How Well Do You Know These Prehistoric Animals?

In one fell swoop, an asteroid ended the Cretaceous period — and even the longer Mesozoic Era — and the resulting Tertiary period saw the rise of mammals. Just as the Great Dying cleared the slate for dinosaurs hundreds of millions years earlier, so too did early mammals benefit from the dinosaurs’ widespread destruction — and life continued on.

The last woolly mammoths that roamed the planet lived in isolation on Wrangel Island, a Russian possession in the Arctic Ocean just northwest of Alaska. Scientists think the mammoths contained on this island experienced what scientists call a “genetic meltdown” due to a limited gene pool and mutations that made it harder to mate.

Although Earth is still home to some massive land mammals — the biggest being African bush elephants — the largest land mammal to roam the planet was Paraceratherium, the ancestor of today’s rhinoceroses. Roaming Eurasia around 35 million years ago, Paraceratherium is said to have been 26 feet long and weighed the equivalent of five elephants.

The shrew-like morganucodontids are the earliest-known mammals from which today’s mammals, including humans, descended. They lived among the dinosaurs around 210 million years ago. Although several mammalian lineages existed at the time, morganucodontids are the only ones that survived and gave rise to a species group that became the preeminent animal class on Earth.

Despite their ubiquity throughout Australia, evidence suggests that marsupials first evolved in North America. The animals’ journey to the Land Down Under is a circuitous one, with scientists believing that early marsupial ancestors traveled a land bridge connecting South America, Antarctica, and Australia.

While whales now live in the ocean, 50 million years ago, the common ancestor of whales, dolphins, and porpoises walked on land. This goat-like mammal, called Pakicetus, lived on the riverbanks of India and Pakistan. Slowly, the animal’s descendants got used to water and evolved into what we know as whales.

With four-inch velociraptor-like talons, the cassowary is one of the most direct descendants of dinosaurs in the world. A cassowary’s legs can push them upwards of 30 mph, and their helmet-like protuberance is their most stunning feature (its exact purpose still stumps scientists today). The cassowary has survived millions of years but now teeters on the edge of extinction.

Think snakes are scary today? Behold the Titanoboa (Titanoboa cerrejonensis), a native of northeastern Colombia that roamed the planet 60 million years ago. This mammal stretched to around 47 feet long, making it the largest-known snake to ever live. Luckily, scientists think it had more of an appetite for fish than warm-blooded mammals.

Although early mammals were cold-blooded for tens of millions of years as they evolved from reptiles, today, no mammal is cold-blooded. All mammals — and also birds — are capable of generating internal heat and are known as “homeotherms.”

The megalodon was the textbook definition of an apex predator. With 7-inch teeth and a jaw over 6 feet wide, this mega-shark could lunch on any size ocean creature, including whales and giant sea turtles. Because these sharks were mostly all cartilage, they only left behind pieces of vertebrae and very, very large teeth — hence their name.

Around 65 million years ago, an apocalyptic, asteroid-induced extinction event known as the K-T Extinction wiped out 75% of all living species, including the dinosaurs. Although it’s the most well-known extinction event, the K-T Extinction is actually the fifth large-scale extinction in Earth’s history.

The ancestors of horses were once no bigger than deers. However, by the time tribes of the Eurasian steppe (modern-day Ukraine and Kazakhstan) domesticated horses 6,000 years ago, the animals stood at their full stature. Horses gave the steppe peoples a major military advantage that persisted for thousands of years.

Only 4,000 years ago, these ancient beasts walked the earth. They were widespread, living on every continent except South America and Australia. The hairy beasts started dying out roughly 10,000 years ago, as the climate began to warm toward the end of the last ice age.

Megatherium americanum, literally meaning “great beast from America,” was a species of giant ground sloth from southwestern South America. Megatherium was 10 times the size of modern sloths and weighed up to 4.4 tons — the average weight of an African elephant. Because it could stand and walk on its legs (at a towering 12 feet), Megatherium remains the biggest bipedal mammal that ever lived.

Dogs are man’s best — and oldest — friend. Because the domestication of wolves into modern-day dogs happened in the distant past, scientists can’t be sure exactly when or where dogs first became our friends. Estimates place the animal’s domestication as far back as 30,000 years ago, long before the domestication of the first livestock animal: goats.

We don’t know everything about how prehistoric humans and animals interacted, but we do know that mammals shrank as human beings evolved and expanded their territory. Scientists attribute much of this to hunting, but humans also changed the landscape, and the largest animals were no longer able to survive.

Both birds and crocodilians descend from archosaurs, which means “ruling reptiles.” From this group came dinosaurs — the ancestors of birds — and crocodilians. However, over time, birds underwent a drastic evolution, whereas crocodiles remained relatively similar throughout the centuries.

On August 12, 1990, paleontologist Sue Hendrickson was walking the Hell Creek Formation in South Dakota when she came across pieces of fossilized bone embedded in a rock face. Hendrickson discovered the world’s most complete T. rex fossil, which now bears her name. Many T. rex fossils at the time were around 50% complete, but Sue was a staggering 90% complete, giving an unprecedented glimpse into the age of dinosaurs.

Recovered ancient tusks are an excellent tool for learning about the lives of woolly mammoths. However, it's not the tusks' length that clues scientists in — it's their rings. Much like in the trunk of a tree, interior lines can tell us about the animals' lifespan and health, even lending clues to weather conditions of the time.

Smilodon, a genus of cat whose name literally means “scalpel tooth,” flourished on the American continent and stalked large megafauna throughout the region. This monstrous cat had 8-inch fangs and could open its mouth 120 degrees (twice as wide as modern-day cats). But with the arrival of warmer weather and Homo sapiens, these cats’ days were numbered. The very last of the genus died out around 10,000 years ago.