Lecture 18 Cenozoic Mammals
Focus Question-How long did it take mammals to fill the niches left empty by the dinosaurs?
1. After seeing how the dinosaurs, pterosaurs and marine reptiles radiated into the ecological niches in the Mesozoic, the pattern observed in the history of the mammals will be very familiar.
a. natural selection and genetic drift are once again driven by environmental pressure and isolation
b. once again, we see niche partitioning, gigantism, and the evolution of flight (this time, the bats)
c. once again, we see adaptation to the sea (the marine mammals)
2. The fossil record is really very rich in Cenozoic mammals, in part because there is such a rich record of fossil bearing sediments of Cenozoic age, and in part because the bones tend to be well preserved. There are some big differences between reptiles and mammals in the skulls that can be seen right away-
a. jaw-reptiles have several bones, mammals have one
b. teeth-reptiles have undifferentiated teeth, mammals have differentiated teeth
c. ear bones - reptiles have one, mammals have 3
3. Cynodonts of the Triassic (early mammal-like reptiles) were the ancestors to all the mammals-
a. one line led to the Monotremes (the egg laying mammals)
b. one line led to the Marsupials and the Placentals
Marsupials and Placentals are more closely related to eachother than they are to the Monotremes
4. Mammals diversified in the Eocene, so about 15 million years after the K/T boundary, the mammals moved into all those available niches. What was special about the Eocene?
a. The Eocene began warm and ended cool-it was the warmest epoch of the Cenozoic, but it ended with a big cooling event.
b. The mammal diversification occurred in the late Eocene, during that cooling episode.
5. The cooling episode of the late Eocene and Oligocene had a botanical link-it coincided with the spread of grasslands-so successful that oxygen actually started to increase again.
a. the evolution of the large placental mammals coincides with the grassland expansion, the slight rise in oxygen, and the cool climate.
b. flight develops during the Eocene-this time, in the bats. This is the fourth time flight has evolved.
6. Looking briefly at three successful Cenozoic mammalian groups-Carnivores, Perissodactyls (odd-number of toes- herbivores) and Whales, we can see some familiar stories of species radiation and niche partitioning
7. Perissodactyls-odd# of toes-such as horses and rhinos. Expansion of grazers and niche partitioning again-some became very large, including Indricotherium, the largest land mammal known.
a. Horse evolution-from Eocene to Pleistocene time: legs lengthen, toes reduce, teeth become high crowned, size increases
8. Carnivores-descendents of Miacids, small mammals of the Paleocene
a. carnivores radiated into different niches, tracking the expansion of species in the grassland grazers and browsers. One famous group is the sabertooth cats. The famously large canines of the sabertooth cats has actually appeared several times in the cat lineage. They are just best known from the Pleistocene.
b. the large flightless birds that were top predators of the early Cenozoic give way to the dominant predators of the mammalian group by mid-Cenozoic time.
9. Whales (Cetaceans)-
An example of convergent evolution is nicely shown in the Cenozoic-
a. Remember
Ichthyosaurs in the Mesozoic? We
see the whole story again, from coastal shallow water to open ocean lifestyle,
only this time, it’s the Mammals.
The whales are marine mammals that have a fossil record beginning in the Eocene.
b.
Small, four legged carnivorous land animals,
about the size of a large dog, occur in the Eocene fossil record
c.
From coastal, shallow water environments to open
ocean lifestyles occurs in less than 20 million years, a little faster than the
Ichthyosaurs
d.
Fossil record shows the decrease of the rear limb
bones, and the shortening and fusing of the forelimb bones, the lengthening of
the head, and numerous other skeletal changes.
e.
We saw this pattern before with the Mesozoic
Ichthyosaurs. Both the whales and
the ichthyosaur body plans converge, as they adapted to the open ocean
lifestyle.