The only mammal branches to survive to this day are the basal monotremes, marsupials, and placentals. Other branches of early mammals evolved and diversified, but most went extinct. Figure 6.4.1 shows an example of echidnas (Fig. The entire group consists of the platypus and four species of echidnas. One branch of basal mammals that survives to this day is the monotremes, a group of mammals that lay eggs. Most reptiles give birth to their offspring by laying eggs externally. The term tetrapods is introduced in Amphibians the term amniotes is introduced in Reptiles. Mammals and reptiles (including birds) are the only two extant groups of amniotes. Amniotes are tetrapods with that produce an amnion or watertight membrane surrounding the embryo developing inside an egg. Snakes and whales lack four limbs but are still considered tetrapods because they evolved from animals with four limbs. Tetrapods are vertebrate animals with four limbs as well as their evolutionary descendants. Early mammals, like their reptile ancestors, were tetrapods and amniotes (Fig. These early mammals evolved from a common ancestor they shared with reptiles (Fig. The first mammals evolved on Earth during the early Jurassic period approximately 200 to 175 million years ago.
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