April 25, 2013 report
Computer simulations show evolution of birds' crouch likely due to increase in forelimb size (w/ video)
(Phys.org) —An international team of researchers working together to discover how, when and why birds have evolved to stand in a crouching position, have come to the conclusion that it was due much more to the growth of forelimbs than a reduction in size of the tail. The team describes in their paper published in the journal Nature, how they built computer simulations to recreate in a virtual sense, the evolution process that led to the crouching position and possibly the evolution of flight.
For many years, the consensus among those who study dinosaurs was that the crouch seen in modern birds was most likely due to a shift in center of balance as tails grew smaller over time. To find out if this was actually the case, the researchers fed data from several types of dinosaurs (mostly archosaurs), modern birds and their closet living relative, crocodiles, into a computer model. Using that information, they built skeletons and then manually covered them with muscle and skin. The computer was then directed to simulate changes in body structure over millions of years of evolution to see how they impacted the center of gravity of evolving dinosaurs. Surprisingly, they found that it wasn't slowly diminishing tails that caused the animals to shift their stance, it was the development of larger forelimbs, which of course, over many more millions of years, for some, led to the development of wings.
The simulation isn't able to show definitively if the increase in forelimb size, or the shift in stance was responsible for the development of flight, but it certainly seems logical to conclude that it was all part of the same gradual process. Developing a crouched position was necessary the researchers note, to support the increased mass that developed towards the front part of the dinosaurs. But that, they add, also means adding more hind leg muscle to support the crouch—it's a far less energy efficient posture. They point to real life dinosaur examples, such as Archaeopteryx, Microraptor and the Velociraptor as proof of their theory—each sported shorter tails, bigger forelimbs and a crouched position due to a more forward center of gravity.
Locomotion in living birds (Neornithes) has two remarkable features: feather-assisted flight, and the use of unusually crouched hindlimbs for bipedal support and movement. When and how these defining functional traits evolved remains controversial. However, the advent of computer modelling approaches and the discoveries of exceptionally preserved key specimens now make it possible to use quantitative data on whole-body morphology to address the biomechanics underlying this issue. Here we use digital body reconstructions to quantify evolutionary trends in locomotor biomechanics (whole-body proportions and centre-of-mass position) across the clade Archosauria. We use three-dimensional digital reconstruction to estimate body shape from skeletal dimensions for 17 archosaurs along the ancestral bird line, including the exceptionally preserved, feathered taxa Microraptor, Archaeopteryx, Pengornis and Yixianornis, which represent key stages in the evolution of the avian body plan. Rather than a discrete transition from more-upright postures in the basal-most birds (Avialae) and their immediate outgroup deinonychosauria5, 6, our results support hypotheses of a gradual, stepwise acquisition of more-crouched limb postures across much of theropod evolution, although we find evidence of an accelerated change within the clade Maniraptora (birds and their closest relatives, such as deinonychosaurs). In addition, whereas reduction of the tail is widely accepted to be the primary morphological factor correlated with centre-of-mass position and, hence, evolution of hindlimb posture, we instead find that enlargement of the pectoral limb and several associated trends have a much stronger influence. Intriguingly, our support for the onset of accelerated morpho-functional trends within Maniraptora is closely correlated with the evolution of flight. Because we find that the evolution of enlarged forelimbs is strongly linked, via whole-body centre of mass, to hindlimb function during terrestrial locomotion, we suggest that the evolution of avian flight is linked to anatomical novelties in the pelvic limb as well as the pectoral.
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