the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen. Later discoveries revealed earlier transitional forms between Acanthostega and completely fish-like animals. Unfortunately, there is then a gap (Romer's gap) of about 30 Ma between the fossils of ancestral tetrapods and Middle Carboniferous fossils of vertebrates that look well-adapted for life on land, during which only some fossils are found, which had five digits in the terminating point of the four limbs, showing true or crown tetrapods appeared in the gap around 350 Ma. Some of the fossils after this gap look as if the animals which they belonged to were early relatives of modern amphibians, all of which need to keep their skins moist and to lay their eggs in water, while others are accepted as early relatives of the amniotes, whose waterproof skin and egg membranes enable them to live and breed far from water. The Carboniferous Rainforest Collapse may have paved the way for amniotes to become dominant over amphibians. Reptiles Possible family tree of dinosaurs, birds and mammals Amniotes, whose eggs can survive in dry environments, probably evolved in the Late Carboniferous period (). The earliest fossils of the two surviving amniote groups, synapsids and sauropsids, date from around . The synapsid pelycosaurs and their descendants the therapsids are the most common land vertebrates in the best-known Permian () fossil beds. However, at the time these were all in temperate zones at middle latitudes, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians. The Permian–Triassic extinction event wiped out almost all land vertebrates, as well as the great majority of other life. During the slow recovery from this catastrophe, estimated to have taken 30 million years, a previously obscure sauropsid group became the most abundant and diverse terrestrial vertebrates: a few fossils of archosauriformes ("ruling lizard forms") have been found in Late Permian rocks, but, by the Middle Triassic, archosaurs were the dominant land vertebrates. Dinosaurs distinguished themselves from other archosaurs in the Late Triassic, and became the dominant land vertebrates of the Jurassic and Cretaceous periods (). Birds During the Late Jurassic, birds evolved from small, predatory theropod dinosaurs.