The earliest forms of life—microbes—date back to roughly 3.7 billion years ago. They slowly transformed, presenting more and more complex organisms that eventually led to formation of the first animal species. You might be surprised to learn that sponges, also known as poriferans, are considered to be some of the first animals there are.

Most of the pictures on this list represent something slightly bigger and more threatening than sponges, though. Take dinosaurs, for instance. Scientists believe they lived roughly 245 to 66 million years ago, during the Mesozoic Era, and could weigh tens of tons each.

The American Museum Of Natural History (AMNH) revealed that some dinosaurs weighed up to 80 tons and were roughly 120 feet long. To put things in perspective, try to imagine a dinosaur upright by the Leaning Tower of Pisa, being nearly two-thirds its height. 

AMNH also pointed out that not all dinosaurs are extinct, though. Birds, also known as avian dinosaurs, actually share an ancestor with the non-avian ones, which makes them the last surviving group of the kind.

When it comes to non-avian dinosaurs, there are roughly 300 genera and 700 species that have been discovered and named. But these numbers are likely to go even higher, as scientists continue to find new fossils.

Thanks to paleontologists, we can get a better understanding of what creatures roaming the Earth millions of years ago looked like. They study the history of life based on fossils—rocks that contain remains of plants, animals, or other forms of life. 

It takes 10,000 years for such preserved remains to become fossils. In their hardened layers, they provide scientists with information about the organism’s environment, living conditions, and other details.

Some of the first fossils to be recognized as dinosaur remains date back to the 1830s. That’s when, according to the American paleontologist Mark Norell, people in the English countryside found what they believed to be giant extinct reptiles.

When it comes to the first person to study fossils, Georges Cuvier is often referred to as the founding father of paleontology. Back in the 19th century, he was a member of the faculty at the National Museum of Natural Sciences in Paris, which allowed him to access the most extensive collection of fossils available at the time.

The images of remains on this list are just a tiny fraction of what’s been dug out in the past years. However, they show just how exceptional in size and other features the creatures were.

Some more examples of such fascinating excavations were presented in an article by Geology Science. In addition to the well-known T-rex, it also covered types of feathered dinosaurs and extinct fish, as well as human remains from millions of years ago.

The Smithsonian Channel also covered the process of restoring an incredible 110-million-year-old fossil. Found in an oil sand mine, it turned out to be the remains of a 18-foot-long nodosaur—a nearly 3000-pound type of armored animal with spikes on its shoulders and a long tail.

Not all animal remains are encapsulated in fossils, though. Some are often found perfectly preserved in other formations, such as amber, for instance. (Earth Archives have covered some pretty impressive examples of that, which you can find here.)

The colorful tree resin traps certain organisms, which become fully enclosed as it solidifies. Despite usually being smaller in size, pieces of amber conserve the remains of different species, in this way saving the information about them as well.

Most fossilized remains are of beings that have been long gone for ages, be it dinosaurs or other creatures. However, some species of animals became extinct as recently as just a few years ago. 

Some examples include the Spix’s macaw, also known as the blue macaw, featured in the animated movie ‘Rio’, the smooth handfish that used to roam the coastal waters of Tasmania, and splendid poison frog, a red-colored frog formerly found in Western Panama.

An astonishingly large number of nature’s creations are on the verge of extinction today. According to Animals Around The Globe, nearly 41,500 species are included in the International Union for Conservation of Nature’s Red List, more than 16,000 of which are threatened with extinction.

For example, there are only 67 Javan rhinos in the entire world. The numbers of mountain gorillas and Asian elephants are dropping rapidly, and so are the ones of Atlantic blue tuna.

The United States Geological Survey pointed out that some of the reasons for the unfortunate changes in such numbers are habitat loss and over-exploitation of wildlife for commercial purposes. Sadly, human activity has made the process of extinction way more rapid than it would have been naturally. Scientists believe that the 20th-century rate of animal species dying is a hundred times higher than it would have been without people’s interference.