Mammals are a diverse group of animals with unique characteristics like mammary glands and hair. They're divided into three main types: monotremes, marsupials, and placentals. Each has its own reproductive strategy, from egg-laying to pouch-rearing to placental development.

Mammals evolved from synapsids, developing features like differentiated teeth and endothermy. Their physiology includes efficient respiratory and circulatory systems, homeothermy, and a neocortex for complex thinking. Placental mammals are further divided into groups like Xenarthra, Afrotheria, Euarchontoglires, and Laurasiatheria.

Mammalian Diversity and Characteristics

Monotremes vs marsupials vs placentals

• Monotremes are egg-laying mammals with only two extant species (platypus and echidna)
  • Possess a cloaca, a single opening for reproductive, digestive, and urinary tracts
  • Lack nipples; young lap milk from mammary patches
• Marsupials give birth to altricial young that complete development in a pouch (marsupium)
  • Have a shorter gestation period compared to placental mammals
  • Examples include kangaroos, koalas, and opossums
• Placental mammals give birth to precocial young after a prolonged gestation period
  • Fetus is nourished by a placenta during development
  • Most diverse and abundant group of mammals, including humans, rodents, bats, and whales
  • Exhibit viviparity, giving birth to live offspring

Evolutionary path of mammals

• Synapsids, the lineage leading to mammals, diverged from sauropsids (reptiles and birds) in the Carboniferous period
• Early synapsids (pelycosaurs) had a single temporal fenestra behind the eye socket
• Therapsids, a later group of synapsids, evolved mammal-like characteristics
  • Differentiated teeth into incisors, canines, and molars
  • Developed more efficient jaw musculature
  • Adopted an upright posture and possibly endothermy
• Cynodonts, a group of therapsids, were the direct ancestors of mammals
  • Further developed mammalian features, such as a secondary palate and a dentary-squamosal jaw joint
• Mammal-like synapsids survived the Permian-Triassic extinction event and gave rise to the first true mammals in the Triassic period

Mammalian Physiology and Diversity

Unique Mammalian Characteristics

• Mammary glands: Specialized exocrine glands that produce milk for offspring nutrition
• Hair: Keratinous structures providing insulation, sensory function, and protection
• Homeothermy: Ability to maintain a constant body temperature regardless of environmental conditions
• Neocortex: Expanded region of the cerebral cortex responsible for higher-order thinking and complex behaviors

Features for mammalian metabolism

• Efficient respiratory system with highly branched bronchi, small alveoli for increased gas exchange, and a diaphragm for efficient ventilation
• Circulatory system with a four-chambered heart for complete separation of oxygenated and deoxygenated blood, high cardiac output, and blood pressure
• Insulation from fur or hair for thermal insulation and a subcutaneous fat layer for additional insulation
• Thermoregulation through sweat glands for cooling and brown adipose tissue for non-shivering thermogenesis
• High-energy diet consisting of energy-rich foods to fuel high metabolic rates, supported by an efficient digestive system with specialized teeth and an elongated gut

Groups of placental mammals

• Xenarthra includes armadillos, sloths, and anteaters
  • Characterized by reduced or absent teeth and unique articulations between vertebrae
• Afrotheria includes elephants, manatees, and hyraxes
  • Diverse group with varying adaptations, such as trunks (elephants), aquatic lifestyle (manatees), or small size (hyraxes)
• Euarchontoglires includes rodents, rabbits, primates, tree shrews, and flying lemurs
  • Adaptations for various lifestyles, such as gnawing teeth in rodents and grasping hands in primates
• Laurasiatheria includes bats, carnivores, pangolins, even-toed and odd-toed ungulates, and cetaceans
  • Adaptations for flight (bats), carnivory (carnivores), hooved locomotion (ungulates), and aquatic life (whales and dolphins)