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
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:
- 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 cheek teeth (premolars and molars)
- Lower jaw comprised only of dentary; jaw joint is dentary-squamosal, not
- Respiration using diaphragm (so that dorsal vertebrae are divided into thoracic and lumbar sections)
- And more
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:
- Evidence for parental care in basal synapsids, as well as
more derived therapsids
- Division of teeth into incisiors, canines, and cheek teeth in early therapsids
- Diaphragm breating in more advanced therapsids
- At around the same level, possible evidence for fur
- Predator-prey ratios of therapid communities suggest elevated (or at least non-cold blooded) metabolisms
Living mammals are divided into three clades:
- Monotremes: the last surviving egg-laying mammals. Rare and surviving
only in Australasia
- Marsupials: reproduce by young born live but very poorly
developed, which then suckle and grow within a pouch. Dominate Australasia, common in South America, present in early Cenozoic Europe and North
- Placentals: stay in the womb until more developed, fed by
a placenta. The dominant group in the Americas, Africa, and Eurasia.
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):
- Generally relatively stumpy legs and tails, even in modern forms
- Oldest from Middle Jurassic onward
- Oldest monotremes proper from Early Cretaceous (perhaps actually a platypus! it has the same electrosensory apparatus); survive today in
Australasia as platypus and echidna
- Prototheres were never a dominant group, but were once moderately common in Gondwana (present in Asia, too)
- Retain ancestral sprawlling posture and egg-laying reproduction
- Range from tiny to
- Relatively common mammals from the Jurassic and Cretaceous in Laurasia and northern Africa
- Omnivores and carnivores: at least one has gut contents containing the remains of
- In their time, were major parts of the larger mammal fauna: ecologically comparable to opposums and raccoons in the modern U.S.
- Wiped out at end of Cretaceous
- Mode of reproduction unknown
Comprosed of the poorly known Late Triassic-Late Jurassic haramiyids and the diverse
- Oldest multituberculate fossils Middle Jurassic; survived into early Cenozoic (about 35 Ma) when they became extinct
- Mode of reproduction unknown
- Many were good climbers
- Extremely common in Laurasia; a possible Gondwanan group may belong to a different clade
- Specialized molars and gnawing teeth: convergent with rodents, and a major
radiation in this way of life before the true rodents evolved
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):
- Oldest fossils Early Cretaceous
- Survive today in opossums of the Americas and great diversity in Australasia (and in Cenozoic were even more diverse in South America)
- In marsupials, birth barely-formed young that develop in pouch: not certain yet when non-marsupial metatheres evolved that form of
- During Mesozoic, metatheres were very common mammals in both Gondwana and Laurasia
- Some very small but some were among the largest mammals
of the Mesozoic (badger-sized)
- Recent phylogenetic analyses show that
there are no definite members of Marsupalia during the Mesozoic, although they are present so early in the Cenozoic that they probably had
evolved before the end of the Cretaceous
Eutheria (placentals and our extinct relatives):
- Oldest fossils Early Cretaceous; survive today as most diverse
group of mammals (including us!)
- Placental mammals reproduce by keeping young in womb until birth, fed by placenta: not certain how non-placental eutheres reproduced
- Mesozoic eutheres were small; many were herbivorous, omnivorous, and insectivorous
- True placentals are not yet known older than the end of the Cretaceous,
but it is quite possible that the major divergences had already happened before the end of the Age of Dinosaurs
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