Re: Floating Dinosaurs

["Richard W. Travsky" <rtravsky@uwyo.edu>]

On Sun, 26 Oct 2003, Dann Pigdon wrote:
> Somebody probably beat me to it, but...
> 
> PALEONTOLOGY: Hulking Dinosaurs Were Buoyant but Unseaworthy
> http://www.sciencemag.org/cgi/content/summary/302/5645/549a?etoc
> Science 302:549

(subscription required)

http://www.sciencenews.org/20031025/fob7.asp

Some of the heftiest four-legged dinosaurs ever to walk the Earth
occasionally left sets of footprints that include only the imprints of
their front feet. New laboratory and computer studies may explain what
those animals were doing with their hind legs.

The sauropod group of dinosaur species consisted of large herbivores, some
weighing up to 100 metric tons. These behemoths spent most of their time
on all fours but may have reared up on their hind limbs to defend
themselves or browse on high foliage. That posture can't explain the
trails of sauropod footprints with no traces of hind feet.

Adding water to the equation, however, may solve the puzzle. Computer
analyses of sauropod buoyancy conducted by Donald M. Henderson, a
paleontologist at Canada's University of Calgary, suggest that floating
sauropods of some species could indeed have made forefoot-only trackways.

Henderson's model divides a sauropod's body into many thin slices and
calculates both the downward-acting weight and the upward-acting buoyancy
of each slice. The model also accounts for body cavities, such as the
lungs, and for appendages, such as the neck, tail, and limbs. 

Brachiosaurus and Camarasaurus, sauropods that had relatively long front
limbs and a balanced weight distribution, floated with their forefeet
deeper than their back feet, Henderson found. So, they could have left
prints of only their front feet as they moved through shallow water.
However, Diplodocus and Apatosaurus, sauropods that had long tails and
carried most of their weight over their rear legs, floated with their hind
feet deeper than their front feet. That makes it almost impossible for
them to produce forefoot-only trackways while partially floating, says
Henderson. He presented results of his analyses last week at the annual
meeting of the Society of Vertebrate Paleontology in St. Paul, Minn.

However, there still may have been a way for even those sauropods to have
left no-hind-foot tracks, argue Jeffrey A. Wilson and Daniel Fisher of the
University of Michigan in Ann Arbor. By placing 1/40-scale models of
various sauropods on sensitive balances, the scientists measured the
changes in weight borne by each creature's front and rear feet as the
models were immersed in slowly rising water. When it reached wading depth
for the sauropods, the water partially buoyed the models' tails and
bodies. That would have shifted the animals' weights toward their front
feet, Wilson says.

At certain water depths, all the sauropod models that Wilson and Fisher
analyzedboth those with balanced weight distributions and those that were
hip-heavyexerted footprint pressures with their front feet that were more
than twice those exerted by their rear feet. Therefore, says Wilson, it's
possible that some sediments would record only the imprints of a wading
dinosaur's front feet. He presented these findings at last week's meeting
in St. Paul. 

References:

Henderson, D. 2003. Sauropod dinosaurs were the colossal corks of the
Mesozoic. Society of Vertebrate Paleontology annual meeting. October
15-18. St. Paul, Minn. Abstract available at
http://www.vertpaleo.org/meetings/VRPA2303supp_all.pdf

Wilson, J., and D. Fisher. 2003. Are manus-only trackways evidence of
swimming, sinking, or wading? Society of Vertebrate Paleontology annual
meeting. October 15-18. St. Paul, Minn. Abstract available at
http://www.vertpaleo.org/meetings/VRPA2303supp_all.pdf