Earliest evidence of limb bone marrow in 370-mln-yr-old fish

Scientists have found the earliest fossil evidence for the presence of bone marrow in the fin of a 370 million-year-old fish.
Long bones, which are found in the limb of tetrapods, are not only important for locomotion and supporting the weight of the body, but also host the bone marrow.
The latter plays a major role in haematopoiesis, the formation of blood cells. In a healthy adult human, about a hundred billion to one trillion new blood cells are produced every day to maintain the stable blood circulation.
Researchers from Uppsala University in Sweden and the European Synchrotron Radiation Facility (ESRF) in France decided to look for the origin of the bone marrow within vertebrates, using synchrotron microtomography to investigate the interior structure of fossil long bones without damaging them.

They discovered that Eusthenopteron, a Devonian (370-million-year-old) lobe-finned fish from Miguasha in Canada that is closely related to the first tetrapods, exhibited typical marrow processes inside its humerus (upper arm bone).
These processes are longitudinal, larger than blood vessel canals, and connect to the shoulder and elbow joint surfaces of the humerus.

The researchers were able to accurately reconstruct the 3D arrangement of the long-bone microanatomy of this close relative of tetrapods.
"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," said Sophie Sanchez, a researcher from Uppsala University and the ESRF.

"This intimate relationship, which has been demonstrated by molecular experiments in extant mammals, is actually primitive for tetrapods," Sanchez said.
This discovery is very important for understanding the evolutionary steps that built up the distinctive architecture of tetrapod limb bones and created a location for the distinctive, complex and functionally important tissue that is bone marrow, researchers said.
The study was published in the journal Proceedings of the Royal Society B.