Mariana Valéria de Araújo Sena

The enantiornithes, a group of Mesozoic birds, have morphological features suggesting a precocial mode of development; however, their detailed physiological characteristics, particularly hatchling metabolic rates, have yet to be explored. Here, we retrodicted metabolic rates for four Lower Cretaceous enantiornithines: one hatchling, one adult from Las Hoyas, one early post-hatching juvenile and one post-juvenile individual from the Jehol Biota, using phylogenetic comparative analyses and a comprehensive dataset of extant birds spanning the altricial–precocial spectrum. By examining vascular density (related to resting metabolic rate (RMR)) and nutrient foraminal size (proportional to blood flow rate) as proxies, we inferred the metabolic scope of the hatchling enantiornithine and determined that it fell toward the precocial end of the altricial–precocial spectrum. The RMR values of the two enantiornithine chicks were retrodicted as 0.67 mlO2 h–1 g–0.93. The peak metabolic rate (PMR) retrodicted is twice the resting metabolism for the Las Hoyas hatchling. These results are consistent with a semi-precocial/precocial development characterized by independent locomotion by climbing using the wing claws soon after hatching. However, flapping flight may have been delayed, owing to incomplete ossification of the sternum, as observed in one hatchling specimen.

Studies on paleophysiology and thermoregulation are central topics in fossil vertebrate paleobiology because they allow inferences about extinct organismal function beyond anatomical description. Pterosaur thermophysiology remains actively debated. Evidence such as relatively rapid growth and elevated metabolic rate estimates has been used to argue for endothermic-like physiologies consistent with the energetic demands of powered flight, whereas reproductive traits, including burial incubation and precocial development, have led some authors to propose ectothermic strategies more akin to those of extant non-avian diapsids. Here, we provide quantitative estimates of resting metabolic rate (RMR) for the tapejarid pterosaur Caiuajara dobruskii using phylogenetic eigenvector maps and osteocyte lacunar density as proxy. Analysis of six histological sections from three appendicular elements (femur, tibia, humerus) at different ontogenetic stages (adult and juvenile) yielded mass-independent RMR estimates of 0.24–0.76 ml O ₂ h ⁻¹ g ^−0.86 , substantially higher than those of lepidosaurs but lower than typical mammalian and avian values. The retrodicted mass-independent maximum metabolic rate (MMR) of 5.68 ml O ₂ h ⁻¹ g ^−0.87 falls between values for highly active reptiles and birds, suggesting sufficient aerobic capacity to sustain powered flight without fully matching the maximal performance of small endotherms. Together, these results suggest active-flighted pterosaurs fall among a metabolically intermediate continuum, limiting straightforward actualistic comparisons for inferring their thermoregulatory strategy.

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 Rhamphorhynchus, 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 Halszkaraptor escuilliei, 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 ($\dot {\mathrm{}Q}$) through the femoral nutrient canal areas and retrodicted mass-independent MMR for immature specimens of Halszkaraptor and Rhamphorhynchus of 5.68 mLO2 h⁻¹ g^−0.87 and 5.55 mLO2 h⁻¹ g^−0.87 (p < 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 H. escuilliei 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 Rhamphorhynchus 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.

We infer the neonatal metabolic rate at rest (RMR) and at maximum activity levels (MMR) of the hadrosaurid dinosaur Maiasaura peeblesorum from the Two Medicine Formation of Montana (USA) using Phylogenetic Eigenvector Maps applied to the following osteohistological features: the Relative Primary Osteon Area and the size of the femoral nutrient foramen as proxies. We investigate the locomotor/motor activity of the neonates by comparing the difference between maximum and minimum rates of oxygen consumption—referred to as aerobic scope and denoted as ΔMR, as a proxy of their activity levels. Applied to Maiasaura, this novel methodology allows for a quantitative assessment of its neonatal state and to deduce its dependence on parental care. The inferred neonatal RMR values for Maiasaura are similar to those of present-day fast-growing endotherms. As for the aerobic scope, M. peeblesorum neonates have a ΔMR value similar to those observed in present-day altricial birds that need intensive parental care. This result is consistent with the previously proposed hypothesis of nidicolous M. peeblesorum requiring parental care. Finally, based on age-estimations from M. peeblesorum neonate remains found both in and outside nest sites, we estimate that this species remained in the nest for approximately 40–75 days. Maiasaura provides a useful ecological baseline from which to infer neonatal states in an other hadrosaur of similar adult size—Hypacrosaurus stebingeri, whose differing ecological traits point to a relatively more precocial condition. The diversity of post-hatching reproductive strategy partly explains the adaptation of hadrosaurs to a wide range of paleolatitudes and environments.

Powered flight has evolved separately in three tetrapod clades: pterosaurs, birds (avian dinosaurs), and bats. To meet the challenges of powered flight, tetrapods acquired structural, mechanical, and physiological adaptations. Circumferential vascular canals, forming laminar bone, have been proposed to be an adaptation linked to withstanding torsional loading during flight in birds. However, whether laminarity serves as an indicator of biomechanical torsion in pterosaurs, remains to be elucidated. Through a comparative statistical approach, we investigate the association between cortical bone laminarity and skeletal stress in pterosaur flight. For the first time, the presence of laminarity in pterosaur bones was analyzed to observe torsion in bones associated with flight, using thirty-five thin sections of the tapejarid Caiuajara dobruskii, Anhangueridae, and Dsungaripteroidea pterosaurs. We conclude that adaptive relationships arise between bone microstructure and biomechanical function, with forelimb elements (humerus, ulna, and wing phalanges) exhibiting higher laminarity rates compared to hindlimb bones. Additionally, the results provide insights into the possible flight style of these pterosaurs through comparison with bird groups. The presence of laminarity in pterosaurs and birds, but not in bats, highlights that this feature, once considered exclusive to birds, may have convergently evolved in pterosaurs and birds.

In the context of an increasing interest for Pseudosuchia, we have compiled a Special Issue, comprising 14 collaborative studies that deepen our understanding of pseudosuchian evolution. These contributions range from the description of a new taxon to exhaustive reviews of thermometabolism, morphological adaptation, systematics, and detailed investigations into ontogeny, paleoneurology, paleohistology, and paleobiology. Through these papers, we explore the evolutionary history of pseudosuchian archosaurs, spotlighting their rise and diversification following the end‐Permian mass extinction.

Paleophysiology is an emergent discipline. Organismic (integrative) approaches seem more appropriate than studies focusing on the variation of specific features because traits are tightly related in actual organisms. Here, we used such an organismic approach (including lifestyle, thermometabolism, and hunting behavior) to understand the paleobiology of the lower Jurassic (Toarcian) thalattosuchian metriorhynchoid Pelagosaurus typus. First, we show that the lifestyle (aquatic, amphibious, terrestrial) has an effect on the femoral compactness profiles in amniotes. The profile of Pelagosaurus indicates that it was amphibious, with a foraging activity in shallow marine environments (as suggested by the presence of salt glands) and thermoregulatory basking behavior in land (as suggested by the presence of osteoderms with highly developed ornamentation). As for the thermometabolism, we show that the mass‐independent resting metabolic rate of Pelagosaurus is relatively high compared to the sample of extant ectothermic amniotes, but analysis of vascular canal diameter and inferences of red blood cell size refute the hypothesis suggesting incipient endothermy. Finally, the foraging behavior was inferred using two proxies. Pelagosaurus had a mass‐independent maximum metabolic rate and an aerobic scope higher than those measured in the almost motionless Iguana iguana, similar to those measured in the sit‐and‐wait predator Crocodylus porosus but lower than those quantified in the active hunter Varanus gouldii. These results suggest that Pelagosaurus may have had a hunting behavior involving a slow sustained swimming or a patient waiting in shallow waters, and may have caught preys like gharials, using fast sideways sweeping motions of the head.

The clade Pseudosuchia appeared 250 million years ago. The exclusively semi‐aquatic Crocodylia, which includes crocodiles, alligators, caimans, and gharials is the only surviving subgroup. Investigating Crocodylia biology is pivotal for inferring traits of extinct pseudosuchians. Alligator femur length is widely used for modeling pseudosuchian body mass, but the regression is influenced by sex and captivity status, leading to potential accuracy problems. An alternative model results from the correlation between alligator femur volume and body mass, which is unaffected by those covariates. Here, an alligator femur volume‐based regression is applied to estimate the masses of non‐crocodylian pseudosuchians, encompassing goniopholids, dyrosaurs, notosuchians, and thalattosuchians. For each, femur volume as the predictor yields lower body masses than does femur length. Morphological resemblances to existing crocodylians support the inference that extinct goniopholids and dyrosaurs were semi‐aquatic. Therefore, body masses predicted from femur length and volume should be reasonable, although larger body masses obtained from femur length may reflect sensitivity to sex or environmental factors. Fully terrestrial notosuchians had proportionately longer femora for their body sizes compared to semi‐aquatic crocodylians, suggesting that the higher body masses predicted from alligator femur length are overestimates. Fully aquatic thalattosuchians, skeletally adapted for buoyancy and with reduced reliance on the femur for locomotion, pose challenges for both femur length and volume‐based models. The results of this study advocate for the use of femur volume to predict body mass, particularly for semi‐aquatic and terrestrial pseudosuchians, and encourage further exploration of volumetric models as body size predictors for extinct vertebrates.