Sunday, 9 December 2012

On The Origin Of The Dinosaurs

Velociraptor as it is featured in Steven
Spielberg's 1993 masterpiece Jurassic Park
The evolutionary history of dinosaurs is a rich tapestry of strange predators, giant herbivores and events which changed the planet's biota on a global scale. Yet their beginnings are fragmented and incomplete. They evolved at a time when the world was in flux. The giant, cumbersome reptiles of the crumbling Palaeozoic empire were disappearing from the world, while smaller quick-footed forms were on the rise.

The scrub forests and deserts of Pangaea were harsh places to survive in, yet it was in these that the dinosaurs took their first steps on the road to dominance. This part we know, but when and as to which creature came first has been something of an enigma until recently. Attempting to extrapolate the identity of a completely new species from a 243 million year old humerus and a scattering of vertebrae is tricky to say the least, but this fossil may represent the oldest known dinosaur on the planet.

The first specimen (the humerus) was originally collected in the 1930s near Lake Nyasa in Tanzania. It remained in the collections of the Natural History Museum, London, with a brief description by British palaeontologist Alan J Charig, 20 years later. Yet its true identity has only become apparent during a post- doctoral study conducted by Sterling Nesbitt from the University of Washington.

'For 150 years, people have been suggesting that there should be Middle Triassic dinosaurs, but all the evidence is ambiguous,' said Nesbitt.

The fossils and microscope images of the bone structure of the
243 million year old dinosaur  Nyasasaurus parringtoni
Previous fossils of a 231 million year old creature called Eoraptor were originally heralded as the oldest known dinosaur remains on the planet, but this new creature, if it is a dinosaur, now named Nyasasaurus parringtoni, has pushed back the evolutionary origins of the group by 12 million years and even if it is not a true member, it is so closely related to them, that older forms than Eoraptor must have existed.

The single feature which tied this creature to the dinosaurs was a crest of bone running along the humerus which was once held fast the muscles which attached the arm to the torso. Known as an elongated deltopectoral crest, it is a common feature in all early dinosaurs which were bipedal. Later forms such as the giant sauropods were quadrupedal with no such need for the attaching ridge.

'We can tell from the bone tissues that Nyasasaurus had a lot of bone cells and blood vessels,' said Sarah Werning at the University of California, Berkeley and co-author of the paper published on the the discovery. 'In living animals, we only see this many bone cells and blood vessels in animals that grow quickly, like some mammals or birds. The bone tissue of Nyasasaurus is exactly what we would expect for an animal at this position on the dinosaur family tree. It's a very good example of a transitional fossil. ' 

An artist's impression of Nyasasaurus parringtoni,
possibly the oldest dinosaur on the planet
So what was this creature actually like? The researchers believe that it may have been about the same size as a large Labrador, but with a long, rigid tail, giving a total length of 2 to 3 metres, a hip height of a metre and a weight of anything between 20 to 60 kilograms. It would have needed to have been a fast runner to have survived in the Triassic period, so it is likely that it was quite light, despite this rather large range.

The Pangaean scrub forests and deserts were a tough environment and organisms had to be adaptable, a reason why the older Palaeozoic reptiles were out-competed by the dinosaurs and became extinct. Nyasasaurus was almost certainly predatory, but may have subsidized its diet with seeds and plant matter. The specimens on display in museums are impressive, but the truly important fossils are often hidden away in their vaults and back rooms. The discovery of Nyasasaurus demonstrates this perfectly.