L-R: The largest land mammals that ever lived, Indricotherium and Deinotherium, would have towered over the living African Elephant. The extinct Paraceratherium, which lived 30 million years ago, weighed some 15 tonnes

A new research headed by University of New Mexico Biology Associate Professor Felisa Smith revealed that the ancient dinosaurs were not the only gigantic creatures the olden world. The study, the details which have been published in November 26th edition of the journal, Science, explains that the very extinction of dinosaurs 65 million years ago paved the way for the surviving mammals to get, bigger. In fact almost a 1000 times bigger than they had been in the first place! The study titled, "The Evolution of Maximum Body Size of Terrestrial Mammals" involved an international team of paleontologists, evolutionary biologists and macro-ecologists, who carefully studied the mammalian fossils from Africa, Eurasia and South America in order to quantitatively examine the patterns of body size of mammals after the great demise of the mighty dinosaurs. It must be mentioned here that, it's already understood that the creatures that roamed the earth after the dinosaurs were fairly big, but, how they got this "big" was still a subject of speculations.
Before the extinction of the dinosaurs, many mammals were part of the formers food chain if not battling larger reptilians for necessary sustenance. But once the massive beasts became extinct, within 25 million years, which is fast in geologic terms, the overall modest-sized land mammals of these regions grew to their maximum size. And that is from being a mammal weighing 1 to 10 kilograms that is 2 to 22 pounds (when they were sharing the world with the dinosaurs) to a maximum of 17 tons and as tall as 18 feet high at the shoulder. Ancient mammals like the hornless rhinoceros, Indicotherium transouralicum are sure testament of that primeval biological evolution. But how it all happened? Co-author Dr. Jessica Theodor of Dept. of Biological Sciences at the University of Calgary explains, "Basically, the dinosaurs disappear and all of a sudden there is nobody else eating the vegetation. That's an open food source and mammals start going for it, and it's more efficient to be an herbivore when you're big." In other words, these quick-metabolizing mammals no longer had to battle the larger reptilian dinosaurs for food and for the first time ever, they encountered the unlimited and undisrupted access to the prehistoric rations bazaar. And this all eating frenzy and pattern of genetic evolution ended about 40 millions years ago, the time when the mammalian creatures of all continents reached their peak size and just stopped growing. And as for why just stopped growing, scientists tried to answer that as well.
In order to get more detailed insight regarding how big the mammals grew after the dinosaurs' extinction, the researchers collected data on the maximum size for major groups of land mammals on each continent. And these include Perissodactyla, the odd-toed ungulates such as horses and rhinos; Proboscidea, which includes elephants, mammoth and mastodon; Xenarthra, the anteaters, tree sloths, and armadillos; along with a number of other extinct groups. Three years of data assembling process paved the way to the clues on what exactly set the limit on maximum body size of these mammals who once roamed on these ancient of lands. Scientists believe that any further growth was capped because of land availability and the climate they lived in. The study showed that the colder the climate was, the bigger the mammals were, since bigger animals conserve the heat better. Besides, it was also found that no one group of mam¬mals dom¬i¬nates the largest size class; the absolute largest mam¬mal belongs to different groups over time and space. In the words of Assoc. Prof. Smith, "The results were strik¬ing. Global tem¬per¬a¬ture and ter¬res¬trial land area set con¬straints on the upper limit of mam¬mal body size, with larger mam¬mals evolv¬ing when the earth was cooler and the ter¬res¬trial land area greater."
And as for how these giant mammals eventually met their demise, it is scientifically believed that either drastic change in climates or the wrath of early humans ultimately ended the age of these colossal mammals.

The contributor is a freelance science writer.