Mammals

Phylum Chordates Overview

  • Phylum Chordata

    • Defined by the presence of a notochord at some stage in their life cycle.

  • Sub-phylum Vertebrates (Craniata)

    • Possess a vertebral column (backbone) and a cranium.

  • Clade Gnathostomata

    • Characterized by vertebrates with jaws.

  • Clade Osteichthyes

    • Encompasses bony fishes and tetrapods.

  • Clade Tetrapods

    • Refers to four-limbed vertebrates, including mammals.

  • Class Mammals (Mammalia)

    • Recognizes mammals with specific adaptations including hair and mammary glands.

Chordata Taxonomy

  • Key Groups in Chordata:

    • Vertebrata:

      • Defined by the presence of vertebrae and cranium.

    • Gnatostomata:

      • Jawed vertebrates.

    • Osteichthyes:

      • Includes bony fishes and tetrapods (four-limbed animals).

    • Tetrapoda:

      • Four-limbed vertebrates.

    • Amniota:

      • Tetrapods with embryos having extraembryonic membranes.

  • Subdivisions:

    • Protochordata:

      • Key primitive chordates, such as lancelets.

    • Agnatha:

      • Jawless vertebrates (e.g., hagfishes and lampreys).

    • Reptilia:

      • Includes reptiles, birds, and their ancestors.

    • Mammalia:

      • Class including all mammals.

Modern Amniotes Classification

  • Major Groups:

    • Synapsida:

      • Includes mammals.

    • Diapsida:

      • Encompasses birds, crocodilians, and most reptiles.

  • Key Features:

    • Structure and characteristics of the skull.

    • Adaptations such as the presence of beta-keratin in the epidermis and specialized reproductive adaptations like amniotic eggs.

Class Mammalia Characteristics

  • General Traits:

    • Approx. 5,400 species, shows significant size variation from small bats to large whales.

    • Distinctive features such as hair, mammary glands, and a synapsid skull with one temporal opening.

  • Adaptations for Survival:

    • Endothermy: Regulates body temperature internally.

    • Parental care: Ensures the survival of young through feeding directly with milk.

  • Diversity in Subclasses:

    • Prototheria:

      • Egg-laying mammals (monotremes).

    • Theria:

      • Includes marsupials and placental mammals (eutherians).

Specialized Adaptations in Mammals

1. Hair

  • Composition: Made from alpha-keratin.

  • Functions:

    • Insulation and temperature regulation.

    • Camouflage and signaling.

    • Protection (e.g., quills in porcupines).

2. Dermal Glands

  • Types and Functions:

    • Sweat Glands:

      • Secrete watery fluid for thermoregulation.

    • Sebaceous Glands:

      • Keeps skin and hair pliable and glossy.

    • Scent Glands:

      • Used for communication.

3. Reproductive Strategies

  • Mammary Glands:

    • Produce milk except in monotremes where milk is secreted onto the belly.

4. High Metabolic Activity Adaptations

  • Skeletal/Muscular Modifications:

    • Incus, malleus, and stapes (ear ossicles), specialized jaw musculature.

  • Digestion:

    • Heterodont teeth specialization (incisors, canines, premolars, molars) for various diets.

  • Respiratory and Circulatory Systems:

    • Presence of diaphragm, alveoli for gas exchange, and four-chambered heart for efficiency.

  • Excretion:

    • Advanced kidneys that excrete nitrogenous wastes as urea.

Classification within Mammalia

  • Prototheria:

    • Egg-laying mammals, notable members include the Platypus and echidnas.

  • Theria:

    • Characterized by placentas, includes marsupials like kangaroos and eutherians (placental mammals).

Marsupials

  • Key characteristics:

    • Short gestation period with prolonged lactation.

    • Underdeveloped placenta leading to birth of immature young that continue development in the marsupium (pouch).

Eutherians

  • Characteristics:

    • Long gestation periods, referred to as viviparous.

    • More complex placentas compared to marsupials.