Friday, 25 January 2013

Swimming Dinosaurs

In the third episode of the BBC series Walking with Dinosaurs, there is a scene of a theropod called Eustreptospondylus swimming through the shallow waters of a tropical ocean between two small verdant islands. The episode itself was set in Oxfordshire, some 150 million years ago, when sea levels were high and much of the land was reduced to a series of small archipelagos. The episode hypothesised that the residents of such islands survived by making short journeys through the water, too shallow to be patrolled by marine predators, to other islands in search for food and mates.

The 90 million year old river bed trackway
The Eustreptospondylus held its head above the water and methodically clawed its way forwards in a sort of doggy paddle, its legs circling and its arms splashing the surface. While this behaviour was hypothetical, recent evidence has shown that dinosaurs were not completely land-based creatures.

In 1984, a track-way, discovered in 90 million year old rocks in a tennis court- sized quarry in the Australian outback, were interpreted by palaeontologists, Tony Thulborn and Mary Wade, as having been made by a herd of small dinosaurs fleeing from a larger, 4 ton predatory theropod.

Most of the track-ways were small, around the size of chicken and emu footprints, but one set was larger with long toes. All were moving in the same direction, suggesting some kind of chase. The footprints crossed over one another and became a jumbled mess; possibly the creatures were in a rush or panicking at the time. New analysis, however, has thrown up a rather unexpected element to the story.

Instead of examining the 2D surface of the fossil, Anthony Romilio from the University of Queensland used 3D computer modeling to reconstruct the substructure of the tracks, as well as the surface. He found that the long-toed track-way was from a short-toed dinosaur, who had dug its feet into the soft sediment. Traces of vegetation, preserved with the stems, followed the same direction and suggest that the tracks were originally made on the bottom of a fast flowing river.
An artist's impression of a small dinosaur crossing the river

The idea of the dinosaurs digging the ends of their toes into the sediment also suggests that it may have been deep water. 'The animals were going on tippy-toes, kind of like a prima ballerina would dance across the stage,' said Romilio. As well as the tracks moving downstream with the current, their shape and depth shows that they were made over the course of days, as water levels went up and down.

It is likely that the track-ways preserve a section of a migration route or communal pathway. Even if they were made in water, this does not mean that the dinosaurs involved were attempting to escape from a predator. They may have been drinking at the river when the were ambushed or may have attempted to cross the river to gain some measure of safety. Complete fossils of dinosaurs are impressive, but it is often the traces of their everyday lives which are the most fascinating, the most meaningful. Often such specimens go unnoticed. Sometimes for nearly a 100 million years.