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Major discovery regarding the evolution of the bird's wing

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  • Major discovery regarding the evolution of the bird's wing

    Researchers may have found an important clue regarding how some lineages of dinosaur evolved into birds when a pair of researchers from the University of Tokyo's Department of Earth and Planetary Science discovered evidence that some of the non-avian dinosaurs from that line possessed a structure that allows birds to fly.

    The structure is known as the propatagium, a membranous body part that assists not just birds but all animals capable of flight save insects, in obtaining lift when flying or gliding (in flying lemurs the membrane is replaced by a fold of skin). Basically, as Professor Tatsuya Hirasawa explains, it's found at the leading edge of a bird's wing and "contains a muscle connecting the shoulder and wrist that helps the wing flapping and makes bird flight possible."

    Given that the propatagium consists of soft tissue and hence not likely to fossilize, the researchers came up with another stratagem for detecting their presence in fossilized remains tens of millions of years old. The solution that they arrived at was to to collect data about the angles of joints along the arm, or wing, of a dinosaur or bird.

    As Yurika Uno, a graduate student in Hirasawa's lab, explained, modern birds cannot fully extend their wings due to the presence of the propatagium because it limits the range of possible angles it can achieve. They figured that if they "could find a similarly specific set of angles between joints in dinosaur specimens, we can be fairly sure they too possessed a propatagium."

    Based on this clue, the researchers discovered that the propatagium probably evolved in a group of dinosaurs known as the maniraptoran theropods, which includes Velociraptors. Then they were able to, in spite of the odds, apparently identify the remnant of propatagia in the fossils of the feathered oviraptorosaurian Caudipteryx as well as the four-winged dromaeosaurian Microraptor. Importantly, the specimens in which they found it all existed prior to the evolution of flight in that lineage.

    I loathe the title:

    Source: How Did Birds Get Wings? We May Have Found The 'Missing Link' in Dinosaur Fossils


    The evolution of wings powerful enough to lift a vertebrate off the ground is one of the greatest mysteries in paleontology.

    Pterosaurs are famous for being the earliest known vertebrates to achieve true lift-off nearly 200 million years ago. Yet these massive ancient reptiles weren't dinosaurs, leaving the direct ancestors of birds to figure out the whole flying business all on their own.

    Avian dinosaurs would only evolve much later from two-footed, feathered theropods 80 million years or more after pterosaurs had already achieved powered flight.

    Despite these vastly different origin stories, birds use a strikingly similar structure to pterosaurs to stay aloft, one that, like feathers, seems to have evolved long before flight itself.

    Called a propatagium, it's a membrane present in all living vertebrates that flap their wings today, including birds and bats. Some gliding mammals even have a similar structure present across their parachute-like upper limbs.

    The best way to imagine a propatagium is to stick your arm out to the side with a bent elbow and wrist. Now picture a tendon stretching from your shoulder across to your hand, creating a bridge, or the 'leading edge' of a wing.

    This 'bridge' allows flying birds to flex and extend their wrist and elbow in unison during a flapping motion. The structure essentially gives lift to a bird's flight, allowing the animal to control two joints at once.

    Musculoskeletal system of the avian left wing..jpg
    Musculoskeletal system of the avian left wing

    For pterosaurs, its role is less clear, but the propatagium seems to have controlled flight take-off and landing by altering the flow of air over the wing's upper surface.

    Without the tendon present, some scientists think birds, bats, and dinosaurs might never have gotten off the ground.

    "It's not found in other vertebrates, and it's also found to have disappeared or lost its function in flightless birds, one of the reasons we know it's essential for flight," explains paleontologist Tatsuya Hirasawa from the University of Tokyo.

    "So, in order to understand how flight evolved in birds, we must know how the propatagium evolved."

    The problem is, the propatagium is a soft tissue, which means it's rarely preserved in the fossil record. What's more, this tendon is so thin, it doesn't leave much of a mark on the bones it attaches to.

    Thankfully, Hirasawa and his colleague, Yurika Uno, have figured out a way to 'see' the tendon, even when it's no longer there. The clue is to know how the propatagium restricts an animal's movements.

    When a modern bird dies, for instance, this membrane naturally keeps the animal's wrist and elbow in flexion.

    Comparing the conspicuous angle of the elbow to the bend of arms in non-avian theropod fossils, researchers have found evidence that a propatagium-like structure probably stretched across the shoulder and wrist of several terrestrial dinosaurs.

    For instance, the angles observed in the fossils of many 'maniraptorans' (which includes velociraptors) were slightly larger than those seen in modern birds, but they still hinted at the presence of an early propatagium-like structure.

    Comparison of how the propatagium evolved in theropod arms and bird wings.jpg
    Comparison of how the propatagium evolved in theropod arms and bird wings

    To back up these predictions, researchers also identified soft tissue remnants of what may be an early propatagium in two maniraptoran fossils: the turkey-sized Caudipteryx and the four-winged microraptor.

    The Caudipteryx probably couldn't fly, and it's still disputed whether the microraptor could. Yet both of these dinosaurs clearly possessed the structures that would later become necessary for powered flight.

    Evidence of soft tissues that resemble a propatagium (PPT) in dinosaur fossils A) microraptor B) Caudipteryx, and C) an enlarged image of B's inset

    Velociraptors are another maniraptoran that probably couldn't fly, but further research will be needed to see if their fossils hold clues of a long-lost propatagium.

    Based on the recent results, however, researchers at the University of Tokyo think the propatagium may be ancestral to all maniraptorans, a lineage that stretches back roughly 150 million years.

    A possible evolutionary lineage of the propatagium

    The only exception is the contentious Archaeopteryx, which is difficult to place on the dinosaur tree but may or may not be considered a maniraptoran. Whether the famous feathered dinosaur could even fly is hotly debated, with some speculating the animal used its limbs to simply glide.

    The angle of its wrist and elbow in fossils certainly doesn't suggest it had a propatagium-like structure.

    "Therefore," the authors of the new research write, "Archaeopteryx was probably incapable of executing the kinematics of modern avian powered flight."

    Maybe if Archaeopteryx had had a propatagium, its ancestors would still be flying today.


    © Copyright Original Source

    The full paper, Origin of the propatagium in non-avian dinosaurs can be read by clicking the hyperlink, with the Abstract also posted here


    Avian wings as organs for aerial locomotion are furnished with a highly specialized musculoskeletal system compared with the forelimbs of other tetrapod vertebrates. Among the specializations, the propatagium, which accompanies a skeletal muscle spanning between the shoulder and wrist on the leading edge of the wing, represents an evolutionary novelty established at a certain point in the lineage toward crown birds. However, because of the rarity of soft-tissue preservation in the fossil record, the evolutionary origin of the avian propatagium has remained elusive. Here we focus on articulated skeletons in the fossil record to show that angles of elbow joints in fossils are indicators of the propatagium in extant lineages of diapsids (crown birds and non-dinosaurian diapsids), and then use this relationship to narrow down the phylogenetic position acquiring the propatagium to the common ancestor of maniraptorans. Our analyses support the hypothesis that the preserved propatagium-like soft tissues in non-avian theropod dinosaurs (oviraptorosaurian Caudipteryx and dromaeosaurian Microraptor) are homologous with the avian propatagium, and indicate that all maniraptoran dinosaurs likely possessed the propatagium even before the origin of flight. On the other hand, the preserved angles of wrist joints in non-avian theropods are significantly greater than those in birds, suggesting that the avian interlocking wing-folding mechanism involving the ulna and radius had not fully evolved in non-avian theropods. Our study underscores that the avian wing was acquired through modifications of preexisting structures including the feather and propatagium.

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    Good news!
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    go with the flow the river knows . . .


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