How did dinosaurs live?

[[☃ image 2]]Dinosaur fossils have been found on every continent, almost everywhere that Late Triassic, Jurassic, or Cretaceous rocks are exposed on Earth's surface. Because dinosaurs were (and in the form of birds, still are) such a diverse group of animals, they probably lived in nearly every terrestrial environment. Their fossils, including bones, teeth, and footprints, have been found in Mesozoic rocks that were part of deserts, savannahs, forests, beaches, and swamps when the dinosaurs were alive.

The world was quite different from today

While dinosaur fossils have even been discovered in Antarctica and North America, but could they really survive polar conditions? During most of the Mesozoic, the world was quite different than it is today. The global climate was much warmer, and polar ice caps probably did not exist. Also, the continents were not as widely separated by large oceans as they are today. Nonetheless, studies of ancient geography indicate that some dinosaurs that lived in Alaska, Antarctica, and even Australia would have lived very near Earth's poles. These dinosaurs would have experienced extensive periods of darkness, which would have drastically affected the food supply of herbivorous dinosaurs by limiting how much plants could grow. Scientists continue to study how dinosaurs adapted to these conditions, perhaps through hibernation or migration.

Since birds are living dinosaurs, we know that modern dinosaurs often travel in flocks, exhibiting a complex social structure. But did extinct, non-avian dinosaurs do the same? Some exceptional fossil discoveries do provide direct evidence of some dinosaur behaviors, which not only enrich our knowledge of how these animals lived but also foreshadow many of the behaviors seen in birds.

[[☃ image 2]]Trackways

The most definitive evidence for dinosaur herd or pack behavior comes from sequences of fossilized footprints called trackways. Several track sites have been found that suggest herding behavior in some groups of dinosaurs. R. T. Bird of AMNH discovered a set of sauropods tracks in 1940 at the Davenport Ranch site outside San Antonio, Texas. Detailed analyses of these trackways reveal a herd of 23 sauropods rambling over muddy ground at a pace of about 6.5 feet (2 meters) per second. The pattern of overlapping tracks indicates that the largest adult sauropods led the herd, followed by smaller individuals.

Other trackways in Colorado and Korea show herds of large sauropods moving side-by-side in a horizontal front. At one site in Colorado, the animals' paths undulate from left to right in unison.

Trackways of meat-eating theropods are less common, but one occurrence is Bird's specimen from Glen Rose, Texas, a part of which is on display at AMNH. In all, the Paluxy River trackways, some of which were sent to other institutions, reveal the footprints of at least 12 sauropods and at least three, large, carnivorous dinosaurs moving along a mudflat above the waterline. But it is unclear how long after the sauropods passed did the theropods follow, or if the theropods moved in a pack. However, in Bolivia, paleontologist Giusseppe Leonardi discovered eight parallel trackways of six adult and two juvenile sauropods moving along the shore in a front, followed shortly by a pack of at least 32 medium-sized, carnivorous theropods, moving in the same general direction along the same front.

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Some chemical reactions in the body, such as the burning of sugar to produce energy while releasing carbon dioxide and water, are referred to as metabolism. These reactions run most efficiently at a particular optimum temperature. In mammals and birds, the optimum temperature is higher than the normal outside temperature and is regulated internally. So these animals maintain generally higher body temperatures and are called "warm-blooded" or endothermic, for their internal temperature control.

Other animals, such as turtles, crocodiles, and lizards, have more variable body temperatures based on their level of activity, and they regulate their body temperatures through external sources, such as warming up in the sun or cooling off in the shade. They are often called "cold-blooded," or more appropriately, ectothermic, for their external temperature control.

Ectothermic or endothermic?

Were dinosaurs ectothermic or endothermic? Since birds are a group of living dinosaurs, we know that at least some dinosaurs are endothermic. But further evidence comes from cutting open dinosaur bones. By counting the rings on dinosaur bones, paleontologists have determined the yearly growth rates for many species. While growth rates vary within dinosaurs, no dinosaurs grew as slowly as modern reptiles. Small dinosaurs grew approximately as fast as modern marsupial mammals (which grow more slowly than placental mammals like humans, but faster than reptiles), and larger dinosaurs grew even faster, allowing them to attain extremely large body sizes within the span of two decades. However, relatively speaking, birds are the fastest-growing modern vertebrates—they don’t put on very much mass but can reach adult size in weeks. No dinosaurs grew as quickly as modern birds. Baby dinosaurs also have the woven bone texture typical of fast-growing baby mammals.

The size and density of blood vessels within dinosaur bones provides more strong evidence for endothermy. Dinosaur bones are marked by extensive Haversian and Volkmann’s canals, which are spaces for blood vessels crisscrossing the bone tissue to provide it with nutrients and oxygen. Ectotherms also have these structures, but because their metabolic rates are so low, they have few of them; only very large reptiles develop a large quantity of Haversian and Volkmann’s canals. Endotherms of all sizes have extensive canal networks within their bones, and dinosaurs do, too.

Also, the size of the openings where blood vessels enter the bones are proportionally larger in dinosaurs than they are in any living mammal, indicating that dinosaurs required an even greater blood supply to sustain their metabolism. So, their metabolic rates may have been even higher than those found in modern mammals.

Because many extinct, non-avian dinosaurs, such as Tyrannosaurus and Apatosaurus, were so immense, scientists used to assume that they grew slowly and required 50-100 years to attain their great size.

Growth rings

But research involving microscopic analyses of the cellular structure of dinosaur bone has shed further light on these mysteries. Dinosaur bones contain growth rings, somewhat like the rings in tree trunks, which reveal yearly periods of rapid and slow growth. By studying growth rings in Tyrannosaurus and other species, scientists can estimate how fast an animal was growing at different times in its life. They can also estimate how old an individual animal was when it died.

These research studies, done in conjunction with paleontologists at AMNH, document that Tyrannosaurus, which attained a weight of more than 10,000 pounds as an adult, reached sexual maturity at about 20 years of age and lived for up to 28 years. In order to obtain its immense size and weight, T. rex could put on almost five pounds a day during its phase of maximum growth. As one might expect, these rates of relative growth are fairly similar to those found in birds, which are the closest living relatives of Tyrannosaurus.

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The very tiniest dinosaurs (weighing less than a pound) took between three and eight years to reach adult size. Small theropods and ornithischians, weighing 65 to 550 pounds (30 to 250 kg), attained adult size in about 10 to 13 years. Large duck-billed dinosaurs reached adulthood in 10 to 12 years, with the largest specimens living to about 18 years. Ceratopsians (horned dinosaurs) took 6 to 13 years to attain adult size, with the largest individuals living 18 to 20 years. Giant sauropods were adult-size by age 20 to 30, with the oldest known individuals living 41 to 55 years. Although the largest sauropods have yet to be successfully aged, it is unlikely that they—or any non-avian dinosaur—lived 100 years.

Over the last few decades, paleontological artists have often adorned their depictions of extinct, non-avian dinosaurs with a rainbow of picturesque color patterns, in stark contrast to depictions from the earlier part of the 20th Century. In part, this shift reflects the scientific switch from viewing dinosaurs as drab relatives of crocodiles and lizards to viewing dinosaurs as the ancestors of birds. But is there any direct fossil evidence for this change? Well, the answer is no... and yes.

No fossils of extinct dinosaurs, including "mummies" complete with fossilized skin impressions, have yet revealed the true color of the dinosaur's skin, at least in terms of what we usually think of as skin. There are fossils of a couple of turtles from the Cenozoic Era that are thought to preserve the pattern of pigmentation on the shell. Nonetheless, in terms of non-feathered dinosaurs, such as Apatosaurus and duckbills, no evidence for skin color has yet been found.

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Fossilized feathers

However, a great wealth of astounding fossils representing small, feathered dinosaurs that are not birds has been collected in Early Cretaceous rocks in China, especially near Liaoning. Fossilized feathers, basically modified scales that grow in the skin, have been found on numerous specimens. In 2008, researchers discovered microscopic structures, called melanosomes, within fossilized feathers of an ancient bird. Different kinds of melanosomes create different colors in the feathers.

Following this discovery in 2010, two teams of researchers reported finding melanosomes preserved in feathers of two, small, Chinese, non-avian dinosaurs — Sinosauropteryx and Anchiornis. In the more comprehensive study of Anchiornis, in which 29 samples were analyzed from feathers all over the body of one specimen, revealed that this 155-million-year-old, feathered dinosaur, about the size of a chicken, possessed black-and-white striped wings and a rusty brown crest of feathers along the top and back of its skull.

Grab your notebook or computer and write out your answer to each of the following questions. Then compare your answer to the suggested answer found in the “Answers to exploration questions” article. Have fun! (Hint: the information needed to answer these questions can be found in the video and the articles.)

1\. What evidence have scientists found that some groups of dinosaurs traveled in groups? Draw pictures to support your answer.

2\. Explain how scientists estimate a dinosaur’s intelligence.

If you are wondering where the suggested answers came from, you can review the video and the articles in this tutorial.

1\. What evidence have scientists found that some groups of dinosaurs traveled in groups? Draw pictures to support your answer.

**Answer:** The best evidence for how dinosaurs traveled comes from trackways, or sequences of fossilized footprints. From trackways, scientists can learn whether dinosaurs traveled individually or in packs, how many traveled together, which dinosaur led the herd, and even how fast they traveled. Trackways from multiple dinosaurs can reveal that they were in a herd if the tracks appear to have been made at the same time and are in the same direction. Some tracks show where dinosaurs moved side-by-side, or where the animals’ paths undulated back and forth together.

2\. Explain how scientists estimate a dinosaur’s intelligence.

**Answer:** First, scientists estimate the size of a dinosaur’s brain based on the size of its skull. Sometimes they use CAT Scan technology to reconstruct the dinosaur’s brain. Then they estimate the dinosaur’s intelligence based on the size of its brain relative to its body size. They also consider the size of certain parts of its brain, since different parts of the brain relate to different senses or aspects of intelligence.

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