Exceptionally preserved stomach contents of a young tyrannosaurid reveal an ontogenetic dietary shift in an iconic extinct predator

Tyrannosaurids were large carnivorous dinosaurs that underwent major changes in skull robusticity and body proportions as they grew, suggesting that they occupied different ecological niches during their life span. Although adults commonly fed on dinosaurian megaherbivores, the diet of juvenile tyrannosaurids is largely unknown. Here, we describe a remarkable specimen of a juvenile Gorgosaurus libratus that preserves the articulated hindlimbs of two yearling caenagnathid dinosaurs inside its abdominal cavity. The prey were selectively dismembered and consumed in two separate feeding events. This predator-prey association provides direct evidence of an ontogenetic dietary shift in tyrannosaurids. Juvenile individuals may have hunted small and young dinosaurs until they reached a size when, to satisfy energy requirements, they transitioned to feeding on dinosaurian megaherbivores. Tyrannosaurids occupied both mesopredator and apex predator roles during their life span, a factor that may have been key to their evolutionary success.

Exceptionally well-preserved crocodilian coprolites from the Late Eocene of Northern Vietnam: Ichnology and paleoecological significance

This study examines 55 coprolites from the Na Duong Basin to reconstruct the paleoenvironment. Coproecology sheds light on understanding the complex prey-predator relationships, trophic dynamics, and ecosystem evolution. Through quantitative and multidisciplinary analysis, the putative coprolites were attributed to crocodilian producers, leading to the establishment of a new ichnogenus and species, Crococopros naduongensis igen. et isp. nov., based on distinct characteristics and comparisons. The study provides compelling evidence of an ancient river or lake-like environment dominated by diverse crocodilian fauna, indicating a thriving food chain in the Na Duong Basin. The findings also highlight the remarkable richness of ichnofauna, fauna, flora, and the presence of a favorable climate, confirming the area as a significant fossil Lagerstätte in Southeast Asia. Overall, this study offers a unique snapshot of the past, providing valuable insights into the regional ecosystem and significantly contributing to our understanding of paleoenvironmental conditions and biotic interactions.

Exposure to hypoxia during embryonic development affects blood flow patterns and heart rate in juvenile American alligators during digestion

The developmental environment can alter an organism's phenotype through epigenetic mechanisms. We incubated eggs from American alligators in 10% O₂ (hypoxia) to investigate the functional plasticity of blood flow patterns in response to feeding later in life. Digestion is associated with marked elevations of metabolism, and we therefore used the feeding-induced stimulation of tissue O₂ demand to determine whether there are lasting effects of developmental hypoxia on the cardiovascular response to digestion later in life. In all animals studied, digestion elicited tachycardia and an elevation of blood flow in the right aorta, left aorta, and the pulmonary artery, whereas flows in the carotid and subclavian artery did not change. We found that heart rate and systemic blood flow remained elevated for a longer time period in juvenile alligators that had been incubated in hypoxia; we also found that the pulmonary blood flow was elevated at 24, 36, and 48 h. Collectively, our findings demonstrate that exposure to hypoxia during incubation has lasting effects on the hemodynamics of juvenile alligators 4 years after hatching.

Comparative and Functional Anatomy of the Ectothermic Sauropsid Heart

The heart development, form, and functional specializations of chelonians, squamates, crocodilians, and birds characterize how diverse structure and specializations arise from similar foundations. This review aims to summarize the morphologic diversity of sauropsid hearts and present it in an integrative functional and phylogenetic context. Besides the detailed morphologic descriptions, the integrative view of function, evolution, and development will aid understanding of the surprising diversity of sauropsid hearts. This integrated perspective is a foundation that strengthens appreciation that the sauropsid hearts are the outcome of biological evolution; disease often is linked to arising mismatch between adaptations and modern environments.

Effects of change in temperature on the cardiac contractility of broad-snouted caiman (Caiman latirostris) during digestion

In many reptiles, digestion has been associated with the selection of higher body temperatures, the so-called post-prandial thermophilic response. This study aimed to investigate the excitation-contraction (E-C) coupling in postprandial broad-snouted caimans (Caiman latirostris) in response to acute warming within a preferred body temperature range of crocodiles. Isometric preparations subjected to a temperature transition from 25°C to 30°C were used to investigate myocardial contractility of postprandial caimans, that is, 48 h after the animals ingested a rodent meal corresponding to 15% of body mass. The caiman heart exhibits a negative force-frequency relationship that is independent of the temperature. At 25°C, cardiac muscle was able to maintain a constant force up to 36 bpm, above which it decreased significantly, reaching minimum values at the highest frequency of 84 bpm. Moreover, E-C coupling is predominantly dependent on transsarcolemmal Ca²⁺ transport denoted by the lack of significant ryanodine effects on force generation. On the contrary, ventricular strips at 30°C were able to sustain the cardiac contractility at higher pacing frequencies (from 12 to 144 bpm) due to an important role of Na⁺ /Ca²⁺ exchanger in Ca²⁺ cycling, as indicated by the decay of the post-rest contraction, and a significant contribution of the sarcoplasmic reticulum above 72 bpm. Our results demonstrated that the myocardium of postprandial caimans exhibits a significant degree of thermal plasticity of E-C coupling during acute warming. Therefore, myocardial contractility can be maximized when postprandial broad-snouted caimans select higher body temperatures (preferred temperature zone) following feeding.

Notes on the gross anatomy of the heart of the broad-snouted caiman, Caiman latirostris (Daudin, 1802)

The broad-snouted caiman, Caiman latirostris (Daudin, 1802), is one of the six crocodilian species from Brazil. The topography, morphology and morphometry of the broad-snouted caiman heart were studied. Data were obtained from the necropsy of four adult animals, three females and one male. The hearts were removed from the coelomic cavity and fixed in 10% formalin for 48 hr for morphological and morphometric description. The heart is in the cranial mediastinum. It is caudally involved by the liver cranial margins, and ventrally by the ribs, intercostal muscles, and sternum and dorsally by the lungs. The four-chambered morphology is typical with two (right and left) atria and ventricles. Right and left aortic, pulmonary and subclavian arteries branch from the truncus arteriosus. Gubernaculum cordis is present as ligamentous folds uniting the heart apex to the pericardium. Main morphometric means are the apex-to-base length (49.86 mm), circumference (105.25 mm) and heart weight (45.03 g). The right atrium is craniocaudally longer with thicker walls, whereas the left ventricle is narrower. The topography, morphology and morphometry of the heart of C. latirostris are consistent with the anatomy of other crocodilian species.

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

This review explores the long-standing question: 'Why do cardiovascular shunts occur?' An historical perspective is provided on previous research into cardiac shunts in vertebrates that continues to shape current views. Cardiac shunts and when they occur is then described for vertebrates. Nearly 20 different functional reasons have been proposed as specific causes of shunts, ranging from energy conservation to improved gas exchange, and including a plethora of functions related to thermoregulation, digestion and haemodynamics. It has even been suggested that shunts are merely an evolutionary or developmental relic. Having considered the various hypotheses involving cardiovascular shunting in vertebrates, this review then takes a non-traditional approach. Rather than attempting to identify the single 'correct' reason for the occurrence of shunts, we advance a more holistic, integrative approach that embraces multiple, non-exclusive suites of proposed causes for shunts, and indicates how these varied functions might at least co-exist, if not actually support each other as shunts serve multiple, concurrent physiological functions. It is argued that deposing the 'monolithic' view of shunting leads to a more nuanced view of vertebrate cardiovascular systems. This review concludes by suggesting new paradigms for testing the function(s) of shunts, including experimentally placing organ systems into conflict in terms of their perfusion needs, reducing sources of variation in physiological experiments, measuring possible compensatory responses to shunt ablation, moving experiments from the laboratory to the field, and using cladistics-related approaches in the choice of experimental animals.

Does the left aorta provide proton-rich blood to the gut when crocodilians digest a meal?

Reptiles have the capacity to differentially perfuse the systemic and pulmonary vascular circuits via autonomic regulation of the heart and the vascular trees. While this aptitude is widely recognized, the role of 'shunting' as a homeostatic mechanism to match convective transport with tissue demand remains unknown. In crocodilians, it has been hypothesized that a pulmonary vascular bypass of systemic venous blood - a right-to-left (R-L) shunt - serves to deliver CO₂-rich blood with protons needed for gastric acid secretion during digestion. This hypothesis is partially based on the unique crocodilian vascular anatomy where a left aorta (LAo) arises from the right ventricle, and appears to preferentially supply the gastrointestinal system, whereas the right aorta emerges from the left ventricle. Recent theoretical considerations imply that a R-L shunt would have minuscule effects on P _CO2 , but direct measurements of blood gases in both the right and left aortae or both the right and left atria in fed animals have not been conducted. For this reason, we measured blood parameters including P _O2 , P _CO2 , pH_e and [HCO₃ ^-] in the right and left aortae and atria following ingestion of a gavage-fed standardized meal (5% body mass). Blood samples were taken at 3, 6, 12, 24, 36 and 48 h into the digestive period to encompass the period of maximal gastric acid secretion. At no point did P _CO2  or pH differ between the left and right aortae, whereas P _O2  was significantly lower in the left aorta at several time points during digestion. Our findings do not support the hypothesis that a R-L shunt serves to deliver CO₂ for the gastrointestinal system after feeding in crocodilians.

Hemodynamics of tonic immobility in the American alligator (Alligator mississippiensis) identified through Doppler ultrasonography

American alligators (Alligator mississippiensis) held inverted exhibit tonic immobility, combining unresponsiveness with flaccid paralysis. We hypothesize that inverting the alligator causes a gravitationally promoted increase in right aortic blood flowing through the foramen of Panizza, with a concurrent decrease in blood flow through the primary carotid, and thereby of cerebral perfusion. Inverting the alligator results in displacement of the liver, post-pulmonary septum, and the heart. EKG analysis revealed a significant decrease in heart rate following inversion; this decrease was maintained for approximately 45 s after inversion which is in general agreement with the total duration of tonic immobility in alligators (49 s). Doppler ultrasonography revealed that following inversion of the alligator, there was a reversal in direction of blood flow through the foramen of Panizza, and this blood flow had a significant increase in velocity (compared to the foraminal flow in the prone alligator). There was an associated significant decrease in the velocity of blood flow through the primary carotid artery once the alligator was held in the supine position. Tonic immobility in the alligator appears to be a form of vasovagal syncope which arises, in part, from the unique features of the crocodilian heart.

Exceptional dinosaur fossils reveal early origin of avian-style digestion

Birds have a highly specialized and efficient digestive system, but when this system originated remains uncertain. Here we report six gastric pellets attributable to the recently discovered 160-million-year-old troodontid dinosaur Anchiornis, which is among the key taxa for understanding the transition to birds. The gastric pellets contain lightly acid-etched lizard bones or fish scales, and some are associated with Anchiornis skeletons or even situated within the oesophagus. Anchiornis is the earliest and most basal theropod known to have produced gastric pellets. In combination with other lines of evidence, the pellets suggest that a digestive system resembling that of modern birds was already present in basal members of the Paraves, a clade including troodontids, dromaeosaurids, and birds, and that the evolution of modern avian digestion may have been related to the appearance of aerial locomotion in this lineage.

Right-to-left shunt has modest effects on CO2 delivery to the gut during digestion, but compromises oxygen delivery

By virtue of their cardiovascular anatomy, reptiles and amphibians can shunt blood away from the pulmonary or systemic circuits, but the functional role of this characteristic trait remains unclear. It has been suggested that right-to-left (R-L) shunt (recirculation of systemic blood within the body) fuels the gastric mucosa with acidified and CO₂-rich blood to facilitate gastric acid secretion during digestion. However, in addition to elevating P_CO2 , R-L shunt also reduces arterial O₂ levels and would compromise O₂ delivery during the increased metabolic state of digestion. Conversely, arterial P_CO2  can also be elevated by lowering ventilation relative to metabolism (i.e. reducing the air convection requirement, ACR). Based on a mathematical analysis of the relative roles of ACR and R-L shunt on O₂ and CO₂ levels, we predict that ventilatory modifications are much more effective for gastric CO₂ supply with only modest effects on O₂ delivery. Conversely, elevating CO₂ levels by means of R-L shunt would come at a cost of significant reductions in O₂ levels. The different effects of altering ACR and R-L shunt on O₂ and CO₂ levels are explained by the differences in the effective blood capacitance coefficients.

An additional Baurusuchid from the cretaceous of Brazil with evidence of interspecific predation among crocodyliformes

A new Baurusuchidae (Crocodyliformes, Mesoeucrocodylia), Aplestosuchus sordidus, is described based on a nearly complete skeleton collected in deposits of the Adamantina Formation (Bauru Group, Late Cretaceous) of Brazil. The nesting of the new taxon within Baurusuchidae can be ensured based on several exclusive skull features of this clade, such as the quadrate depression, medial approximation of the prefrontals, rostral extension of palatines (not reaching the level of the rostral margin of suborbital fenestrae), cylindrical dorsal portion of palatine bar, ridge on the ectopterygoid-jugal articulation, and supraoccipital with restricted thin transversal exposure in the caudalmost part of the skull roof. A newly proposed phylogeny of Baurusuchidae encompasses A. sordidus and recently described forms, suggesting its sixter-taxon relationship to Baurusuchus albertoi, within Baurusuchinae. Additionally, the remains of a sphagesaurid crocodyliform were preserved in the abdominal cavity of the new baurusuchid. Direct fossil evidence of behavioral interaction among fossil crocodyliforms is rare and mostly restricted to bite marks resulting from predation, as well as possible conspecific male-to-male aggression. This is the first time that a direct and unmistaken evidence of predation between different taxa of this group is recorded as fossils. This discovery confirms that baurusuchids were top predators of their time, with sphagesaurids occupying a lower trophic position, possibly with a more generalist diet.

Comparative cardiovascular physiology: future trends, opportunities and challenges

The inaugural Kjell Johansen Lecture in the Zoophysiology Department of Aarhus University (Aarhus, Denmark) afforded the opportunity for a focused workshop comprising comparative cardiovascular physiologists to ponder some of the key unanswered questions in the field. Discussions were centred around three themes. The first considered function of the vertebrate heart in its various forms in extant vertebrates, with particular focus on the role of intracardiac shunts, the trabecular ('spongy') nature of the ventricle in many vertebrates, coronary blood supply and the building plan of the heart as revealed by molecular approaches. The second theme involved the key unanswered questions in the control of the cardiovascular system, emphasizing autonomic control, hypoxic vasoconstriction and developmental plasticity in cardiovascular control. The final theme involved poorly understood aspects of the interaction of the cardiovascular system with the lymphatic, renal and digestive systems. Having posed key questions around these three themes, it is increasingly clear that an abundance of new analytical tools and approaches will allow us to learn much about vertebrate cardiovascular systems in the coming years.

The alligator gut microbiome and implications for archosaur symbioses

Among vertebrate gastrointestinal microbiome studies, complete representation of taxa is limited, particularly among reptiles. Here, we provide evidence for previously unrecognized host-microbiome associations along the gastrointestinal tract from the American alligator, a crown archosaur with shared ancestry to extinct taxa, including dinosaurs. Microbiome compositional variations reveal that the digestive system consists of multiple, longitudinally heterogeneous microbiomes that strongly correlate to specific gastrointestinal tract organs, regardless of rearing histories or feeding status. A core alligator gut microbiome comprised of Fusobacteria, but depleted in Bacteroidetes and Proteobacteria common to mammalians, is compositionally unique from other vertebrate gut microbiomes, including other reptiles, fish, and herbivorous and carnivorous mammals. As such, modern alligator gut microbiomes advance our understanding of archosaur gut microbiome evolution, particularly if conserved host ecology has retained archosaur-specific symbioses over geologic time.

Ablation of the ability to control the right-to-left cardiac shunt does not affect oxygen consumption, specific dynamic action or growth in rattlesnakes, Crotalus durissus

The morphologically undivided ventricle of the heart in non-crocodilian reptiles permits the mixing of oxygen-rich blood returning from the lungs and oxygen-poor blood from the systemic circulation. A possible functional significance for this intra-cardiac shunt has been debated for almost a century. Unilateral left vagotomy rendered the single effective pulmonary artery of the South American rattlesnake, Crotalus durissus, unable to adjust the magnitude of blood flow to the lung. The higher constant perfusion of the lung circulation and the incapability of adjusting R-L shunt in left-denervated snakes persisted over time, providing a unique model for investigation of the long-term consequences of cardiac shunting in a squamate. Oxygen uptake recorded at rest, during spontaneous and forced activity, was not affected by removing control of the cardiac shunt. Furthermore, metabolic rate and energetic balance during the post-prandial metabolic increment, plus the food conversion efficiency and growth rate were all similarly unaffected. These results show that control of cardiac shunting is not associated with a clear functional advantage in adjusting metabolic rate, effectiveness of digestion or growth rates.

Abdominal contents from two large early cretaceous compsognathids (Dinosauria: Theropoda) demonstrate feeding on confuciusornithids and dromaeosaurids

Two skeletons of the large compsognathid Sinocalliopteryx gigas include intact abdominal contents. Both specimens come from the Jianshangou Beds of the lower Yixian Formation (Neocomian), Liaoning, China. The holotype of S. gigas preserves a partial dromaeosaurid leg in the abdominal cavity, here attributed to Sinornithosaurus. A second, newly-discovered specimen preserves the remains of at least two individuals of the primitive avialan, Confuciusornis sanctus, in addition to acid-etched bones from a possible ornithischian. Although it cannot be stated whether such prey items were scavenged or actively hunted, the presence of two Confuciusornis in a grossly similar state of digestion suggests they were consumed in rapid succession. Given the lack of clear arboreal adaptations in Sinocalliopteryx, we suggest it may have been an adept stealth hunter.

Change of cardiac function, but not form, in postprandial pythons

Pythons are renowned for a rapid and pronounced postprandial growth of the heart that coincides with a several-fold elevation of cardiac output that lasts for several days. Here we investigate whether ventricular morphology is affected by digestive state in two species of pythons (Python regius and Python molurus) and we determine the cardiac right-to-left shunt during the postprandial period in P. regius. Both species experienced several-fold increases in metabolism and mass of the digestive organs by 24 and 48 h after ingestion of meals equivalent to 25% of body mass. Surprisingly there were no changes in ventricular mass or dimensions as we used a meal size and husbandry conditions similar to studies finding rapid and significant growth. Based on these data and literature we therefore suggest that postprandial cardiac growth should be regarded as a facultative rather than obligatory component of the renowned postprandial response. The cardiac right-to-left shunt, calculated on the basis of oxygen concentrations in the left and right atria and the dorsal aorta, was negligible in fasting P. regius, but increased to 10-15% during digestion. Such shunt levels are very low compared to other reptiles and does not support a recent proposal that shunts may facilitate digestion in reptiles.

Prenatal cardiovascular shunts in amniotic vertebrates

During amniotic vertebrate development, the embryo and fetus employ a number of cardiovascular shunts. These shunts provide a right-to-left shunt of blood and are essential components of embryonic life ensuring proper blood circulation to developing organs and fetal gas exchanger, as well as bypassing the pulmonary circuit and the unventilated, fluid filled lungs. In this review we examine and compare the embryonic shunts available for fetal mammals and embryonic reptiles, including lizards, crocodilians, and birds. These groups have either a single ductus arteriosus (mammals) or paired ductus arteriosi that provide a right-to-left shunt of right ventricular output away from the unventilated lungs. The mammalian foramen ovale and the avian atrial foramina function as a right-to-left shunt of blood between the atria. The presence of atrial shunts in non-avian reptiles is unknown. Mammals have a venous shunt, the ductus venosus that diverts umbilical venous return away from the liver and towards the inferior vena cava and foramen ovale. Reptiles do not have a ductus venosus during the latter two thirds of development. While the fetal shunts are well characterized in numerous mammalian species, much less is known about the developmental physiology of the reptilian embryonic shunts. In the last years, the reactivity and the process of closure of the ductus arteriosus have been characterized in the chicken and the emu. In contrast, much less is known about embryonic shunts in the non-avian reptiles. It is possible that the single ventricle found in lizards, snakes, and turtles and the origin of the left aorta in the crocodilians play a significant role in the right-to-left embryonic shunt in these species.

Role of the left aortic arch and blood flows in embryonic American alligator (Alligator mississippiensis)

All embryonic and fetal amniotes possess a ductus(i) arteriosus(i) that allows blood to bypass the pulmonary circulation and the non-functional lungs. The central hemodynamic of embryonic reptiles are unique, given the additional systemic aorta that allows pulmonary circulatory bypass, the left aorta (LAo). The LAo exits in the right ventricle or ‘pulmonary side’ of reptilian hearts in both embryos and adults, but its functional significance in ovo is unknown. This study investigated the role of the LAo in embryonic American alligators by surgically occluding the LAo and measuring oxygen consumption and, in addition, measured hemodynamic responses to hypoxia in embryonic alligators. We measured systemic cardiac output and primary chorioallantoic membrane (CAM) artery blood flow for normoxic and hypoxic-incubated (10% O₂) American alligator embryos (Alligator mississippiensis). Chronic blood flow (1–124 h) in the primary CAM artery for hypoxic-incubated embryos (92 ± 26 ml min⁻¹ kg⁻¹) was elevated when compared with normoxic-incubated embryos (29 ± 14 ml min⁻¹ kg⁻¹, N = 6; P = 0.039). For hypoxic-incubated embryos, acute LAo blood flow (49.6 ± 24.4 ml min⁻¹ kg⁻¹) was equivalent to the combined flow of the three systemic great vessels that arise from the left ventricle, the right aorta, common carotid and subclavian arteries (43.6 ± 21.5 ml min⁻¹ kg⁻¹, N = 5). Similarly, for normoxic-incubated embryos, LAo blood flow (27.3 ± 6.6 ml min⁻¹ kg⁻¹) did not statistically differ from the other three vessels (18.4 ± 4.9 ml min⁻¹ kg⁻¹, N = 5). This study contains the first direct test of LAo function and the first measurements of blood flow in an embryonic reptile. These data support the hypotheses that embryonic alligators utilize the LAo to divert a significant amount of right ventricular blood into the systemic circulation, and that CAM blood flow increases following chronic hypoxic conditions. However, surgical occlusion of the LAo did not affect egg \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{V}_{{\text{O}}_{2}},$$\end{document} supporting the hypothesis that the LAo of reptiles is not critical to maintain in ovo oxygen consumption.

Turning crocodilian hearts into bird hearts: growth rates are similar for alligators with and without right-to-left cardiac shunt

The functional and possible adaptive significance of non-avian reptiles' dual aortic arch system and the ability of all non-avian reptiles to perform central vascular cardiac shunts have been of great interest to comparative physiologists. The unique cardiac anatomy of crocodilians - a four-chambered heart with the dual aortic arch system - allows for only right-to-left (R-L; pulmonary bypass) cardiac shunt and for surgical elimination of this shunt. Surgical removal of the R-L shunt, by occluding the left aorta (LAo) upstream and downstream of the foramen of Panizza, results in a crocodilian with an obligatory, avian/mammalian central circulation. In this study, R-L cardiac shunt was eliminated in age-matched, female American alligators (Alligator mississippiensis; 5-7 months of age). We tested the hypothesis that surgical elimination of R-L cardiac shunt would impair growth (a readily measured proxy for fitness) compared with sham-operated, age-matched controls, especially in animals subjected to exhaustive exercise. While regular exercise caused a decrease in size (snout-to-vent length, head length and body mass), elimination of the capacity for R-L cardiac shunt did not greatly reduce animal growth, despite a chronic ventricular enlargement in surgically altered juvenile alligators. We speculate that, despite being slightly smaller, alligators with an occluded LAo would have reached sexual maturity in the same breeding season as control alligators. This study suggests that crocodilian R-L cardiac shunt does not provide an adaptive advantage for juvenile alligator growth and supports the logic that cardiac shunts persist in crocodilians because they have not been selected against.

Surgical removal of right-to-left cardiac shunt in the American alligator (Alligator mississippiensis) causes ventricular enlargement but does not alter apnoea or metabolism during diving

Crocodilians have complete anatomical separation between the ventricles, similar to birds and mammals, but retain the dual aortic arch system found in all non-avian reptiles. This cardiac anatomy allows surgical modification that prevents right-to-left (R-L) cardiac shunt. A R-L shunt is a bypass of the pulmonary circulation and recirculation of oxygen-poor blood back to the systemic circulation and has often been observed during the frequent apnoeic periods of non-avian reptiles, particularly during diving in aquatic species. We eliminated R-L shunt in American alligators (Alligator mississippiensis) by surgically occluding the left aorta (LAo; arising from right ventricle) upstream and downstream of the foramen of Panizza (FoP), and we tested the hypotheses that this removal of R-L shunt would cause afterload-induced cardiac remodelling and adversely affect diving performance. Occlusion of the LAo both upstream and downstream of the FoP for approximately 21 months caused a doubling of RV pressure and significant ventricular enlargement (average approximately 65%) compared with age-matched, sham-operated animals. In a separate group of recovered, surgically altered alligators allowed to dive freely in a dive chamber at 23 degrees C, occlusion of the LAo did not alter oxygen consumption or voluntary apnoeic periods relative to sham animals. While surgical removal of R-L shunt causes considerable changes in cardiac morphology similar to aortic banding in mammals, its removal does not affect the respiratory pattern or metabolism of alligators. It appears probable that the low metabolic rate of reptiles, rather than pulmonary circulatory bypass, allows for normal aerobic dives.

Development of cardiac form and function in ectothermic sauropsids

Evolutionary morphologists and physiologists have long recognized the phylogenetic significance of the ectothermic sauropsids. Sauropids have been classically considered to bridge between early tetrapods, ectotherms, and the evolution of endotherms. This transition has been associated with many modifications in cardiovascular form and function, which have changed dramatically during the course of vertebrate evolution. Most cardiovascular studies have focused upon adults, leaving the development of this critical system largely unexplored. In this essay, we attempt a synthesis of sauropsid cardiovascular development based on the limited literature and indicate fertile regions for future studies. Early morphological cardiovascular development, i.e., the basic formation of the tube heart and the major pulmonary and systemic vessels, is similar across tetrapods. Subsequent cardiac chamber development, however, varies considerably between developing chelonians, squamates, crocodilians, and birds, reflected in the diversity of adult ventricular structure across these taxa. The details of how these differences in morphology develop, including the molecular regulation of cardiac and vascular growth and differentiation, are still poorly understood. In terms of the functional maturation of the cardiovascular system, reflected in physiological mechanisms for regulating heart rate and cardiac output, recent work has illustrated that changes during ontogeny in parameters such as heart rate and arterial blood pressure are somewhat species-dependent. However, there are commonalities, such as a beta-adrenergic receptor tone on the embryonic heart appearing prior to 60% of development. Differential gross morphological responses to environmental stressors (oxygen, hydration, temperature) have been investigated interspecifically, revealing that cardiac development is relatively plastic, especially, with respect to change in heart growth. Collectively, the data assembled here reflects the current limited morphological and physiological understanding of cardiovascular development in sauropsids and identifies key areas for future studies of this diverse vertebrate lineage.

Functional morphology and patterns of blood flow in the heart of Python regius

Brightness-modulated ultrasonography, continuous-wave Doppler, and pulsed-wave Doppler-echocardiography were used to analyze the functional morphology of the undisturbed heart of ball pythons. In particular, the action of the muscular ridge and the atrio-ventricular valves are key features to understand how patterns of blood flow emerge from structures directing blood into the various chambers of the heart. A step-by-step image analysis of echocardiographs shows that during ventricular diastole, the atrio-ventricular valves block the interventricular canals so that blood from the right atrium first fills the cavum venosum, and blood from the left atrium fills the cavum arteriosum. During diastole, blood from the cavum venosum crosses the muscular ridge into the cavum pulmonale. During middle to late systole the muscular ridge closes, thus prohibiting further blood flow into the cavum pulmonale. At the same time, the atrio-ventricular valves open the interventricular canal and allow blood from the cavum arteriosum to flow into the cavum venosum. In the late phase of ventricular systole, all blood from the cavum pulmonale is pressed into the pulmonary trunk; all blood from the cavum venosum is pressed into both aortas. Quantitative measures of blood flow volume showed that resting snakes bypass the pulmonary circulation and shunt about twice the blood volume into the systemic circulation as into the pulmonary circulation. When digesting, the oxygen demand of snakes increased tremendously. This is associated with shunting more blood into the pulmonary circulation. The results of this study allow the presentation of a detailed functional model of the python heart. They are also the basis for a functional hypothesis of how shunting is achieved. Further, it was shown that shunting is an active regulation process in response to changing demands of the organism (here, oxygen demand). Finally, the results of this study support earlier reports about a dual pressure circulation in Python regius.