The first reptiles are the ancestors of all of life's terrestrial vertebrates (land-based animals with backbones, aside from the amphibians). All of today's reptiles, as well as birds and mammals, owe their origin to the very first reptiles that evolved from their amphibian ancestors in the Late Carboniferous Period. To date, the earliest known confirmed reptile is a small lizard-like creature called Hylonomus, which lived approximately 315 million years ago. With the first reptiles came scaly bodies, as well as the ability to lay hardened and protected eggs that could survive on land without the need for an aquatic larval stage. Reptiles were the first amniotes, which are animals with hardened eggs that posses an amnion (a double membrane that allows the embryo to breathe on land). Reptiles, like their amphibian ancestors and cousins, are also ectothermic (cold-blooded), which means that their bodies require an external source of heat (i.e. the sun), in order to become active. Severe climate change would bring a close to the Carboniferous Period approximately 299 million years ago, and with it came a severe extinction event. While amphibians declined dramatically during this time, reptiles began to fulfill the newly vacated ecological niches. New varieties of reptile would arise in the Permian Period to follow, which would eventually give rise to two more of nature's biggest players in the Triassic Period: mammals and dinosaurs. Today there are four recognized groups of reptiles: the squamates (lizards, worm lizards, and snakes), the sphenodonts (the very lizard-like creatures called tuataras, which have only two surviving species), the chelonians (turtles and tortoises), and the crocodilians (alligators, gharials, caimans, and crocodiles).