Volume 2, Issue 14 September 2005
A Postcard from Jurassic Park
Kevin Padian is the president of the National Center for Science Education (NCSE), a group that defends the teaching of evolution in public schools. (Photo by Jon Blair/National Geographic)
Picture a cockatoo and a crocodile. Which one is closer to a Tyrannosaurus Rex? If you guessed the latter, you were mistaken. The reality is that about 230 million years ago the lineage that produced crocodiles and their relatives split off from the evolutionary line leading to dinosaurs and, eventually, birds. So if birds are related to massive creatures like the T. Rex, how did they ever get off the ground? Answering these kinds of big evolutionary questions requires a trip deep into the fossil record. That's UC Berkeley integrative biologist Kevin Padian's favorite stomping grounds.
Thin sections of bone from a fossil coelophysus femur (above) and a modern moa tibia (below) reveal striking similarities in growth pattern. Both have an abundance of vessels running through the bone, indicating a good metabolism and fast growth. (courtesy the researchers)
"My work has always been about the events, patterns, and processes behind macroevolution," says Padian, curator of UC Berkeley's Museum of Paleontology. "How do things play out on a large stage over the very long term? Why do some species succeed and some die?"
Padian is particularly curious about what life on the planet was like at the end of Triassic period and beginning of Jurassic, more commonly known as the age of dinosaurs.
Perhaps most famously, Padian and his collaborators discovered that dinosaurs "grew like a house on fire." That's contrary to what paleontologists previously thought. In recent years though, the researchers have shown that dinosaurs underwent rapid growth, more akin to today's mammals like birds and elephants than reptiles like crocodiles. Indeed, some dinosaurs reached their massive adult size during their early teenage years.
How do we know? Since there are no living dinosaurs left, Padian and his colleagues looked at dead ones. Working with Museum of the Rockies curator John Horner and professor Armand de Ricqles of the University of Paris, Padian has spent the last dozen years examining the inside of fossil bones. It turns out that bones record their growth history in tissue similar to the way a tree's years can be counted in its rings. As the animal ages, certain kinds of tissue are deposited while others are eroded. Comparing the fossilized bone tissue with samples from juvenile and hatchling dinosaur bones and also samples from modern animals, the researchers pieced together the entire growth history of various dinosaurs.
By cracking open the dinosaur bones, the scientists learned a great deal about the animals' evolution on the macro-scale. Early in their lineage, dinosaurs developed the sustained elevated growth rates that led to their massive size while other lines from a common ancestor did not. According to Padian, that growth rate may have given dinosaurs an advantage 200 million years ago when other animals with more typical reptilian bone structures died off. Also, the research suggests that dinosaurs may have been warm-blooded creatures with basal metabolic rates comparable to today's birds.
Artist's conception of Stegosaurus stenops. (Image courtesy ©Melani McKim)
"Dinosaurs weren't sluggish, lumbering, overgrown crocodiles at all," Padian says. "They were actually closer to what you saw in Jurassic Park."
The biology of dinosaurs' enormity is only one evolutionary riddle that Padian has pondered using the fossil record as his guide. Several years ago, his team pursued the question of why birds are so much smaller than their extinct dinosaur cousins. Studying the bone tissue, the researchers determined that ancient bird species truncated their growth rate after an initial fast spurt.
"We don't know why this happened," Padian says, "but when they became smaller, the proportionately larger feathers gave them an aerodynamic advantage."
An acid-prepped fossil Scelidosaurus scute from the British Museum in which most of the rock matrix has been digested away, revealing the interior lattice-work trabeculae surrounded by relatively thin layers of compact cortical bone. The trabeculae are 'pinched' out at the top, leaving an upper keel of just cortical bone similar to the plates and spikes of Stegosaurus. Inset shows a histological section of a Scelidosaurus scute that exemplifies the light construction and structure of the scute, similar to what is seen in Stegosaurus. (Photos by Russell Main/UC Berkeley)
More recently, Padian and his collaborators' studies of bone histology have clued them in to secrets about the social life of dinosaurs. For instance, the spikes and plates running down the back of stegosaurs and the helmet-like structure on the heads of triceratops did not evolve as protection devices, sexual displays, or heat exchangers to keep the animals cool or warm, a popular assumption. According to the Berkeley research, the bizarre structures simply helped dinosaurs spot members of their own species. Once again, Padian says, it's evolution working in a way that's not obvious on first glance, but crystal clear upon closer inspection.
"We know more about dinosaurs than we've ever known," Padian says. "In fact, we know more about the life history of dinosaurs than that of some living animals because we've worked so hard to reconstruct it."