Evolution


  1. Mammals (Class Mammalia) evolved from stem amniotes. Their lineage split off from the lineage that gave rise to reptiles (Class Reptilia) about 320-315 mya
  1. Synapsids are mammalian ancestors, sometimes also known as proto-mammals

    1. Synapsids were very diverse, and all but the lineage that led to the mammals are now extinct
    2. Characteristics of synapsids (Subclass Synapsida)

      1. Opening in cranium behind eye

        1. Contrasts with the solid "temporal shield" (turtles) or multiple temporal openings of other modern day reptiles (snakes, lizards) and birds
        2. Function

          • May make room for more powerful jaw muscles
          • May make skull lighter (without sacrificing strength)
          • Saves energy by requiring less bone
  1. One group of synapsids, the therapsids, is the lineage that gave rise to mammals

    1. Characteristics of therapsids



      1. Large body size, generally carnivorous
      2. Heterodont dentition
      3. Attachment of jaw muscles to zygomatic arch
      4. Beginnings of a secondary palate
      5. Less spraddle-legged than ancestors (e.g., see Dimetrodon artist's rendering and video from a BBC recreation)

        Mammal-like forelimbs Reptile-like forelimbs (spraddle-legged)
      6. Expansion of the dentary bone to make mammalian articulation w/cranium (presence of dentary-squamosal joint is dividing line between stem-reptiles and stem-mammals)


  1. The transition from proto-mammals to mammals - jaws and middle ears (accumulated changes along the lineage that gave rise to mammals)
    Another diagram summarizing the changes in chewing and hearing anatomy over evolutionary time
Stem amniotes Proto-mammals (used to be referred to as mammal-like reptiles) (e.g., Morganucodon) Mammals (e.g., Didelphis)
Mandible composed of many bones, including the dentary Increase in size of dentary bone, decrease in size of postdentary bones Mandible composed of a single bone, the dentary
Articulation of mandible with cranium occurs between the articular bone (lower jaw) and the quadrate bone (cranium) Articulation varies, and sometimes occurs in two places Articulation of mandible with cranium occurs between the dentary bone and the squamosal
Sound is conducted from the tympanic membrane to the inner ear via a single bone, the stapes Sound is conducted via the stapes, but the articular and quadrate bones also play a role in transmitting sound Sound is conducted from the tympanic membrane by a series of three bones: the malleus (called the articular bone in non-mammals), the incus (called the quadrate in non-mammals) and the stapes. They are collectively known as the ossicles.
Dentition is homodont Dentition is heterodont Dentition is heterodont


  1. Why the obsession with jaws and ears?


  1. Characteristics of the first mammals

    1. Mouse-sized (new fossils...)
    2. Insectivorous
    3. Relied on well-developed senses of smell and hearing
    4. Nocturnal (hiding from dinosaurs)
    5. Had fur
    6. Homeothermic
    7. Laid eggs
    8. Had mammary glands


THINK-PAIR-SHARE: Scientific knowledge is based on a combination of observation and inference. What might the evidence be for the characteristics listed above? Do we have direct, observable evidence (e.g., fossil), or are we inferring this based on shared homology or evolution?


  1. Why did mammals survive, while all other synapsids went extinct?


Concept mapping exercise


  1. Since the early evolution of mammals, there have been adaptive radiations and waves of extinctions as the result of
    1. Breakup of Pangaea and continental drift (200-65 mya)
    2. Extinction of dinosaurs (65 mya)
    3. Impact of human behavior, especially in the last 30,000 years


  1. Recommended readings