It is well known that birds can fly at altitudes where the partial pressure of oxygen is so low that mammals cannot function, but it is not known whether the sister group of birds, the crocodilians, have the physiological responses and pulmonary structures to support activity in hypoxic conditions. Conditions of low oxygen cause animals to breathe excessively, which then reduces the tension of carbon dioxide (CO2) in their blood. Low CO2 tension in turn reduces blood flow to the brain of mammals, leading to disorientation and even death. In contrast birds tolerate low blood CO2 levels without these adverse effects. Furthermore the pattern of air and blood flow in the lungs of birds, a crosscurrent design, facilitates the absorption oxygen by blood under conditions of low oxygen. Birds and crocodilians share a common ancestor dating back to the Mesozoic, a time of environmental hypoxia compared to today's atmosphere. Are the features that allow birds to function in environmental hypoxia a primitive archosaur trait or are they derived? If they were present ancestrally, then an exceptional ability to exercise under conditions of hypoxia may partially explain why archosaurs dominated the Mesozoic terrestrial fauna. To address these questions this project will study how air and blood move in the lungs of American alligators, as well as whether low CO2 tension affects brain blood flow. These studies will shed light on the evolutionary connections between the crocodilian and avian pulmonary systems. This research will test the hypothesis that crocodilians have a simple crosscurrent lung design and, if correct, then only a few small modifications would be requisite for evolution of the more complex avian lung. Thus, this research will potentially impact both the scientific community and the lay community, because it will provide insight into why and how archosaurs dominated the Mesozoic and it may provide a plausible explanation for the evolution of the avian lung. This project will also support the training of students, including those from groups underrepresented in the sciences.
The Earth experienced its most severe extinction event at the end of the Permian Period, approximately 250 million years ago, with up to 96% of marine species and 70% of terrestrial species dying out. It appears that gradual climate change precipitated the extinctions, although we lack a complete understanding of this event. The great lava outflows that make up the Siberian Traps were formed during this period, and provide evidence of massive volcanism. The Traps today cover a landmass that is equivalent to that of Western Europe, but the original flows may have been three or four times that large. The gasses released by the volcanoes caused dramatic changes in the composition of the atmospheric gasses and lead to climatic changes. One of the great mysteries in vertebrate evolution unfolded during this period of massive change. Since the Carboniferous Period until today, two clades of vertebrates have dominated terrestrial assemblages in a reciprocating manner that correlates with fluctuations in the Earth’s atmospheric oxygen levels. In the Carboniferous and Permian Periods oxygen levels were relatively high (about 30%) compared to today’s levels (approximately 21% oxygen). Under these high levels of oxygen the synapsid lineage, a clade of organisms named for the single opening in the skull immediately behind the eye and which includes mammals, dominated terrestrial assemblages in the sense that they are the largest and most common fossils found in these assemblages. With the extinction event oxygen levels dropped to about half of what they were in the Permian and most of these synapsids died out, but about four families persisted and became again the largest and most common members of the Early Triassic assemblage as the disaster taxon. However, by the End of the Triassic their role in the ecosystem had been usurped by a new clade, the archosaurs, and the synapsids, which were composed only of mammals at this point in time, assumed ecological niches of very small body size. The archosaur radiation was one of the most spectacular radiations the Earth has ever known in terms of their great variation in body forms and in terms of their complete dominance of large body size niches. There is a period of about 150 million years in which any terrestrial vertebrate found that is larger than a meter is an archosaurs, while the mammals existed primarily as very small creatures, about the size of shrews. Levels of oxygen slowly increased during the Mesozoic Era until they reached todays levels by the beginning of the Cenozoic Era, 65 million years ago. Correlating with this increase in oxygen the synapsids (mammals) once again gained dominance of large body size niches. My research aims to understand how these changes in environmental oxygen may have interacted with and influenced the evolution of the respiratory system. I am asking basic questions about how the respiratory system of archosaurs (the modern groups include crocodilians and birds) functions in various levels of oxygen and carbon dioxide compared to the respiratory system of mammals. Answering these questions is providing insight into the mystery of why Mesozoic mammals were constrained to small body size. It is also changing the textbook in terms of how we think about the evolution of the respiratory system. Although we have known since the work of Robert Boyle, in 1660, that birds are better than mammals at surviving and even thriving in low oxygen environments, we do not understand the mechanisms underpinning these differences. My research aims to understand the effects of low oxygen environments (hypoxia) on crocodilians, the sister lineage to birds, providing insight into the evolutionary underpinnings of these mechanisms. The results of the research have transformed how we think about the evolution of the respiratory system in mammals and archosaurs. I have discovered that crocodilians have birdlike lungs. Prior to my work it was thought that they had lungs more similar to those of mammals than to birds. This research has added to our scientific capitol. This knowledge is contributing to natural designs, or bio-inspired designs, that aim to capture CO2 that is released as a byproduct of many of the machines of our modern society.
