GEOL 104 Dinosaurs: A Natural History

Fall Semester 2006
In the Shadow of the Dinosaurs: The first 7/10th of Mammalian History

Mammals and their closest relatives (more properly Mammaliformes) appear in fossil record the same time as dinosaurs, in Late Triassic.

Mammals are very advanced therapsids synapsids.

True mammals (Mammalia) found from Middle Jurassic onward.

Like birds, many of the features that characterize modern mammals don't fossilize:

On the other hand, some mammalian features are preservable:

Many features limited to Mammalia among living amniotes were probably found in their closest non-mammalian therapsids relatives.

Mesozoic mammals very small (shrew-to-house cat sized, with a badger-sized marsupial from the very end of the Cretaceous); mammals only become large AFTER extinction of non-avian dinosaurs.

Many groups of Jurassic and Cretaceous mammaliforms; four main clades within Mammalia:

Monotremata (monotremes):

Marsupalia (marsupials):

Multituberculata (multituberculates):

  • Oldest fossils Late Jurassic; survived into Early Cenozoic but are now extinct
  • Uncertain if closer to monotremes, closer to Marsupalia + Placentalia (Theria), or closer to marsupials than to all other mammals
  • Not known if egg layers, pouched, placental birth, etc.
  • Many were good climbers
  • Specialized molars and gnawing teeth: rodent-like

    Placentalia (placentals):

    Monotremes, marsupials, multituberculates, and placental mammals all survived the great extinction event at the end of the Cretaceous.

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    Last modified: 14 July 2006