Amphibians are an ancient class of cold-blooded animals that first evolved in the Devonian period, approximately 370 million years ago. The word "amphibian" means "both life," so named for their natural (and necessary) ability to thrive on both land and water. In fact, most species of amphibian start their lives in a water-based larval stage, undergoing a metamorphosis from a gill-breathing form into a lung-breathing form (though some can even breathe through their skin). Today's amphibians are survived by a sub-class called Lissamphibians, comprising the small Anurans (frogs and toads), the Caudatans (salamanders), and the worm-like Apodans (caecilians), though some prehistoric varieties of amphibian could get to enormous sizes, such as the 30' long Prionsuchus. The ancestors of amphibians are lobe-finned fishes like Tiktaalik, of which have multi-jointed, leg-like fins. These fins are a revolutionary adaptation, as they allowed these fish to haul themselves out of the water and onto the land. In time these fish would develop more efficient lungs, denser bones, and fully functional hands and feet, giving rise to the early amphibians, such as the Devonian-age, 360 million year old Ichthyostega. Amphibians would remain at the top of the food chain, feasting on the land-based arthropods that had crawled out of the ocean millions of years before. From the amphibians evolved the reptiles, which possessed the first amniotic eggs. This variety of egg could be laid on land and prevent the embryo from drying out, unlike amphibian eggs which require a water source for the young to develop. This huge advantage would allow for reptiles to become the dominant vertebrates on land, causing amphibian varieties to decrease steadily in numbers. Despite the takeover of reptiles over amphibians in the Triassic period, amphibians would still survive extinction, with the modern sub-class of Lissamphibia evolving in the Jurassic period, and persisting to this day.