The High-Pressure Mesozoic (HPM) Hypothesis: A Physical Constraint on Terrestrial Gigantism and Its Implications for Atmospheric Evolution and the K–Pg Extinction

The High-Pressure Mesozoic (HPM) Hypothesis proposes that surface atmospheric pressure during the Mesozoic was 1.8–2.3 times higher than today, resolving the biophysical impossibility of sauropod gigantism and pterosaur flight under modern conditions. This planetary-scale parameter explains cardiovascular, biomechanical, and aerodynamic paradoxes as natural consequences of elevated oxygen availability and air density, and links the K–Pg mass extinction to rapid atmospheric decompression. The hypothesis is physically motivated and generates testable predictions for paleobarometry, isotope geochemistry, and biomechanical modeling.