The discovery, which is the first of its kind, is being heralded as a potentially groundbreaking find. Intact bone marrow has never before been discovered in a fossil of that age, according to study leader Maria McNamara.

The finding, detailed in the upcoming issue of the journal Geology, was made in the Tereul province of north eastern Spain. The Tereul province was once a deep lake, which has led to many ancient species of frog and salamanders being uncovered in the region.

Researchers from UCD were examining fossils in a local museum when they came across what they suspected was marrow tissue in the bones of a 10m year-old frog. According to McNamara, the research team, which included scientists from the UK, USA and Spain, did not believe that they could have stumbled upon a discovery of such major importance so accidentally.

"We saw a bit of red inside the bone but thought that it would be too good to be true for it to be bone marrow", she said. "We brought it back to Ireland for analysis and we really couldn't believe it when we found out that it was."

Although preserved red blood cells were discovered in the cells of a Tyrannosaurus Rex's leg bone last year, this is the first time fully preserved marrow has been uncovered.

According to McNamara, the discovery will allow researchers to build-up unprecedented profiles of how extinct species lived. If scientists could find bone marrow from dinosaurs, it could help resolve the debate about whether the creatures were warm-blooded or not, she said.

The species of frog from which the marrow was extracted is now extinct, although the families to which they belonged are still alive.

McNamara says that testing on fossilised bone is difficult because researchers must get to the centre of the bone before they can test for marrow. Marrow may exist in already exhumed fossils but may have been missed because detection requires breaking the bones apart.

"We were lucky in that the bones were fractured already, which allowed us to examine them", she says. "We can't go around smashing up bones in museums so we will have to use x-ray equipment if we want to examine any more specimens."

The red blood cells discovered in the Tyrannosaurus Rex leg last year were found only after researchers were forced to break off parts of the bone in order to fit it into a helicopter. After breaking the bone, the scientists noticed the cells.