Preliminary painting of Cretaceous Baja California Dinosaur mural
at the San Diego Natural History Museum
Give those Dinosaurs
By Lynett Gillette Illustrations by William Stout
If you gaze at the splendid, newly installed dinosaur mural in San Diego Natural History Museumís Fossil Mysteries exhibition, youíll note right away the 14,000-foot-high mountains, the predatory saurischian theropods Albertosaurus and Troodon, and the nesting group of crested hadrosaurs. Look a little harder and youíll see a sparrow-sized, rare toothed bird, the enantiothornine Alexornis. Underfoot, the small mammals wait for their chance to shine, which in this case will be in about 10 million years hence.
Perhaps you can appreciate the role of plate tectonics in thrusting up and eroding this Late Cretaceous Baja California mountain terrain next to the ocean. If youíre a dinosaur fan, you might ponder the revelation that the muralís diminutive Alexornis has a common ancestor with the 27-foot-long, scaled Albertosaurus and the pack of smaller, feathered Troodon.
All evidence points to a very warm world for these Cretaceous creatures. If youíve been paying attention to climate change studies, you may know that new circulation models of the ocean and atmosphere can now describe climates all the way back to the time when life began on Earth.
The Cretaceous Period shown in this mural, for example, had global mean annual surface temperatures at least 18? Fahrenheit (10? Celsius) warmer than today, according to Robert DeConto, geologist at the University of Massachusetts. Baja California fossils of both alligators and crocodiles support this model because they presumably didnít tolerate cold temperatures.
Heavy clouds pictured rolling over the mountains in the Museumís mural find support in the Earthís rock record of powerful storm deposits in the world at this time. Surprisingly, snow on the high mountaintops would be plausible, says DeConto, even in such a warm climate, given how quickly the temperature would drop at altitude.
Even if youíre a paleontologist or geologist, you may not have given serious thought to the actual gas composition of the Earthís atmosphere in past ages. Frankly, Fossil Mysteries overlooks the importance of air in its graphic panels of text. Evolving landscapes and life pose the traditional challenges for scientists to explain and museums such as ours to interpret. Yet what about the makeup of the atmosphere enveloping it all?
Biologist Peter Ward, in his provocative new book Out of Thin Air, suggests that research in paleontology and geology has matured to the point that we now can link the composition of the atmosphere with many patterns observed in the fossil record. In particular, he considers oxygen that sustains most (but not all) life on Earth. Until recently scientists havenít had the tools to understand how oxygen and carbon dioxide vary over the long sweep of time. Researcher Robert Berner and students at Yale University among others are making great strides towards this goal by gathering data on the burial rates on Earth of organic carbon and sulfur minerals. How slow or fast this burial happens determines the volume of available oxygen in the atmosphereónow and in the past. Knowledge of plate tectonics, fossils, and chemistry all feed data into models that researchers create to allow us to understand this history of our planetís air.
One of the remarkable outcomes of all this modeling is the discovery that quantities of oxygen and carbon dioxide in the Earthís oceans and atmosphere have varied significantly over time. Todayís atmosphere, in terms of gas volume, measures 21% oxygen, 78% nitrogen, plus a small yet important contribution of trace gases including carbon dioxide and water vapor. It hasnít always been so.
Around 300 million years ago, oxygen content of the atmosphere rose to an unprecedented 30%. It probably was no coincidence that dragonflies of the time were five times larger than todayís. By 200 million years ago, oxygen in the atmosphere plummeted to an all-time low of 12%.
Early dinosaurs like the small theropod Coelophysis just happened to live then, in Late Triassic times. Coelophysis must have been adapted to exceedingly low levels of oxygen. Why did dinosaurs evolve when they did: were they simply more energy efficient than earlier reptiles in low-oxygen conditions? Since birds are their descendants, do they have similar efficiency in low-oxygen conditions?
Thereís no doubt about birdsí phenomenal respiration. They fly regularly over mountain ranges at altitudes that would incapacitate a mammal. These feats are made possible by a flow-through breathing system supported by air sacs as well as lungs. Air sacs grow into bones of the vertebrae, ribs and limbs, which accommodate the sacs by thinning. The same thinning can be seen in the same places on fossil bones of early saurischian dinosaurs. Birds and their dinosaur ancestors may thus owe their physiology to dynamics of Earth systems some 200 million years ago.
Can we learn anything about dinosaurs and other members of their ecosystem by knowing about oxygen specifically in the Late Cretaceous Period shown in our mural? Oxygen had risen significantly from its Late Triassic low, but at 18%, it still was below modern levels. Intriguingly, ornithischian dinosaursóthe other kind of dinosaursóproliferate at this time.
In contrast with the saurischian dinosaurs, fossils of ubiquitous hadrosaurs and armored nodosaurs and ankylosaurs donít seem to have the thinning bones that would indicate air sac respiration. Perhaps such efficiency wasnít necessary under higher-percent oxygen skies. Mammals underfoot may have remained relatively small until another precious 3% oxygen became available during the Tertiary Period.
Further studies will test these powerful new models. In a way, what we need to do is add some appropriate air to our dinosaursí worlds and to other extinct ecosystems. Many of the great mysteries of evolution may soon be solved.
SAN DIEGO NATURAL HISTORY: FIELD NOTES, May 2007