The evolution of bone marrow and its interaction with surrounding bones has remained partially mysterious, but scientists have now discovered the earliest fossil evidence for the presence of bone marrow in the fin of a 370 million-year-old fish.

 

 A team of researchers discovered that Eusthenopteron, a Devonian (370-million year old) lobe-finned fish from Miguasha in Canada that is closely related to the first tetrapods, already 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. Researchers from Uppsala University in Sweden and the European Synchrotron Radiation Facility (ESRF) in France looked for the origin of the bone marrow within vertebrates.

 

 They used synchrotron microtomography (imaging technique based on radiography) to investigate the interior structure of fossil long bones without damaging them.

 

 Thanks to the beam power of the ESRF, they were able to reach submicron resolutions and 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, she added.

 

 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.

 

 The study appeared in the journal Proceedings of the Royal Society B.