I.Introduction
A. Falling sea level due to collisional events involving Eurasia and Africa, and India and Eurasia, as well as other factors help in the emergence of a series of tropical islands and peninsulas.
B. The beginning of the Panamanian land bridge between North America (NAM) and South America (SAM) forms at the end of the Tertiary. The Panamanian arc was completed by the end of the Pliocene. The general date associated with the completion is approximately 2.5 million years ago. The results of the land bridge are far-reaching. NAM was only slightly affected when compared to SAM. In SAM, all of the large endemic land mammals became extinct.
II. SAM--pre-connection
A. Oligocene--Very few mammals existed. They were descendants from one species of monkey, and one species of rodent. These radiated into giant mammalians.
B. Dominant Fauna: most is endemic with a wide range of marsupials, edentates, ungulates, and condylarths.
1. The big predators roaming SAM were 10-15 feet tall flightless birds. They had strong claws which they could use to hold prey while they attacked with their razor-sharp bills.
2. Marsupials, animals with pouches as opposed to placental mammals, are also common. This includes armadillos and porcupines.
3. Ungulates native of SAM including Notoungulata.
C. Flora present (based on microfossil assemblages)--97% of avian genera are restricted to SAM.
III. NAM--pre-connection
A. Dominant Fauna are large, but not excessively large, placental mammals including: lions, tigers, camels, horses, bisons, elephants, bears, raccoons, sloths. Most are immigrants from Eurasia.
B. Flora present (based on microfossil assemblages)--99% of avian genera are restricted to NAM.
IV. The complete closing of the Central American Seaway occurred at approximately 3.7-3 million years ago.
A. This prevented the warm currents from reaching the Pacific. They were instead directed into the Gulf Stream, which brought the warm water into the Arctic where it evaporated and fed the glaciers, further contributing to the decrease in sea level as the land bridge formed.
B. The surface salinity increased in the waters east of the land bridge.
C. Sedimentation rates and marine climate conditions were changed on both sides of the land bridge.
1. Coastal upwelling caused the accumulationof biosiliceous sediments along the coast of Peru, but did not reach northern Chile. However these sediments occur in northern Chile in the Pliocene and Pleistocene showing a general incease.
2. The ratio of tropical-subtropical planktonic foraminifrea to the total assemblage recorded fromthe Pacific coast of Ecuador indicate an abrupt cooling of seasurface temperature.
V. The interchange associated with the land bridge.
A. The effects on NAM life
1. Introduction of the following:
a. edentates-Glossotherium, Glyptotherium, Kaglieuichia, Dasypus
b. rodents-Neochoerus, Erethizon
c. marsupials-armadilla, porcupine
2. Extinction of Dipoides, Ogmodontomyus, Nebraskamyus, Pliolemmus, Pliopotamyus, Practilepus.
3. The general effect is that 10% of the genera after completion of the bridge are from SAM.
B. The effects on SAM life
1. Introduction of the NAM placentals
2. Extinction of the large SAM mammals and birds due to competition with the smaller, fitter NAM mammals.
3. Of the 32 comparable taxa of palynomorohs, 27 are similar to those of NAM. (e.g. Cecropia, Gramineae, Sapium, Typha)
4. The general effect is that 50% of the genera after completion of the bridge are from NAM.
C. Southern Cental America by the Mid-Pliocene.
1. Increase in grass pollen to a maximum of 7.5% (formerly near 0%)
2. Appearance of open tropical dry forest, fossil oak pollen has been found
3. Wetter Atlantic and dryer Pacific side
4. Browsing and grazing mammals are common as the environment became more suitable to them.
Bibliography
Condie, K.C., and R.E. Sloan. 1998. Origin and Evolution of the Earth: Principles of Historical Geology. Upper Sadle River, New Jersey: Prentice-Hall Inc.
Graham, A. 1992. Utilization of the isthmian land bridge during the Cenozoic-paleobotanical evidence for timng, and the selective influence of altitudes and climate. Review of Palaeobotany & Palynology. v. 72, n. 1-2. p. 119-128.
Ibaraki, Masako. 1997. Closing of the Central American Seaway and Neogene coastal upwelling along the Pacific Coast of South America. Tectonophysics. v. 281, n. 1-2, p. 99-104.
Woodburne, M.O. 1987. Cenozoic Mammals of North America. Berkeley and Los Angeles, California: University of California Press.