Data from: 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 1) the environment and the capillary beds of the gas exchanger,
and 2) 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 Prolacerta 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.