Most people can stand comfortably under the jawline of a mounted Tyrannosaurus rex or walk under the rib cage of a Brachiosaurus without bumping their head. T. rex is as big as the largest known African elephant, and Brachiosaurus, like other great sauropods, was much larger than any land animal alive today. We are so used to the enormous size of dinosaurs that we almost forget to think about how they grew to be so large. How long did it take them, and how long did they live? And does the way they grew tell us about the way their bodies worked?

Until relatively recently, we had no way to measure age in a dinosaur. Paleontologists had generally assumed that because dinosaurs were reptiles, they probably grew much as reptiles do today—that is, rather slowly. Thus, the thinking went, large dinosaurs must have reached very old ages indeed, but no one knew how old, because no living reptiles attain anything near the size of a dinosaur.

This attitude can be traced back to English paleontologist Sir Richard Owen. When he named the Dinosauria in 1842, he was putting a label on a very small, poorly known group of very large, unusual reptiles. Not only were they big, he said, but they were terrestrial, unlike the seagoing ichthyosaurs and plesiosaurs that had been known since the early 1800s. They had five vertebrae (backbones) connected to the hips, not two like living reptiles. And they held their limbs underneath their bodies, not sprawled out to the sides. Despite these differences, he continued, the anatomical features of their bones—the shapes, joints and muscle attachments—showed them to be reptiles. So they must have had a reptilian physiology—that is, a typically “cold-blooded,” slow metabolism. The image stuck, and well into the 1960s dinosaurs were portrayed as sluggish, lumbering animals that must have grown slowly to great size in a sort of benign hothouse where huge beasts reigned and raged.

Yet evidence for the ages of dinosaurs, and so for how they must have grown, was there all the time—locked inside the bones themselves. Although paleontologists had known for many years that the bones of dinosaurs contain growth lines, something like the circumferential growth rings we see in trees, it was only in the second half of the 20th century that they began to use these growth lines and other structures inside the bones to figure out how these extinct animals actually grew.

The Bones Tell the Story

Like the rings in trees, the lines in the bones of dinosaurs were annual. But they aren't quite as simple to interpret. A tree carries nearly the entire record of its growth inside its trunk. Cut it down, and you can count the rings one by one from the center to the bark. Only the outer layer is making new wood; the inside is really deadwood. The center of a bone, in contrast, is a busy place. Cells called osteoclasts hollow out the center of a long bone, such as the femur (thigh) or tibia (shin), by breaking down existing bone and allowing its nutrients to be recycled. This center, or marrow cavity, is also the factory that produces red blood cells.

To accomplish these tasks, the whole bone constantly grows and changes throughout life. As a bone grows, new tissue is deposited on the outside, and in the long bones growth also occurs at the ends of the shafts. Meanwhile, in the marrow cavity, osteoclasts are eroding the bone that was deposited early in life, and other cells are making secondary bone tissue along the perimeter of the cavity or invading the cortex (outer layer) of the remaining bone to remodel it.

This activity at the center of the bone often erodes the record of growth during the youngest stages of an individual's life. Consequently, it is difficult to cut open the bone of a dinosaur and find a complete record of growth just by counting the rings. So we reconstruct the early history of the bone in several ways. One is to use the bones of younger individuals to fill in the record. These younger bones contain the tissues that have been eroded in older bones. By examining these tissues and counting the growth lines, we can approximate the number of years that are missing from the older bones. When we have no juveniles available, we can “retrocalculate” the number of growth lines by examining distances between preserved growth lines.

In 2004 we tried retrocalculation on the most famous dinosaur of all: T. rex. The Museum of the Rockies at Montana State University has a dozen specimens of this giant carnivore, and seven of them have reasonably well preserved hind-limb bones that allowed us to make thin slices that can be examined under a microscope.

The microscopic slides of T. rex bones revealed only four to eight preserved growth lines. Others, near the center, had been obscured by the growth of secondary bone tissue. Even more striking, the marrow cavity is so large in these dinosaurs that two thirds of the original bone cortex is eroded away. We also noticed that in some individuals the space between the growth lines suddenly became very small toward the outermost surface of the bone. We had seen this before in other dinosaurs, such as the plant-eating duckbill Maiasaura. It signifies the end of active growth, when the animal reached full size.

Our retrocalculations estimated that T. rex took 15 to 18 years to attain full size, which is to say a hip height of three meters (10 feet), a length of 11 meters (34 feet), and a weight of 5,000 to 8,000 kilograms (five to eight tons). (We were pleased to see that our estimates matched those of Gregory M. Erickson of Florida State University and his colleagues, which were completed at about the same time.) If that seems like rapid growth, it is. At least, for a reptile. It turns out that dinosaurs grew much faster than other living or extinct reptiles do.

For example, Erickson and Christopher A. Brochu of the University of Iowa charted the growth of the giant crocodile Deinosuchus, which lived during the Cretaceous period, some 75 million to 80 million years ago [see bottom illustration on page 10]. These huge reptiles reached estimated lengths of 10 to 11 meters. Examining the growth lines in the skin armor of the neck, Erickson and Brochu determined that such an animal required nearly 50 years to achieve this length—three times as long as it took T. rex to get to the same size. A closer comparison for T. rex proves to be the African elephant, which reaches about the same mass (5,000 to 6,500 kilograms) in 25 to 35 years. So T. rex grew to its adult size even faster than an elephant does.

Further research showed that T. rex is not unusual for dinosaurs—except that it actually grew a little bit more slowly for its size than other large dinosaurs did. Anusuya Chinsamy-Turan, now at the University of Cape Town in South Africa, found that the plant-eating Massospondylus took about 15 years to reach a length of two to three meters. Erickson and Tatyana A. Tumanova of the Paleontological Institute in Moscow found that the small ceratopsian (horned) Psittacosaurus was mature at 13 to 15 years. And we calculated that Maiasaura reached adulthood at between seven and eight years, by which time it was seven meters long. The giant sauropods (“brontosaur” types) outdo all the others, though: Martin Sander of the University of Bonn in Germany discovered that Janenschia reached maturity at about 11 years, although it continued to grow substantially after that. Frdrique Rimblot-Baly and her colleagues, then at the University of Paris VII, determined that Lapparentosaurus attained full size before it was 20 years old. Kristina A. Curry Rogers, now at Macalester College, found that Apatosaurus (more familiarly known as Brontosaurus) matured in eight to 10 years—an annual weight gain of nearly 5,500 kilograms.

Inside a Dinosaur Bone

Why should dinosaurs grow more like elephants than like giant crocodiles? And what does this mean for other aspects of their biology? To answer these questions, we have to look inside a dinosaur bone at the kind of tissues it laid down.

The tissue in a typical long bone of a dinosaur is primarily a type called fibro-lamellar: it is highly fibrous or “woven” in texture, and it forms around a matrix of poorly organized collagenous fibers that is well supplied with blood vessels. In contrast to what we would expect in conventional reptiles, this is the same kind of tissue that predominates in the bones of large birds and large mammals, which grow to full size faster than typical reptiles do. A crocodile bone, on the other hand, is formed mostly of lamellar-zonal tissue—compact, highly mineralized bone that contains more regularly organized fibers and much sparser, smaller vascular canals. Furthermore, the growth lines in crocodile bones are more tightly spaced than those in dinosaur bones, another indication that crocodile bones grow more slowly.

The late Rodolfo Amprino, then at the University of Turin in Italy, recognized in the 1940s that the type of tissue laid down in a bone at any given place or time during growth was mainly a function of how fast the tissue was growing at that point. Fibro-lamellar tissue, no matter where or when it is deposited, reflects locally rapid growth, whereas lamellar-zonal tissue signals slower growth. An animal can lay down either of these tissues at different times—as the growth strategy warrants. The type of tissue that predominates through the animal's life provides the best guide to its growth rate.

One difference between dinosaurs, on the one hand, and crocodiles and other reptiles, on the other, is that dinosaurs deposit fibro-lamellar tissue all through growth to adult size, whereas other reptiles switch very soon to lamellar-zonal bone. We inferred from this that dinosaurs sustained more rapid growth until the adult stage because there would be no other good explanation for the persistence and predominance of fibro-lamellar tissue.

The pace at which dinosaurs grew was assessed in a different way by Erickson, Rogers and Scott A. Yerby, then at Stanford University. Using estimates of body mass of dinosaurs, they plotted the animals' mass against time to derive growth curves for a variety of species and compared the curves with those for other groups of vertebrates. They found that all dinosaurs grew faster than all living reptiles, that many dinosaurs grew at rates comparable to those of living marsupials, and that the largest dinosaurs grew at rates comparable to those of rapidly maturing birds and large mammals. We confirmed their results for body mass with our own studies using length.

In one sense, such findings were not unexpected. Many years ago Ted J. Case, then at the University of California, Los Angeles, showed that within any group of vertebrates, larger species reach adult size in a longer time but grow more quickly to do so. What was surprising is that dinosaurs grew as fast as they did.

We were curious about when in the course of their evolution dinosaurs acquired this habit of rapid growth, so we plotted our estimated growth rates on a cladogram, or diagram of relationships, that was built on hundreds of independent characteristics from all parts of the skeleton. We added the estimated growth rates for pterosaurs (flying reptiles closely related to dinosaurs, which grew much like them), crocodiles and their extinct relatives, and lizards. We put birds among the dinosaurs because birds evolved from dinosaurs and so are technically included with them.

For added help in estimating the growth rates of dinosaurs, we looked at living birds, which show the same range of tissues expressed in dinosaur bones. Jacques Castanet and his colleagues, then at Pierre and Marie Curie University in Paris, injected mallard ducks with solutions that would stain the growing bones. By using different colors at different times, they were able to measure rates of weekly growth in the sacrificed birds. Using these calibrations, we determined that dinosaurs and pterosaurs grew at much higher rates than other reptiles.

Unconventional Reptiles

The study of dinosaur bones has told us a great deal about the evolution of some of the major features of these animals. About 230 million years ago, in the early part of the Triassic period, the lineage that would produce dinosaurs, pterosaurs and their relatives split from the lineage that would produce crocodiles and their relatives. The dinosaurian lineage soon acquired sustained elevated growth rates that set them apart from other reptiles. This speedy growth may have played a role in the success that dinosaurs and pterosaurs enjoyed toward the end of the Triassic, when so many species with more typical reptilian bone structure became extinct.

The high growth rates of dinosaurs also give us a firmer idea about their metabolic features. The higher the metabolic rate—that is, the more energy devoted to building up and breaking down bone and other tissues—the faster the tissues will grow. So evidence of sustained rapid growth, even into late juvenile and subadult stages, implies that the animals in question had relatively high basal metabolic rates, probably more like those of birds and mammals than like those of today's reptiles. This suggests that they were much more likely to have been warm-blooded, in a general sense, than cold-blooded, but it is difficult to know the details, such as body temperature and how much it varied. Clearly, many questions remain. Dinosaurs were perhaps even more unusual creatures than we had previously thought—not exactly like any animals of today and certainly not conventional reptiles. If anyone ever discovers a five-ton living bird, a lot of these questions will be settled.