Femoral metadiaphyseal and nutrient foramen perfusion suggests comparable maximal metabolic rates in a pterosaur and in a semi-aquatic maniraptoran dinosaur

Ornithodirans represent a diverse and highly successful clade that encompasses a wide array of morphologies and ecological adaptations. This group includes volant forms such as <i>Rhamphorhynchus</i>, a medium-sized, non-pterodactyloid long-tailed pterosaur from the Jurassic Solnhofen lagoons, characterized by prow-shaped lower jaw and forward-pointing teeth consistent with a piscivorous diet. In addition, the ornithodiran group included theropod dinosaurs such as <i>Halszkaraptor escuilliei</i>, a dromaeosaurid from Mongolia that exhibit morphological traits indicative of a semi-aquatic lifestyle. In this study, we retrodicted the aerobic performance of these two extinct ornithodirans by using femoral metadiaphyseal nutrient foramina dimensions as a proxy for maximal metabolic rate (MMR), based on an extant-amniote dataset of reference. We estimated femoral blood flow ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover><mml:mrow><mml:mi></mml:mi> <mml:mi>Q</mml:mi></mml:mrow> <mml:mrow><mml:mo>˙</mml:mo></mml:mrow> </mml:mover> </mml:math> ) through the femoral nutrient canal areas and retrodicted mass-independent MMR for immature specimens of <i>Halszkaraptor</i> and <i>Rhamphorhynchus</i> of 5.68 mLO2 h^-1 g^-0.87 and 5.55 mLO2 h^-1 g^-0.87 (<i>p</i> < 0.001), respectively. Our findings revealed that femoral blood flow rates and mass-independent MMR values were similar in the two taxa, despite their extreme differences in phylogenetic affinity, locomotory behavior and ecology. The predicted mass-independent MMR for <i>H. escuilliei</i> aligned with values observed in extant ground-dwelling emus and migratory shorebirds such as gulls and terns, but fall below the high MMR value of ducks. Further investigation into adult specimens is needed to refine our understanding of aerobic capacity in mature individuals, particularly with regards to the ability of <i>Rhamphorhynchus</i> for achieving the energetic demands of flight. Our research enhances understanding of the physiological strategies of extinct taxa and helps address key gaps in paleophysiological reconstructions.