information:extraction
Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibian ancestors during the Carboniferous period and further diverged into two groups, namely the sauropsids and synapsids, an event that marks the appearance of Amniota, according to the definition established under the PhyloCode. This basal divergence within Amniota has been dated by molecular studies at 310–329 Ma or 312–330 Ma, but the presence of Hylonomus at Joggins implies a minimal age of about 317 Ma. A fossilized birth-death process study of early amniotes suggested an age of 322–340 Ma. Amniotes are distinguished from the other living tetrapod clade — the non-amniote lissamphibians — by the development of three extraembryonic membranes, thicker and keratinized skin, and costal respiration. Additional unique features are the presence of adrenocortical and chromaffin tissues as a discrete pair of glands near their kidneys, which are more complex, the presence of an astragalus for better extremity range of motion, the diminished role of skin breathing, and the complete loss of metamorphosis, gills, and lateral lines. All three main amniote features listed above, namely the presence of an amniotic buffer, water-impermeable cutes and a robust air-breathing respiratory system, are very important for living on land as true terrestrial animals — the ability to survive and procreate in locations away from water bodies, better homeostasis in drier environments, and more efficient non-aquatic gas exchange to power terrestrial locomotions, although they might still require regular access to drinking water for rehydration like the semiaquatic amphibians do. Because the amnion and the fluid it secretes shields the embryo from environmental fluctuations, amniotes can reproduce on dry land by either laying shelled eggs or nurturing fertilized eggs within the mother, unlike anamniotes that have to spawn in or closely adjacent to aquatic environments. Among the earliest known crown group amniotes, the oldest known sauropsid is Hylonomus and the oldest known synapsid is Asaphestera, both of which are from Nova Scotia during the Bashkirian age of the Late Carboniferous around 318 million years ago. Basal amniotes resembled small lizards and evolved from semiaquatic reptiliomorphs during the Carboniferous period. After the Carboniferous rainforest collapse, amniotes spread around Earth's land and became the dominant land vertebrates, and soon diverged into the synapsids and sauropsids, whose lineages both still persist today. Older sources, particularly before the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the antiquated idea of the evolutionary great chain of being.