Archaeopteryx, the famous feathered fossil, was probably the oldest and most primitive bird after all.

For 150 years the creature occupied top spot on the avian evolutionary tree until this summer when the discovery of a close relative suggested it was a mere bird-like dinosaur. Now it looks to have regained its previous perch thanks to a more sophisticated anatomical analysis.

"This shows that when you look at the data with a higher degree of analytical rigour it supports the traditional view that Archaeopteryx is a bird," said Dr Paul Barrett, a dinosaur researcher at London's Natural History Museum.

The first complete specimen of Archaeopteryx was discovered in Germany in 1861, two years after the publication of Charles Darwin's On the Origin of Species.

It lived around 150 million years ago, had sharp teeth, three fingers with claws, a long bony tail, feathers, broad wings, could grow to about 0.5 metres in length and could fly.

This combination of avian and reptilian characteristics saw it positioned at a key spot in the branching off of birds from dinosaurs in the tree of life, and provided hard evidence to back Darwin's theory of evolution by natural selection.

Since then palaeontologists have largely taken it as the starting point for bird life.

But in July, researchers led by Xing Xu at Linyi University, China, announced the unearthing of Xiaotingia zhengi, a previously unknown chicken-sized dinosaur. The group carried out a statistical analysis of its anatomical traits that placed it in a group of bird-like dinosaurs called deinonychosaurs.

Archaeopteryx was so closely related to the new arrival that the consequent tweaking of the tree of life saw it shifted into this grouping as well.

Now Dr Michael Lee of the South Australian Museum in Adelaide, Australia, has repeated the exercise using the same technique, known as phylogenetic analysis, only this time applying a more sophisticated statistical method.

Instead of considering all anatomical traits he examined as equally informative, Dr Lee placed greater weight on slow-evolving characteristics, in order to minimise the effect of biological traits that evolve independently in unrelated lineages.

"When we did this for Archaeopteryx we found this pulled it away from dinosaurs such as Velociraptor and nestled it back with the birds," said Dr Lee.

The research was published on Wednesday in the Royal Society journal Biology Letters.

As a greater number of related species and specimens have been discovered, the distinctions between them have become smaller, leading to several species flipping between groups.

"There is now a very fine line between birds and bird-like dinosaurs, with only some subtle differences between them," added Dr Barrett. "As a result it's not surprising that the positions of these animals will occasionally flip around in the tree as they are really very similar."