The Devonian period was crucial to the evolution of life on earth: this was when the first tetrapods climbed onto dry land, and the world's oceans were populated by the bizarre armored fish called placoderms. Here's an overview of prehistoric life during the Devonian period.

Illustration of Acanthostega: Nobu Tamura

Stegosaurus and Allosaurus lived in the same place (western North America) at the same time (the late Jurassic period), and these two dinosaurs undoubtedly had a predator-prey relationship. The question is, could an adult Allosaurus hope to take down a full-grown Stegosaurus? Here's what we can surmise about an Allosaurus vs. Stegosaurus showdown.

Illustration: Alain Beneteau

Many people mistakenly believe that Stegosaurus was the only dinosaur of its kind. The fact is, though, that the stegosaur family included about 20 genera native to late Jurassic North America and Eurasia. Here's an article explaining everything you ever needed to know about stegosaurs.

Photograph of Stegosaurus: Senckenberg Museum

Modern dogs were only domesticated toward the end of the Pleistocene epoch, but their evolutionary history goes back further than that--as witness one of the earliest canines yet discovered, Hesperocyon, which lived in North America 40 million years ago, during the late Eocene epoch. As you might expect from such a distant ancestor, Hesperocyon didn't look much like any dog breed alive today, and was more reminiscent of a giant mongoose or weasel. However, this prehistoric mammal did have the beginnings of specialized, meat-shearing teeth, as well as noticeably dog-like ears. There's some speculation that Hesperocyon (and other late Eocene dogs) may have pursued a meerkat-like existence in underground burrows, but the evidence for this is sparse at best.

Read more about prehistoric mammals like Hesperocyon: Prehistoric Dogs - The Story of Dog Evolution

Photograph of Hesperocyon: American Museum of Natural History