39.  Cenozoic Mammal Radiations           Geology 2024: Historical Geology

I.  Lecture Outline
   Placental Radiations and success

II.  Lecture Discussion

Placental Radiations and Success

A. Review of Placental Ancestry and general success of Placentals

                            - placental mammals appeared during the Cretaceous as small, unspecialized
                                                insectivores -- modern moles are members of the order Insectivora, but the
                                                tiny shrew (see text) is more representative of the kind of animal
                                                from which other orders of Cenozoic placentals evolved
                            - primary placental radiation occurred in northern hemisphere and primarily in North
                                                America
                            - primary placental radiation occurred during the Cenozoic
                            - also the end of the Cretaceous extinctions of the dinosaurs also opened habitat,
                                                niches, and a daylight lifestyle to mammals

B. Cenozoic Placental Radiations (see discussion in text and especiallydiagrams showing
                            diversification)

                            - by Paleocene, 18 different mammalian orders were known
                            - Lifestyles of Cenozoic Placentals

                                    -- marine carnivores such as seals and walrus and whales
                                    -- large browsers -- elephants, mammoths, mastodons
                                    -- large grazers -- horse, camels, deer, goats, sheep
                                    -- rodents
                                    -- terrestrial carnivores -- wolves, dogs, big cats
                                    -- insectivores
                                    -- flying placentals -- bats, squirrels
                                    -- marine herbivores -- manatees
                                    -- primates
                                    -- and others

C. Important North American Lines of Cenozoic Placental Descent

Although all types of placentals have been found in the Cenozoic rocks of North
                        America, placental grazers were especially successful in North America.  The most
                        successful of the grazers belonged to the following two ancestral lines:

                        - Perissodactyls -- odd number of toes on each foot :

                                        horses -- previously discussed in discussions on modes and evidence of  evolution
                                        rhinos and rhino similar such as Titantotheres and Brontotheres
                                        tapirs

                        - Artiodactyls -- even number of toes on each foot

pigs
                                        camels
                                        deer and pronghorn (antelope)
                                        bison
                                        hippopotamus

D. Placental Gigantism:

                        - During Pleistocene, some varieties of Placental mammals became quite large:

                                    -- sloths (nearly size of small elephants)
                                    -- Dire Wolf -- 6 foot at shoulder
                                    -- beavers -- 6 foot
                                    -- Mammoths
                                    -- Bears

                        - Large size has been postulated to be an adaptation to cold -- less surface area per
                                    body weight in larger animals rather than small.
                        - Large forms died out at end of the Pleistocene -- theories:
- overhunted by primitive native Americans
  - stress during warm-ups during interglacial periods

E. Placental Success during the Cenozoic

- During the Cenozoic, placental mammals became quite successful. Possible reasons include:

                        - end of competition with large, successful dinosaurs
                        - during Cenozoic a slow cooling of world climates would have favored homeothermic
                                    animals
                        - variation in geological settings of the Cenozoic and potential isolation of many
                                    populations (punctuated equilibrium) probably also favored the diversification
                                    of mammals