Opisthoptera (lit. "rear wings") are a clade of Elastospondyli, and the first animals to develop powered flight on the planet.
Opisthopterans first appeared around 4222 ma, following the decrease of carbon dioxide and hydrogen sulfide in the atmosphere as a result of the spread of chemophytes contributing to lowered temperatures. Evolving from a branch of Elastospondyli, the Opisthoptera are the first clade of animals to develop flight on the planet.
During the evolutionary process, Opisthopterans have developed a greater internal support system, active respiration, and localized endothermy (in their case, at the base of their wing membranes), ensuring they can function even in lower temperatures.
Ancestral Opisthopterans initially resembled their ancestors, excluding their newly formed patagium and somewhat lengthened legs. Over time, however, their patagium connected limbs developed into two pairs of wings, while their simple spiracles became more akin to the lung-like organs of the various Osteopod descendants that colonized land. While they initially relied on wing-beats to actually power their lungs alone, the development of thoraic muscles around the lungs increased their efficiency noticeably, enabling them to become true fliers, not to mention developing true metabolic control over their body temperature, and potentially conquer any environment given enough time.