An open access paper published in eLife a couple of days ago, sheds light on a mystery surrounding the evolution of the limb bones of terrestrial tetrapods; in particular the evolution of bone marrow and the transfer of red blood cell production into the bone marrow and away from the liver and kidneys, where they are normally produced in most aquatic vertebrates such as fish.
Using the powerful x-ray beam produced by the European Synchrotron Radiation Facility, an internationa team of scientists led by Jordi Estefa of Department of Organismal Biology, Evolution and Development, Uppsala University, Uppsala, Sweden, examined the limb bones of fossil lobe-finned fish and early terrestrial tetrapods.
The eLife Digest explains:
For many aquatic creatures, the red blood cells that rush through their bodies are created in organs such as the liver or the kidney. In most land vertebrates however, blood-cell production occurs in the bone marrow. There, the process is shielded from the ultraviolet light or starker temperature changes experienced out of the water.
It is possible that this difference evolved long before the first animal with a backbone crawled out of the aquatic environment and faced new, harsher conditions: yet very little fossil evidence exists to support this idea. A definitive answer demands a close examination of fossils from the water-to-land transition including lobe-finned fish and early limbed vertebrates. To support the production of red blood cells, their fin and limb bones would have needed an internal cavity that can house a specific niche that opens onto a complex network of blood vessels.