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