Mammals

Mammals evolved around 220 million years ago alongside dinosaurs.
Early mammals were small, likely nocturnal, while dinosaurs dominated the terrestrial landscape.
A significant mass extinction event 65 million years ago allowed mammals to thrive, with critical insulation provided by hair possibly aiding their survival.
Post-extinction, mammals diversified, filled ecological niches left by dinosaurs, with maximum diversity reached around 15 million years ago.

Key ancestors:
- Therapsids: Early synapsids whose limbs were positioned beneath their bodies and featured enhanced jaw muscle attachments.
- Cynodonts: Showed stronger jaws and a higher metabolic rate, some characteristics of early mammals.
- Early mammals: Exhibited features such as hair, mammary glands, and specialized molars for shearing food.

Diversity: Approximately 6495 species, including humans.
Habitat: Mammals are found in all habitats across the globe.
Defining Features:
- Mammary Glands: Present in all females; specialized tissue secreting milk, with newborns born toothless and reliant on suckling for nourishment.
- Hair: Almost all mammals have hair, serving multiple functions such as insulation, camouflage, or sensory functions (e.g., whales and dolphins have reduced but present hair).
- Endothermy: Mammals are warm-blooded, allowing them to maintain consistent body temperature and be active in varied climates.

Three Major Groups:
1. Placental Mammals: Nourish embryos fully in utero; the placenta forms from the chorion and allantois, facilitating nutrient and waste exchange between maternal and fetal blood.
2. Marsupials: Includes around 350 species; fertilized eggs develop briefly before the immature young crawl into a pouch to nurse and continue development.
3. Monotremes: Only five species (including the platypus and echidnas); they lay shelled eggs and show reptile-like characteristics while possessing mammary glands without nipples. Milk is excreted onto fur for the young to lap up.

Mammary glands are unique to mammals, providing nutrition to young post-birth.
They consist of specialized tissue that secretes milk, essential for the growth and development of newborns.

Hair is formed of dead cells filled with keratin, distinct from the skin structure.
Functions include temperature regulation and sensory perception, as well as camouflage in various environments.

Endothermy allows mammals to regulate their body temperature independently of the environment.
This capability enables activity throughout the day and in a wider range of environments, granting a survival advantage.

Placenta: Provides ongoing nourishment and waste management during the critical development phase before birth.
Amniotic Egg: Historical reproductive method; provided a way for embryos to develop in a controlled aqueous environment, crucial for early vertebrate life.

Placental Mammals: Extended gestation periods with significant maternal care.
Marsupials: Short gestation with additional nurturing in the pouch.
Monotremes: Oviparous (egg-laying) with minimal postnatal care; young are highly dependent on maternal care.