The world’s earliest evidence of bone marrow has been discovered in the fin of a 370-million-year-old fish.

The findings, published in the journal Proceedings of the Royal Society B, reveal how researchers analyzed the long bone from a Eusthenopteron – a lobe-finned fish from Canada believed to be related to the first tetrapod. Long bones, which are found in the limbs of tetrapods, are known to contain bone marrow, the flexible tissue inside bones that is responsible for blood cell production.  

Using synchrotron microtomography, a high-resolution imaging technique, researchers examined the internal structure of the fish’s long bones. They found that within the upper arm bone called the humerus, the tetrapods were already exhibiting typical marrow processes.

“We have discovered that the bone marrow certainly played a major role in the elongation of fin bone through complex interactions with the trabecular bone,” Sophie Sanchez, a researcher from Uppsala University in Sweden and the ESRF said in a statement. “This intimate relationship, which has been demonstrated by molecular experiments in extant mammals, is actually primitive for tetrapods.”

Researchers were able to reconstruct a 3D model of the microanatomy of the fish’s long bones. They found the marrow processes were longitudinal, larger than blood vessel canals, and connect to the shoulder and elbow joint surfaces of the humerus.

“Without the 3D information provided by the synchrotron, we could never have understood the internal organization of the marrow space,” Per Ahlberg from Uppsala University said. “If you cut a slice through a bone like this, which would damage it irreparably, you would only see an uninformative pattern of holes in the cut surface. With the synchrotron we can image the whole internal structure and understand how the marrow processes are organized, without doing any damage to the bone at all.”

The latest discovery is a significant one to understand the evolutionary history behind tetrapod limb bones and why bone marrow is located where it is.   

Eusthenopteron, a genus of extinct lobe-finned fish, has been preserved as fossils in rocks belonging to the late Devonian Period, about 370 million years ago. They are considered the main line of evolution leading to tetrapods, the first terrestrial vertebrates. The skeletal structure of their fin bones is similar to what is found in the femur bones of land animals. The fish also had primitive internal nostrils, enameled teeth and a two-part cranium.