Dinosaur bites back

Big Al: Like a blow from a hatchet

By BBC News Online's Ivan Noble

The dinosaur known as Big Al, or more formally Allosaurus fragilis, was a "slash and tear" feeder, not a bone cruncher like T. rex, a new study says.

It has a weird combination of features

Emily Rayfield, Cambridge University

It used its powerful neck muscles to drive its skull down into its prey like a hatchet and then tear away pieces of flesh, report researchers at Cambridge University, UK.

They borrowed an engineering technique to analyse the forces at work in Big Al's skull and found it too light and unnecessarily strong to have belonged to a bone cruncher.

A. fragilis probably tore at its food like a modern, giant lizard counterpart: the Komodo 'dragon' (Varanus komodoensis).

Big Al - so called because the fossil skeleton found in the United States was almost complete - featured in the BBC programme The Ballad of Big Al, made by the Walking With Dinosaurs team.

'Weird features'

The biggest of its kind were 12 metres (39 feet) long, stood three metres (10 feet) tall and lived during the late Jurassic period, around 150 to 145 million years ago.

Emily Rayfield and her colleagues at Cambridge's Department of Earth Sciences wanted to try to explain the peculiar structure of its skull.

The Komodo dragon is a modern "slasher"

"It has a weird combination of features. The upper part of skull is strong but the front of the lower jaw is weak, and the skull has a series of struts at the back," she told BBC News Online.

Dr Rayfield used a technique called FEA, or finite element analysis.

"We built a digital model of the structure of the skull and then divided it into 3D blocks."

She explained that once a computer knows the properties of each block - the elasticity for instance - it can calculate how the whole skull would behave under stress.

Heavy hatchet

The technique has been used for many years in engineering and orthopaedic design, but the idea of trying it on dinosaurs came when a colleague heard of a study on horses' hooves, she said.

Modern cow bones are a near equivalent to dinosaur bones, so she used elasticity data from them, then calculated how strong the dinosaur's muscles would have been.

Allosaurus generally used a high velocity impact of the skull into its prey

Emily Rayfield, Cambridge University

It turned out that the Allosaurus skull was "overengineered": it could withstand much higher forces than it would need to cope with simply biting and crunching bones. And though it had a relatively tame bite, it could withstand high forces applied upwards to its teeth.

"There were two ways to explain this. Either it was overdesigned for day-to-day basic use or it was an adaptation to its feeding strategy: opening its jaws really wide and slamming its upper tooth row down into its prey," Dr Rayfield said.

"Allosaurus generally used a high-velocity impact of the skull into its prey. An analogue would be a person wielding a large, heavy hatchet," she writes in the scientific journal Nature.