Evolution and Radiation of Mammals after the Great American Interchange

1. Mammals evolved from reptile-like ancestors about 220 mya • 65 mya the dinosaurs went extinct • Extensive mammalian radiation began after the dinosaur extinction

2. Marsupial mammals evolved 100 mya in North America • Marsupials then dispersed to South America, Antarctica, Australia, and Europe • By about 140-175 mya, South America was becoming isolated

3. Placental mammals likely evolved in Asia sometime after the marsupials and had dispersed by 50 mya to Europe, North America, Africa, India and South America

4. From 65 to 30 mya, mammals evolved and speciated in South America, both marsupials and placentals

5. South America's biota during the Tertiary included • Litopterns (placental)--similar to horses and camels • Toxodonts (placental)--similar to rhinos and hippopotamuses • Pyrotheres (placental)--some similarities to elephants

6. 30 mya other species moved into South America from North America across a series of islands • early primate species • early guinea pigs • early porcupines • early capybaras

7. Panamanian landbridge connected North to South America about 3 mya • Great American Interchange occurred

8. Species from NA that moved into SA • Jaguars, peccaries, tapirs, coatis, kinkajous, giant otters, llamas, mastodons

9. Species from SA that moved into NA • Giant sloths, armadillos, possums, glyptodonts, porcupines, anteaters, toxodonts, Titanis

10. Equilibrium number of families? • In SA before the interchange--32 families of mammals • after the landbridge—39 families • then levelled out at 35 families • In NA before the interchange--30, then 35, then 33 families

11. Marsupial cats of SA were replaced by placental cats • Toxodonts of SA were replaced by tapirs and deer • Some unique groups of SA persisted--monkeys, tree sloth, anteaters

12. Many more genera evolved after the interchange in NA and SA • In SA there was an increase from 70 to 170 genera • This increase has resulted from the radiation of the NA mammals in SA

13. 50% of mammal species of SA have NA ancestors • 10-20% of NA species have SA origins • Why?

14. Northern mammals were better migrators? • Northern mammals more prone to speciation? • Northern mammals outcompeted Southern mammals? • North mammals more tested? • South American forms larger?

15. Did other vertebrates show similar patterns? • Fossil records for birds, reptiles, amphibians and fish are poorer

16. Some patterns for other taxonomic groups • Communities evolved gradually • Bird species exchange between NA and SA more balanced than that for mammals • Herptiles and fish tended to go to NA from SA

17. Climate on the earth has varied tremendously over time and one major reason for this is plate tectonics • For example, when large landmasses are near or over the poles, this may trigger glaciation events

18. While glacial events have come and gone over the history of the earth, we have a much more detailed picture of the events that have occurred in the recent epoch, the Pleistocene

19. During much of the Mesozoic (248-65 mya) and the Cenozoic Eras (65 mya to today) the climate was relatively warm

20. So the glacial and interglacial cycles the earth has experienced in the last few million years have been a new type of phenomenon for the many groups of organisms that had evolved since the beginning of the Mesozoic

21. Unlike in the earlier periods, when glaciations were caused primarily by continents moving over polar regions, we believe the Pleistocene glaciations were caused by changes in the earth's orbit which led to changes in the solar radiation the earth absorbed

22. Three characteristics influence the earth's orbit and the discoverer of these characteristics, Milankovitch, a Yugoslavian physicist, gives the cycles their name, Milankovitch cycles

23. Milankovitch cycle 1 • Earth's orbit is ellipitical and changes in shape over time, this change being referred to as its eccentricity • When orbit is more circular, solar input during winter and summer is somewhat more even. When orbit is more elliptical, difference between winter and summer is greater • Complete cycle of changes takes 100,000 years

25. Milankovitch cycle 2 • Tilt of the earth on its axis, its obliquity, cycles every 41,000 years from 22.1 to 24.5 degrees • More pronounced angle leads to greater contrast in seasons; less of an angle means less contrast.

27. Milankovitch cycle 3 • Earth's orientation, or precession, changes, with the north pole pointing one way at one point during the cycle and another way at another point in the cycle • This factor also affects seasonal contrasts. • This cycle lasts 22,000 years.

29. Minimum summer insolation in northern hemisphere occurs when • Eccentricity is high (revolution is elliptical) although eccentricity affects insolation only a little • Tilt of earth’s axis is a minimum (obliquity) • When earth is farthest from the sun in the summer (pattern of precession)

30. When these cycles lead to periods when insolation in the northern latitudes is low, glacial periods are triggered. • The northern latitudes are particularly important because they have much more land than the southern latitudes, where ice sheets can form

31. When summers in the northern latitudes are cool, snow that has accumulated in the winter will not melt, leading to glaciers

32. Feedback effects • Cool earth with much ice tends to become cooler (ice reflects solar radiation) • Warm earth tends to become warmer (production of CO2 and methane)

33. Why have these Milankovitch cycles been stronger in the last 2 my than at previous times in the earth's history? • We don't know • Some people have suggested that processes that cause feedbacks have been more common in the last 2 my than previously, for example, volcanic activity • Volcanic activity may enhance the glaciation process by reducing solar radiation reaching earth because of dust produced

34. Much evidence suggests a rapid cooling in the earth's climate about 1.8 mya (2 mya Pliocene/Pleistocene boundary) • Fossils of cold-water animals become more common at this point in time

35. During the Pleistocene, parts of the Palearctic and the Nearctic were covered with ice sheets which retreated and advanced • Ice sheets were sometimes more than one mile thick and deformed the earth's crust in some instances • Up to one third of the earth's land surface may have been covered by ice at one point • 18,000 ya, most recent glacial maxima, ice stretched down to the middle of North America.

36. During glacial maxima, temperate regions tended to be cooler and wetter than they are today and tropical regions tended to be drier

37. Most of the ice during the Pleistocene glaciations occurred in the temperate and polar regions of the Northern Hemispheres • However, in the equatorial regions and the southern hemisphere, changes also occurred, although primarily in high elevation regions • Mountains in New Zealand were glaciated, as were the Andes, and very high mountains in East Africa.

38. Interglacials--warm episode between two colder episodes • We are currently living in an interglacial period which we refer to as the Holocene epoch that began about 10,000 years ago.

39. Evidence that is used to reconstruct the past climatic history • Landforms--glacially carved valleys tend to be rounded while water-carved valleys tend to be v-shaped.

40. Evidence that is used to reconstruct the past climatic history • Arrangements of soil particles and types of soil particles found in different areas • Tills are heterogeneous with many different particle sizes (clay, sand, gravel, boulders) and they are deposited directly by ice • Peaty soils are indicative of warmer periods and are organic soils with no or slight decomposition.

41. Evidence that is used to reconstruct the past climatic history • Data from ocean and lake bottoms--fossil pollen can show the types of plant species found in an area at different points in time • If pollen shows primarily cold-adapted species, like many conifers, for a particular stratum of the core, we can assume the surrounding environment was cold at that point in time

42. Evidence that is used to reconstruct the past climatic history • Oxygen isotope studies--O16 is lighter than O18 • O16 builds up in atmosphere while O18 builds up in oceans • When climate is cold (essentially more condensation than evaporation is taking place), O16 remains frozen in ice while O18 becomes relatively more abundant in the oceans • Foraminiferans, which build their skeletons with material from the environment, become more O18 rich in periods of cold • Hence, when they die and fall to the bottom of the ocean, the sediments they rest in will have high concentrations of O18 and reflect that the atmosphere was cold at this point in time

43. Ocean sediment O18 levels

44. During the height of the Wisconsin glaciation, about 18,000 years ago, a great deal of ocean water was tied up in ice, causing a 100 m drop in sea level • At other points when glaciers were at a maximum, drops may have been 160 m

45. The reverse also occured--during interglacials, sea levels may have climbed to 70 m higher than today. • Overall, the total amount of sea level change that occurred during the Pleistocene could have been 230 m (approx. 700 feet)

46. When sea levels were low, many areas were connected that weren't previously connected, like North America and Asia and areas of southeastern Asia and Australia. • These created connections for terrestrial organisms but barriers for aquatic organisms.

47. Toward the end of the Pleistocene numerous species of terrestrial vertebrates went extinct in North America--mastodonts and mammoths, camels, llamas, ground sloths, cave bears, hyenas and saber-toothed tigers. • Large birds went extinct, including Teratornis incredibilis, which had a wingspan of approximately 5 m. • The glaciers had already retreated from much of the area where the extinctions took place by the time the extinctions occurred.

49. Large species were particularly likely to go extinct--100% of mammalian herbivores 1000 kg or more in body mass went extinct in North and South America, Europe and Australia during the late Pleistocene

50. Hypotheses to explain the extinctions • Overkill hypothesis----humans with relatively efficient hunting techniques arrived in North America from Asia toward the end of the Pleistocene and slowly spread southward • They caused the extinction of the large herbivorous mammals as they went, as well as the scavengers and predators of the herbivorous mammals • Humans may have entered North America sooner than the dates of the extinctions but these populations may not have been as dense and technologically sophisticated as those that caused the extinctions.