Expanding the osteohistological toolbox for palaeophysiological studies

One way to study the thermometabolism of extinct organisms is to infer 2D measurements of red blood cells (erythrocytes), namely their width and area, using histological data from skeletal elements and incorporating them into palaeophysiological models. However, this method suffers from a major issue, limiting its usefulness: the model is based on femoral sections, and this element is known to be reduced or even missing in several groups of amniotes, especially marine ones. Moreover, recent studies questioned the use of erythrocyte width and area and instead proposed to use the mean corpuscular volume (MCV), which would also significantly expand the available data. In order to incorporate MCV in our studies, we: 1) tested if a significant correlation exist between the harmonic mean of vascular canals (HMC) and MCV; and 2) tested if there is a significant difference between HMC from femur and humerus, in order to use humeral data when femoral ones are missing. Our results demonstrate a significant correlation between HMC and MCV, and the absence of a significant difference in the inferences using the two different bones. This supports the modification of the current model towards the use of MCV as the response variable and the incorporation of humeral data, greatly expanding the available data for many animal groups.