Evolution of Red Blood Cells in Extant and Extinct Tetrapods

Several authors showed that red blood cell volume (Mean Corpuscular Volume, MCV) is inversely correlated with metabolic rate; therefore, reconstruction of MCV can give useful hints on the physiology of extinct tetrapods. In the present study, the usefulness of histological parameters as proxies of MCV were evaluated. It was found that osteocyte size and vascular minimum diameter are significantly correlated with MCV in non-mammals, but are not correlated with MCV in mammals. This is due to the fact that mammalian red blood cells are enucleated. Using literature data, osteocyte size and vascular minimum diameter were used to reconstruct MCV in extinct non-synapsid tetrapods. Moreover, it was found when enucleation of red blood cells probably appeared in the synapsid lineage, and thus MCV was reconstructed also in Synapsida. My hypothesis is that enucleation of RBC in Eucynodontia allowed to efficiently oxygenate tissues with high metabolic demand, even in the presence of high affinity hemoglobins. This would not have been the case for dinosaurs, whose hemoglobin could have retained, like in birds, a low affinity (perhaps linked to their unidirectional respiratory system) and could easily release oxygen in tissues with high metabolic rates. Finally, a trend of MCV decrease over time (probably indicating an increase in metabolic rates) was found both in the synapsid and the ornithodiran lineages.