Eutherians, also known as placental mammals, are a group of mammals characterized by their complex placenta, which allows for a longer gestation period. This adaptation enables the developing young to receive nutrients and oxygen directly from the mother, leading to more advanced offspring at birth. Examples of eutherians include humans, dogs, and whales. Eutherians are distinguished from other mammals, such as marsupials and monotremes, which have different reproductive strategies. They are the most diverse group of mammals, with over 5,000 species found worldwide. Their adaptability to various environments has contributed to their success in many ecosystems.