Wednesday, 14 March 2012

An Ancient Ancestor To The Dinosaurs

An artist's impression of Asilisaurus kongwe with
the sail backed Hypselorhachis in the background
The dinosaurs are, without a doubt, the most iconic group of extinct animals on Earth. Every child by the age of five can name the Cretaceous hypercarnivore Tyrannosaurus rex. Others tend towards the more herbivorous side of things, looking at the giant sauropods such as Diplodocus or Brachiosaurus. Yet their origins are a mystery. Over the years, palaeontologists have found fossils of very primitive dinosaurs and their very close ancestors.

One such creature is Asilisaurus kongwe, which was discovered in 2001 from 240 million year old, mid-triassic rocks on Tanzania, Africa. Unlike many different proto-dinosaurs and late ancestors, fossil remains of this creature were common, with bones from over 14 different individuals recovered from a single bone bed, allowing a skeleton to be constructed, minus parts of the hand and skull. This new creature stood around 1 to 1.5 metres tall at hip height, was around 20 to 30 kilograms and 1 to 3 metres in length.
A skeletal reconstruction of Asilisaurus kongwe as the fossil of the creature

It falls just outside of the dinosaur family tree by just 10 million years. What makes this discovery interesting is the way the creature moved. It was a quadruped, walking and running on four legs. The earliest dinosaurs were bipeds, meaning that the change in locomotion occured in a short period of time. While we have fossils of the very last ancestors of the famous reptiles which show signs of being able to move both on four and two legs, the change was actually more complex than one might think.

The proposed relationship of Asilisaurus to the dinosaurs
Firstly the tail had to become longer and heavier in order to provide a counterbalance to the head and torso. The muscles also had to become denser in order to provide counter thrust to the legs. Secondly, the legs had to become more powerful in order to provide enough energy to stay upright and move. Finally the body and head had to become lighter and more streamlined to increase efficiency and balance. All of this had to occur within 10 million years.

The question is what drove this sudden change in locomotion which would ultimately lead to the evolution of the dinosaurs? 'The research suggests that at least three times in the evolution of dinosaurs and their closest relatives, meat-eating animals evolved into animals with diets that included plants' said Randall Irmis, curator at the Utah Museum of Natural History.

'These shifts all occurred in less than 10 million years, a relatively short time by geological standards, so we think that the lineage leading to silesaurs and dinosaurs might have had a greater flexibility in diet, and that this could be a reason for their success.' Silesaurs were a group of reptiles which lived through the mid to late Triassic. They were most closely related to the dinosaurs, the relationship being analogous to that between chimpanzees and humans. 

It is likely that they diverged from the last common ancestor of the dinosaurs. Yet their mainly herbivorous lifestyle may have been their undoing in contrast to the mainly carnivorous dinosaurs, which explains the early success of the latter.