Wednesday, 12 May 2010

Long-necked dinosaurs

Among many questions I've never thought to ask is this: why did sauropods (Diplodocus, Cetiosaurus, etc) develop those enormously long necks?

Turns out a group of researchers in Germany asked that very quetion about 6 years ago, and they've just published their results.

..an elephant hardly has time to sleep. It spends 18 hours every day satisfying its huge appetite. 'This led us to one of the many riddles that gigantism of dinosaurs puts before us,' Professor Martin Sander from the University of Bonn explains.
Sauropods, having been very much bigger than elephants, must have had a big problem simply ingesting enough food, but the Germans have worked out a couple of key characteristics which helped them.

Sauropods had simple teeth and jaw mechanisms, so couldn't chew. They just nipped off mouthfuls of food and swallowed it. This meant their heads could be small and light, whereas an elephant needs a big, robust skull to house all the chewing muscles.

A tiny skull means you can stick it at the end of a long neck, so you can stand in one place and still reach a large area of vegetation. So the evolutionary pressure was to get bigger to be resistant to predation by large carnivores, and one helpful feature was a longer neck to reach more food without expending any energy in actually moving around. Neato!
However, the herbivorous giant dinosaurs had relatively small, light skulls. Only this fact enabled them to grow extremely long necks. And these again helped them to make food intake as efficient as possible. So they did not constantly have to heave their 80-ton body over the Jurassic savanna while looking for their greens. They just remained on the spot and used their agile neck to browse their surroundings. This was particularly relevant for the heavy-weights. Smaller dinos simply had far smaller necks compared to their body length.
The other thing which helped them along was a clever feature of their lungs, these days found in birds. About 230 million years ago there was a dip in atmospheric oxygen levels (I don't know why yet) to about 2/3 current levels. So the dinosaur's precursers were gasping for oxygen and evolved this neat trick.

Our lungs are relatively simple balloons that we inflate and deflate. Dinosaurs and birds have air sacs in various other parts of their bodies and when they breathe in, the air flows through the lungs and into these air sacs, so some oxygen exchange can take place on the in-breath. Then when they breathe out again, it all flows through the lungs again, giving a second chance for gas-exchange. The whole system is much more efficient than that in mammals.

So the sauropods could eat without moving around much, and breathed really efficiently. Such a shame about that damned asteroid!

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