Diverging trends in erythrocyte size elucidate cardiovascular evolution in stem dinosaurs and crocodilians

Red blood cell (RBC) size constrains the rate of diffusion of gases between (i) the environment and the capillary beds of the gas exchanger and (ii) the blood and organs. In birds, small RBCs with a high surface area to volume ratio permit a high O₂ diffusion capacity and facilitate sustained, vigorous exercise. Unfortunately, our knowledge of archosaur cardiovascular evolution is incomplete without fossilized RBCs and blood vessels. However, muscle capillary diameters closely match RBC width and, importantly, these microvessels leave a signature in bone in the fossil record. Here, we ask: do fossilized, histological indicators of RBC size, combined with phylogenetic information, support divergent patterns of cardiovascular evolution in Mesozoic crocodile-line and bird-line archosaurs? Building on a published dataset, we used vasculo-lacunar histometrics and phylogeny to retrodict RBC sizes in 20 extinct and 20 extant tetrapods. Our results indicate decreases in RBC size within the archosauromorph <i>Prolacerta</i> and in bird-line archosaurs (Avemetatarsalia). Conversely, crocodile-line archosaurs (Pseudosuchia) that transitioned to an aquatic environment demonstrated increases in RBC size. These patterns offer an opportunity to probe physiological hypotheses regarding archosaur cardiovascular evolution and can explain, in part, the contrasting aerobic capacities of extant species in these two major archosaur lineages.