The Mesozoic Era, often called the Age of Dinosaurs, spanned approximately 186 million years (252 to 66 million years ago). This era is divided into the Triassic, Jurassic, and Cretaceous periods. Today, the Earth’s atmosphere contains about 21% oxygen, which serves as the modern baseline. Determining the precise composition of the air that dinosaurs breathed requires analyzing chemical and geological clues preserved in the rock record. Mesozoic oxygen levels fluctuated significantly, ranging from lower than modern air to noticeably higher levels.
Atmospheric Oxygen Levels Across the Mesozoic Era
The dinosaurs first appeared during the Late Triassic period, characterized by generally low atmospheric oxygen. Following the massive Permian-Triassic extinction event, oxygen levels had plummeted and remained suppressed. Some reconstructions suggest oxygen may have dipped to around 15%, or even 12%, significantly lower than today’s 21% concentration. This low-oxygen environment may have favored the evolution of highly efficient respiratory systems.
Oxygen concentrations began a slow, fluctuating recovery as the Triassic period continued. Around 215 million years ago, a rapid increase saw levels jump from approximately 15% to about 19%. This rise coincided with the diversification and expansion for the early dinosaur groups across Pangaea. The Jurassic period saw oxygen levels stabilize, remaining near or slightly below the modern concentration.
The Cretaceous period experienced the highest atmospheric oxygen levels of the entire Mesozoic Era. During the mid-Cretaceous, oxygen concentration is estimated to have peaked, potentially reaching values as high as 25% to 30%, a substantial increase over the current 21%. This “oxygen maximum” supported widespread, lush vegetation. However, some studies suggest a much lower range for the Cretaceous (10% to 15%), highlighting the ongoing debate in paleo-atmospheric science. The oxygen concentration began to decline toward the end of the Cretaceous, settling back closer to modern levels.
Scientific Methods for Estimating Paleo-Oxygen
Reconstructing the atmosphere of the Mesozoic Era requires scientists to examine chemical signals and physical residues preserved in the ancient rock and fossil record. One approach involves analyzing the ratios of stable isotopes found in sedimentary minerals. Isotopic geochemistry links the concentration of oxygen in the atmosphere to the global burial rates of organic carbon and sulfur.
The ratio of specific sulfur isotopes, for example, changes depending on the oxygenation state of the environment where sedimentary rock is formed. When organic matter and sulfur are buried without being oxidized, the corresponding oxygen is effectively left behind in the atmosphere, increasing its concentration. By studying these isotope signatures in ancient sedimentary pyrite and sulfate, scientists can model the long-term balance between oxygen production and consumption. This allows for a reconstruction of the historical trends in atmospheric oxygen levels.
Another technique relies on the analysis of fossilized charcoal, known as fusain. Widespread wildfires require a minimum concentration of atmospheric oxygen to ignite and sustain combustion. By analyzing the abundance and nature of fossil charcoal deposits, researchers can establish a threshold for the minimum oxygen level present at that time. A high prevalence of charcoal suggests frequent, intense fires, which indicates a higher-than-average minimum oxygen concentration.
A more direct, though rare and often debated, method involves the analysis of tiny air bubbles trapped within fossilized tree resin. As the resin dripped from ancient trees, it sometimes encapsulated small samples of the air. Scientists can attempt to extract and analyze the gases within these bubbles to measure the actual atmospheric composition. While this technique provides a direct sample, the results are often viewed with caution due to the potential for gas exchange or chemical alteration over geological timescales.
The Biological Impact of Changing Oxygen Concentrations
The fluctuating oxygen levels of the Mesozoic Era influenced the life forms, particularly the terrestrial vertebrates. Dinosaurs, closely related to modern birds, possessed a highly efficient, avian-like respiratory system featuring air sacs that allowed for a unidirectional flow of air through the lungs. This superior respiratory design meant that dinosaurs were better equipped than many other animals to thrive during the Late Triassic, when oxygen levels were at their lowest. Their efficient breathing apparatus enabled them to dominate terrestrial niches even in an atmosphere difficult for less efficient lung systems to handle.
The subsequent rise in oxygen, culminating in the Cretaceous peak, is linked to the appearance of the largest dinosaur species, known as gigantism. Higher oxygen concentrations would have made it easier to supply the vast metabolic demands of massive bodies, such as those of the long-necked sauropods. However, the relationship is complex, as gigantism in vertebrates is not solely limited by oxygen availability, but also by factors like food availability and bone structure. The efficient dinosaur lung system may have simply allowed them to fully exploit the environmental opportunity provided by elevated oxygen.
In stark contrast to the vertebrates, the body size of invertebrates like insects and other arthropods is directly limited by atmospheric oxygen concentration. Arthropods rely on passive diffusion through a network of tubes called tracheae and spiracles to deliver oxygen to their tissues. A higher oxygen level increases the partial pressure, allowing oxygen to diffuse farther into the body, which directly enables a larger maximum body size. This is why the Carboniferous period, which had the highest oxygen levels, featured giant insects, and why modern insect size is restricted by the current 21% concentration.