DOI: 10.1017/pab.2026.10110 ISSN: 0094-8373
Blood flow rates to long bones of archosaurs reveal high aerobic metabolic rates and endothermy throughout most of their evolutionary history
Roger S. Seymour, Case Vincent Miller, Qiaohui Hu, Martin D. Ezcurra, Sterling J. Nesbitt, Marc E. H. Jones, Susannah C. R. Maidment, Daniela Schwarz, Eric W. Wilberg, Nicolás E. Campione, Gordon Grigg Abstract
The origin and evolution of endothermy among Archosauria are uncertain because living species are represented by two lineages: ectothermic crocodylians (Pseudosuchia) and endothermic birds (Avemetatarsalia). Endothermy is characterized by high metabolic rates (tachymetabolism) and a physiological ability to control body temperature (thermoregulation), partly by using heat generated within the body. Ectothermy is characterized by low metabolic rates (bradymetabolism) and behavioral body temperature regulation. Levels of metabolic rate can be indirectly assessed by measuring the size of primary nutrient foramina of long bones. Foramen radius is related to blood flow rate (
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$ \dot{\mathrm{Q}} $
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) into the bone, which supports growth, repair, and remodeling of bone tissue that increase in more active vertebrates. Extant endothermic mammals are known to have profoundly higher
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$ \dot{\mathrm{Q}} $
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than extant ectothermic reptiles of the same body size. This study measures nutrient foramen minor radius, mid-shaft circumference, and length of femora of archosaur fossils, focusing on the earliest lineages. It reports
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$ \dot{\mathrm{Q}} $
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and body mass for 81 extinct archosauriform taxa and compares the data allometrically with extant endothermic and ectothermic vertebrates. The extinct archosauriforms yield generally higher
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$ \dot{\mathrm{Q}} $
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values than occur in extant mammals and non-varanid reptiles. The study indicates that tachymetabolic archosauriforms dominated the Mesozoic from the Triassic onward. Their high basal metabolic rates produced heat potentially used for thermoregulation. With few exceptions, both Pseudosuchia and Avemetatarsalia exhibit high
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$ \dot{\mathrm{Q}} $
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values from the Triassic to the Cretaceous. Avemetatarsalians apparently remained endothermic until the present, while some tachymetabolic pseudosuchians became bradymetabolic relatively recently, as evident in extant crown crocodylians.