Sctratch marks found in a Cretaceous lakebed suggest a large dinosaur clawed the sediment as it swam against a current

Credit: Guillaume Suan

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PARIS: Twelve footprints found in the bed of an ancient lake in northern Spain have thrown up the first compelling evidence that some land dinosaurs could swim.

The 15-metre-long track in sandstone "strongly suggests a floating animal clawing the sediment" as it swam against a current, according to a team of researchers led by Ruben Ezquerra of the Foundation for Palaeontological Patrimony in La Rioja, Spain.

Therapod tracks

The swimmer is believed to have been a therapod - the vast family of carnivorous dinosaurs that included the mighty Tyrannosaurus rex - which lived in the Early Cretaceous, some 125 million years ago.

The trackway in the former lakebed consists of six asymmetrical pairs of two or three S-shaped scratch marks. Each set measures some 50 centimetres in length and 15 centimetres in width, the researchers report in the June issue of the U.S. journal Geology.

The prints paint a beguiling picture of a large, buoyant dinosaur whose clawed feet raked the sediment as it swam in a depth of some 3.2 metres of water. Ripple marks on the surface of the site indicate the dinosaur was swimming against a current, struggling to maintain a straight path.

Pelvic paddle

"The dinosaur swam with alternating movements of the two hind limbs, a pelvic paddle swimming motion," said co-author Loic Costeur of the Laboratory for Planetology and Geodynamics at the University of Nantes, western France. "It is a swimming style of amplified walking, with movements similar to those used by modern bipeds, including aquatic birds."

The question as to whether dinosaurs could swim has been debated for years. Until now, however, no firm evidence had come to light – only mysterious "ghost traces" at various sites.

Asked to speculate as to which dinosaur may have made the tracks, Costeur cautiously pointed to the allosaurus - a bipedal carnivorous dinosaur with a large skull balanced by a long, heavy tail. Some allosauruses could reach more than 10 metres in length.

The discovery opens up new avenues in dinosaur research, said Costeur. Computer modelling will be able to reveal more about anatomy and biomechanics, "as well as our view of the ecological niches in which they lived."

The set of footprints – dubbed the Virgen del Campo track – is located at the Cameros Basin in La Rioja, northern Spain, at the site of a delta to a former lake. The basin is already known as a treasure trove of footprints made by walking theropods.