Chapter 15: Life in the Cenozoic

 

Life in the Marine Realm

1. After the Cretaceous extinctions, the microplankton recovered and again thrived in the Paleocene oceans. Globergerinid forams, calcareous nannofossils, diatoms and radiolarians continued to be the dominant planktonic groups. The rudistid reef communities of the Cretaceous were replaced by modern scleroctinian corals in the Paleocene.

2. The ammonites were replaced by teleost (bony) fishes as the new predators in the marine environment. Sea urchins (such as sand dollars) evolved and became adapted for burrowing on sandy beaches.

3. Figure 15.40: The coin-sized nummulitid forams were the most common animals in the warm Tethys seaway during the early and middle Eocene. Their size was remarkable considering that their large, flat, spiral-shaped shells (reaching several centimeters across) were secreted by single-celled protozoans. Nummulitic limestone was used to build the great pyramids of Egypt. Most nummulitids became extinct in the late Eocene as a result of worldwide cooling.

4. Figure 15.35: During the Cenozoic, most of the common invertebrates we find today such as clams, echinoids and gasteropods, were abundant since the beginning of the Cenozoic.

5. The Mesozoic marine reptiles were replaced by Cenozoic marine mammals. Whales, seals, sea lions, walruses, and their relatives had evolved from bear-like ancestors by the late Oligocene and early Miocene.

 

Birds

1. The Cenozoic record for birds is generally poor because they are rarely preserved. The rather fragmental record indicates, however, that birds have been essentially modern in basic skeletal structure since the early Cenozoic. Other characteristic of early Cenozoic birds include a body covering of feathers, jaws in the form of a toothless horny beak and light, porous bones.

2. Figure 15.49: The fossil record also includes large terrestrial flightless birds. The Eocene Diatryma stood more than 2 meters (7 feet) tall. The Cenozoic dodo bird lived on a volcanic island near Madagascar until about 1700 AD when they were exterminated by sailors searching for provisions. The moas of New Zealand were also exterminated by humans.

3. The only surviving flightless land birds remaining today are the African ostrich, South American rhea, and theAustralian emus and cassowaries.

 

Age of Mammals

1. After evolving from Synapsids in the Late Triassic, mammals spent nearly 150 million years as shrew-sized beasts living primarily in the underbrush, many only coming out at night to search for food.

2. Figure 15.45: The Multituberculates were rodent-like mammals that appeared in the Jurassic and may have been the first entirely herbiverous mammals. Multituberculites survived until the Eocene.

3. Figure 15.43: The most primitive of all living mammals were the monotremes which appeared in the Jurassic and are represented today only by the platypus and spiny anteaters of Australia, Tasmania and New Guinea. Monotremes lay eggs in the reptilian manner but, unlike reptiles, have primitive mammary glands and provide newly hatched offspring with nourishment.

4. During the Early Cretaceous, mammals diverged into two major groups, theMarsupials (which nurture their young in a special pouch) and the Placentals (which include most modern mammals, including us).

5. Figure 15.43: When the K/T event wiped out the dinosaurs, a whole new world was opened to mammals and they came to dominate the earth in much the same way that reptiles had dominated the Mesozoic.

 

Early Cenozoic Mammals

1. Figure 15.43: During the early Paleocene, mammals began an explosive evolutionary radiation, diversifying and adapting to many different ecological niches. 18 mammalian orders had appeared by Paleocene time and diversified into 22 orders by the Eocene, including bats, whales, hoofed mammals, carnivores, rodents and rabbits. Some Eocene mammals had evolved to the size of a rhinoceros or elephant, others were small rodent-like creatures and most looked very different from those living today.

2. Figure 15.44: The Paleocene-Eocene subtropical world required that most mammals adapt to living in dense forest. Many early Cenozoic mammals were tree dwellers and fed on abundant leaves and seeds in the forest canopy or the floor beneath.

3. Lemur-like primates (15.45 A & B) and rodent-like multituberculates (15.45 C & D) were common. The multituberculates and most lemur-like primates became extinct by the end of the Eocene, their ecological niche taken over by rodents and rabbits.

 

Marsupial Mammals

1. Figure 3.9: Marsupials are mammals that nurture their young in a special pouch. The Marsupials emerged in the Early Cretaceous and are represented today by kangaroos, wallabies, wombats, Tasmanian wolf, phalangers, koalas of Australia and opossum.

2. The fossil record suggests that marsupials probably either originated in North America and migrated into South America during the Cretaceous, or vice versa. From South America, they apparently migrated via Antarctica to Australia before the end of the Cretaceous when these two continents separated from one another.

3. Marsupials had their greatest success in Australia and South America since both continents were relatively isolated during most of the Tertiary, allowing marsupials to evolve without excessive competition from placentals.

4. Figure 15.50: Then in the middle Pliocene, the Panamanian land bridge reconnected South America and North America, allowing North American placentals to migrate south. South American marsupials fared poorly in competition with placental immigrants from the North and many have since become extinct.

 

Placental Mammals

Figure 15.44: Early Cenozoic placentals included leaf-eating mammals which browsed along the ground. The pantodonts were the largest browsing ground dwellers of the Paleocene and Eocene, but afterwards became extinct. The uintatheres were the largest animals of the middle Eocene, had six knob-like horns on their heads and huge tusks.

 

Carnivores

1. Figure 15.44: The history of flesh-eating placental mammals began in the Cretaceous with the advent of creodonts that included small, weasel-like animals and bear-like or wolf-like varieties. Most creodonts became extinct by the end of the Eocene.

2. During the Late Cretaceous, creodonts were joined by members of the order Carnivora which have a common ancestry with the creodonts and are represented today by dogs, cats, bears, weasels, hyenas, seals and walruses.

3. Expansion of the Carnivora accelerated during the late Tertiary. Canines (e.g. dire wolf) were much in evidence during the Pleistocene as was the most famous of the extinct cats, the saber-toothed Smilodon.

4. Some carnivores, such as seals, sea lions, and walruses gathered their food at the edge of the sea. Others roamed the forests, rivers and grasslands either alone or in packs.

 

Cetaceans

1. Figure 15.41: Placentals also include cetaceans (whales and porpoises).

2. The whales probably evolved from a group of bear-like mammals, the mesonychids, which had skulls and teeth like primitive whales although they were not yet aquatic. Other scientists dispute this and claim the ancestors of whales were Artiodactyls (see below).

3. Figure 15.41: The earliest whales appeared in the Eocene and had already developed streamlined bodies with front flippers, although they lacked a blowhole on the top of their heads. The Eocene Basilosaurus was over 20 meters long.

4. In the Oligocene, whales evolved into the two main groups found today. The baleen whales (which include the gigantic blue whale) specialized in filter feeding while the other group, the toothed whales, ate squid and fish.

 

The Ungulates

1. The largest group of Cenozoic plant eaters were the ungulates, animals that walk upon hoofs and feed upon plants.

2. Figure 15.44: Ungulates of the Paleocene and Eocene were browsing ground dwellers that fed on plants and seeds from the floor of forests. The uintatheres were the largest of the middle Eocene browsing ground dwellers, had six knob-like horns on their heads and also huge tusks.

3. Figure 15.46: By Late Oligocene time, vast forests were replaced by grassland. The archaic browsing hoofed mammals were at a disadvantage and gradually became extinct. The archaic, hoofed mammals were replaced by early horses, rhinos, camels, deer, and elephants.

4. The Ungulates are subdivided into two orders, the Perissodactyls (odd-toed hoof mammals) and Artiodactyls (even-toed ungulates).

 

Ungulates (Perissodactyls)

1. The perissodactyls (or odd-toed mammals), include the modern horses, rhinoceroses, and the extinct chalicotheres and titanotheres.

2. Titanotheres existed from the Early Eocene to Early Oligocene. They evolved from small, sheep-sized Eocene ancestors (inset) into large beasts that stood about 2.5 meters high at the shoulder.

3. The Chaliocotheres appeared in the Miocene, having evolved from small ancestral mammals into large animals that superficially resembled horses, except that their feet had claws rather than hoofs. The claws were possibly used to hook and pull down branches.

Evolution of Horses

4. Figure 3.15: The oldest known perissodactyl, Hyracotherium, has been found in upper Paleocene and Eocene strata of both North America and England. Hyracotherium was the earliest member of the horse family, not much larger than a fox, had four hoofed toes on the front feet and three on the hind feet.

5. The horse family progressively evolved towards an increase in overall size, length of legs, and brain size. There were many different kinds of early horses, some specialized for forest browsing while others were fast grazers.

6. With the spread of grass-covered prairies during the Miocene, the family tree of horses became more complicated as more conservative species stayed behind in the forests while others took advantage of open environments by grazing and running.

7. Several different species of horses thrived during the Pleistocene. Some were the size of small ponies while others were larger than the great draft horses of today. Horses suffered extinction in North America a few thousand years ago, but have since been restocked by early European settlers.

8. The perissodactyls reached their peak of evolutionary history during the Miocene and have declined steadily since then.

Rhinoceroses

9. Figure 15.47: Among other surviving perissodactyls are the tapirs and rhinoceroses. The rhinoceroses began their evolution in the Eocene as small, swift-running creatures and eventually evolved into giants like the Oligocene/Miocene hornless rhinoceros Paraceratherium (also known asBaluchitherium.) which reached 6 meters (18 feet) at the shoulders and weighed over 10,000 kg.

10. During the Miocene, some rhinos became specialized for browsing leaves and bushes while others became fat amphibious grazers. Rhinos vanished from North America in the early Pliocene, possibly because of climatic changes.

 

Ungulates (Artiodactyls)

1. The even-toed artiodactyls have two or four toes on each foot. Some groups such as the pig-like mammals have changed little since the Eocene while others, such as the camels, deer, pronghorns, and cattle have changed dramatically.

2. The major radiation of artiodactyls began in the Eocene and ultimately produced three major surviving catagories: swine, camels and ruminants.

3. Much of the development of the camel lineage occurred in North America. Camels and llamas appeared in Eocene time as tiny creatures about the size of a small goat. As they evolved through the Tertiary, they lost their side toes and increased the length of their legs and neck.

4. By Pleistocene time, numerous modern-looking camels and llamas roamed North America. The llamas eventually migrated to South America via the Isthmus of Panama while the camel migrated across the Bering Land Bridge to Eurasia and Africa. Those left behind in North America mysteriously became extinct.

Ruminants

5. The ruminants have multichambered stomachs and are mostly cud chewers. The earliest ruminants were small, delicate, four-toed animals and are represented today by the mouse deer of Africa and Asia.

6. Deer originated as small, hornless creatures that lived in North America during the Miocene. Subsequent evolution involved increases in size and development of antlers. The Pleistocene Megaloceros, the remains of which are found in peat bogs in Ireland, had antlers reaching 3.6 meters in width.

7. The giraffe family probably branched off from the deer lineage sometime during the Miocene and became specialized in browsing on leaves of trees. The subsequent evolutionary trend of giraffes emphasized leg and neck elongation.

8. Cattle, sheep and goats are the most numerous of modern day ruminants. Several species of bison lived in North America during the Pleistocene. Some were truly giants with horns that measured 2 meters from end to end.

 

Proboscideans

1. Figure 15.48: The Proboscideans are mammals with trunks and include the modern elephants as well as older relatives, the mammoths and mastodons.

2. The earliest fossil record of proboscideans includes a trunkless, tapir-like animal named Moeritherium, whose bones were found in upper Eocene and lower Oligocene beds in Egypt.

3. Figure 15.48: Proboscidians eventually separated into two branches. One branch led toward a group of Miocene-Pliocene animals called dinotheres that had tusks only on the lower jaws. These tusks were curved downward, presumably useful for uprooting plants and digging for roots.

4. The other branch produced mastodons and elephants.

5. Figure 15.48: Proboscidean evolution also produced animals with shovel-like jaws.

6. The Asian elephant, African elephant and mammoth evolved from Gomphotheres during the Miocene. By Miocene time, Gomphotherium made its way into North America by way of the Bering isthmus and later evolved into the wooly mammoths of North America.

7. Mammoth is a term loosely applied to the Ice-Age elephants of North America, Europe and Africa that lived mostly during the Pleistocene. The great imperial mammoth of North America reached heights of 4.5 meters and roamed across California, Mexico and Texas. Pictures of wooly mammoths were drawn on the walls of caves by early humans.

8. The late Pleistocene proboscideans were hunted by humans in North America, Europe and Africa. About 8,000 years ago, all but two genera of elephants became extinct.

9. The decimation of proboscideans has been attributed to overkill by humans. Some scientists, however, also blame the warming climate that followed the ice age. Climatic warming caused retreat of the ice, leading to excessive aridity in many regions which placed severe stress on the large mammals.