Theropod Speed

The speeds at which theropods could run has been a source of speculation and debate for many years. As they consider the question of potential speeds, paleontologists draw on fossil trackways for direct evidence of dinosaur locomotion, on an understanding of dinosaur limb anatomy, and on assumptions regarding metabolism and potential energy expenditure as compared to the physiology of extant animals.

Fossil footprints, and especially trackways, left behind by dinosaurs are the best evidence about the locomotory behavior of dinosaurs.

It is usually not possible to identify the maker of such tracks; but tracks made by theropod dinosaurs, with their three birdlike talons, are easily distinguishable from those of herbivorous dinosaurs. Most trackways are not long and almost always show an animal walking rather than running; this makes sense because running through soft sediment, a surface that is good for capturing footprints, puts an animal in danger of slipping and falling much more than does running on hard ground. There is, however, some trackway evidence for running theropods that has been studied to determine speed.

It is also worth noting, however, that it is impossible to associate a particular set of footprints with any species of dinosaur unless the tracks lead directly to the body of the track-making animal. (This has never yet happened with a dinosaur.) Also, determining the leg length involves either unproveable guesswork as to which animal made the tracks or using some statistics to find generalizations about the group of track-making animals. For theropods, hip height is generally perceived as somewhere between four and five times the length of the footprint. Of course, there always are the possibilities of exceptions: Among birds, for example, jacanas have huge feet for their relatively short legs, so hip height and footprint length do not fall into the same range. Still, this sort of generalization from statistics is the best technique presently available.

To calculate dinosaur speed from trackways, paleontologists use a formula that takes into consideration the length of the stride and the length of a dinosaur's leg from the ground to the hip. Zoologist R. McNeill Alexander is an expert on the biomechanics of animals. In 1976, he first worked out a formula to calculate speed from trackways that is widely used today. Alexander applied his formula to several kinds of theropod trackways. The top theropod speed he calculated from the trackways of a small theropod, probably of the ostrich-dinosaur variety, was 27 mph (16.8 km/hr). This is faster than a human can run, somewhat slower than a racehorse, and about the same speed as a galloping antelope. How long a theropod could maintain such speed is, however, unknown.

Trackways for large running theropods are scarce, and none have been found for a creature the size of Tyrannosaurus. Educated guesses are all that can be made about the speed of the largest theropods. In the 1980s, paleontologists Gregory Paul and Robert Bakker proposed that tyrannosaurs and ornithomimosaurs ("ostrich-mimic dinosaurs") were fast runners, with top speeds ranging from 30 to 45 mph (18.7 to 28 km/hr). Several anatomical features of those dinosaurs led Paul and Bakker to that conclusion. These features included the long limbs, powerful thigh and calf muscles, shock-absorbing and flexible knees and ankles, and long, narrow, three-toed feet shared by such theropods.

Variation in theropod forelimbs (from left): Struthiomimus, Tyrannosaurus, Carnotaurus (not to scale)

Most other paleontologists are not as convinced about fast-running theropods, however. Much of this lack of conviction is due to uncertainty over the metabolic rate of the large theropods. Most paleontologists think that a top speed for Tyrannosaurus of 45 mph was unlikely except, perhaps, for a very short period of time. Paleontologist James O. Farlow went a step further by suggesting that a large theropod risked serious injury if it fell while running at high speed. Farlow questioned whether it made sense for such animals ever to do so.

The recent work of mechanical engineer John Hutchinson of Stanford University has also dampened enthusiasm for fast-running giant theropods. Using a computer model to simulate the anatomy of a running T. rex, Hutchinson and his colleagues calculated how much leg muscle a terrestrial animal would require to support running at various speeds. "As animals get really enormous," explained Hutchinson, "eventually to support their weight

(continues on page 102)

THINK ABOUT IT

Dinosaur Intelligence

Is it possible to determine the intelligence of dinosaurs? If intelligence is defined as the ability to reason and learn, it would seem impossible to know this about dinosaurs without having the opportunity to observe them in life. Surprisingly, paleontologists are not entirely without some clues about the intelligence of extinct animals—clues based on evidence from the fossil record. Chief among these is the relative size of the dinosaur brain, with relative meaning "compared to the size of the body the brain controlled."

The size of an animal does not determine how intelligent it is. A bigger animal is not necessarily a smarter one. Similarly, the size of the brain does not determine how intelligent an animal is, and a bigger brain is not necessarily a smarter one. What is more important is the weight of the brain in proportion to the weight of the body. By this measure, people have a large brain in proportion to a relatively small body size. Birds and mammals score higher than reptiles, amphibians, and fish when it comes to brain-weight-to-body-weight ratios. This places mammals at the top of the "intelligence" pyramid in today's animal kingdom, a conclusion that most people can agree with if one loosely defines intelligence as the ability to reason and learn.

The brain, like other soft tissues and organs, does not fossilize. The approximate size of a dinosaur's brain can be determined either by casting, or by computed tomography (CT) scanning, and then measuring the cavity inside the skull that once held the brain. Having a dinosaur skull, however, does not guarantee that the brain cavity inside the skull is preserved well enough to measure. Most dinosaur skull material is fragmentary, often missing part or all of the braincase bones that surround the cavity. Even when a complete braincase is present, skulls are often distorted due to compression and crushing of the bones during fossilization, thus making measurements of the braincase less accurate. The advent of CT scanning during the past 10 years has provided a noninvasive way for paleontologists to measure the braincase inside a skull. As a result, knowledge about dinosaur brains is slowly expanding for those dinosaurs represented by good skull material. Even so, measurements of dinosaur braincases have thus far been made for less than 5 percent of all known taxa.

The size of the brain alone is not a measure of intelligence. When studies of brain size are combined with studies of living animals, however, scientists can establish a rough link between the weight of the brain, the weight of the body, and observable intelligence. In the largest kinds of living animals, such as the elephant, the ratio of brain weight to body weight is predictably lower than that seen in smaller animals—such as a dog or cat—even though a high level of intelligence is still present. This means that the brain itself does not have to be huge in a huge animal for the animal to be smart. Overall body size is an important consideration when dealing with dinosaurs because of their tendency toward gigantism.

In the world of dinosaur brain sizes, theropods generally had a higher brain-weight-to-body-weight ratio. Small theropods, such as Troodon, had the largest known brains in comparison to body weight. This made them comparable to some modern birds and mammals. Many other dinosaurs, although maybe not as gifted, were still comparable to modern crocodyl-ians and other reptiles when it came to the sizes of their brains compared to body weight.

In considering the intelligence of dinosaurs it is also instructive to keep in mind that the parts of the brain come in many different sizes, and that not all parts of the brain are (or even could be) involved in reasoning. Many dinosaurs had highly developed parts of their brains that were involved instead in smell, sight, and other senses. It is also important to keep in mind that intelligence itself is not the be-all and end-all of things, and that the longevity of a species does not depend on intelligence. That is, the ability to reason is by no means a requirement for an animal to function or survive; the animal only has to have enough brain function

(continues.)

(continued)

to enable it to function in its ecological niche. Humans are by no means "better" than other animals because they have bigger brains. Humans are better at reasoning and abstract thought but are not better at, for example, doing what elephants do, or what voles or owls do. The brains of those creatures are adapted for doing what they do. It is not a matter of "better" or "worse."

(continued from page 99)

their muscles have to be bigger and bigger and bigger. But as they get bigger, they add more mass." Eventually, the large size of an animal prevents it from adding more muscle to support its growing weight, and a ratio between muscle mass and body weight is achieved. Hutchinson concluded that a biomechanically reasonable speed range for Tyrannosaurus was between 10 and 25 mph (6 to 16 km/hr). It is important to note that Hutchinson's conclusions were based strictly on mechanical aspects of tyrannosaur anatomy and did not take into consideration the possible metabolic rate of giant theropods.

Was this article helpful?

0 0

Post a comment