Biology

Mammals: Evolution & Classification

Big Picture

Over time, reptiles evolved into mammals. Mammalian ancestors such as pelycosaurs, therapsids, and cynodonts started developing mammalian traits. These gave rise to early mammals that still had some reptilian traits. After the extinction of dinosaurs, these early mammals flourished and evolved into modern mammals.

Key Terms

Pelycosaur: Reptile-like land vertebrate.

Therapsids: Evolved from pelycosaurs and have more mammalian traits.

Cynodonts: Evolved from therapsids and later evolved into mammals.

Monotreme: Earliest mammal that reproduces by laying eggs.

Marsupial: Early mammal in which the embryo is born at an early, immature stage and completes its development outside the mother’s body in a pouch on her belly.

Placental Mammal: Mammal with a placenta that sustains the fetus while it develops.

Evolution of Mammals

Mammalian Ancestors

Pelycosaurs (275 million years ago): reptile-like land vertebrate

  • Had a few mammalian traits such as different types of teeth

Some pelycosaurs evolved into therapsids

  • Therapsids developed lots of mammalian traits After mass extinction, some therapsids evolved into cynodonts
  • Cynodonts were nocturnal, developed even more mammalian traits (such as ability to regulate body temperature)
  • Cynodonts later evolved into mammals

Evolution of Early Mammals

Monotremes evolved 150 million years ago–still had some non-mammalian traits such as laying eggs

  • Marsupials evolved from monotremes
  • Marsupials evolved into placental mammals

Evolution of Modern Mammals

  • After mass extinction at the end of the Cretaceous Period, dinosaurs went extinct
  • Conditions were ideal for mammals to take over:
  • More resources for mammals
  • Temperature changes favored mammals since they were warm-blooded
  • More flowering plants more food for mammals’ preys more food for mammals

Mammalian Classification

Mammals were traditionally grouped into 17 orders. Recently, scientists have started to classify mammals based on their ancestors. The new classification method, phylogenetic classification, separates mammals into 4 superorders.

Classification of Placental Mammals

Traditional classification puts mammals into 17 orders:

Order
Example
Sample Trait
Insectivora
mole
small sharp teeth
Edentata
anteater
few or no teeth
Pholidota
pangolin
large plate-like scale
Chiroptera
bat
digits support membra-nous wings
Carnivora
coyote
long pointed canine teeth
Rodentia
mouse
incisor teeth grow con-tinuously
Lagomorpha
rabbit
chisel-like incisor teeth
Perissodactyla
horse
odd-toed hooves
Artiodactyla
deer
even-toed hooves
Cetacea
whale
paddlelike forelimbs
Primates
monkey
five digits on hands and feet
Proboscidea
elephant
tusks
Hyracoidea
hyrax
rubbery pads on feet
Dermoptera
colugo
membrane of skin be-tween legs for gliding

Biology

Mammals: Evolution & Classification Cont.

Classification of Placental Mammals (Cont.)

Order
Example
Sample Trait
Pinnipedia
seal
feet with fins
Sirenia
manatee
paddle-like tail
Tubulidentata
aardvark
teeth without enamel

Phylogenetic Classification

Phylogenetic classification groups mammals by common ancestor.

Superorder
Example
Afrotheria
aardvarks, elephants, manatees
Xenarthra
anteaters, sloth, armadillos
Laurasiatheria
bats, whales, hoofed mammals, carnivores
Supraprimates
primates, rabbits, rodents

Notes