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:
- Warm-blooded
- Covered with fur
- Sweat glands
- Mammary glands
- Parental care of young
On the other hand, some mammalian features are preservable:
- Only two sets of teeth: deciduous (“baby”) and permanent
- Highly specialized teeth divided into incisors, canines, premolars,
and molars
- Teeth highly distinctive, can recognize a species from only one or two teeth
- Lower jaw comprised only of dentary; jaw joint is dentary-squamosal, not
quadrate-articular
- Post-dentary lower jaw bones moved up into middle ear
- And more
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):
- Oldest fossils Early Cretaceous; survive today in Australasia as platypus and echidna
- Still lay eggs (only living mammals to do so)
- Very simple mammary glands
- Today's monotremes have lots of primitive features, but many specializations of their own
- No evidence that monotremes were ever a dominant group of mammals
Marsupalia (marsupials):
- Oldest fossils Late Jurassic; survive today in opossums of the Americas and great
diversity in Australasia (and in Cenozoic were even more diverse in South America)
- Birth barely-formed young that develop in pouch
- During Mesozoic were very common mammals in both Gondwana and Laurasia
- Largest mammals of Mesozoic were North American marsupials (badger-sized)
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):
- Oldest fossils Early Cretaceous; survive today as most diverse group of mammals
(including us!)
- Reproduce by keeping young in womb until birth, fed by placenta
- Mesozoic placentals were small
- Placentals had diversified during Cretaceous, including early hoofed mammals and
primate-relatives
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
