The Therian group of mammals includes both marsupials and placentals. Marsupials, such as kangaroos and koalas, give birth to relatively underdeveloped young, which often continue their development in a pouch. Placentals, on the other hand, give birth to more fully developed young after a longer gestation period, thanks to the presence of a placenta that nourishes the fetus during development. Together, marsupials and placentals make up the vast majority of mammalian species, showcasing a wide array of adaptations and lifestyles.
Prototheria
Prototheria, on the other hand, includes the egg-laying mammals, also known as monotremes. Examples of monotremes are the platypus and echidnas. These mammals lay eggs instead of giving birth to live young, differentiating them from Therians.
Diapsids
Diapsids are a group of reptiles that includes lizards, snakes, and birds (aves). The defining characteristic of diapsids is the presence of two temporal fenestrae, which are openings in the skull behind the eyes. These openings allow for the attachment of larger and stronger jaw muscles, providing these animals with a powerful bite. Diapsids are incredibly diverse and can be found in a variety of habitats across the world. Birds, a subgroup of diapsids, are particularly notable for their adaptations to flight, although some have evolved to live primarily on land or in water.
Synapsids
Synapsids include mammals and other primitive reptiles. The defining characteristic of synapsids is the presence of a single temporal fenestra in the skull. This opening allows for the attachment of jaw muscles, similar to diapsids, but with a different configuration. Synapsids were the dominant terrestrial vertebrates during the Permian and early Triassic periods. Over time, they evolved into modern mammals, retaining the single fenestra characteristic. This group includes all living mammals, characterized by traits such as hair, mammary glands, and endothermy (warm-bloodedness).
