Mammalian Characters, Cynodonts, and Early Mammals

Vertebrate Evolution: Mammalian Characters, Cynodonts, and Early Mammals

Basal Amniote Groups

  • Neodiapsids: Includes turtles, dinosaurs (leading to birds), pterosaurs, marine reptiles, crocodiles, snakes, and lizards.
  • Synapsida: Also known as 'mammal-like reptiles', characterized by synapsid skulls. This group includes cynodonts and mammals.

Mammalian Characters

Soft Part and Physiological Characters
  • Endothermy:
    • High energy requirement.
    • Allows for living in polar environments.
    • Also present in birds, pterosaurs, and possibly some dinosaurs.
  • Hair:
    • Keratin-derived from the skin.
  • Mammary Glands:
    • Secrete milk for nourishing young.
  • Large Brains:
    • Generally larger brains compared to other groups.
  • Extended Care of Young:
Acquisition of Mammalian Characters
  • Acquisition occurred gradually over 30-40 million years during the late Permian and Triassic periods in the cynodonts.
  • Cynodonts are synapsid amniotes.
  • The exact point at which cynodonts became mammals is somewhat arbitrary.
Hard Part Characters
  • Dentary-Squamosal Jaw Articulation:
  • Three Small Bones in Middle Ear:
  • Three Cusped (Tribosphenic) Molar Teeth:

Triassic Cynodonts

  • Mammalian ancestors.
  • Key groups:
    • Procynosuchidae
    • Galesauridae
    • Thrinaxodontidae
    • Cynognathidae
    • Diademodontidae
    • Traversodontidae
    • Chiniquodontidae
    • Probainognathidae
    • Tritheledontidae
    • Ritylodontidae

Jaw Evolution in Synapsids

  • Reduction in post-dentary jaw bone size and change in position from Permian to Triassic synapsids.

Jaw Joints and Middle Ears

  • Early cynodonts had a joint formed between the quadrate in the skull and the articular in the back of the lower jaw, similar to modern reptiles and birds (diapsids).
  • Some later cynodonts developed 'double' jaw joints.
  • Mammals have a joint between the squamosal and dentary.
  • The jaw bones involved in the 'old' joint pushed behind the 'new' joint into the middle ear.
    • Malleus = articular.
    • Incus = quadrate.
    • Tympanic = angular.
  • Consequence: we can hear our jaws moving while we eat!

Cynodonts

  • First appeared in the Permian period.
  • Survived the Permian-Triassic extinction event and radiated in the Triassic period.
  • Carnivorous (ancestral state) and herbivorous examples.

Thrinaxodon

  • A Triassic carnivorous cynodont found in South Africa and Antarctica.
  • Features:
    • Lower jaw formed mostly of the dentary bone.
    • Semi-erect gait.

Later Cynodont Changes

Posture
  • Fully erect gait achieved by the articulating head of the femur bent inwards; deeper acetabulum (similar to archosaurs, dinosaurs, and birds).
Tooth Occlusion in Herbivorous Cynodonts
  • Expansion and occlusion of cheek teeth.
  • Chewing: greater food processing abilities.
    • Jaw closing.
    • Tooth movement.
Reduction and Loss of Lumbar Ribs
  • Related to the evolution of the diaphragm, which improves breathing efficiency.

Cynodont Changes Related to Chewing

Jaw Mechanics
  • The synapsid external adductor muscle gave rise to new temporalis and masseter muscles.
  • Increase in adductor muscle volume.
  • Increase bowing of zygomatic arch.
  • All allow back-to-front and side-to-side chewing motion.
Reduction of Tooth Replacement Cycles
  • Mammals: once.
  • Reptiles: continuous.

Early 'Mammaliaform' Groups and Mesozoic Mammals

  • First true mammals from the Late Triassic, very poorly known.
  • Several lineages by the Late Jurassic, but almost all fossils are isolated jaws and teeth of tiny animals (20 – 30 mm skulls).
  • Most were very small, carnivorous, shrew-like.

Morganucodontids

  • A big group of Mesozoic mammals that were common in the early Jurassic in Europe, China, and South Africa; the last member was in the mid-Jurassic in England.
  • Features:
    • Expanded braincase enclosed entirely by bone.
    • Eye socket and temporal fenestra linked - no postorbital bar.
    • Cheek teeth divided into premolars and molars.
    • Teeth replaced once: juvenile (milk) and adult set.
    • Loss of lumbar ribs.
    • Probably warm-blooded insectivores with mammary glands.

Multituberculates

  • Largest group of Mesozoic mammals.
  • Belong to crown group mammals.
  • Late Jurassic - Eocene.
  • Global distribution.
  • Features:
    • Small rodent-like omnivores.
    • Large incisors; no canines; long gap to cheek teeth (as in modern rodents).
    • The last premolar is modified into a shearing blade.
    • One species has epipubic or 'marsupial' bones in front of the pelvis (as have modern marsupials and monotremes).

Changing Views of Jurassic and Cretaceous Mammals

  • Traditional view: mammal diversity was suppressed in the 'dino-dominated' Mesozoic.
  • New Jehol discoveries indicate that Mesozoic mammals inhabited many ecological niches: from swimming and burrowing to gliding.

Modern Mammal Groups

  • Three groups form a reproductive continuum:
    • Monotremes: lay eggs.
    • Marsupials: give birth to tiny young; later development in an external pouch.
    • Placentals: advanced development in the uterus.
    • The latter two are sister groups, known as the therians.

Monotremes - Egg Laying Mammals

  • Two modern groups: duck-billed platypus in Australia only, echidnas (spiny anteaters) in Australia and New Guinea.
  • Features:
    • Lay eggs; echidnas brood these in a pouch.
    • Lots of 'primitive' skeletal elements, including epipubic bones in the pelvis and small ribs on the neck (cervical) vertebrae.
    • No teeth as adults; juvenile platypus has molars.

Fossil Monotremes

  • Sparse record in the Southern Hemisphere only - Gondwanan distribution.
    • Opalized early Cretaceous jaws and teeth in Australia.
    • Platypus-like teeth in the Paleocene, Argentina.
    • Obdurodon, Oligocene and Miocene, Australia - fossil platypus.

Next Lecture

  • Marsupials and early placentals