Prehistoric giants populated the Earth. These behemoths included mighty dinosaurs, airplane-sized pterosaurs, massive crocodiles and snakes, and even car-sized armadillos. But today there are only a few large animals on our planet.
What happened? Why aren’t there many giants anymore?
First, there is plenty of fossil evidence that the ancient past really did have larger animals – beasts that were huge but also larger, on average, than creatures today, Greg Erickson, a paleobiologist from vertebrates at Florida State University in Tallahassee which specializes in ancient reptiles, Live Science says. Ever since scientists discovered the first known cache of dinosaur bones in the 19th century, researchers have come up with ideas as to why giants were common millions of years ago, but less so today. But no one can point to a definitive answer, Erickson said. “It’s so multifactorial.”
Related: Why are there so many giants in the deep sea?
However, several major differences between dinosaurs and today’s largest animals, mammals, may help explain the loss of the mastodons. Along with other giant reptiles, dinosaurs could adapt to different niches as they grew through life, hunting smaller prey as juveniles and larger victims as adults. In part, they might do this because they’ve swapped sets of teeth over their lifetime. “They’re constantly replacing their teeth, just like sharks. But along the way, they might change the type of teeth,” Erickson said. Crocodiles, for example, go from “needle-like teeth to more robust teeth. Mammals don’t have that luxury.”
In other words, as some young reptilians matured into large adults, they traded in their relatively puny juvenile teeth for larger weapons, allowing them, in turn, to hunt larger meals to fuel their larger bodies.
In dinosaurs, too, the air sacs probably extended from their lungs to their bones, creating a strong but lightweight scaffold, said University of Edinburgh paleontologist Steve Brusatte. American scientist (opens in a new tab). This gave the dinosaurs skeletons “still strong and still flexible, but light. It helped them grow bigger and bigger and bigger,” Brusatte said. “The same way skyscrapers get bigger and bigger because of internal support structures.” (Of course, although the air sacs helped create strong, light bones, no animal could actually grow as big as a skyscraper. This is because body weight increases much faster than body strength. bones as animals increase in size, as physicist Neil deGrasse Tyson put it. Explain (opens in a new tab).)
Mammals lack such air sacs, however, “which can invade the bone and lighten the bone,” Brusatte said, “So the size of an elephant or a little bigger, that might be the limit of l ‘place where mammals, at least on land’ can get. … You can’t really have mammals, it seems, the size of dinosaurs.”
As warm-blooded or endothermic creatures, mammals also need a lot of fuel. “Elephants are complete endotherms, and dinosaurs, at least herbivorous dinosaurs, probably weren’t for the most part,” Geerat Vermeij, professor of geobiology and paleobiology at the University of California, Davis, told AFP. LiveScience. “So the food requirements for, say, a gigantic elephant would be…maybe 5 times higher than those of the largest dinosaurs.”
Paleontologists have debated whether dinosaurs were cold or hot blooded. But current science places many animal species on a cold-blooded to warm-blooded gradient, and the dinosaurs were likely “at the low end of the warm-blooded scale,” Erickson said. This made a large body less energetically expensive for the dinos.
Huge size also requires the right environment. In a 2016 study published in the journal PLOS A (opens in a new tab), Vermeij concluded that gigantism depends primarily on sufficient resources being produced and recycled by “a highly developed ecological infrastructure”. In other words, the ecology must produce enough oxygen, food, and habitat to grow a truly giant creature. Such ecologies had undergone a great development during the Middle Triassic period, towards the beginning of the age of dinosaurswrites Vermeij.
In a potentially significant environmental change, ancient atmospheres had higher concentrations of oxygen. This may have played a role in gigantism, especially in insects. Wingspans among the largest prehistoric insects tracked ancient increases in oxygen concentration, a 2012 study in the journal Proceedings of the National Academy of Sciences (opens in a new tab) reported.
Nor should brewers of gigantism forget the crucial ingredient of time. Although animal lines tend to get bigger over generations, it takes a long evolutionary time to reach giant sizes, Erickson said. And mass extinction events tend to wipe out larger creatures, Vermeij said, so those events can leave giant animal slots empty for tens or hundreds of millions of years. “It took about 25 million years for the first mammals to reach a weight of one tonne,” he said. In the case of woolly mammoths, decimated by climate change and human hunters a mere 10,000 years ago, it may not be a coincidence that we modern humans do not see such huge creatures: our own ancestors helped kill them not so long ago.
For Vermeij, the most complete explanation for the decrease in height comes not from physiology or the environment, but from social structure. “The evolution of … organized social behavior, not just herding but really organized hunting” in mammals introduced a new form of dominance, he said. “Group hunting by relatively small predators makes even very large prey vulnerable. Individual gigantism has in effect been replaced on earth by gigantism at the group level,” he wrote in the 2016 study. with wolves and hyenas for example, can be a more efficient way to get big. than to build a huge body. As a result, “gigantism has lost its luster on earth,” Vermeij wrote.
Social organization may also help explain a rather, uh, giant exception to the timeline plotted here: In the ocean, the largest animals that ever lived still exist today: blue whale. Marine life, Vermeij said, makes long-distance communication more difficult, hampering the development of complex hunting groups. The evolution of these groups “has happened much more on land than, at least until recently, in the ocean”, as with killer whaleshe said.
Originally posted on Live Science.