GEOL 102 Historical Geology

Spring Semester 2011
The Mesozoic Era IV: Flowers and Mammals

Plants of the Mesozoic:
Main groups in the Triassic and Jurassic include:

All but ferns are "gymnosperms": seed plants with indirect fertilization. During the Early Cretaceous, origin of the angiosperms (flowering & fruiting plants). The basic angiosperm life cycle hinges on co-evolution with animals:

Possible angiosperm body fossils are known from the Jurassic, and close relatives of the angiosperms go back to the Permian, but the oldest definite angiosperms are from the Early Cretaceous. Early Cretaceous angiosperm pollen and leaves are known from far off Prince George's County, Maryland, and similar fossils are known from earlier in the Cretaceous in China.

Angiosperms remain small herbaceous weeds for most of the Cretaceous, although during the Late Cretaceous some became arborescent. Some "gymnosperns" (including the Permian "seed-fern" Glossopteris and the bennettitalians) were more closely related to angiosperms than to other gymnosperms.

Insects of the Mesozoic:
Continued insect diversification throughout the Mesozoic, including:

Terrestrial Vertebrates of the Mesozoic I: Mammals and their Ancestors
In Early Triassic, advanced therapsids remain the dominant group. However, rising in diversity are various groups of reptiles (see next lecture).

Early Triassic therapsids included large bodied herbivores and smaller carnivores and omnivores. Circumstantial evidence suggests some of the latter were furry and whiskered.

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.

    To Next Lecture.
    To Previous Lecture.
    To Syllabus.

    Last modified: 14 January 2011