Tuesday, 20 September 2011

The Cretaceous Asteroid May Have Killed The Archaic Bird Families

An artist's impression of Confuciusornis in life.
It was about the size of a crow
In the light of the recent discovery of Xiaotingia, scientists believe that birds first evolved in the early Cretaceous around 145 million years ago, represented by primitive and mysterious forms such as Confuciusornis and Hesperonis. Such creatures form a group of avians generally known as the archaic birds. These bizarre specimens inhabited a range of habitats from shallow tropical lakes to the canopies of vast jungles. The real mystery is what happened at the end of the Cretaceous?

There is a conflicting argument as to whether the bird population was in decline in the first place, and simply returned with renewed vigour after the removal of the dinosaurs, or suffered a similar catastrophe to their cold blooded, dominant cousins. A lack of fossil evidence is the real problem. Bird bones are light and filled with air sacks to maintain strength, but reduce in weight in order to fly. Yet they are fragile and do not preserve easily, leaving scientists with little insight into what happened to them during the defeat of the dinosaurs.

Samples of the bird remains discovered by Dr Longrich and his team
This has now changed. A team of scientists from Yale University led by Dr Longrich have discovered fragmentary avian remains dating to just 1.5 million years before the K-T boundary extinction in fossil deposits, from Dakota and Wyoming in the US and Saskatchewan in Canada. The most common bone found was the coracoid or shoulder bone. The bone does not vary much between species of specific groups and therefore can be used to chart avian diversity.

The team found that birds were bottle- necked into their modern day forms, leaving the archaic forms behind. Out of the 24 species analysed, 17 became extinct during the Cretaceous boundary, of which 7 were archaic forms. While these fossils were difficult to date due to poor stratigraphic distinction, more bird remains from China, Mongolia, Madagascar and Europe, from around the same age, have been discovered which point to the same conclusion.

The sheer geographical range of specimens only supports this theory. It is discoveries such as these that shed the most slight on evolution. Genetic evidence is wonderful, but fossil evidence is the real icing on the cake or nail in the coffin for many scientific theories. 'It is similar to what happened with mammals following the age of the dinosaurs' said Dr Longrich. After the extinction, birds would diversify and establish a brief dominant empire until mammals took over. Yet birds still remain top predators even today.