Most people can stand 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 was much larger than any land animal alive today.
We’re 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 how their bodies worked?
Until relatively recently, we had no way to measure age in a dinosaur. Paleontologists generally assumed that because dinosaurs were reptiles, they probably grew the way reptiles do today—slowly. So, the thinking went, large dinosaurs must have reached very old ages. But no one knew just how old, since no living reptiles reach anything near the size of a dinosaur.
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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 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, 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. Owen’s image stuck, and well into the 1960s dinosaurs were portrayed as sluggish, lumbering animals that grew slowly to great size in a sort of benign hothouse where huge beasts reigned and raged.
But evidence for the ages of dinosaurs and how they must have grown was there all along—locked inside the bones themselves. Although paleontologists had long known that dinosaur bones contain growth lines, 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 are 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, by contrast, is a busy place. In fact, the whole bone constantly grows and changes throughout an individual’s life.
We can see the annual growth lines in this cross-section of the femur of a Troodon, a small carnivorous dinosaur. The lines become more closely spaced toward the outside of the bone, which was deposited later in the dinosaur’s life—when the animal was growing at a slower rate, as we all do with advancing age.
As a bone grows, new tissue is deposited on the outside. In the long bones, like the femur and tibia, growth also occurs at the ends of the shafts. The cortex, or outer layer, of bone is built of minerals such as calcium phosphate and proteins such as collagen. which are carried by blood vessels. When the vascular canals, which contain the blood vessels, begin to deposit bone along their insides in concentric layers, they are called osteons.
Meanwhile, cells called osteoclasts hollow out the center of a long bone by breaking down existing bone and allowing its nutrients to be recycled. This center—or marrow cavity, as it’s called—is also the factory that produces red blood cells.
But in the process of doing all this, osteoclasts end up eroding the bone that was deposited early in life, thereby destroying the record of growth during the youngest stages of an individual’s life. Because of this, it’s difficult to cut open the bone of a dinosaur and determine its age just by counting the rings. So we must use different methods to reconstruct the early history of the bone.
One method is to study the bones of younger individuals to fill in the record. These younger bones contain the tissues that have been eroded in older specimens. 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 juvenile specimens available, an alternate method is to “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 had a dozen specimens of this giant carnivore, and seven of them had reasonably well preserved hind-limb bones.
The microscopic slides of thinly sliced T. rex bones revealed only four to eight preserved growth lines. Other lines near the center had been obscured by the growth of secondary bone tissue. Even more striking, the marrow cavity was so large in these dinosaurs that two-thirds of the original bone cortex was 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’d seen this before in other dinosaurs, such as the plant-eating duckbill Maiasaura. It signifies the end of active growth, when the animal has reached full size.
Our retrocalculations estimated that T. rex took 15 to 18 years to attain full size—that is, a hip height of 3 meters, a length of 11 meters, and a weight of 5,000 to 8,000 kilograms. If that seems like rapid growth, well, that’s because it is. At least, for a reptile. It turns out that dinosaurs grew much faster than other reptiles, living or extinct.
For example, research by Gregory Erickson and Christopher A. Brochu of the University of Iowa shows that the giant crocodile Deinosuchus, which lived during the Cretaceous period, some 75 million to 80 million years ago, required nearly 50 years to reach a length of 10 to 11 meters. That’s three times as long as it took T. rex to get to the same size.
And further research shows that T. rex is not unusual for dinosaurs—in fact, it actually grew a little bit slower for its size than other large dinosaurs. Even the largest dinosaurs were still teenagers when they reached their huge sizes.
The plant-eating Massospondylus took about 15 years to reach a length of two to three meters. The small horned Psittacosaurus was mature at 13 to 15 years. Maiasaura reached adulthood at between 7 and 8 years, by which time it was 7 meters long.
The giant sauropods outdo all the others, though: Janenschia reached maturity at about 11 years, and continued to grow substantially after that. Lapparentosaurus attained full size before it was 20. And the Apatosaurus (more commonly known as Brontosaurus) matured in 8 to 10 years—an annual weight gain of nearly 5,500 kilograms!
Inside a Dinosaur Bone
Strangely enough, a closer comparison for T. rex is the African elephant, which reaches a similar mass of about 5,000 to 6,500 kilograms in 25 to 35 years. So T. rex grew to its adult size even faster than an elephant.
But 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 again have to look inside a dinosaur bone.
The tissue in an average dinosaur long bone is composed mostly of a type called fibro-lamellar. Highly fibrous or “woven” in texture, it forms around a matrix of poorly organized collagenous fibers that’s well supplied with blood vessels. This is actually 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.
The differences don’t stop there. A crocodile bone is formed mostly of lamellar-zonal tissue—compact, highly mineralized bone that contains more regularly organized fibers and much sparser, smaller vascular canals. Also, the growth lines in crocodile bones are more tightly spaced than those in dinosaur bones, another indication that crocodile bones grow more slowly.
The Italian scientist Rodolfo Amprino recognized in the 1940s that the type of tissue laid down in a bone at any given place or time during growth is mainly a function of how fast the tissue was growing at that point. Fibro-lamellar tissue, no matter where or when it’s 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 and crocodiles and other reptiles is that dinosaurs deposit fibro-lamellar tissue throughout their growth to adult size, whereas other reptiles very soon switch 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.
Gregory Erickson, Kristina Curry Rogers, and Scott A. Yerby, then at Stanford University, assessed the pace of dinosaurs’ growth in a different way. They plotted the dinosaurs’ estimated body mass against time to derive growth curves for a variety of species. They then compared the curves with those for other groups of vertebrates.
What they found was 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.
We were curious about when in the course of their evolution dinosaurs acquired this 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), as well as 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, too, 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.
Early Birds
The question of how dinosaurs got to be so big raises another interesting question: Why are birds, the living descendants of dinosaurs, so much smaller than their extinct forebears?
It turns out that our new understanding of dinosaurs’ rapid growth gives us new insights into the evolution of birds, as well.
We began looking into these connections by examining the bone tissues of Confuciusornis, a 125-million-year-old bird from the Early Cretaceous period of China that appears on the avian family tree shortly after Archaeopteryx, the first known bird.
The inner part of the bone tissues of the crow-sized Confuciusornis is of a fast-growing, fibro-lamellar type (like those of other dinosaurs). But toward the outside it becomes a slower-growing type—a sign that the growth rate waned after a short, youthful spurt.
We compared these tissues with those of Troodon, a small raptorlike dinosaur about 1.5 meters long. Troodon tissues indicate faster growth overall.
As Confuciusornis shows, to become small these ancient bird species truncated the juvenile growth spurt that was most rapid in other dinosaurs, which caused the birds in effect to become miniaturized.
Miniaturization had an important influence on locomotion because the feathers present on the forelimbs of the closest dinosaurian relatives of birds would have been more likely to help these smaller animals become airborne. Small animals can flap their wings faster than large ones, and in a smaller animal the wing loading (the ratio of weight to wing area) will be proportionally smaller and therefore aerodynamically more advantageous.
But birds today usually reach full size in weeks to months. So what changed?
It appears that after slowing early in their evolution, birds over time sped up their growth rate again—to rates that are often even faster than those of extinct dinosaurs.
Close to the Cretaceous-Paleogene boundary, about 66 million years ago, growth rates increased substantially, so much so that all living birds—even the ostrich—attain full size within less than a year (seven days in the case of the sparrow).
Only examination of birds from the Early Paleogene will tell us whether the living groups of birds acquired their rapid growth to adult size gradually or relatively suddenly.
Unconventional Reptiles
The study of dinosaur bones has told us a great deal about the evolution 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 of dinosaurs and pterosaurs 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 these animals had relatively high basal metabolic rates, probably more like those of birds and mammals than like those of today’s reptiles. This suggests they were much more likely to have been warm-blooded, in a general sense, than cold-blooded. But it’s 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 previously thought—not exactly like any animals that live today and certainly not conventional reptiles. Perhaps if we one day discover a five-ton living bird, these mysteries will be finally be solved.
This article was originally published with the title “How Dinosaurs Grew So Large and So Small” in SA Special Editions 23, 2s, 4-11 (May 2014). doi:10.1038/scientificamericandinosaurs0514-4
