How long did dinosaurs live?

Scientists do not know the exact lifespan of the dinosaurs, but they estimate that dinosaurs lived about 75 to maybe as much as 300 years. This educated guess is based on examining the microstructure of dinosaur bones, which indicate that the dinosaurs matured slowly. This is similar to ancestors of the dinosaur, including crocodiles, whose eggs take about 90 days to hatch, and whose lifespans can extend from 70 to 100 years.

Does the microstructure of dinosaur bones indicate the age of these animals?

Yes, paleontologists have discovered that the bones of dinosaurs have growth rings similar to those found in the trunks of trees that possibly represent each year of a dinosaurs growth. The ring features are very small; to see them, the bones have to be cut into thin sections and examined under a microscope using polarized light. These growth rings are known scientifically as lines of arrested growth (LAG). Although they are assumed to form at the rate of one ring per year, no one truly knows if this is correct. Thats because the distances between the growth rings between dinosaurs species vary greatly.

Using this technique, scientists have estimated the age of certain dinosaurs. For example, the bones of a ceratopsian dinosaur, Psittacosaurus, indicated that this particular animal was about 10 to 11 years old when it died; a second analyzed dinosaur, a Troodon, was three to five years old; the bones of a sauropod called Bothriospondylus showed it was 43 years old; a Massospondylus, a prosauropod, was 15 years old; and a ceratosaur called Syntarsus was seven years old.

What is another way to estimate a dinosaurs life span?

Another way to estimate a dinosaurs life span is to compare it to the known life spans of living animals, based on body size. In general, larger animals tend to live longer than smaller ones. Using this method, the life span of very large sauropods like Apatosaurus and Diplodocus was probably on the order of 100 years. Smaller dinosaurs would likely have had shorter life spans.

Do we definitely know the growth pattern of dinosaurs?

No, we really dont know for certain what kind of growth pattern the dinosaurs had, and there are several reasons why. First, not all dinosaurs shared the same growth rate. Different types of dinosaurs probably grew at varying rates, complicating the subject. The second reason for the uncertainty in growth rates has to do with climate. Dinosaurs inhabiting warmer regions of the planet probably grew more rapidly than those living in colder climates. Another reason has to do with the metabolic rate of dinosaurs, which is an area of intense speculation. Warm-blooded vertebrates with higher metabolic rates can grow up to ten times faster than cold-blooded vertebrates. Dinosaurs with potentially higher metabolic rates, such as the small theropods, might have grown faster than the slow moving sauropods, though not larger.

It might be possible to estimate the age of a dinosaur (here, a T rex) by analyzing the growth pattern in its bones (Big Stock Photo).

What are the estimated growth rates for a ceratopsian and sauropod dinosaur?

Even though there are concerns with determining growth rates, some scientists have tried to estimate the numbers. In particular, the growth rates were calculated for species where there is fossil evidence from both eggs and adults, using the maximum growth rate of living reptiles as a guide.

For example, an adult Protoceratops, a ceratopsian of the Cretaceous period in Mongolia, weighed approximately 390 pounds (177 kilograms); the hatchling weighed approximately 0.95 pounds (0.43 kilograms) (its assumed the hatchling weight was 90 percent of the egg weight). From this data, the researchers calculated the time the young Protoceratops needed to reach adulthood was approximately 26 to 38 years. On the other end of the size scale was an adult Hypselosaurus, a sauropod of the Cretaceous period in France. This large dinosaur weighed approximately 5.8 tons (5.3 metric tons) at adulthood; the hatchling weighed approximately 5.2 pounds (2.4 kilograms). The scientists calculated the time need to reach adulthood for this animal at 82 to 188 years.

Are there problems with estimating dinosaur growth rates based on modern reptiles?

Yes, there are problems with using growth rates of modern reptiles as a guide to determine those of dinosaurs. If we look to the modern relatives of the dinosaurs to determine growth patterns, we are again baffled. All these animals also show different types of growth patterns. Reptiles continue to grow as long as they live, though the rate slows with age; this is called indeterminate growth. On the other hand, birds cease growing as they reach adulthood; this is called determinate growth.

Some, if not all, dinosaurs probably had a much different metabolism than modern, cold-blooded reptiles, which would drastically alter the growth rate calculations. Also, the actual growth rate depends on the climate where the dinosaur lived. Animals that live in warmer climates grow faster than those in colder ones.

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