Chapter 18 Life of the Cenozoic Era This

Chapter 18 Life of the Cenozoic Era This mural from the Smithsonian Institution depicts what life was like at Agate Beds National Monument, Nebraska, during the Miocene Epoch Cenozoic Biota Recall that when Earth formed it was hot, barren, and waterless, the atmosphere was noxious,

it was bombarded by meteorites and comets and no organisms existed During the Precambrian, and Paleozoic and Mesozoic eras, Earth and its organisms took on their present-day appearance Geologically Short Even though the Cenozoic constitutes only 1.4% of all geologic time,

this comparatively brief episode of only 66 million years of Earth and life history was certainly long enough for considerable change to take place Earths biota continues to change, although most of the changes are minor from our perspective, but nevertheless important Evolution of Mammals

Mammals evolved during the Late Triassic, so they were contemporaries with dinosaurs Some of the earliest mammals differed little from their ancestors, the cynodonts but as they evolved they became increasingly familiar During the Cenozoic,

they diversified into numerous types that adapted to nearly all terrestrial habitats as well as aquatic environments, and one group, the bats, became fliers Other Biological Events Other equally important biological events were taking place

Angiosperms, or flowering plants, evolved during the Cretaceous and soon became the dominant land plants, and now constitute more than 90% of all land plant species Their geographic distribution varied during the Cenozoic, depending on changing climates The first birds evolved during the Jurassic, but the families now common appeared during the Paleogene and Neogene,

reached their maximum diversity during the Pleistocene Epoch, and have declined slightly since then Increasingly Familiar Following the Mesozoic extinctions, marine invertebrates diversified giving rise to the present-day familiar marine fauna Overall, we can think of the Cenozoic Era as a time during which Earth's flora and fauna became more and more like what it is today

Cenozoic rocks are more easily accessible at or near the surface, so we know more about Earth and life history for this time than for any previous eras Good Fossil Records Cenozoic rocks are especially widespread in western North America, although they are also found along the Gulf and Atlantic coasts

As a result, we have a particularly good fossil record for many organisms Several of our national parks and monuments in the west feature displays of fossil mammals, including Agate Fossil Beds National Monument, Nebraska, Badlands National Park in South Dakota, and John Day Fossil Beds National Monument in Oregon John Day Fossil Restoration

Restoration of Clarno Formation fossils from Eocene age rocks in John Day Fossil Beds National Monument Oregon The climate at this time was subtropical and the lush forests of the region were occupied by early rhinoceroses tapirs Titanotheres standing 2.5m high at the shoulder ancient horses carnivores

Eastern Mammal Fossils Terrestrial deposits with land-dwelling fossil mammals are not nearly as common in the east, but Florida is a notable exception Nevertheless, some eastern and southern states such as Maryland, South Carolina, and Alabama have deposits with the fossils of Cenozoic marine mammals as well as fossil invertebrates and sharks

The Alabama state fossil is Basilosaurus cetoides, a fossil whale that lived during the Eocene Messel Fossil Beds Mammal fossils are found on other continents, too, but certainly one of the most remarkable fossil sites anywhere in the world is the Messel fossil beds in Germany Marine Invertebrates and Phytoplankton

The Cenozoic marine ecosystem was populated mostly by those plants, animals, and single-cell organisms that survived the terminal Mesozoic extinction Gone were the ammonites, rudists,

and most of the planktonic foraminifera Especially prolific Cenozoic invertebrate groups were the foraminifera, radiolarians, corals, bryozoans, mollusks, and echinoids More Provincial The marine invertebrate community in general became more provincial during the Cenozoic because of changing ocean currents and latitudinal temperature gradients

In addition, the Cenozoic marine invertebrate faunas became more familiar in appearance Species Diversified Entire families of phytoplankton became extinct at the end of the Cretaceous with only a few species in each major group surviving into the Paleogene

These species diversified and expanded during the Cenozoic, perhaps because of decreased competitive pressures Coccolithophores, diatoms, and dinoflagellates all recovered from their Late Cretaceous reduction in numbers to flourish during the Cenozoic Diatoms Diatoms were particularly abundant

during the Miocene, probably because of increased volcanism during this time Volcanic ash provided increased dissolved silica in seawater which diatoms used to construct their skeletons Massive Miocene diatomite rocks are present in several western States Diatomite Outcrop of diatomite from the Miocene

Monterey Formation, Newport Lagoon, California Diatoms A pinnate diatom and a centric diatom from the Monterrey Formation, California Foraminifera The foraminifera were a major component of the Cenozoic marine invertebrate community

Although dominated by relatively small forms, it included some exceptionally large forms that lived in the warm waters of the Cenozoic Tethys Sea Shells of these larger forms accumulated to form thick limestones, some of which ancient Egyptians used to construct the Sphinx and the Pyramids of Giza Cenozoic Foraminifera Benthonic

foraminifera of the Cenozoic Era Cibicides americanus Early Miocene, California Cenozoic Foraminifera A planktonic form Globigerinoides fistulosus

Pleistocene, South Pacific Ocean Cenozoic Foraminifera The numerous disc-shaped objects are Nummulites, a benthonic foraminifera in the limestone used to construct the pyramids on the Giza Plateau, Egypt CoralsReef Builders Again

Corals were perhaps the main beneficiary of the Mesozoic extinctions Having relinquished their reef-building role to rudists during the mid-Cretaceous, corals again became the dominant reef builders They formed extensive reefs in the warm waters of the Cenozoic oceans and were especially prolific in the Caribbean and Indo-Pacific regions

Coral The dominant reef-building animals of the Cenozoic Era are corals such as this modern colonial scleractinian Other Suspension Feeders

Other suspension feeders such as bryozoans and crinoids were also abundant and successful during the Paleogene and Neogene Bryozoans, in particular, were very abundant Perhaps the least important of the Cenozoic marine invertebrates were brachiopods, with fewer than 60 genera surviving today Brachiopods never recovered

from their reduction at the end of the Paleozoic Mollusks Just as during the Mesozoic, bivalves and gastropods were two of the major groups of marine invertebrates during the Cenozoic, and they had a markedly modern appearance After the extinction of ammonites and belemnites at the end of the Cretaceous,

the Cenozoic cephalopod fauna consisted of nautiloids and shell-less cephalopods such as squids and octopuses Bivalves The bivalve Chlamya sp. encrusted with barnacles from the Miocene of Virginia Gastropods

The gastropod Busycon contrarium from Pliocene rocks in Florica Echinoids Echinoids continued their expansion in the infaunal habitat and were particularly prolific during the

Cenozoic New forms such as this sand dollar evolved during this time from biscuitshaped ancestors Cenozoic Vegetation and Climate Angiosperms continued to diversify during the Cenozoic as more and more familiar types of plants evolved

although seedless vascular plants and gymnosperms were also present in large numbers In fact, many Paleogene plants would be familiar to us today but their geographic distribution was not what it is now because changing climatic conditions along with shifting plant distributions were occurring Changing Climatic Patterns Climate was a strong control on plant distribution in the past

Furthermore, leaves with entire or smooth margins, many with pointed drip-tips, are dominant in areas with abundant rainfall and high annual temperatures Smaller leaves with incised margins are more typical of cooler, drier areas Leaf Structure

For instance, leaves with entire or smooth margins, many with pointed drip-tips, are found mostly in areas with abundant rainfall and high annual temperatures Smaller leaves with incised margins are more typical of cooler, drier areas

Plant Leaves as Climatic Indicators Fossil floras with mostly smooth-margined leaves with drip-tips indicate warm, wet conditions while predominance of small leaves with incised margins and no drip-tips indicate a cool, dry climate Paleocene Flora Paleocene rocks in the western interior of

North America have fossil ferns and palms, both indicating a warm, subtropical climate In a Paleocene flora in Colorado with about 100 species of trees, nearly 70 percent of the leaves are smooth margined and many have drip-tips This nature of these leaves

coupled with the diversity of plants is much like that in todays rain forests Early Oligocene fossil plants indicate that a warm, wet climate persisted then Late Paleocene Thermal Maximum Seafloor sediments and geochemical evidence indicate that about 55 million years ago an abrupt warming trend took place During this time,

known as the Paleocene-Eocene Thermal Maximum, large-scale oceanic circulation was disrupted so that heat transfer from equatorial regions to the poles diminished or ceased As a result, deep oceanic water became warmer, resulting in extinctions

of many deep-water foraminifera Release of Methane Some scientists think that this deep, warm oceanic water released methane from seafloor methane hydrates, contributing a greenhouse gas to the atmosphere and either causing or contributing to the temperature increase at this time

Subtropical Conditions Subtropical conditions persisted into the Eocene in North America, probably the warmest of all the Cenozoic epochs Fossil plants in the Eocene John Day Beds in Oregon include ferns, figs, and laurels, all of which today live in much more humid regions, as in parts of Mexico and Central America

Warm Eocene Climate Yellowstone National Park in Wyoming has a temperate climate now with warm dry summers and cold snowy winters, certainly not an area where one would expect avocado, magnolia, and laurel trees to grow Yet their presence there during the Eocene indicates the area then had a considerably warmer climate than it does now

Major Climatic Change A major climatic change took place at the end of the Eocene when mean annual temperatures dropped as much as 7 degrees C in about 3 million years

Climatic Change Since the Oligocene, mean annual temperatures have varied somewhat worldwide, but overall have not changed much in the middle latitudes except during the Pleistocene Epoch Decrease in Precipitation A general decrease in precipitation during the last 25 million years took place in the midcontinent region

of North America As the climate became drier, the vast forests of the Oligocene gave way first to savannah conditions grasslands with scattered trees and finally to steppe environments short-grass prairie of the desert margin Herbivores Adapted Quickly Many herbivorous mammals

quickly adapted to these new conditions by developing chewing teeth suitable for a diet of grass Cenozoic Birds Birds today are diverse and numerous, making them the most easily observed vertebrates But the first members of many of the living orders, including owls, hawks, ducks, penguins, and vultures, evolved during the Paleogene

Beginning during the Miocene a marked increase in the variety of songbirds took place, and by 5 to 10 million years ago many of the existing genera of birds were present Recently Birds Diversity Decreased Birds adapted to numerous habitats and continued to diversify into the Pleistocene, but since then their diversity has decreased slightly

Birds Vary Considerably Today, birds vary considerably in habitat, adaptations, and size Nevertheless, their basic skeletal structure has remained remarkably constant throughout the Cenozoic Adaptations for Flying Given that birds evolved from a creature very much like Archaeopteryx

this uniformity is not surprising because adaptations for flying limit variations in structure Other Adaptations Penguins adapted to an aquatic environment, and in some large extinct and living flightless birds the skeleton became robust

and the wings shrank to vestiges Many authorities on prehistoric life are now convinced that birds are so closely related to dinosaurs that they refer to them as avian dinosaurs In fact, following the demise of the non-avian dinosaurs

at the end of the Mesozoic, the dominant large, land-dwelling predators during the Paleocene and well into the Eocene were flightless birds Diatryma Among these predators were giants such as Diatryma This remarkable bird

stood more than 2 m tall, had a huge head and beak, toes with large claws, and small vestigial wings Diatryma and Related Genera Its massive, short legs indicate that Diatryma was not very fast, but neither were the early mammals it preyed on

This extraordinary bird and related genera were widespread in North America and Europe and in South America they were the dominant predators until replaced by carnivorous mammals during the Oligocene Epoch Moas and Elephant Birds Two of the most notable large flightless birds were the now extinct moas of New Zealand and elephant birds of Madagascar

Moas were up to 3 m tall, elephant birds were shorter but more massive, weighing up to 500 kg They are known only from Pleistocene-age deposits, and both went extinct shortly after humans occupied their respective areas Fliers Large flightless birds are truly remarkable

creatures, but the real success among birds belongs to the fliers Even though few skeletal modifications occurred during the Cenozoic, a bewildering array of adaptive types arose If number of species and habitats occupied is any measure of success, birds have certainly been at least as successful as mammals

The Age of Mammals Begins Mammals coexisted with dinosaurs for more than 140 million years, yet during this entire time they were not very diverse, and even the largest was only about 1 m long Even at the end of the Cretaceous Period there were only a few families of mammals, a situation that was soon to change

Mammal Diversification With the demise of non-avian dinosaurs and their relatives, mammals quickly exploited the adaptive opportunities, beginning a diversification that continued throughout the Cenozoic Era The Age of Mammals had begun Easier to Study Cenozoic Fossils We have already mentioned

that Cenozoic deposits are easily accessible at or near the surface, and overall they show fewer changes resulting from metamorphism and deformation when compared with older rocks In addition, because mammals have teeth fully differentiated into various types, they are easier to identify and classify than members of the other classes of vertebrates Mammal Teeth

In fact, mammal teeth not only differ from front to back of the mouth, but they also differ among various mammalian orders and even among genera and species This is especially true of the chewing teeth, the premolars and molars A single chewing tooth is commonly enough to identify the genus from which it came

Class Mammalia All warm-blooded vertebrates with hair and mammary glands belong to the class Mammalia, which includes the monotremes, or egg-laying mammals the platypus and spiny anteater of the Australian region the marsupials, commonly called the pouched mammals kangaroos, opossums, wombats

and the placental mammals, about 18 orders Monotreme Mammals Female monotremes do secrete a milky substance that their young lick from the mothers hair, but both marsupials and placentals have true mammary gladns and milk to nourish their young. Monotremes have the requisite features

to be called mammals, but they appear to have had a completely separate evolutionary history from the marsupials and placentals Unfortunately they have a very poor fossil record Marsupial Mammals When the young of marsupials mammals are born,

they are in a very immature, almost embryonic state and then undergo further development in the mother's pouch Marsupials probably migrated to Australia, the only area in which they are common today, via Antarctica before Pangaea fragmented completely South American Marsupials They were also common in South America

during much of the Cenozoic Era until only a few million years ago When a land connection was finally established between the Americas, most of the South American marsupials died out as placental mammals from North America replaced them Now the only marsupials outside Australia and nearby islands are various species of opossums

Placentals Like marsupials, placental mammals give birth to live young, but their reproductive method differs in important details In placentals, the amnion of the amniote has fused with the walls of the uterus, forming a placenta

Marsupial Placenta Less Efficient Nutrients and oxygen flow from mother to embryo through the placenta, permitting the young to develop much more fully before birth Actually, marsupials also have a placenta, but it is less efficient, explaining why their newborn are not as fully developed

Success of Placental Mammals A measure of the success of placental mammals is related in part to their method of reproduction More than 90% of all mammals, fossil and extinct, are placentals Mammals Several orders of

mammals existed during the Mesozoic but most placental mammals diversified during the Paleocene and Eocene epochs Bold lines are actual Among the living

geologic orders of mammals ranges all are placentals except thinner for lines are the monotremes and inferred marsupials branching Diversification of Placentals

In our following discussion of placental mammals, we emphasize the origin and evolution of several of the 18 or so living orders During the Paleocene Epoch several orders of mammals were present, but some were simply holdovers from the Mesozoic or belonged to new but short-lived groups that have no living descendants

Archaic Mammals These so-called archaic mammals, such as various marsupials, insectivores, and the rodentlike multituburculates, occupied a world with several new mammalian orders, including the first rodents, rabbits, primates, and carnivores, and the ancestors of various hoofed mammals But most of these Paleocene mammals, even those belonging to orders that still exist, had not yet become clearly differentiated

from their ancestors, and the differences between herbivores and carnivores were slight Mammals Mammals Archaic Mammals of the Paleocene The archaic mammals of the Paleocene Epoch included such animals as

Protictus, an early carnivore Insectivores the pantodont Pantolambda, which stood about 1 m tall the treedwelling multiturbuculate Ptilodus

Mostly Small Creatures The Paleocene mammalian fauna was also made up mostly of small creatures By Late Paleocene time, though, some rather large mammals were around, although giant terrestrial mammals did not appear until the Eocene Giant Mammals Evolved With the evolution of a now extinct order known as the Dinocerata,

better known as uintatheres, and the strange creature known as Arsinoitherium, giant mammals of one kind or another have been present ever since Arsinoitherium Skull of Arsinoitherium a rhinoceros to elephant-sized Early Oligocene

animal with hollow horns more than 0.5 m long Uintatheres Scene from the Eocene showing the rhinocerossized mammals known as Uintatherium They had three pairs of bony protuberances

on the skull and saberlike upper canine teeth Mammalian Orders by the Eocene Many mammalian orders that evolved during the Paleocene died out, but of the several that first appeared during the Eocene only one has become extinct Thus by Eocene time

many of the mammalian orders existing now were present yet if we could go back for a visit we would not recognize most of these animals Only Vaguely Familiar Surely we would know they were mammals and some would be at least vaguely familiar, but the ancestors of horses, camels, elephants, whales, and rhinoceroses bore little resemblance

to their living descendants Archaic Paleocene Mammals Died Out Warm, humid climates persisted throughout the Paleocene and Eocene of North America but by Oligocene time drier and cooler conditions prevailed Most of the archaic Paleocene mammals as well as several groups

that originated during the Eocene had died out by this time The large, rhinoceros-like titanotheres died out, and the uintatheres were also extinct Other Groups Died Out In addition, several smaller groups of mammals suffered extinctions, including several types of herbivores loosely united as condylarths,

carnivorous mammals known as creodonts, most of the remaining multituburculates, and some primates All in all, this was a time of considerable biotic change Oligocene By Oligocene time, most of the existing mammalian orders were present, but they continued to diversify

as more and more familiar genera evolved If we were to encounter some of these animals we might think them a bit odd, but we would have little difficulty recognizing rhinoceroses, although some were hornless, elephants, horses, rodents, and many others Some Unfamiliar However, large horselike animals with claws known as chalicotheres, and the large piglike entelodonts

would be unfamiliar, and others would be found in areas where today we would not expect them elephants in the western United States, for instance Miocene and Pliocene By Miocene and certainly Pliocene time, most of the mammals were quite similar to those existing now On close inspection, though,

we would see horses with three toes, cats with huge canine teeth, deerlike animals with forked horns on their snouts, and very tall, slender camels We would still see a few odd mammals, but overall the fauna would be quite familiar Pliocene Pliocene mammals of western North

American grasslands Amebeledon, a shovel-tusked mastodon Teleoceras, a short-legged rhinoceros Cranioceras, a horned hoofed mammal

Pliocene a rodent a rabbit Merycodus, an extinct pronghorn Synthetoceras, a hoofed mammal with a horn on its snout Pliohippus, a one-toed grazing

horse Mammals of the Tertiary Period We know mammals evolved from mammal-like reptiles called cynodonts during the Late Triassic, and diversified during the Cenozoic, eventually giving rise to the present-day mammalian fauna

Now more than 5000 species exist, ranging from tiny shrews to giants such as whales and elephants Most Mammals Are Quite Small Nevertheless, when one mentions the term mammal, what immediately comes to mind are horses, pigs, cattle, deer, dogs, cats, and so on, but most often we do not think much about small mammals, rodents, shrews, rabbits, and bats Yet most species of mammals,

probably 70%, are quite small, weighing less than 1 kg Small Mammals Insectivores, rodents, rabbits and bats are all placental mammals and accordingly share a common ancestor, but they had separate evolutionary histories since they first evolved With the exception of bats, the oldest of which is found in Eocene rocks,

the others were present by the Late Mesozoic or Paleocene Small Mammals The main reason we consider them together is that with few exceptions they are small and have adapted to the microhabitats unavailable to larger mammals In addition to being small, bats are the only mammals capable of flight

An Insectivore Likely Gave Rise to Diverse Placental Mammals As you would expect from the order name Insectivora, members of this group, today's shrews, moles, and hedgehogs, eat insects Insectivores have probably not changed much since they appeared during the Late Cretaceous

In fact, an insectivore-like creature very likely lies at the base of the great diversification of placental mammals Rodents More than 40% of all existing mammal species are rodents (order Rodentia), most of which are very small A few, though, including beavers and the capybara of South America,

are sizable animals, the latter measuring more than 1 m long and weighing in excess of 45 kg Miocene Rodents One Miocene beaver known as Paleocastor was not particularly large, but it constructed some remarkable burrows, The Miocene rodent called ratzilla weighed an estimated 740 kg

Insectivora These spiral burrows in Miocene rocks in Nebraska were made by a land-dwelling beaver known as Paleocastor, which was about 30 cm long Insectivora This restoration of Phoberomys from Miocene rocks in South America

shows a huge rodent that weighed about 700 kg. Rodents Eat Almost Anything Rodents evolved during the Paleocene diversified rapidly and adapted to a wide range of habitats One reason for their phenomenal success is that they can eat almost anything Rabbits

Rabbits (order Lagomorpha) superficially resemble rodents but differ from them in several anatomic details Furthermore, since they arose from a common ancestor during the Paleocene rabbits and rodents have evolved independently Like rodents, rabbits are gnawing animals, although details of their gnawing teeth differ

Powerful Hind Limbs The development of long, powerful hind limbs for speed is the most obvious evolutionary trend in this group Fossil Bats The oldest fossil bat (order Chiroptera) comes from the Eocene-age Green River Formation of Wyoming, but well-preserved specimens are known from several other areas, too

Apart from having forelimbs modified into wings, bats differ little from their immediate ancestors among the insectivores Indeed, with the exception of wings they closely resemble living shrews Bat Wings Unlike pterosaurs and birds,

bats use a modification of the hand in which four long fingers support their wings A Brief History of the Primates The order Primates includes the lower primates tarsiers, lemurs, and lorises, and the living monkeys, apes, and humans,

collectively referred to as higher primates Primates Evolved Primates may have evolved by Late Cretaceous time, but by the Paleocene they were undoubtedly present Small Paleocene primates closely resembled their contemporaries, the shrewlike insectivores By the Eocene, though,

larger primates had evolved lemurs and tarsiers that resemble their present descendants lived in Asia and North America Primate Evolution By Oligocene time primitive New World and Old World monkeys had developed in South America and Africa, respectively

The Hominoids, the group that includes apes and humans, evolved during the Miocene The Meat Eaters Carnivorous Mammals The order Carnivora is extremely varied, consisting of present-day animals as different as bears, seals, weasels, skunks, dogs, and cats All are predators and therefore meat eaters,

but their diets vary considerably For example, cats rarely eat anything but meat, whereas bears, raccoons, and skunks have a varied diet and are thus omnivorous Carnivore Specialties Most carnivores have well-developed, sharp, pointed canine teeth and specialized shearing teeth known as carnassials for slicing meat

Some land-dwelling carnivores depend on speed, agility, and intelligence to chase down prey, but others employ different tactics Badgers, for instance, are not very fast they dig prey from burrows, and some small cats depend on stealth and pouncing to catch their meals Jaws of Large Cats This present-day skull and jaw of a large cat

show the specialized sharp-crested shearing teeth of carnivorous mammals The canine teeth are also large, but several Cenozoic saber-tooth cats had huge canine teeth The one shown here is the Oligocene Eusmilus specialized sharp-crested shearing teeth called carnassials

huge canine saber tooth Carnivores Are Less Common Fossils of carnivorous mammals are not nearly as common as those of many other mammals Why should this be so? First, in populations of warm-blooded (endothermic) animals,

carnivores constitute no more than 5% of the total population, usually less Carnivores Are Solitary Animals Second, many, but not all, carnivores are solitary animals, so the chance of large numbers of them being preserved together is remote Nevertheless, fossil carnivores are common enough for us to piece together

their overall evolutionary relationships with some confidence Carnivores Began to Diversify The order Carnivora began to diversify when two distinct lines evolved from creodonts and miacids during the Paleocene Both had well-developed canines and carnassials, but they were rather short-limbed and flat-footed

Certainly they were not very fast but neither was their prey Creodonts Died Out but Miacids Evolved Into Existing Carnivores The creodont branch became extinct by Miocene time, but the other branch evolving from miacids led to all existing carnivorous mammals Miacids

Todays carnivorous mammals evolved form a primitive group known as miacids This miacid, Tapocyon, was a coyote-sized animal that leaved during

the Eocene Carnivora Relationships among some carnivores Cats, Hyenas and Dogs Cats, hyenas, and viverrids (civits and mongooses) share a common ancestry,

but dogs are rather distantly related to the somewhat similar appearing hyenas In fact, dogs (family Canidae) and hyenas (family Hyenadae) not only are similar in appearance but also, with few exceptions, are pack hunters Cats, Hyenas and Dogs Nevertheless, the fossil record and studies of living animals clearly indicate hyenas

are more closely related to cats and mongooses Their similarity to dogs is another example of convergent evolution Saber-Toothed Cats One of the most remarkable developments in cats was the evolution of huge canines in the saber-toothed cats

Saber-toothed cats existed throughout most of the Cenozoic Era and are particularly well known from Pleistocene-aged deposits Aquatic Carnivores The aquatic carnivores, seals, sea lions, and walruses, are most closely related to bears, but unfortunately their ancestry is less well known

than for other families of carnivores Aquatic adaptations include a somewhat streamlined body, a layer of blubber for insulation, and limbs modified into paddles Most are fish-eaters and have rather simple, single-cusped teeth, except walruses which have flattened teeth for crushing shells

The Ungulates or Hoofed Mammals Ungulate is an informal term referring to several groups of living and extinct mammals particularly the hoofed mammals of the orders Artiodactyla and Perissodactyla Artiodactyla and Perissodactyla The artiodactyls, or even-toed hoofed mammals,

are the most diverse and numerous with about 170 living species of cattle, goats, sheep, swine, antelope, deer, giraffes, hippopotamuses, camels, and several others In marked contrast, the perissodactyls or odd-toed hoofed mammals, have only 16 existing species of horses, rhinoceroses, and tapirs Defining Characteristics of Hoofed Mammals

During the Early Cenozoic, though, perissodactyls were more abundant than artiodactyls Some defining characteristics of these groups are the number of toes and how the animal's weight is borne on their toes Their teeth are also distinctive Artiodactyls

Artiodactyls have either two or four toes, and their weight is borne along an axis that passes between the third and fourth digits For those artiodactyls with two toes, such as today's swine and deer, the first, second, and fifth digits have been lost or remain only as vestiges Toes of Hoofed Mammals Perissodactyls

Perissodactyls have one or three toes, although a few fossil species retained four toes on their forefeet Nevertheless, their weight is borne on a axis passing through the third toe Even today's horses have vestigial side toes and rarely they are born with three

toes Depend on Speed Many hoofed mammals such as antelope and horses depend on speed to escape from predators in their open-grasslands habitat As a result they have long slender limbs, giving them a greater stride length

The bones of the palm and sole have become very long Long Slender Limbs Long slender limbs in hoofed mammals evolved as the bones between the wrist and toes and ankle and toes became longer Fewer Bony Elements In addition, these speedy runners have fewer bones in their feet, mostly because they have fewer toes

Not all hoofed mammals are long-limbed, speedy runners Some are quite small and dart into heavy vegetation or a hole in the ground when threatened by predators Size for Protection Size alone is adequate protection in some very large species such as rhinoceroses

In contrast to the long, slender limbs of horses, antelope, deer, and so on, these animals have developed massive, weight-supporting legs Hoofed Herbivores All artiodactyls and perissodactyls are herbivorous animals with their chewing teeth, premolars and molars,

modified for a diet of vegetation One evolutionary trend in these animals was molarization, a change in the premolars so that they are more like molars, thus providing a continuous row of grinding teeth Grazers Versus Browsers Some ungulates, horses for example,

are characterized as grazers because they eat grass, as opposed to browsers, which eat the tender leaves, twigs, and shoots of trees and shrubs Grasses are very abrasive because as they grow through soil they pick up tiny particles of silt and sand that quickly wear teeth down

Low- and High-Crowned Teeth Once grasses had evolved, many hoofed mammals became grazers and developed high-crowned, abrasion-resistant teeth Low-crowned teeth are typical of many mammals with varied diets High-crowned, cement-covered

chewing teeth are adapted for grazing ArtiodactylsEven-Toed Hoofed Mammals The oldest known artiodactyls were Early Eocene rabbit-sized animals that differed little from their ancestors Yet these small creatures were ancestral to the myriad living and several extinct families

of even-toed hoofed mammals Relationships Among Artiodactyls Extinct Artiodactyls Among the extinct families are the rather piglike oreodonts so common in North America until their extinction during the Pliocene

and the peculiar genus Synthetoceras with forked horns on their snouts Camels During much of the Cenozoic Era, especially in North America, camels of one kind or another were quite common The earliest were small four-toed animals,

but by Oligocene time all had two toes Among the various types were very tall giraffe camels, slender gazelle-like camels, and giants standing 3.5 m high at the shoulder Camel Evolution Most camel evolution took place in North America, but during the Pliocene they migrated to Asia and South America,

where the only living species exist now North American camels went extinct near the end of the Pleistocene Epoch Bovidae Among the artiodactyls the family Bovidae is by far the most diverse, with dozens of species of cattle, bison, sheep, goats, and antelope

This family did not appear until the Miocene, but most of its diversification took place during Pliocene time on the northern continents Bovids in North America Bovids are now most numerous in Africa and southern Asia North America still has its share of bovids such as bighorn sheep and mountain goats,

but the most common ones during the Cenozoic were bison, which migrated from Asia, the pronghorn, and oreodonts, all of which roamed the western interior in vast herds Ruminants Most living artiodactyls are ruminants, which are the cud-chewing animals with complex three- or four-chambered stomachs

in which food is processed to extract more nutrients Perissodactyls lack such a complex digestive system Artiodactyls ruminants, cudchewing animals Most artiodactyls are ruminants, that is, cud-chewing animals Artiodactyls More Effective

Perhaps the fact that artiodactyls use the same resources more effectively than do perissodactyls explains why artiodactyls have flourished and mostly replaced perissodactyls in the hoofed mammal fauna Perissodactyls Odd-Toed Hoofed Mammals Earlier we noted that if we examine the fossil record of presumably related organisms

we should find that they were quite similar when they diverged from a common ancestor but became increasingly different as their divergence continued Perissodactyls Odd-Toed Hoofed Mammals Fossil records for horses, rhinoceroses, tapirs, and their extinct relatives the titanotheres and chalicotheres provide this kind of evidence

When these animals first appeared in the fossil record they differed slightly in size and structure of teeth, but as they evolved differences between them became more apparent Perissodactyls Diverged Then Declined Perissodactyls evolved from a common ancestor during the Paleocene

reached their greatest diversity during the Oligocene, and have declined markedly since then Fossil Record of Horses With the possible exception of camels, probably no group of mammals has a better fossil record than do horses Indeed, horse fossils are so common, especially in North America where most of their evolution took place,

that their overall history and evolutionary trends are quite well known Earliest Member of the Horse Family The earliest member of the horse family (family Equidae) is the fox-sized animal known as Hyracotherium This small forest-dwelling animal had four-toed forefeet and three-toed hind feet, but each toe was covered by a small hoof

Otherwise it possessed few of the features of present-day horses Hyracotheriums Link to Horses So how can we be sure Hyracotherium belongs to the family Equidae at all? Horse evolution was a complex, branching affair, with numerous genera and species existing at various times during the Cenozoic

Nevertheless, their exceptional fossil record clearly shows Hyracotherium is linked to the present-day horse, Equus, by a series of animals possessing intermediate characteristics Progressive Trend in the Development Late Eocene and Early Oligocene horses, followed by more recent ones show a progressive development

of various characteristics found in present-day Equus Horse Evolution Branched Horse evolution proceeded along two distinct branches One led to three-toed browsing horses, all now extinct, and the other led to three-toed grazing horses and finally to one-toed grazers The appearance of grazing horses,

with high-crowned chewing teeth coincided with the evolution and spread of grasses during the Miocene Evolution of Horses Summary chart showing genera of horses During the Oligocene two separate lines emerged

one leading to threetoed browsers and the other to onetoed grazers including the presentday horses Pony-Sized Merychippus Speed was essential in this habitat, and horses legs became longer and the number of toes was reduced finally to one Pony-sized Merychippus is a good example of the early grazing horses; it still had three toes,

but its teeth were high-crowned and covered by abrasion-resistant cement Horse Evolution Some evolutionary trends in horses include an increase in size lengthening of the limbs reduction in the number of toes

and development of high-crowned teeth with complex chewing surfaces Other Perissodactyls The other living perissodactyls, rhinoceroses and tapirs, increased in size from Early Cenozoic ancestors, and both became more diverse and widespread than they are now

Most rhinoceroses evolved in the Old World, but North American rhinoceroses were common until they became extinct at the end of the Pleistocene Largest Land-Dwelling Mammal Ever At more than 5 m high at the shoulder and weighing perhaps 13 or 14 metric tons, a hornless Oligocene-Miocene rhinoceros in Asia was the largest land-dwelling mammal ever

For the remaining perissodactyls, chalicotheres and titanotheres, only the titanotheres have a good fossil record Chalicotheres Chalicotheres, although never particularly abundant, are interesting though because the later members of this family were the size of large horses but their feet had claws on their feet

rather than hooves The prevailing opinion is that these claws were used to hook and pull down branches Titanotheres Titanotheres existed only during the Eocene, giving them the distinction of being the shortest lived perissodactyl family They evolved from small ancestors

to giants standing 2.5 m high at the shoulder Giant Land-Dwelling Mammals Elephants Some of todays elephants, order Proboscidea, are also giants The largest one on record weighed in at nearly 12 metric tons and stood 4.2 m at the shoulder, and some extinct mammoths were equally as large

or perhaps slightly larger Giant Land-Dwelling Mammals Elephants In addition to their size, a distinctive feature of elephants is their long snout, or proboscis; During much of the Cenozoic, proboscideans of one kind or another mastodons, mammoths, and todays elephants

were widespread on the northern continents, but now only three species exist, one in southeast Asia and two in Africa Early Elephant The earliest members of the order, was a 100- to 200-kg creature known as Moeritherium from the Eocene, that possessed few elephant characteristics It was probably aquatic

Elephant Tusks By Oligocene time, elephants existed that showed the trends toward large size, and had developed a long proboscis and large tusks which are enlarged incisors Most elephants developed tusks in the upper jaw only, but a few had them in both jaws, and one, the deinotheres, had only lower tusks

Elephants Phylogeny of elephants and some of their relatives Trends were increased size development of large tusks and a long

proboscis Several fossil elephants are not shown Mastodons and Mammoths The most familiar elephants, other than living ones, are the extinct mastodons and mammoths

Mastodons evolved in Africa, but from Miocene to Pleistocene time they spread over the Northern Hemisphere continents and one genus even reached South America These large browsing animals died out only a few thousands of years ago During the Pliocene and Pleistocene mammoths and living elephants diverged

Mammoths Mammoths were about the size of elephants today but they had the largest tusks of any elephant In fact, mammoth tusks are common enough in Siberia that they have been and continue to be a source of ivory Until their extinction near the end of the Pleistocene mammoths lived on all Northern Hemisphere continents

as well as in India and Africa Giant Aquatic Mammals Whales Our fascination with huge dinosaurs should not overshadow the fact that by far the largest animal ever is alive today At more than 30 m long and weighing an estimated 130 metric tons blue whales greatly exceed the size of any other living thing,

except some plants such as redwood trees But not all whales are large Not All Whales Are Large Consider, for instance, dolphins and porpoises both are sizable but hardly giants Nevertheless, an important trend in whale evolution has been increase in body size

Aquatic Mammals Several kinds of mammals are aquatic or semiaquatic, but only sea cows and whales, order Cetacea, are so thoroughly aquatic that they cannot come out onto land Land-Dwelling Ancestors Fossils discovered in Middle Eocene rocks in Pakistan indicate that land-dwelling ancestors of whales

were among the mesonychids, wolf-sized, meat-eating mammals Transition from Land Dweller During the transition from land-dwelling animals to aquatic whales, the front limbs modified into paddlelike flippers, the rear limbs were lost, the nostrils migrated to the top of the head and a large, horizontal tail fluke used for propulsion developed

Whale Cladogram Cladogram showing the relationships among some fossil and living whales and their land-dwelling ancestors Pakicetus had well-developed hind limbs, but only vestiges remain in Protocetus and Basilosaurus

Years of Little Fossil Evidence For many years, paleontologists had little fossil evidence that bridged the gap between land-dwelling animals and fully aquatic whales This important transition took place in a part of the world where the fossil record was poorly known But beginning 15 or 20 years ago,

paleontologists have made some remarkable finds that resolved this evolutionary enigma Eocene Whales The Early Eocene whale Ambulocetus still had limbs capable of support on land, whereas Basilosaurus, a 15 m long Late Eocene whale, had only tiny, vestigial rear limbs Basilosaurus still had teeth similar to those of their ancestors

and its nostrils were on the snout, but it was truly a whale, although very differently proportioned from those living now Eocene Whales Restoration of Protocetus and Basilosaurus Although Basilosaurus was a fully aquatic whale it differed considerably from todays whales Baleen and Toothed Whales By Oligocene time,

both presently existing whale groups baleen whales and toothed whales had evolved An interesting note on fossil whales is that during the 1840s, Albert Koch claimed he found a sea serpent in Eocene rocks in Alabama The nearly complete skeleton was of an ancient whale Koch used vertebrae from 5 different animals

to render a sea serpent nearly 35 m long, but the scientific community was not fooled Pleistocene Faunas As opposed to fauna of the Paleocene Epoch with its archaic mammals, unfamiliar ancestors of living mammalian orders, and large predatory birds, the fauna of the Pleistocene consists mostly of quite familiar animals Even so, their geographic distribution

might surprise us because rhinoceroses, elephants, and camels still lived in North America, and a few unusual mammals, such as chalicotheres and the heavily armored glypotodonts, were present Pleistocene Avian Fauna In the avian fauna, giant moas were in New Zealand and elephant birds were in Madagascar Mammals of the Ice Age

The most remarkable aspect of the Pleistocene mammalian fauna is that so many very large species existed Mastodons, mammoths, giant bison, huge ground sloths, immense camels, and beavers 2 m tall at the shoulder were present in North America South America had its share of giants, too, especially sloths and glyptodonts

Mammals in Pleistocene Europe, Asia, and Australia Elephants, cave bears, and giant deer known as Irish Elk lived in Europe and Asia, and Australia had 3 m tall kangaroos and wombats the size of rhinoceroses Megaloceros The giant deer Megaloceros

giganteus commonly called Irish Elk lived in Europe and Asia during the Pleistocene Large males had antler spreads of nearly 4 m Cooler ConditionsLarger Sizes Of course, many smaller mammal species

also existed, but one obvious trend among Pleistocene mammals was large body size Perhaps this was an adaptation to the cooler conditions that prevailed during that time Large animals have less surface area compared to their volume and thus retain heat more effectively than do smaller animals

Frozen Mammals Some of the world's best-known fossils come from Pleistocene deposits You probably have heard of the frozen mammals found in Siberia and Alaska, such as mammoths, bison, and a few others These extraordinary fossils, although very rare, provide much more information than most fossils do

Frozen Baby Mammoth Frozen baby mammoth found in Russia in 1989 Recovered From Permafrost Contrary to what you might hear in the popular press, all of these frozen animals were partly decomposed, none were fresh enough to eat, and none were found in blocks of ice or icebergs

All were recovered from permanently frozen ground known as permafrost Pleistocene Fossil Localities Paleontologists have recovered Pleistocene animals from many places in North America, but two noteworthy areas are Florida and the La Brea Tar pits at Rancho La Brea in southern California

In fact, Florida is one of the few places in the eastern United States where fossils of Cenozoic land-dwelling animals are common Pliocene and Pleistocene Mammals of Florida Among the diverse Pliocene and Pleistocene mammals of Florida were 6-m-long giant sloths

armored animals known as glyptodonts that weighed more than 2 metric tons La Brea Tar Pits The tar is naturally formed asphalt, whereas tar is a product manufactured from peat or coal At the La Brea tar pits at least 230 kinds of vertebrate animals were found trapped in the sticky residue

where liquid petroleum seeped out at the surface and then evaporated Many of the fossils of are carnivores, especially dire wolves, saber-toothed cats, and vultures that gathered to dine on various animals mired in the tar Pleistocene La Brea Tar Pits Restoration of a mammoth trapped in the sticky tar at a present-day oil seep

Pleistocene Extinctions During the Pleistocene, the continental interior of North America was teeming with horses, rhinoceroses, camels, mammoths, mastodons, bison, giant ground sloths, glyptodonts, saber-tooth cats, dire wolves, rodents, and rabbits Beginning about 14,000 years ago, many of these animals become extinct, especially the larger ones

Pleistocene Extinctions These Pleistocene extinctions were rather modest compared to those of the Paleozoic and Mesozoic eras, but they were unusual in that they had a profound effect on large, land-dwelling mammals Lost Genera Particularly hard hit were Australia and the

Americas In Australia, 15 of the continent's 16 genera of large mammals died out, North America lost 33 of 45 large-mammal genera, and in South America 46 of 58 such genera went extinct In contrast, Europe lost only 7 of 23 such genera, and in Africa south of the Sahara only 2 of 44 died out

Extinctions These data bring up three questions: (1) What caused Pleistocene extinctions? (2) Why did these extinctions eliminate mostly large mammals? (3) Why were extinctions more severe in Australia and the Americas? Scientists are currently debating two competing hypotheses Extinction Hypotheses

One, the climatic change hypothesis holds that rapid changes in climate at the end of the Pleistocene caused extinctions, whereas another hypothesis called prehistoric overkill holds that human hunters were responsible Climate and Vegetation Changes Rapid changes in climate and vegetation

did occur over much of Earth's surface during the Late Pleistocene, as glaciers began retreating The North American and northern Eurasian open-steppe tundras were replaced by conifer and broadleaf forests as warmer and wetter conditions prevailed Climate and Vegetation Changes The Arctic region flora changed from a productive herbaceous one

that supported a variety of large mammals, to a relatively barren water-logged tundra that supported a much sparser fauna The southwestern U.S. region also changed from a moist area with numerous lakes, where saber-toothed cats, giant ground sloths, and mammoths roamed, to a semiarid environment unable to support a diverse fauna of large mammals Why Didn't

Large Mammals Migrate? Rapid changes in climate and vegetation can certainly affect animal populations, but the climate hypothesis presents several problems First, why didn't the large mammals migrate to more suitable habitats as the climate and vegetation changed? After all, many other animal species did Mammal Migration in Europe

For example, reindeer and the Arctic fox lived in southern France during the last glaciation and migrated to the Arctic when the climate became warmer Argument Against the Climatic Hypothesis The second argument against the climatic hypothesis is the apparent lack of correlation between extinctions

and the earlier glacial advances and retreats throughout the Pleistocene Epoch Previous changes in climate were not marked by episodes of mass extinctions Arrival of Humans Proponents of the prehistoric overkill hypothesis, argue that the mass extinctions in North and South America and Australia coincided closely with the arrival of humans

Perhaps hunters had a tremendous impact on the faunas of North and South America about 11,000 years ago because the animals had no previous experience with humans The same thing happened much earlier in Australia soon after people arrived about 40,000 years ago Scattered Communities No large-scale extinctions

occurred in Africa and most of Europe, because animals in those regions had long been familiar with humans One problem with the prehistoric overkill hypothesis is that archaeological evidence indicates the early human inhabitants of North and South America, as well as Australia, probably lived in small, scattered communities, gathering food and hunting

Extinctions on Oceanic Islands How could a few hunters decimate so many species of large mammals? However, it is true that humans have caused major extinctions on oceanic islands For example, in a period of about 600 years after arriving in New Zealand, humans exterminated several species of the large, flightless birds called moas

Hunters Concentrate on Small Animals A second problem is that present-day hunters concentrate on smaller, abundant, and less dangerous animals The remains of horses, reindeer, and other smaller animals are found in many prehistoric sites in Europe, whereas mammoth and woolly rhinoceros remains are scarce

Other Arguments Finally, few human artifacts are found among the remains of extinct animals in North and South America, and there is usually little evidence that the animals were hunted Countering this argument is the assertion that the impact on the previously unhunted fauna was so swift as to leave little evidence

Multiple Reasons The reason for the extinctions of large Pleistocene mammals is still unresolved and probably will be for some time It may turn out that the extinctions resulted from a combination of different circumstances Populations that were already

under stress from climatic changes were perhaps more vulnerable to hunting, especially if small females and young animals were the preferred targets Intercontinental Migrations The mammalian faunas of North America, Europe, and northern Asia exhibited many similarities throughout the Cenozoic Even today, Asia and North America

are only narrowly separated at the Bering Strait, which at several times during the Cenozoic formed a land corridor across which mammals migrated Land Connection During the Early Cenozoic, a land connection between Europe and North America, allowed mammals to roam across all the northern continents Many did;

camels and horses are only two examples Separate Island Continents However, the southern continents were largely separate island continents during much of the Cenozoic Africa remained fairly close to Eurasia, and, at times, faunal interchange between those two continents was possible For example, elephants first evolved in Africa,

but they migrated to all the northern continents South America South America was isolated from all other landmasses from the Late Cretaceous until a land connection with North America formed about 5 million years ago Before the connection was established,

the South American mammalian fauna was made up of marsupials and several orders of placental mammals that lived nowhere else These animals thrived in isolation and showed remarkable convergence with North American placental mammals Isthmus of Panama

When the Isthmus of Panama formed, migrants from North America soon replaced most of the indigenous South American mammals whereas fewer migrants from the south were successful in North America As a result of this great American interchange today about 50% of South Americas mammalian fauna came from the north, but in North America only 20% of its mammals

came from the south The Great American Interchange When the Isthmus of Panama formed during the Pliocene, many placental mammals

migrated south Many South American mammals became extinct Marsupials Most of the living species of marsupials are restricted to the Australian region

Marsupials occupied Australia before its complete separation from Gondwana, but apparently placentals, other than bats and a few rodents, never got there until they were introduced by humans

Unlike South America, Australia has remained isolated and its fauna is unique Summary 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

During much of the Early Cenozoic, North American was covered by subtropical and tropical forests but the climate became drier by Oligocene and Miocene time, especially in the midcontinent region Summary 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 Evolutionary history is better known for mammals than for other classes of vertebrates because mammals have a good fossil record Their teeth are so distinctive and Cenozoic deposits are easily accessible Summary

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 shrewlike ancestors that existed from Late Cretaceous to Paleogene time

Summary Small mammals such as insectivores, rodents, and rabbits occupy the microhabitats unavailable to larger mammals Bats, the only flying mammals, have forelimbs modified into wings but otherwise differ little from their ancestors Summary Most carnivorous mammals

have well-developed canine teeth and specialized shearing teeth, although some aquatic carnivores such as seals have peglike teeth The most common ungulates are the even-toed hoofed mammals (artiodactyls) and odd-toed hoofed mammals (perissodactyls), both of which evolved during the Eocene Summary Many ungulates show evolutionary trends

such as molarization of the premolars as well as lengthening of the legs for speed During the Paleogene, perissodactyls were more common than artiodactyls but now their 16 living species constitute less than 10% of the world's hoofed mammal fauna Summary 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 Even though horses, rhinoceroses, and tapirs as well as the extinct titanotheres and chalicotheres do not closely resemble one another, fossils show that they diverged from a common ancestor during the Eocene

Summary The fossil record for whales verifies that they evolved from land-dwelling ancestors Elephants evolved from rather small ancestors, became quite diverse and abundant, especially on the Northern Hemisphere continents, and then dwindled to only three living species Summary

Horses, camels, elephants, and other mammals spread across the northern continents during the Cenozoic because land connections existed between those landmasses at various times South America was isolated during most of the Cenozoic and its mammal fauna was unique Summary A land connection was established

between the Americas during the Late Cenozoic and migrations in both directions took place One important evolutionary trend in Pleistocene mammals and some birds was toward giantism Many of these large species died out beginning about 40,000 years ago

Summary Changes in climate and prehistoric overkill are the two hypotheses explaining Pleistocene extinctions

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