Image caption Baryonyx used its very long snout to catch fish

A 5cm-wide (2in) fossil may have something big to say about how dinosaurs ranged across the Earth.

The 125-million-year-old neck vertebra belonged to a spinosaurid - an animal with a crocodile-like snout that it probably used to prey on fish.

The specimen is the first such dinosaur identified in Australia but one that is nearly identical to a UK creature.

This suggests northern and southern hemisphere dinos had a lot more in common than previously thought.

The traditional idea has been that these ancient animals could be placed into distinctive, geographically separated, groups. This small vertebra undermines that view, says Dr Paul Barrett from London's Natural History Museum.

"After looking at this specimen and having been forced to re-assess the distribution of spinosaurids, we took a look at other dinosaur groups from Australia, including the Tyrannosaur our team announced last year," he told BBC News.

"Taking all this evidence into account, we started to realise that a lot of dinosaur groups we'd thought of as either northern specialists or southern specialists actually had more cosmopolitan distributions."

It may be just one bone, but the team says it displays features that are unmistakably those of a spinosaurid.

Image caption A comparison of the neck bone from the Australian dino (L) and its UK counterpart (R). They share very distinctive spinosaur features

The vertebra, which is almost certainly from a juvenile, was unearthed on the coast of Victoria state.

It is hugely reminiscent of the neck bones in the well-known British dinosaur Baryonyx walkeri.

The remains of this UK creature were found in southern England, which in Cretaceous times was a lot warmer and covered by lagoons.

It had the classic crocodile-like skull with conical teeth that were ideally suited for catching fishy prey in the expanses of shallow water.

The assumption is that the Australian version pursued a very similar lifestyle.

Its discovery location was probably a flood plain in a rift valley created as Australia and Antarctica were breaking apart.

The dinosaur's Baryonyx-style snout would have been ideal for pulling fish out of the waters on this plain.

"The evidence is very limited - we'd be the first to admit that, but we're very confident that this Australian specimen belongs to the spinosaurids," Dr Barrett told BBC News.

"And because all the other members of this group share the same skull features we're pretty sure this dinosaur behaved in much the same way."

Jonathan.Amos-INTERNET@bbc.co.uk