Femoral growth plate is sensitive to loss of cardiac shunt in the American alligator

Longitudinal growth rate (LGR) has been shown to correlate with calcified cartilage column height (CCC) in the growth plates of birds. In order to infer limb bone growth patterns in non‐avian dinosaurs, this correlation needs to be documented in crocodilians. We tested the LGR‐CCC correlation in femora of juvenile alligators raised for two years under laboratory conditions, and injected with fluorochrome dyes (alizarin, calcein). Some alligators were constrained to in‐series circulation by left aortic ablation (n=24), whereas sham‐operated controls retained in‐parallel circulation (n=36). Animals either remained sedentary or were subjected to rigorous walking or swimming exercise (n=20). We found CCC correlated with LGR and was significantly augmented (+36%) in alligators with in‐series circulation compared to those with in‐parallel circulation. Exhaustive exercise reduced thickness (−15%) of the distal, but not proximal, growth plate, suggesting local effect of biomechanic loading. We propose loss of the pulmonary bypass shunt results in higher systemic arterial oxygen tension, which allows for greater oxygen diffusion distance through the growth plate. Among archosaur ancestors, thicker growth plates may have appeared concurrently with the origin of in‐series circulation, and thus set the stage for later acquisition of fast growth and endothermic metabolism. Supported by NSF IOB 04445680 to JWH.