Ancient mammals often end up overshadowed by dinosaurs in science stories. But overlooking the massive rhinos that dominated the landscape more than 23 million years ago would be a tall feat.
Pistols packed, camels loaded and Fulton trucks rumbling, the "biggest scientific expedition ever to leave the United States" exited a gate in China's Great Wall and headed for Mongolia.
On the expedition on that day, April 22, 1922, was its leader, the explorer Roy Chapman Andrews, and a team of veteran fossil hunters, including paleontologist Walter Granger. Sent by the American Museum of Natural History in New York, their mission was to travel 250 miles into badlands and find the fabulous "missing link" fossil connecting humankind to our earlier mammal ancestors. Little did they know that people seem to have originated in Africa.
"That was how the museum sold the expedition to the rich folks who paid for everything back then," says paleontologist Donald Prothero of the Natural History Museum of Los Angeles County. "Luckily, they made some pretty spectacular finds instead, dinosaur nests and all kinds of ancient mammals, including some wonderfully large specimens."
In a soon-to-be-released look at the biggest of these wonders, Rhinoceros Giants: The Paleobiology of the Indricotheres, Prothero delves into these overlooked big guys, the most massive mammals to ever walk the Earth. These giant rhinos or Indricotheres (IN-drick-oh-theers), roamed arid scrublands that stretched from what is today Turkey to India to Mongolia from 37 million to 23 million years ago.
First discovered in modern-day Pakistan more than a century ago, they were massive, weighing around 14 tons with the biggest ones weighing in at 22 tons. Elephants today typically weigh 5 to 6 tons. And they were tall, with thick skulls that were lifted 17 feet high atop thick, strong necks to dine on treetops, making them the giraffes of their day. By most estimates, they were about as big as mammals can possibly grow, based on sheer physical limits to animal metabolism.
Dinosaurs, of course, are the glamour queens of fossils, but Indricotheres were pretty amazing. They lived like elephants most likely, even sporting the same over-sized ears to radiate away heat, wallowing in waterholes and trudging about to dine on tree leaves. They lacked the horns we associate with rhinos today, but had the same teeth and leg bones, gifts from their ancient common ancestor, a deer-sized fleet-footed creature that lived more than 50 million years ago.
In his short book, Prothero cuts across every aspect of paleontology to tell the story of these vanished giants. Chapters range from entertaining recounting of discoverers such as Chapman and Sir Clive Forster Cooper, who saved the British Museum of Natural History from destruction during World War II, to scholarly descriptions of the rules that biologists must follow to name a new species or estimate a vanished rhino's weight. "It is part of paleontology's charm that we have some strange, entertaining characters in our past," Prothero says. "I wanted to tell their stories, too."
That story includes how the giant rhinos vanished. Most likely, Prothero concludes, they lost out to the ancestors of elephants, mastodon-like beasts that migrated from Africa about 23 million years ago along with their predators, ones far tougher than anything the Indricotheres faced before (aside from the occasional 30-foot-long crocodile whose teeth marks are sometimes found on giant rhino fossil bones and who lived in marshes of the era). These "mastodon bulldozers" likely uprooted the stands of trees that the giant rhinos depended on for food, just as elephants break up forests in Africa today. And their predators likely feasted on unwary giant rhinos.
"The one remarkable thing they tell us is about how evolution will often lead to similar solutions, long necks like the ones seen in sauropod dinosaurs and giraffes today to eat from treetops," Prothero says. "While at the same time evolution is boxed in by the body shape of ancestors, where these giant rhinos had the long toes of older, smaller, rhinos, despite their tremendous weight."
So, the explorers led by Andrews never found their "missing link," but they did make remarkable discoveries, some of which can still be seen at their museum. The whole idea of a fossil missing link between man and ape is an idea now seen as well, extinct, by paleontologists, who have recovered numerous fossils of species that might fit the bill and who also find the notion of a chain of progress in evolution of any species, man or rhino, as obsolete as well.
Indricotheres, after all, thrived for 14 million years, far longer than the 6 million years that anything faintly human has walked around, a fairly successful run for most creatures. "I really hadn't realized how much we can say about them until I started putting the book together," Prothero says. "We can always learn more, though."