Computer simulations show evolution of birds' crouch likely due to increase in forelimb size (w/ video)

Apr 25, 2013 by Bob Yirka report
Credit: Concept art by Luis Rey

(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, , into a . 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.

This video is not supported by your browser at this time.
Movie showing the 3D modelling procedure that we used; here featuring the large dinosaur Allosaurus. Credit: John R. Hutchinson

This video is not supported by your browser at this time.
Movie showing all 17 of our computer models, in evolutionary sequence from furthers to closest in relationship to living birds. Credit: John R. Hutchinson

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 . They point to real life dinosaur examples, such as Archaeopteryx, Microraptor and the Velociraptor as proof of their theory—each sported shorter tails, bigger and a crouched position due to a more forward center of gravity.

Animal standing or at the midpoint of a step (a). For the animal to balance, forces applied by the feet (red) must match the force of body weight (blue) pointing downwards from the centre of mass (yellow/black). If the centre of mass moves forward (b), then the feet must move forward (and thus the limb must get more crouched) to maintain balance, as in (c). Credit: John R. Hutchinson


Explore further: New hadrosaur noses into spotlight

More information: Linking the evolution of body shape and locomotor biomechanics in bird-line archosaurs, Nature (2013) doi:10.1038/nature12059

Abstract
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.

Press release

Related Stories

Muscle reconstruction reveals how dinosaurs stood

Jun 21, 2012

(Phys.org) -- Much is known about the dinosaurs that walked on 4 legs like Stegosaurus and Triceratops, but their stance has been a topic of debate, until now. Scientists at the Natural History Museum have ...

Archaeopteryx and the dinosaur-bird family tree

Sep 15, 2011

The magpie-sized Archaeopteryx had bird and dinosaur features and helped show that birds evolved from dinosaurs. However, recent research in the journal Nature questions its position in the dinosaur-bird family ...

Early birds had an old-school version of wings

Nov 21, 2012

In comparison to modern birds, the prehistoric Archaeopteryx and bird-like dinosaurs before them had a more primitive version of a wing. The findings, reported on November 21 in Current Biology, lend support ...

Mysterious feathered dinosaur was agile flyer

Oct 23, 2012

(Phys.org)—In 2003, a mysterious and surprising dinosaur was discovered that possessed not only wings on its arms but also long feathers on each leg forming a "hind wing." This was a completely new and unexpected body plan ...

Dinosaur shook tail feathers for mating show

Jan 16, 2013

(Phys.org)—A University of Alberta researcher's examination of fossilized dinosaur tail bones has led to a breakthrough finding: some feathered dinosaurs used tail plumage to attract mates, much like modern-day ...

Recommended for you

New hadrosaur noses into spotlight

Sep 19, 2014

Call it the Jimmy Durante of dinosaurs – a newly discovered hadrosaur with a truly distinctive nasal profile. The new dinosaur, named Rhinorex condrupus by paleontologists from North Carolina State Univer ...

Militants threaten ancient sites in Iraq, Syria

Sep 19, 2014

For more than 5,000 years, numerous civilizations have left their mark on upper Mesopotamia—from Assyrians and Akkadians to Babylonians and Romans. Their ancient, buried cities, palaces and temples packed ...

New branch added to European family tree

Sep 17, 2014

The setting: Europe, about 7,500 years ago. Agriculture was sweeping in from the Near East, bringing early farmers into contact with hunter-gatherers who had already been living in Europe for tens of thousands ...

User comments : 0