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

We use bone histology to model evolutionary trends in red blood cell size in extinct archosaurs. The early-diverging archosauromorph Prolacerta
and all bird-line (Avemetatarsalia) archosaurs show significant reductions in red blood cell size compared to other non-avian reptiles, suggesting that the
last common ancestor of archosaurs may have exhibited higher exercise capabilities and aerobic thresholds. Greater aerobic capacities may have allowed bird-line
archosaurs to be adaptable to tumultuous climatic extremes. In contrast, red blood cell size increased in the croc-line (Pseudosuchia) archosaurs with a
secondarily aquatic lifestyle, consistent with relaxed demands on oxygen diffusive capacity and lower oxygen needs during aquatic exercise.

Raw RBC and Histometric Data: Byrne_ESM
Code Language: R
Code Location: Byrne_ESM.txt
*Raw Histometric Data Unit Information*: Our histome...,