings became a very popular one.

However, it was not long before some researchers pointed out that the ability to splay the hindlimbs in the manner originally reconstructed would be very unusual for a dinosaur, and there was little to indicate that Microraptor was much different from other dinosaurs in that regard. Several competing hypotheses were proposed for how the hindlimb feathers may have functioned with the legs held beneath the body in a more typical dinosaurian posture, with a "biplane-like" model receiving much press. Some of these potential gliding postures were notably shown being experimentally tested on an episode of Nova, "The Four-Winged Dinosaur", which aired in 2008.

Microraptor got another makeover in 2012, when it was inferred to have had largely glossy, iridescent feathers across its body based on analyzing the melanosomes preserved in the feathers of a new specimen (BMNHC PH881). This specimen additionally preserved a pair of especially elongate feathers at the tip of the tail, and the authors of the study reinterpreted the "feather mohawk" that had been shown on the head of previous depictions as a taphonomic artifact.

It was also around this time that several publications came out disputing the interpretation of Microraptor as a primarily arboreal animal, with some researchers even arguing for a mostly terrestrial lifestyle instead. One study comparing its limb proportions and overall anatomy to extant animals in detail found that it had more in common with terrestrial mammals and birds than with those that habitually climb. At the very least, the discovery of fish remains preserved as gut contents as well as analyses of gliding performance suggesting that Microraptor was a decent but not particularly specialized glider seemed to indicate that it was not a "dinosaurian flying squirrel" that was strictly tied to the trees.

Our knowledge about the life appearance of Microraptor hasn't changed much since then, but further studies have shed light on its possible lifestyle and life history. Perhaps most notably, a growing number of analyses have found evidence that Microraptor could have taken off under its own power; in other words, it was likely capable of powered flight instead of having been limited to gliding. This was not a total surprise; it had already been previously observed that Microraptor was as similar or more similar to flying birds in just about every flight-related part of its anatomy than was Archaeopteryx, for which powered flight abilities have also been debated but nonetheless considered. However, the flight apparatus of Microraptor still had notable differences from that of extant flying birds, so how well it could fly once it got into the air requires more research.

Some more findings about Microraptor that have been published in the last few years include additional gut contents revealing that it also fed on lizards, possible evidence of molting implying that it replaced its wing feathers in a specific sequence (similar to many flying birds today), and analyses of its hindlimb proportions suggesting that it was likely a faster runner than most other theropods of its size, contrary to previous assumptions that it would have been clumsy on the ground.

To be fair, there are a number of well-respected paleontologists who still consider an arboreal lifestyle for Microraptor plausible. However, I personally find the evidence that it was not an arboreal specialist to be fairly convincing, and I suspect that the pendulum will swing further in that direction in the future. I certainly wouldn't go as far as to claim that Microraptor never ventured into trees, but I do think that essentially all of the arguments for the idea that it spent most of its time living and feeding in trees are at least questionable.

The possible grasping function of its foot, for example, could equally be interpreted as a predatory adaptation , and its first toe is still unreversed and much smaller than in most birds that regularly perch in trees (even primarily terrestrial species that mostly use trees as roosts, like galliforms). It has also been found that ecology is very difficult to infer from claw curvature alone in extant animals, with large amounts of overlap in claw shape among different functional categories.

How the hindlimb feathers might have affected walking and running requires study, as their orientation and the potential muscular mechanisms controlling them in Microraptor remain uncertain. However, it can be said that domestic pigeon breeds with large hindlimb feathers are still quite capable of moving on the ground, especially individuals in which the longer feathers on the toes have worn away (which appears to be more similar to the condition seen in the preserved feathering of Microraptor). Furthermore, even if the hindlimb feathers were a hindrance in locomotion, an arboreal lifestyle would be unlikely to solve the problem, as tree climbing promotes crouched postures that lower the body's center of gravity, which, if anything, would bring the hindlimb feathers into closer and more frequent contact with the substrate. Ironically, the simplest way to keep the hindlimb feathers clear would have probably been to stand with the metatarsals raised high off the substrate, a stance more typical of terrestrial birds than arboreal ones.

Lastly, if Microraptor was capable of powered flight, that would remove climbing as a requirement for it to have gotten airborne.

A non-exhaustive selection of papers that have been published on Microraptor is listed below:

Anatomy

Pei, R., Q. Li, Q. Meng, K.-Q. Gao, and M.A. Norell. 2014. A new specimen of Microraptor (Theropoda, Dromaeosauridae) from the Lower Cretaceous of western Liaoning, China. American Museum Novitates 3821: 1–28. doi: 10.1206/3821.1

Xu, X., Z. Zhou, and X. Wang. 2000. The smallest known non-avian theropod dinosaur. Nature 408: 705–708. doi: 10.1038/35047056

Xu, X., Z. Zhou, X. Wang, X. Kuang, F. Zhang, and X. Du. 2003. Four-winged dinosaurs from China. Nature 421: 335–340. doi: 10.1038/nature01342

Plumage

Hone, D.W.E., H. Tischlinger, X. Xu, and F. Zhang. 2010. The extent of the preserved feathers on the four-winged dinosaur Microraptor gui under ultraviolet light. PLoS ONE 5: e9223. doi: 10.1371/journal.pone.0009223

Kiat, Y., A. Balaban, N. Sapir, J.K. O'Connor, M. Wang, and X. Xu. 2020. Sequential molt in a feathered dinosaur and implications for early paravian ecology and locomotion. Current Biology 30: 3633–3638. doi: 10.1016/j.cub.2020.06.046

Li, Q., K.-Q. Gao, Q. Meng, J.A. Clarke, M.D. Shawkey, L. D'Alba, R. Pei, M. Ellison, M.A. Norell, and J. Vinther. 2012. Reconstruction of Microraptor and the evolution of iridescent plumage. Science 335: 1215–1219. doi: 10.1126/science.1213780

Aerial locomotion

Chatterjee, S. and R.J. Templin. 2007. Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui. PNAS 104: 1576–1580. doi: 10.1073/pnas.0609975104

Dececchi, T.A., H.C.E. Larsson, and M.B. Habib. 2016. The wings before the bird: an evaluation of flapping-based locomotory hypotheses in bird antecedents. PeerJ 4: e2159. doi: 10.7717/peerj.2159

Dececchi, T.A., H.C.E. Larsson, M. Pittman, and M.B. Habib. 2020. High flyer or high fashion? A comparison of flight potential among small-bodied paravians. Bulletin of the American Museum of Natural History 440: 295–320.

Dyke, G., R. de Kat, C. Palmer, J. van der Kindere, D. Naish, and B. Ganapathisubramani. 2013. Aerodynamic performance of the feathered dinosaur Microraptor and the evolution of feathered flight. Nature Communications 4: 2489. doi: 10.1038/ncomms3489

Evangelista, D., G. Cardona, E. Guenther-Gleason, T. Huynh, A. Kwong, D. Marks, N. Ray, A. Tisbe, K. Tse, and M. Koehl. 2014. Aerodynamic characteristics of a feathered dinosaur measured using physical models. Effects of form on static stability and control effectiveness. PLoS ONE 9: e85203. doi: 10.1371/journal.pone.0085203

Pei, R., M. Pittman, P.A. Goloboff, T.A. Dececchi, M.B. Habib, T.G. Kaye, H.C.E. Larsson, M.A. Norell, S.L. Brusatte, and X. Xu. 2020. Potential for powered flight neared by most close avialan relatives, but few crossed its thresholds. Current Biology 30: 4033–4046. doi: 10.1016/j.cub.2020.06.105

Non-aerial locomotion

Dececchi, T.A. and H.C.E. Larsson. 2011. Assessing arboreal adaptations of bird antecedents: testing the ecological setting of the origin of the avian flight stroke. PLoS ONE 6: e22292. doi: 10.1371/journal.pone.0022292

Dececchi, T.A., A.M. Mloszewska, T.R. Holtz, Jr., M.B. Habib, and H.C.E. Larsson. 2020. The fast and the frugal: divergent locomotory strategies drive limb lengthening in theropod dinosaurs. PLoS ONE 15: e0223698. doi: 10.1371/journal.pone.0223698

Persons, W.S. IV and P.J. Currie. 2016. An approach to scoring cursorial limb proportions in carnivorous dinosaurs and an attempt to account for allometry. Scientific Reports 6: 19828. doi: 10.1038/srep19828

Diet

O'Connor, J., Z. Zhou, and X. Xu. 2011. Additional specimen of Microraptor provides unique evidence of dinosaurs preying on birds. PNAS 108: 19662–19665. doi: 10.1073/pnas.1117727108

O'Connor, J., X. Zheng, L. Dong, X. Wang, Y. Wang, X. Zhang, and Z. Zhou. 2019. Microraptor with ingested lizard suggests non-specialized digestive function. Current Biology 29: 2423–2429. doi: 10.1016/j.cub.2019.06.020

Xing, L., W.S. Persons IV, P.R. Bell, X. Xu, J. Zhang, T. Miyashita, F. Wang, and P.J. Currie. 2013. Piscivory in the feathered dinosaur Microraptor. Evolution 67: 2441–2445. doi: 10.1111/evo.12119