T. rex owes its giant size to the ultimate teenage growth spurt

Sue was discovered in South Dakota in 1990 and has been The Field Museum's most popular exhibit since being unveiled in May 2000. With a total of approximately 321 bones, the skeleton is over 90% complete and exquisitely preserved. Photo courtesy of The Field Museum Full size image available here

CHICAGO--For the first time, scientists have determined the lifelong growth pattern for the Tyrannosaurus rex. T. rex reached its massive adult size due to an extraordinary growth spurt that stretched from about 14 to 18 years of age. Furthermore, the surge was followed by about 10 years of little or no growth as an adult, according to a study that will be the cover story of Nature Thursday, Aug. 12, 2004.

During the peak in its growth spurt, T. rex gained 2.1 kilograms (4.6 pounds) per day, developing into a more than 5,000-kilogram (11,000-pound) giant, one of the largest terrestrial carnivorous animals ever.

"Knowing the lifeline is important because we now understand the evolution of T. rex's giantism, one of the most fascinating aspects of dinosaurs," said Peter Makovicky, PhD, Dinosaur Curator at The Field Museum and a coauthor of the study. "With the life history parameters, we can better understand T. rex evolution, biology, biomechanics and population dynamics."

One of Sue's ribs shows growth lines corresponding to its 15th through 19th years of age. Inset: nine thin growth lines condensed at the outer edge of Sue's rib indicate that once Sue reached 19 years old it stopped growing. Copyright The Field Museum Full size image available here

Makovicky and his colleagues determined T. rex's growth pattern and lifespan using an innovative technique that could be applied to many other dinosaurs. They determined the age by counting growth lines in T. rex bones and calculated the corresponding body size from circumference measurements of the femur. Correlating these two sets of data results in a growth curve.

It has long been known that large weight-bearing dinosaur bones display growth lines, but they are hard to read. This is due to the way these large bones form: their marrow cavity expands and changes shape as they grow, erasing some of the internal lines.

This study, however, focuses on smaller, nonweight-bearing dinosaur bones, including ribs, gastralia, and fibulas. These bones do not develop hollow cavities or remodel as they grow, so growth lines are clear when viewed under a microscope.

"Our work was based on counting lines in a very large number of tyrannosaurid bones that correspond to annual growth cycles, just as the lines on bones of modern snakes, lizards and crocodiles reveal the age of those animals," Dr. Makovicky said.

Using smaller bones in this way will greatly enhance dinosaur research since they are more plentiful and typically do not have to be cut since many of these smaller bones are already fragmented, he added.

The study in Nature examined more than 60 bones from 20 different fossils of four closely related Late Cretaceous North American tyrannosaurids. This includes T. rex (seven different fossils), Albertosaurus (five), Gorgosaurus (five) and Daspletosaurus (three). Three tyrannosaurid specimens used in the study came from Field Museum collections, including the largest and oldest specimen studied (FMNH PR 2081, also known as Sue) as well as one of the youngest individuals in the study (FMNH PR 2211).

"Drawing on such a large number of fossil specimens provides the first evidence of its kind pointing to major differences in whole body growth rates among a non-avian group of dinosaurs," said Gregory Erickson, PhD, Assistant Professor of Biological Sciences at Florida State University, a Field Museum Research Associate and lead author of the paper. Other coauthors work at the Royal Tyrrell Museum of Palaeontology, American Museum of Natural History (Mark Norell), Stanford University and The University of Iowa.

Press conference

Growth curves for T. rex and three closely related tyrannosaurids. Age was determined by counting the growth lines in bones; the corresponding body size was calculated by measuring the circumference of the femur. Copyright Gregory M. Erickson Full size image available here

Drs. Makovicky and Erickson will present their findings at a 12:00 noon press conference on Aug. 11 at The Field Museum in front of Sue, the world's largest, most complete and best preserved T. rex fossil. [Free parking for the media will be available in The Museum's west lot, immediately adjacent to the building.] Seven million people have seen Sue at The Field Museum since it was unveiled in May 2000, and millions more around the country have seen traveling casts of Sue.

This research determines that Sue was 28 years old at the time of death, although the fossil is 67 million years old. For more information on Sue, visit http://www.fmnh.org/exhibits/exhibit_sites/sue/default.html.

Of the four types of tyrannosaurids studied in this research, T. rex demonstrated the largest by far exponentially accelerated growth curve. Albertosaurus, Gorgosaurus and Daspletosaurus showed a similar four-year growth spurt, but much less pronounced, gaining an average of 310 to 480 grams (11 to 17 ounces) per day. Also, they reached adulthood at 14 to 16 years of age (a little earlier than T. rex). Sue was instrumental in the study, by revealing that she had reached her full size nine years before her death, thus indicating that T. rex could have a life span of about thirty years, one-third of which would be spent at adult size. T. rex grew faster but had a shorter lifespan than an African elephant, the only living land animal of a comparable size.

"We can now stop guessing about how T. rex and some of its closest relatives grew," Dr. Makovicky said. "We expect that this empirically based method of developing life patterns from small, amedullar bones [those without a hollow cavity] will be applied to many other types of dinosaurs."