Historical Geology
INTRODUCTION
MARINE INVERTEBRATES AND PHYTOPLANKTON
¬Perspective The Messel
Pit Fossil Site in Germany
CENOZOIC VEGETATION AND CLIMATE
CENOZOIC BIRDS
THE AGE OF MAMMALS BEGINS
Monotremes and Marsupial Mammals
Diversification of Placental Mammals
PALEOGENE AND NEOGENE MAMMALS
Small Mammals—Insectivores, Rodents, Rabbits, and Bats
A Brief History of the Primates
The Meat Eaters – Carnivorous Mammals
The Ungulates or Hoofed Mammals
Giant Land-Dwelling Mammals—Elephants
Giant Aquatic Mammals—Whales
PLEISTOCENE FAUNAS
Ice Age Mammals
Pleistocene Extinctions
INTERCONTINENTAL MIGRATIONS
SUMMARY
The following content objectives are presented in Chapter 18:
¬ Survivors of the Mesozoic extinctions evolved and gave rise to the present-day invertebrate marine fauna.
¬ Angiosperms continued to diversify and to dominate land plant communities, but seedless vascular plants and gymnosperms are still common.
¬ Many of todayÕs families and genera of birds evolved, and large flightless birds were important Early Cenozoic predators.
¬ If we could visit the Paleocene, we would not recognize many of the mammals, but more familiar ones evolved during the following epochs.
¬ Small mammals such as rodents, rabbits, insectivores, and bats adapted to the microhabitats unavailable to large mammals.
¬ Carnivorous mammal fossils are not as common as those of herbivores, but there are enough to show their evolutionary trends and relationships to one another.
¬ The evolutionary histories of odd-toed and even-toed hoofed mammals are well documented by fossils.
¬ TodayÕs giant land mammals (elephants) and giant marine mammals (whales) evolved from small Early Cenozoic ancestors.
¬ Extinctions at the end of the Pleistocene Epoch were most severe in the Americas and Australia, and the animals most affected were large land-dwelling mammals.
¬ As Pangaea continued to fragment during the Cenozoic, intercontinental migrations became increasingly difficult.
¬ A Late Cenozoic land connection formed between North and South America, resulting in migrations in both directions.
To exhibit
mastery of this chapter, students should be able to demonstrate comprehension
of the following:
¬ the composition of Cenozoic marine invertebrate communities
¬ the distribution of Paleogene and Neogene vegetation and its climatic significance
¬ the continuing evolution of birds
¬ the diversification of mammals, especially placental mammals
¬ the nature of archaic Paleocene-Eocene mammalian faunas compared to those of the later Cenozoic
¬ adaptive radiation and evolutionary trends in hoofed animals
¬ the significance of gigantism in Pleistocene mammals
¬ the distribution of mammals as related to plate tectonics
¬ the hypotheses for the Pleistocene extinction event
1. The marine invertebrate groups that survived the Mesozoic extinctions diversified throughout the Cenozoic. Bivalves, gastropods, corals, and several kinds of phytoplankton such as foraminifera proliferated.
Figure 18.1 Restoration
of Fossils from the Eocene-aged Clarno Formation in
John
Day Fossil Beds National Monument, Oregon
Figure
18.2 Miocene
Diatomite and Diatoms
Figure
18.3 Cenozoic
Foraminifera
Figure
18.4 Cenozoic Fossil
Invertebrates
2. During much of the Early Cenozoic, North America was covered by subtropical and tropical forests, but the climate became drier by Oligocene and Miocene time, especially in the midcontinent region.
Figure 18.5 Cenozoic
Vegetation and Climate
3. Birds belonging to living orders and families evolved during the Paleogene Period. Large, flightless predatory birds of the Paleogene were eventually replaced by mammalian predators.
Figure 18.6 Restoration of Diatryma
Enrichment Topic 1.
Shorebird Population in Rapid Decline.
Since records have been kept of bird populations, 20% of species have gone extinct. In March 2008, a 24-year survey reported the decline in Australian and Asian shorebirds, beginning in the mid-1980s. Migratory shorebirds of Australia dropped to 73%, while resident Australian shorebirds had been reduced to 81%. Wetland loss from damming and river diversion is partly responsible for decline. Wright, ÒShorebird Population in Rapid Decline,Ó Discover, January 2009, p. 42-43.
4. Evolutionary history is better known for mammals than for other classes of vertebrates, because mammals have a good fossil record and their teeth are so distinctive.
Paleobiologist Malcolm McKenna proposed a proto-Iceland that once connected North America to Europe, 56 million years ago. While most researchers acknowledge a land bridge between Siberia and Alaska, McKenna proposed that 55 million years ago, it was possible to Òwalk from New Mexico to Paris, without changing latitude.Ó This land bridge may have allowed species to migrate between continents, but researchers pointed out that the land bridge would also have blocked circulation between the Arctic and Atlantic oceans. Furthermore, changes in ocean temperature would have occurred when the land bridge broke, which may have led to intense global warming in the Paleocene-Eocene boundary. The hypothesis also explains an extinction of benthonic foraminifera. Other geologists question the new hypothesis, however. ÒVertebrates and Tectonics,Ó Geotimes, December 2003, v.48 n.12 p.8-9.
5. Egg-laying mammals (monotremes) and marsupials exist mostly in the Australian region. The placental mammals—by far the most common mammals—owe their success to their method of reproduction. All placental and marsupial mammals descended from shrew-like ancestors that existed from Late Cretaceous to Paleogene time.
Figure
18.7 Mammals
Existed During the Mesozoic, but Most
Placental
Mammals
Diversified During the Paleocene and Eocene Epochs
Figure 18.8 Archaic Mammals of the Paleocene Epoch
Figure
18.9 Some of the
Earliest Large Mammals
Figure 18.10 Pliocene Mammals of the Western North American Grasslands
6. Small mammals such as insectivores, rodents, and rabbits occupy the microhabitats unavailable to larger animals. Bats, the only flying mammals, have forelimbs modified into wings but otherwise differ little from their ancestors.
Figure 18.11 Fossil
Rodents
Enrichment Topic 3.
Move Over, Ratzilla.
A rat skull dating 4 million years old was found in 2008 in Uruguay. This skull belonged to a creature as large as a bull, and as heavy as a small car. The new ÒchampionÓ rat, Josephoartigasia monesi, lived in forests near river deltas or estuaries. This large vegetarian probably survived until 3 million years ago, when the intercontinental migration brought North American bears and cats into its range. (Dickinson, ÒAncient Rat as Big as a Bull,Ó Discover, January 2009, p. 79.
7. Most carnivorous mammals have well-developed canines and specialized shearing teeth, although some aquatic carnivores such as seals have peglike teeth.
Figure 18.12 Teeth of Carnivorous Mammals
Figure 18.13 TodayÕs Carnivorous Mammals Evolved from a Primitive Group
Known as Miacids
Richard Fox, paleontologist, found some fossils of a 60 million-year-old mammals. The fragments featured a long, pointed canine tooth that resembled venom-delivery structures. In fact, the groove in the canine tooth parallels channels found in some venomous snakes. Few modern mammals are known for venoms: Only the two species of solenodons living in Haiti and Cuba have grooves in their teeth for venom delivery. Mark Dufton, biochemist, suggested that early mammals may have been limited by their size, and oral venom would have been an asset. ÒKiller Bite,Ó Science News, June 25, 2005, v.164 n.20 p.403.
8. The most common ungulates are the even-toed hoofed mammals (artiodactyls) and odd-toed hoofed mammals (perissodactyls), both of which evolved during the Eocene. Many ungulates show evolutionary trends such as molarization of the premolars as well as lengthening of the legs for speed.
Figure
18.14 Characteristics and Evolutionary
Trends in Hoofed Mammals
9. During the Paleogene, perissodactyls were more common than artiodactyls but now their 16 living species constitute less than 10% of the worldÕs hoofed mammal fauna.
Figure 18.15 Relationships
among the Artiodactyls (Even-Toed Hoofed
Mammals)
10. Although present-day Equus differs considerably from the oldest known member of the horse family, Hyracotherium, an excellent fossil record shows a continuous series of animals linking the two.
Figure 18.16 Evolution of Horses
Table 18.1 Trends
in the Cenozoic Evolution of the Present-Day Horse Equus
11. Even though horses, rhinoceroses, tapirs, as well as the extinct titanotheres and chalicotheres, do not closely resemble one another, fossils show they diverged from a common ancestor during the Eocene.
12. During much of the Cenozoic Era, proboscideans of one kind or another were
widespread on northern continents, but now only three species exist.
Figure 18.17 Phylogeny
of Elephants and Some of Their Relatives
13. The fossil record for whales is now complete enough to verify that they evolved
from land-dwelling ancestors.
Figure 18.18 Relationships among Whales and Their Land-Dwelling Ancestors
Paleontologists look at the skeletal morphology of modern organisms and ancient fossils to find similarities and differences. Molecular biologists look at genes to see how many the modern and fossil organism had in common. Until recently these two groups had very different answers for the ancestor of whales. They did agree that cetaceans evolved from four-legged animals that roamed the land, with nostrils near the tips of their noses and teeth of several sizes and types. Cetaceans moved into the sea during the Eocene, almost 50 million years ago. Scientists, however, disagreed on other issues. The paleontologists thought that ancient whales were members of an extinct group of highly-specialized ungulates (hoofed mammals) called mesonychians. Molecular biologists favored artiodactyls (even-toed ungulates such as cows, pigs, camels, deer, and hippos) as the ancestors of cetaceans. Recently discovered complete cetacean fossils, 49 million years old, have brought paleontologists over to the side of the molecular biologists. This peace-making fossil was wolf-sized Pakicetus attocki, a meat eater, similar to modern dogs but with more a powerful tail, longer snout and smaller eyes. Pakicetus was a landlubber; adapted to walking and running on land, but also may have fed while wading in streams. Pakicetids are thought to be ancestral to cetaceans because of the structure of their ear bones and anklebones. ÒFrom Where the Whale?Ó Oct. 2001, www.studyworksonline.com.
Enrichment Topic 6.
Blue Whale Ancestor
A ferocious fossil whale with sharp, 3 cm long teeth has been found in Australia. The 25 million year fossils are from an early type of baleen whale, although the new fossil discovery did not have baleen. Instead, the whale was a powerful predator that captured its prey, perhaps sharks. The recent discovery has caused the rewriting of baleen whale evolution. In addition to sharp teeth, the whale also had large eyes, suitable for hunting. ÒReport: Blue Whale Ancestor Was No Gentle Giant,Ó Reuters, August 2006, http://www.cnn.com/2006/TECH/science/08/16/blue.whale.ancestor.reut/index.html
14. One important evolutionary trend in Pleistocene mammals and some birds was toward gigantism. Many of these large species died out, beginning about 40,000 years ago. Pleistocene mammalian fauna is remarkable in that so many large species existed.
Horses, camels, elephants, and other mammals spread across the northern continents during the Cenozoic because land connections existed between those landmasses at various times
Figure 18.19 The Irish Elk
Figure 18.20 Pleistocene Fossils from Florida and California
15. Changes in climate and prehistoric overkill are the two hypotheses explaining
Pleistocene extinctions.
Enrichment Topic 7.
Were Humans Responsible for Mammoth Extinction?
Dale Guthrie reported in 2006 that climate shifts were probably responsible for the mammoth extinctions, and not over-hunting by humans. He used radiocarbon dating of bones from bison, moose, and humans that survived the extinction, and bones of mammoth and wild horses that did not survive. Guthrie stated that radiocarbon dates showed that numbers of bison were actually expanding—before and during human colonization. He concluded that climate shifts were responsible, because they resulted in vegetation changes to which the wild horses and mammoths could not adapt. ÒScientist: Humans Not Responsible for Mammoth Extinction,Ó Reuters, May 13, 2006, http://www.cnn.com/2006/TECH/science/05/10/mammoth.extinction.reut/index.html
16. During most of the Cenozoic, South America was isolated, and its mammal fauna was unique. A land connection was established between the Americas during the Late Cenozoic, and migrations in both directions took place.
Figure 18.21 The Great American Interchange
Enrichment Topic 8. Did
Blond Mammoths Have More Fun?
An analysis of 43,000-year-old DNA from prehistoric mammoths indicated that some of them possessed blond fur, a finding that supported previous visual analyses of mammoth hair, which appears to come in a variety of colors. When investigating the DNA, researchers found a key pigmentation gene, Mc1r, which exists in two varieties. When researchers investigated further, they determined that one of the gene versions functioned better than the other for production of brown pigment. The researchers proposed that the weaker gene version produced lighter brown, blonde, or red shades. ÒMammoths Get Lighter: DNA Analysis Says There Were Probably Woolly Blondes,Ó News@Nature.com, July 10, 2006, http://www.nature.com//news/2006/060703-14.html
Life after the Extinction
Evolution of the Horse
The evolution of the horse is a great example to review evolution by gradualistic changes. There are several fossil intermediates of the horse. Discuss the evolutionary trends surrounding body size, teeth, elongation of the skull, length of limbs, and number of digits. Why might each of these changes have given the horse an advantage via natural selection?
CopeÕs Rule
1.
Is
the term ÒAge of Mammals" a fair assessment of the Cenozoic Era? Is there a better term that should be
used?
2.
Why
do you think the top marine predators of the Mesozoic were reptiles, but they
relinquished that role in the Cenozoic?
Artiodactyla Hyracotherium premolar
browser molar Primates
carnassials molarization Proboscidea
Carnivora Paleocene-Eocene Thermal ruminant
Cetacea Maximum ungulate
grazer Perissodactyla
Videos
Demonstration Aids and Slides
|
1. b |
5. b |
9. a |
|
2. a |
6. e |
10. b |
|
3. c |
7. b |
|
|
4. d |
8. c |
|
11. South America was isolated from all other landmasses from Late Cretaceous until a land connection with North America formed about 5 million years ago. Before the connection, South American fauna was made up of marsupials and several orders of placental mammals that lived nowhere else. However, when the Isthmus of Panama formed, migrations took place in both directions. As a result of the great American interchange, about 50% of South AmericanÕs modern mammals came from the north.
12. As they became grazers, the animalsÕ teeth became high-crowned and abrasion-resistant for chewing grasses. They evolved long, slender limbs as the bones between the wrist and toes and ankles and toes became longer. Their long limbs allowed them to see over grasses and to run fast with greater stride length. Their toes became weight-bearing.
13. Western North America has rocks of the right ages for fossil mammals but eastern rocks are predominantly older.
14. Large leaves with smooth,
entire margins characterize rainy, warm climates, while small leaves with
incised margins typify cooler, drier regions
15. In the early Cenozoic, one of the early adaptations in birds was the evolution of large, flightless predators such as Diatryma, which stood more than 2 meters tall. These early predators were widespread in North America and Europe during the Paleogene, and in South America they were the dominant predators until about 25 million years ago. Eventually they died out and were replaced by carnivorous mammals.
16. The three trends in whale evolution are (a) increase in body size, (b) modification of the front limbs to paddlelike flippers, and loss of the rear limbs (vestigial organs indicate their former presence and function), and (c) migration of the nostrils to the top of the head.
17. There are four evolutionary trends in
horses: (a) a decrease in the
number of toes, (b) an increase in size, (c) lengthening of the limbs, and (d)
the development of high-crowned teeth with complex chewing surfaces.
18. The Paleocene fossil ferns and palms in the western interior of North America indicate a warm, subtropical climate. About 55 million years ago a large scale oceanic circulation was disrupted, and an abrupt warming trend took place, known as the Paleocene-Eocene Thermal Maximum. Subtropical conditions persisted in the Eocene, probably the warmest of all Cenozoic epochs. A major climate change took place at the end of the Eocene, when mean annual temperatures dropped as much as 7oC in 3 million years. A general decrease in precipitation during the last 25 million years took place in the mid-continent of North American, and the vast forests of the Oligocene gave way to savannah conditions, then steppe conditions. Our Pleistocene Ice Age ended about 10,000 years ago, and temperatures have risen since then.
19. The Paleocene fauna are considered archaic because, although several orders of mammals were present, some were simply holdovers from the Mesozoic or belonged to new, but short-lived groups that have no living descendants.
20. Although dogs and hyenas are similar in appearance and are pack hunters, they are
only distantly related. Hyenas are more closely related to cats and mongooses and
their similarity to dogs is because of convergent evolution.
Apply Your Knowledge
1. One anomalous feature of this assemblage of fossils is the number of carnivores. With endothermic animals, the ratio of predator to prey is a low number (usually less than 5%), because endothermic animals have a large energy requirement. ÒSeveralÓ horses, camels, and mastodons would not be enough animals to sustain ÒdozensÓ of saber-toothed cats, dogs, and vultures (all predators). The organic mudstones and claystones indicate a shallow environment, perhaps a swampy area. If large animals came to this paleoenvironment to drink, they might have become mired or trapped in the area. Trapped animals and decaying carcasses would attract predators to the site, and these animals likely became trapped as well. The large occurrence of fossil carnivores is also noted at La Brea Tar Pits in Los Angeles, California, where a similar scenario likely played out in the Pleistocene.
2. Carnivorous mammals can be identified by their specialized shearing teeth, or carnassials, and sharp pointed canine teeth. In herbivorous mammals, the canines are not well developed, but instead the teeth show molarization to provide a continuous row of grinding teeth. Grazers, or animals that eat grass, evolved high-crowned cement-covered chewing teeth. Browsers, or animals that ate bushes and shrubs, have molars, but do not have the high-crowned teeth found in grazers. Speedy runners can be detected by the long, slender limb bones. As hoofed mammals evolved for speed, the bones between the wrist and toes and the ankle and toes became longer. In contrast, mammals such as rhinoceroses have sturdy limb bones to support their large weight. Plant fossils can be used to indicate ancient climates. The smaller leaves with incised margins are more indicative of cooler, drier areas, while leaves with entire or smooth margins and drip-tips dominate in areas with abundant rainfall and high annual temperatures.
3. Student answers will vary greatly. Example: The first vertebrates to evolve were fish. Eventually, fish with articulating bones in their fins evolved to survive in swampy environments. From these fish, limbs evolved via natural selection, and amphibians eventually moved onto the land. However, these amphibians, just like the frogs we have today, had to return to the water to lay their eggs. When reptiles evolved from amphibians, they had tough skin and were able to reproduce with shelled eggs. They did not need to return to the water to reproduce. From early reptiles, a group of mammal-like reptiles evolved before the dinosaurs. Although mammals did co-exist with dinosaurs, they did not dominate their area until the dinosaurs went extinct. Then, the mammals that survived the extinction rapidly diversified to fill the different environments on Earth.
4. Student answers will vary. Data that would help resolve the overkill hypothesis might include the discovery of large deposits of butchered extinct animalsÕ bones, whose dates would coincide with the extinction of these organisms. Conversely, if large deposits of extinct animalsÕ bones were found in areas with no signs of human habitation, then scientists would have to rethink the overkill hypothesis. Data that would support or refute the climate change hypothesis might be more difficult, because climate changes would affect more organisms, instead of a selective group (large North and South American mammals). If faunal and floral data could show that the continents most affected by extinction had a more severe climate change, this would support the hypothesis. Conversely, if data show that the climate fluctuations on the continents most affected by the extinctions were milder, this would help to refute the climate change hypothesis.
All Pages Copyright © GeoClassroom. All Rights Reserved.