Woolly Mammoth and Prehistoric Mammals

When the asteroid hit 66 million years ago and wiped out non-bird dinosaurs, tiny mammals that had spent millions of years hiding in burrows and under leaves suddenly had a world of empty niches to fill. What followed was one of the greatest evolutionary explosions in history. Within 10 million years, mammals had diversified into everything from giant carnivores to enormous browsers to fast runners to ocean-going whales. The Cenozoic era, which we are still living in today, is sometimes called the Age of Mammals.

Some of the most dramatic prehistoric mammals lived during the Eocene epoch, 56 to 34 million years ago. Uintatherium was a rhino-sized browser with three pairs of blunt horns and saber-like canine teeth. Andrewsarchus, possibly the largest land carnivore ever, had a skull over 33 inches long. Basilosaurus, one of the first whales, reached 60 feet long and had tiny, useless hind legs as a remnant of its land-walking ancestors. These animals would be utterly unrecognizable to anyone expecting the familiar mammals of today.

The Pleistocene epoch, from about 2.6 million to 11,700 years ago, gave us the iconic Ice Age megafauna most people picture when they think of prehistoric mammals: woolly mammoths, saber-toothed cats, giant ground sloths, cave bears, dire wolves, and giant beavers as big as modern bears. These animals roamed alongside the first modern humans, and their extinction at the end of the Ice Age is one of the most studied and debated events in all of paleontology.

Woolly mammoths are the most recognizable prehistoric mammals in the world. Several complete frozen specimens have been discovered in the Siberian permafrost, including animals with their fur, skin, stomach contents, and even blood still preserved. These specimens have allowed scientists to sequence the mammoth genome, reconstruct what they ate, and determine that the last population, on Wrangel Island in the Arctic Ocean, survived until about 4,000 years ago, meaning mammoths were still alive when the ancient Egyptians were building pyramids.

Smilodon fatalis is the most famous predator of the Ice Age and the official state fossil of California, where over 2,000 specimens have been recovered from the La Brea Tar Pits in Los Angeles. The La Brea collection is extraordinary because Smilodon individuals apparently tried to prey on other animals already trapped in the tar, only to become trapped themselves. This has given scientists an unprecedented look at Smilodon population biology, including evidence that the animals formed social groups and cared for injured individuals.

Megatherium americanum, the giant ground sloth of South America, could reach 20 feet long and 8,000 pounds, making it the size of a modern elephant. It used its powerful claws to pull branches down or dig up roots, and some scientists have proposed it reared up on its hind legs to reach even higher. Dire wolves (Aenocyon dirus) were not simply bigger versions of gray wolves but a completely separate species. They had larger teeth adapted for crushing bone and lived across North and South America before going extinct about 9,500 years ago.

Prehistoric mammal fossils are found on every continent. The Fayum Depression in Egypt has produced some of the most important early whale and early elephant fossils ever found, revealing how those groups transitioned from land to sea and from small hoofed mammals to giants. The Badlands of South Dakota and Nebraska contain some of the richest early horse, rhinoceros, and camel fossil beds in the world, documenting 50 million years of mammal evolution across the Great Plains.

Siberia and Alaska together form one of the richest zones for Ice Age mammal discoveries because permafrost preserves organic material for tens of thousands of years. Baby woolly mammoths, woolly rhinoceroses, cave lions, and even ancient horses have been found intact, with DNA preserved well enough to sequence. In 2021, scientists recovered a 1.2-million-year-old mammoth tooth from Siberian permafrost and successfully extracted ancient DNA from it, making it the oldest DNA ever recovered from any organism.

The La Brea Tar Pits in Los Angeles remain one of the world's most productive Ice Age fossil sites. Since the 1800s, excavations have recovered over 3.5 million individual specimens representing over 600 species. Active excavation continues today, and in 2006, construction workers discovered a cache of over 16 fossil deposits while excavating for an underground parking garage, including a nearly complete Columbian mammoth skeleton nicknamed Zed. New fossil mammals are being discovered worldwide at an increasing pace as more remote regions are systematically surveyed.