McNamara, who is undertaking the research as part of work for a doctoral degree, said there were a number of reasons why fossilised bone marrow has never been found before.
"Because bone marrow rots away so quickly when people die, people never thought that it could be preserved.
"Also, because you have to fracture the bones before you can see inside, obviously if you have nice fossil specimens in a museum you are not going to get permission to break them up," she said.
The researchers accessed the marrow from the fossils because they were found in rock that had split, and some of the bones had fractured as a result.
McNamara said the original red and yellow colours of the marrow had been preserved.
"The fatty bone marrow is yellow and is a sort of energy reserve in terms of starvation or something and you also have red bone marrow and that is where your blood cells are produced," she said.
The main finds of marrow have been made in fossilised frogs and salamanders.
"We did find some very small fragments of bone marrow in one tadpole," she added.
