GEOL 104 Dinosaurs: A Natural History

Fall Semester 2007
In the Shadow of the Dinosaurs: The First 7/10th of Mammalian History

Mammals and their closest relatives (more properly Mammaliformes, or sometimes "Mammaliaformes") 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. For example, we can't say for certan when warm-blood, fur, sweat & mammary glands show up. We can determine a few of these, though:

Living mammals are divided into three clades:

However, mammal diversity in the Mesozoic was MUCH different. Many groups of Mesozoic mammals have long since died out. And some Mesozoic mammal groups survived the end of the Cretaceous, but have since died out.

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

Oldest mammaliforms of the Late Triassic and Early Jurassic were fairly small. But by Middle and Late Jurassic, there were already some specialized mammals:

Some major groups of Jurassic and Cretaceous mammals:

Prototheria (sometimes called "Australosphenida"; monotremes and their extinct relatives):

Eutriconodonta (eutriconodonts):

Allotheria (allotheres):

  • Comprosed of the poorly known Late Triassic-Late Jurassic haramiyids and the diverse Multituberculata:

    There are other branches of early mammals (docodonts, symmetrodonts, etc.), but the most important remaining two are joined together as the clade Theria. Therians are united by various skeletal (parasaggital stance, some dental, etc.) and soft-tissue (nipples, external ears, etc.) features. Therians include the metatheres and eutheres, which diverged in the Early Cretaceous.

    Metatheria (marsupials and their extinct relatives):

    Eutheria (placentals and our extinct relatives):

    Prototheres, allotheres (as multitubercultates), metatheres (including the first marsupials), and eutheres (including the first placentals) all survived the great extinction event at the end of the Cretaceous.

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    Last modified: 26 November 2007