The largest land mammal of today, the african elephant, is certainly no featherweight. The largest males weigh in at over 6 1/2 tons, can reach heights of up to 13 feet, and are perhaps the only mammal that cannot jump, since their immense weight would injure their legs upon landing. And yet, in the grand scheme of earth's backboned, land-dwelling occupants, the african elephant is rather underwhelming. The largest mammal ever, Paraceratherium, had a shoulder height of 16 ft and weighed up to 18 tons. Argentinosaurus is thought to have been about 100 feet long from nose to tail, and would have weighed around 80 tons. Some incomplete finds point to even larger dinosaurs, but the fundamental question remains: why did dinosaurs get so much larger than mammals? During my time at the museum, I was presented with a number of different viable hypotheses on the cause of the dinosaurs' giganticism. However, all of them faling under either external or internal factors.
First, let's look at the external factors. The earth was a very different place 200 million years ago, when dinosaurs first appeared in the fossil record. For starters, the Mesozoic climate was warmer than ours today. For dinosaurs, who were presumed to be cold-blooded, this was huge. A warmer climate meant that dinosaurs were less at the mercy of the sun than today's reptiles, and compared to mammals of comparable size, being exothermic in a warm environment means saving energy on maintaining a huge body and its metabolism. Less energy spent on movement and metabolism means more energy for growth and reproducion. Second, the Mesozoic era had higher levels of oxygen: we can assume higher levels of oxygen fostered growth in dinosaurs. In a 2010 study, Arizona State University researchers raised dragonflies(who are themselves survivors of the dinosaur age) in environments with higher oxygen. On average, these dragonflies grew 15% larger than the control group. Multiply this effect on small dinosaurs over the course of millions of years, and it's not suprising the sizes they grew. Third, plants such as cycads, conifers, and ferns were plentiful, meaning there was less competition for food.
As far as internal factors go, dinosaurs had a few things working for them as well. To complement the higher oxygen levels, dinosaurs are thought to have had air sacs similar to modern day birds', allowing for more effective breathing thanks to the air sacs' billows-like assistance in breathing. These air sacs could have also filled some of the space in dinosaurs' skeletons to offset weight: a hollow spot in some dinosaurs' pelvises is thought to have housed an air sac. Second, dinosaurs had very dense bones. As philosophers and thinkers like Socrates and Galileo explained with the Square Cube Law, increases in height have exponential growth on a structure's weight and volume. Dinosaurs weren't immune to this law, but the density of their bones(coupled with the airs sacs) compensated for their immense girth.Third(and this is just my own conjecture), dinosaurs were able to grow new teeth throughout their life. The largest land animal today, the elephant, is remarkable in that its death in old age can almost invariably be linked to its teeth. An elephant goes through six pairs of massive molars in its life that it uses to chew all kinds of tough plant material. Once an elephant goes through these six sets, though, it is no longer able to effectively chew its food and dies. Elephants are especially long-lived mammals--some live into their seventies--if they were able to replace their teeth constantly, how much longer could they live? Perhaps dinosaurs are the answer to that question. Perhaps the largest dinosaurs we find are simply ancient specimens that were able to elude predators for decades, allowing them to reach their truly behemoth massive sizes.
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