Placental mammals originated in the Cretaceous. The extinct family Leptictidae includes the earliest known placentals.
The most recent classification of placentals separates the Edentata (sloths, armadillos, and anteaters) sharply from all the rest.
Most of the evolution of the Edentata took place in South America, but a few of them migrated to North America during the Pleistocene.
The remaining placentals (other than leptictids and edentates) can be arranged in five large groups: Archonta (primates, bats, etc.),
Glires (rodents, rabbits, etc.), Lipotyphla (moles, shrews, etc.), Ferae (Carnivora and related orders, and Ungulata (hoofed mammals and whales).
- Leptictids and placental beginnings
- Order Edentata
- Characteristics:
- Largely confined to South America, with Pleistocene incursions into North America
- Stapes imperforate (no arterial foramen)
- Variation in number of cervical vertabeae (as many as 9; other mammals have only 7)
- Extra articulations (metapophyses) in lumbar vertabrae ("Xenarthra")
- Tendency to form armor in dermis of skin
- suborder Palaeanodonta (early, primitive)
- suborder Cingulata: armadillos and glyptodonts
- suborder Pilosa: tree sloths, ground sloths, and anteaters
- Do pangolins (order Pholidota) belong here?
- What about tillodonts and taeniodonts?
- Classification of the remaining placentals:
- Grandorder Insectivora or Lipotyphla
- Grandorder Archonta: tree shrews, primates, bats, and colugos
- Grandorder Glires: rodents, rabbits, elephant shrews, etc.
- Grandorder Ferae: Carnivora and related orders
- Grandorder Ungulata: a great diversity of hoofed mammals and whales
- Insectivora or Lipotyphla:
- Characteristics:
- always small
- no auditory bulla
- short caecum
- no pubic symphysis
- Erinaceomorpha: hedgehogs
- Soricomorpha: moles and shrews (and Caribbean solenodonts)
- Chrysochloroidea: "golden moles" of South Africa (and possibly tenrecs as well)
Illustrations
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