dinosaurs -- 2/23/22
Today's selection -- from A Brief History of Earth by Andrew H. Knoll. Why were dinosaurs big?:
"Now we can return to basic questions about dinosaurs. The definition of a dinosaur is actually fairly prosaic. Beginning in the early 1800s, paleontologists discovered a number of enormous fossils, distinct from any tetrapod alive today, and gave them an evocative name: dinosaurs (from the Greek for 'terrible lizards'). Today the group is defined in terms of genealogy: dinosaurs include the last common ancestor of these first-found behemoths and its descendants. Fortunately, this definition aligns pretty well with the image conjured up when someone mentions the term, but as we shall see, it has a surprising consequence.
"When we think of dinosaurs, most of us envision gigantic creatures, with even the herbivores looking more than a little forbidding. That's broadly correct, although the smallest known dinosaurs weighed only about 15 pounds (7 kilograms), the size of a miniature schnauzer. A recent compilation of body size among vertebrate species shows that for most groups, be they mammals, birds, amphibians, or fish, size distribution is weighted toward smaller bodies, with a long, low distributional tail extending to larger species -- lots of rodents but not many elephants. Dinosaurs, however, are different; their sizes are actually skewed toward large bodies.
|Scale diagram comparing a human and the largest-known dinosaurs of five major clades|
"So as any eight-year-old could tell you, most dinosaurs really were big. But why should this be the case? Why were dinosaurs different from other tetrapods that have roamed the Earth through time? There is no consensus on the answer, but German paleontologist Martin Sander and his colleagues have articulated a hypothesis that strikes me as sound.
"The first dinosaur giants, and indeed, the largest dinosaurs of all time, were the sauropods, long-necked herbivores whose largest representatives, the titanosaurs, reached lengths up to 120 feet (37 meters) and weights of 70 to 90 tons. (The magnificent specimen on display at New York's American Museum of Natural History is cleverly mounted so that its head sticks out into the hallway, underscoring its immensity.) Sander and his colleagues draw particular attention to those long necks.
"The remarkable necks of sauropods enabled them to reach food not available to other plant eaters and to forage over large areas with minimal motion -- the larger they grew, the more effective they became at garnering food resources. Long necks were possible because sauropods have very small heads -- the necks of sauropods couldn't have supported heads the size of those on hadrosaurs or tyrannosaurs. Small heads, in turn, were possible because, unlike obedient children, sauropods didn't chew their food. They simply-and rapidly-nipped and stripped branches, swallowing leaves and seeds more or less whole.
"Unlike crocodiles, dinosaurs had a bird-like respiratory system, facilitating the efficient transport of oxygen through a mammoth body and, importantly, leading to vertebrae with numerous air cavities that lightened the weight of the neck. Also, sauropods had high metabolic rates, enabling them to grow rapidly, a necessity for species whose adults could be 100,000 times larger than hatchlings. Today animals are usually classified as warm-blooded, maintaining high body temperature by burning a lot of calories, or cold-blooded, relying on the environment to regulate body temperature. Warm-blooded mammals and birds use a lot of the energy they take in as food to maintain those high internal temperatures. It appears that while dinosaurs weren't warm-blooded in the sense we associate with birds and mammals today, they were able to maintain elevated body temperature in a distinct way that promoted efficient metabolism, while devoting more of their food intake to growth. The key, not surprisingly, was size. As an animal grows larger, the heat it generates increases as a function of its volume (the cube of length), while body heat is dissipated as a function of surface area (the square of linear dimensions). Thus, at the large sizes attained by dinosaurs, high internal temperatures could be maintained passively. Recent support for this view comes from chemical analyses of sauropod bones, which indicate a body temperature of 97-100 degrees Fahrenheit (36-38 °C), much like living mammals.
"For sauropods, size provided a strong defense against predators (elephants seldom fear leopards). In response predators grew larger, setting off an evolutionary arms race across dinosaurs as a whole. In consequence, the ecological and physiological sweet spot on land became that of the terrible lizards. Early mammals lived in the same communities as dinosaurs, but were unable to attain similarly large size. To survive, most simply stayed out of the dinosaurs' way, living nocturnally, or in trees or burrows, as many mammal species do today. For those who favor David over Goliath, it is worth noting that at least some of these early mammals ate dinosaur eggs."