Wednesday, 17 June 2009
A bird in the hand...
There are lots of reasons to think that birds evolved from therapod dinosaurs but there are also lots of questions. One of those questions appears to have been answered by a new fossil find.
Early therapods have a hand with five digits. Birds have three. As the shape of the birds 'fingers' look like numbers 1, 2 and 3 of their dino predecessors it was assumed that they had simply lost numbers 4 & 5 through time.
Then enter the developmental biologists to throw a spanner into the works! They observed that in bird embryos all five digits start to develop, but then numbers 1 & 5 wither away leaving 2, 3 & 4. That kind of vestige is powerful evidence for evolution but it did leave a bit of a puzzle. This new find helps sort it out. The developmentalists were right.
Limusaurus inextricabilis is a therapod dinosaur of a 160 million year vintage. It has 4 digits but number 1 is dramaticaly reduced in size whilst 5 is absent completly, and there are shape changes too... in other words this is a snapshot in the process that starts with 5 fingered therapods and ends with modern day birds.