A team of French and Swedish researchers has discovered earliest fossil evidence of limb bone marrow in the fin of a 370-million-year-old fish. Researchers studied the long bone from Eusthenopteron, a lobe-finned fish from Canada that is believed to be closely related to the first tetrapod.

Long bones in the limbs of tetrapods contain bone marrow - the soft flexible tissue inside bones that plays a crucial role in haematopoiesis/ formation of blood cells. In humans, about a hundred billion to one trillion new blood cells are generated every day to facilitate steady blood flow.

In the study, researchers from Uppsala University in Sweden and the European Synchrotron Radiation Facility (ESRF) in France aimed to determine the origin of the bone marrow within vertebrates. They employed synchrotron microtomography to examine the internal structure of the fish's long bones.

The researchers found typical marrow processes in the fish's upper arm bone (humerus). The processes were longitudinal, larger than blood vessel canals, and connect to the shoulder and elbow joint surfaces of the humerus.

 "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 press release. "This intimate relationship, which has been demonstrated by molecular experiments in extant mammals, is actually primitive for tetrapods."

"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," Per Ahlberg from Uppsala University said.

The finding is published in the journal Proceedings of the Royal Society B.

Recently, an international team of researchers found fossils of an ancient sea creature, Tamisiocaris, which filtered food like modern whales. The predators had flaps on either side of the body and large appendages in front of their mouth to probably capture larger prey like trilobites. A 3D analysis showed that Tamisiocaris gradually evolved into suspension feeders, where their gasping appendages camouflaged into a filtering tool to catch small crustaceans and other organisms as small as half a millimeter in size.