Life of the Mesozoic Era main points

1
Chapter 15
Life of the Mesozoic Eramain points..1.marine
invertebrates that survived end Permian
extinction diversified and repopulated the
seas2. flowering plants evolve in Cretaceous3.
reptile diversification continued4. Mesozoic
Age of Dinosaurs lasted 140 million years!!5.
flying reptiles and marine reptiles develop,
become extinct at end Cretaceous6. birds evolved
from reptiles.7. mammals evolve from early
reptile, coexist along with reptiles8. fossil
evidence for transition from reptiles to mammals
exists9. Mesozoic mammalssmall, low
diversity10. proximity of continents and mild
climates allowed plants and animals to spread
widely. Plate tectonics would lead to population
isolation
2
Survivors

• Recall the Paleozoic extinctions
• that decimated the marine invertebrate faunas,
• causing a phenomenal decrease in biotic diversity
• The survivors of this crisis in life history
• diversified during the Triassic
• and repopulated the seas, accounting
• for the success of several types of
• cephalopods, bivalves, and several other
  invertebrates

3
Mesozoic Life Fascinates

• The animals existing during the Mesozoic Era
• fascinate nearly everyone
• Ever since Sir Richard Owen
• first used the term dinosaur in 1842,
• dinosaurs have been the objects of intense
  curiosity
• No other group of animals
• has so thoroughly captured the public
  imagination,
• but dinosaurs were only one type of Mesozoic
  reptile

4
The Age of Reptiles

• Other Mesozoic reptiles include
• flying reptiles
• marine reptiles,
• as well as turtles, crocodiles, lizards, and
  snakes
• Geologists informally call the Mesozoic
• "The Age of Reptiles,"
• calling attention to the importance of reptiles
• among land-dwelling animals

5
Many New Discoveries

• Scientists have investigated Mesozoic animals
• for more than 150 years,
• yet much knowledge of dinosaurs and their
  relatives
• comes from studies beginning during the 1970s
• Paleontologists make so many new discoveries
• augmenting knowledge of Mesozoic life
• that it is difficult to keep up with the current
  literature

6
Dinosaurs Movies

• Of course Mesozoic animals,
• especially dinosaurs,
• have been popularized in numerous books,
• TV specials, and movies
• such as Jurassic Park (1993)
• and its two sequels,
• The Lost World (1997)
• and Jurassic Park III (2001),
• as well as Dinosaur (2000)

7
Mammals Too

• The evolution and diversification of Mesozoic
  reptiles was certainly important,
• but so were several other events
• such as the origin of mammals during the Triassic
• Thus mammals and dinosaurs were contemporaries
• throughout the Mesozoic,
• but mammals were not particularly diverse
• and all were small creatures

8
Birds

• Birds also made their appearance,
• most likely evolving
• from small carnivorous dinosaurs
• during the Jurassic

9
Land Plants

• Important changes took place
• in land plant communities
• as the flowering plants evolved
• during the Cretaceous
• and soon became widespread and numerous
• The major groups of Paleozoic land plants
  persisted,
• but now they constitute less than 10 of all
  species

10
Systems Approach

• Here we continue to emphasize
• the systems approach to Earth and life history
• The distribution of land and sea
• profoundly influences oceanic circulation,
• which in turn partly controls climate
• The proximity or separation of landmasses
• partly determines the geographic distribution of
  organisms

11
Isolation

• Pangaea began fragmenting
• during the Triassic and continues to do so
• Organisms had increasing difficulty
• migrating between continents as a result
• In fact, South America and Australia
• became isolated island continents
• their faunas evolving in isolation
• became quite different from those elsewhere

12
Mesozoic Mass Extinctions

• Mass extinctions at the end of the Mesozoic,
• second in magnitude only to the Paleozoic
  extinctions,
• had a tremendous impact on the biosphere
• But because dinosaurs were among the victims,
• these extinctions have received
• much more attention than any other extinction
• So just as at the end of the Paleozoic Era,
• biotic diversity was sharply reduced,
• but once again many survivors evolved rapidly,
• giving rise to the Cenozoic fauna

13
Marine Invertebrates and Phytoplankton

• Following the Paleozoic mass extinctions,
• the Mesozoic was a time
• when marine invertebrates repopulated the seas
• The Early Triassic invertebrate fauna
• was not very diverse,
• but by the Late Triassic the seas
• were once again swarming with invertebrates
• from planktonic foraminifera
• to cephalopods

14
Brachiopods Never Fully Recover

• The brachiopods,
• that had been so abundant during the Paleozoic,
• never completely recovered from their near
  extinction
• Although they still exist
• the bivalves
• have largely taken over their ecological niche

15
Mollusks

• Mollusks such as
• cephalopods, bivalves, and gastropods
• were the most important elements
• in the Mesozoic marine invertebrate fauna
• Their rapid evolution
• and the fact that many cephalopods were nektonic
• make them excellent guide fossils
• Cephalopods were present throughout the Mesozoic
• but they were most abundant during the Jurassic
  and Cretaceous

16
Ammonoidea

• The Ammonoidea,
• cephalopods with wrinkled sutures,
• constitute three groups
• the goniatites, ceratites, and ammonites
• Ammonites, while present during the
• entire Mesozoic,
• were most prolific during the Jurassic
• and Cretaceous
• Most ammonites were coiled,
• some attaining diameters of 2 m,
• whereas others were uncoiled
• and led a near benthonic existence
• Cephalopods
• such as the Late Cretaceous ammonoids Baculites
• and Helioceros
• were important predators
• and excellent guide fossils

17
Surviving Cephalopods

• Ammonites became extinct
• at the end of the Cretaceous,
• but two related groups of cephalopods
• survived into the Cenozoic
• the nautiloids,
• including the living pearly Nautilus,
• and the coleoids, represented by extinct
  belemnoids
• which are good Jurassic and Cretaceous guide
  fossils
• as well as by the living squid and octopus

18
Bivalves

• Two Cretaceous bivalves Oysters and clams
• epifaunal suspension feeders
• Bivalves were particularly diverse and abundant
  during the Mesozoic
• Even today they remain important elements in the
  marine invertebrate fauna

19
Mesozoic Reef-Builders

• Where shallow marine waters were warm and clear,
• coral reefs proliferated, as they do today
• An important reef-builder throughout the Mesozoic
  
• was a group of bivalves known as rudists
• Rudists are important
• because they displaced corals
• as the main reef-builders during the later
  Mesozoic
• and are excellent guide fossils
• for the Late Jurassic and Cretaceous

20
Mesozoic Reef-Builders Rudistsvery large
clams..
21
Familiar Coral

• A new and familiar type of coral
• also appeared during the Triassic,
• the scleractinians
• Whether sclaractinians evolved from rugose corals
  
• or from an as yet unknown soft-bodied group of
  anthozoans
• with no known fossil record is still unresolved

22
Burrowing Organisms

• One of the major differences
• between the Paleozoic and Mesozoic
• marine invertebrate faunas
• was the increased abundance and diversity
• of burrowing organisms
• Paleozoic burrowers, with few exceptions,
• were soft-bodied animals such as worms
• The bivalves and echinoids,
• which were epifaunal elements
• during the Paleozoic,
• evolved various means of entering infaunal
  habitats

23
Escaping from Predators

• This trend toward an infaunal existence
• may reflect an adaptive response
• to increasing predation
• from the rapidly evolving fish and cephalopods
• Bivalves, for instance,
• expanded into the infaunal niche
• during the Mesozoic,
• and by burrowing
• they escaped predators

24
Mesozoic Primary Producers

• The primary producers in the Mesozoic seas
• were various types of microorganisms
• 1. Coccolithophores are an important group
• of phytoplankton
• that first evolved during the Jurassic
• and became extremely common during the Cretaceous

25
Coccolithophores

• Coccolithophores from the Gulf of Mexico
• of Miocene age

• of Miocene-Pliocene age

26
Diatoms

• 2. Diatoms
• which build their skeletons of silica,
• made their appearance during the Cretaceous,
• but they are more important
• as primary producers during the Cenozoic
• Diatoms are presently most abundant
• in cooler oceanic waters
• and some species inhabit freshwater lakes

27
Diatoms

• Diatoms from Upper Miocene rocks in Java

28
Dinoflagellates

• 3. Dinoflagellates were common during the
  Mesozoic and today are the major primary
  producers in warm water

• One of Eocene age from Alabama

• of Miocene-Pliocene age from the Gulf of Mexico

29
Foraminifera

• 4. The foraminifera
• single-celled consumers
• underwent an explosive diversification
• during the Jurassic and Cretaceous
• They are still diverse and abundant today
• The planktonic forms
• in particular
• diversified rapidly,
• but most genera
• became extinct at the end of the Cretaceous

30
Increasing Complexity

• The Mesozoic was a time of
• generally increasing complexity
• of the marine invertebrate fauna
• At the beginning of the Triassic,
• diversity was low and food chains were short
• Near the end of the Cretaceous, though,
• the marine invertebrate fauna was highly complex
• with interrelated food chains
• This evolutionary history
• reflects changing geologic conditions
• influenced by plate tectonic activity

31
PlantsPrimary Producers on Land

• Plants practice photosynthesis
• and thus lie at the base of the food chain on
  land,
• so we discuss them as a prelude
• to consideration of land-dwelling animals
• Just as during the Late Paleozoic,
• seedless vascular plants and gymnosperms
• dominated Triassic and Jurassic land-plant
  communities,
• and, in fact, representatives of both groups
• are still common

32
Seedless Vascular Plants and Gymnosperms

• Seedless vascular plants and gymnosperms
• were prolific
• until angiosperms
• replaced many of them
• during the Mesozoic

33
Gymnosperms

• Among the gymnosperms,
• the large seed ferns became extinct
• by the end of the Triassic,
• but ginkgos remained abundant
• and still exist in isolated regions,
• Conifers continued to diversify
• and are now widespread in some terrestrial
  habitats,
• particularly at high elevations and high latitudes

Ginkgo
34
Cycads

• A new group of gymnosperms
• known as cycads made its appearance
• during the Triassic
• These palm-like plants
• became widespread
• and now exist in tropical
• and semi-tropical areas

35
Angiosperms

• The long dominance of seedless plants and
  gymnosperms
• ended during the Early Cretaceous,
• perhaps the Late Jurassic,
• when many were replaced
• by angiosperms,
• or flowering plants
• Angiosperms probably evolved
• from specialized gymnosperms

Sapindopsis, Cecil County, Maryland
36
Angiosperms Success

• Several factors account for their phenomenal
  success,
• but chief among them is their method of
  reproduction
• Two developments were particularly important
• the evolution of flowers,
• which attract animal pollinators, especially
  insects
• and the evolution of enclosed seeds

37
More Than 90

• Seedless vascular plants and gymnosperms
• are important and still flourish in many
  environments
• in fact, many botanists regard ferns
• and conifers as emerging groups
• Nevertheless, a measure of the angiosperms'
  success is
• that today with 250,000 to 300,000 species
• they account for more than 90 of all land plant
  species,
• and they occupy some habitats
• in which other land plants do poorly or cannot
  exist

38
The Diversification of Reptiles

• Reptile diversification began
• during the Mississippian Period
• with the evolution of the protorothyrids,
• apparently the first animals to lay amniotic eggs
• From this basic stock of so-called stem reptiles,
  
• all other reptiles
• as well as birds and mammals evolved

39
Reptiles and Birds

• Relationships among fossil and living reptiles
  and birds

40
The Story of Reptile Diversification

• Recall that pelycosaurs
• were the dominant land vertebrates
• of the Pennsylvania and Permian
• Here we continue our story
• of reptile diversification
• with a group called archosaurs

41
Dinosaurs Orders

• All dinosaurs possess
• a number of shared characteristics,
• yet differ enough for us to recognize
• two distinct orders,
• the Saurischia
• and Ornithischia
• A distinctive pelvic structure
• characterizes each order

42
Distinctive Pelvic Structure

• Saurischian dinosaurs
• have a 1izardlike pelvis
• and are thus called lizard-hipped dinosaurs
• Ornithischians
• have a birdlike pelvis
• and are called bird-hipped dinosaurs

43
Dinosaur Cladogram

• Cladogram showing dinosaur relationships
• showing Pelvises of ornithischians and
  saurischians
• Among the several subgroups of dinosaurs

• theropods were carnivores
• and all others were herbivores

44
Common Ancestor

• For decades, paleontologists thought
• each order evolved independently
• during the Late Triassic,
• but they now conclude
• that both orders
• had a common ancestor
• much like archosaurs
• known from Middle Triassic rocks in Argentina

45
Dinosaur Ancestors

• These dinosaur ancestors were small
• less than 1 m long
• long-legged carnivores that walked
• and ran on their hind limbs,
• so they were bipedal,
• as opposed to quadrupedal animals
• that move on all four limbs

46
Dinosaurs

• Sir Richard Owen
• proposed the term dinosaur in 1842
• to mean "fearfully great lizard"
• although now "fearfully" has come to mean
• "terrible," thus the characterization of
  dinosaurs as
• "terrible lizards"
• But of course they were not terrible,
• or at least no more terrible
• than animals living today,
• and they were not lizards

47
Misconceptions about Dinosaurs

• It is true that many were quite large,
• in fact, dinosaurs varied from giants
• weighing several tens of metric tons
• to those no larger than a chicken
• To consider them poorly adapted
• is to ignore the fact
• that dinosaurs were extremely diverse
• and widespread for more than 140 million years!

48
Active and Cared for Their Young

• Although various media
• now portray dinosaurs as more active animals,
• the misconception that they were lethargic,
• dim-witted beasts persists
• Evidence now available indicates
• that some were quite active
• and perhaps even warm-blooded
• Some species probably cared for their young
• long after hatching,
• a behavioral characteristic most often associated
  
• with birds and mammals

49
Care of the Young

• Maiasaura, a Late Cretaceous ornithopod, nested
  in colonies in northern Montana

• In this scene a female leads her young to a
  feeding area

50
Questions Remain

• While many questions remain unanswered about
  dinosaurs,
• their fossils and the rocks containing them
• are revealing more and more
• about their evolutionary relationships and
  behavior

51
Saurischian Dinosaurs

• The Saurischians,
• or lizard-hipped dinosaurs,
• include two distinct groups
• known as theropods and sauropods
• All theropods
• were carnivorous bipeds
• ranging in size from tiny Compsognathus
• to giants such as Tyrannosaurus
• and similar species
• that might have weighed
• as much as 7 or 8 metric tons

52
Dinosaur Cladogram
53
Small Theropod Dinosaur

• Compsognathus weighed only 2 or 3 kg
• Bones found within its ribcage indicate it ate
  lizards

54
Tyrannosaurus

• The skull of Tyrannosaurus,
• another theropod,
• measured more than 1 m long

55
Velociraptor and Deinonychus

• The movie Jurassic Park and its sequels
• popularized some of the smaller theropods
• such as Velociraptor,
• a 1.8-m-long predator
• with large sickle-like claws on the hind feet
• This dinosaur and its somewhat larger relative
• Deinonychus probably used their claws
• in a slashing type of attack

56
Deinonychus

• Lifelike restoration of Deinonychus in its
  probable attack posture
• It was about 3 m long

57
Sauropods

• Included among the sauropods
• are the truly giant,
• quadrupedal herbivorous dinosaurs
• such as Apatosaurus, Diplodocus, and
  Brachiosaurus,
• the largest known land-animals of any kind
• Brachiosaurus,
• a giant even by sauropod standards,
• might have weighed as much as 75 metric tons,
• and partial remains
• indicate that even larger sauropods
• may have existed

58
Ornithischian Dinosaurs

• Scientists recognize five distinct groups of
  ornithischians
• ornithopods,
• pachycephalosaurs,
• ankylosaurs,
• stegosaurs,
• and ceratopsians

59
Dinosaur Cladogram
60
Ornithopod Dinosaurs

• Ornithopods include the duck-billed dinosaurs
• with flattened bill-like mouths
• They were especially numerous during the
  Cretaceous
• and several genera had head crests
• which might have served a variety of functions
• All ornithopods were herbivores
• and primarily bipedal
• with well-developed forelimbs
• that allowed them to walk
• in a quadrupedal fashion, too

61
Duck-Billed Dinosaurs

• Two dinosaurs from the Late Cretaceous
• with head crests,
• hollow, bony extensions of the skull

• Parasaurolophus
• Corythosaurus

62
Pachycephalosaurs

• The most distinctive feature
• of the pachycephalosaurs
• is their thick-boned,
• dome-shaped skull
• Perhaps males used these thick skulls
• in butting contests for dominance and mates
• The few known genera of pachycephalosaurs
• lived during the Late Cretaceous

63
Horned Dinosaurs

• Ceratopsians or horned dinosaurs
• have a good fossil record
• indicating that small Early Cretaceous animals
• were ancestors to large Late Cretaceous genera
• such as Triceratops
• which was a very common dinosaur in North America
• The later ceratopsians had huge heads,
• a large bony frill over the neck for muscle
  attachment,
• and a horn or horns on the skull

64
Triceratops

• Skeleton of the ceratopsian Triceratops
• in the Natural History Museum, London, England

65
Triceratops
66
Fossil Herds

• Fossil trackways
• and bone beds
• of ceratopsians
• indicate that these large,
• quadrupedal herbivores
• moved in herds

67
Stegosaurs

• The stegosaur known as Stegosaurus
• was a medium-sized, quadrupedal herbivore
• from the Jurassic Period
• Its most distinctive features include
• a spiked tail,
• used almost certainly for defense,
• and triangular plates on the back
• The exact arrangement
• of these plates is debated,
• but most paleontologists are convinced
• that they functioned
• as a device to absorb and dissipate heat

68
Dinosaur Cladogram
69
Ankylosaurs

• Finally the ankylosaurs
• were the most heavily armored of all dinosaurs
• All were quadrupedal herbivores
• and some were quite large
• Bony armor
• protected the animal's back, flanks,
• and top of the head
• The tail ended in a bony club
• that could undoubtedly deliver a crippling blow
• to an attacking predator
• Some ankylosaur species
• lacked this bony club

70
Ankylosaur

• The ankylosaur Euoplocephalus
• Note the heavy armor and bony club at the end of
  the tail

• The ankylosaur Sauropelta

71
Warm-Blooded Dinosaurs?

• Were dinosaurs endotherms
• warm-blooded
• like today's mammals and birds,
• or were they ectotherms
• cold-blooded
• as are all of today's reptiles?
• Almost everyone now agrees
• that some compelling evidence exists for dinosaur
  endothermy,

72
Opinion Is Divided

• but opinion is still divided among
• (1) those holding that all dinosaurs were
  endotherms
• (2) those who think only some were endotherms
  and
• (3) those proposing that dinosaur metabolism,
• and thus their ability to regulate body
  temperature,
• changed as they matured
• Bones of endotherms
• typically have numerous passageways that,
• when the animals are alive,
• contain blood vessels,
• but considerably fewer passageways are present in
  bones of ectotherms

73
Higher Metabolic Rates

• Endotherms must eat more
• than comparable sized ectotherms
• because their metabolic rates are so much higher
• Consequently, endothermic predators
• require large prey populations
• and thus constitute a much smaller proportion
• of the total animal population than their prey,
• usually only a few percent

74
Predators to Prey Proportion

• In contrast, the proportion
• of ectothermic predators to prey
• might be as high as 50
• Where data are sufficient
• to allow an estimate,
• dinosaur predators made up 3 to 5
• of the total population
• Nevertheless, uncertainties in the data
• make this less than a convincing argument
• for many paleontologists

75
Large Brain

• A large brain in comparison to body size
• requires a rather constant body temperature
• and thus implies endothermy
• Some dinosaurs were indeed rather brainy,
• especially the small- and medium-sized theropods

76
Insulation

• So brain size might be a convincing argument for
  these dinosaurs,
• but even more compelling evidence for theropod
  endothermy
• comes from their probable relationship to birds,
• and the rather recent discoveries in China
• of dinosaurs with feathers or a feather-like
  covering
• Today, only endotherms have
• hair, fur, or feathers for insulation

77
Flying Reptiles

• Paleozoic insects
• were the first animals to achieve flight,
• but the first among vertebrates were pterosaurs,
• or flying reptiles,
• which were common in the skies
• from the Late Triassic
• until their extinction at the end of the
  Cretaceous

78
Pterodactyls

• Pterodactyls, a long-tailed Late Jurassic
  pterosaur

• Among several known species,
• wingspan ranged from 50 cm to 2.5 m

79
Adaptations for Flight

• Adaptations for flight include
• a wing membrane
• supported by an elongated fourth finger
• light hollow bones,
• and development of those parts of the brain
• associated with muscular coordination and sight
• Because at least one pterosaur species
• had a coat of hair or hair-like feathers,
• possibly it was an endotherm
• as perhaps all pterosaurs were

80
Wings

• In all flying vertebrates,
• the forelimb has been modified into a wing
• A long 4th finger supports the pterosaur wing
• whereas in birds the 2nd and 3rd fingers are
  fused together
• and in bats, fingers 2 through 5 support the wing
• Are these wings analogous, homologous, or both?

81
Too Weak for Sustained Flapping

• Experiments and studies of fossils
• indicate that the wing bones of large pterosaurs
• such as Pteranodon
• were too weak for sustained flapping
• These comparatively large animals
• probably took advantage of rising air currents
• to stay airborne,
• mostly by soaring
• but occasionally flapping their wings for
  maneuvering

82
Pteranodon

• The short-tailed pterosaur
• known as Pteranodon
• was a large Cretaceous animal
• with a wingspan of more than 6 m

83
Mesozoic Marine Reptiles

• Several Mesozoic reptiles
• adapted to a marine environment
• including turtles and some crocodiles,
• as well as the Triassic mollusk-crushing
  placodonts
• Here, however, we concentrate on
• the ichthyosaurs, plesiosaurs, and mosasaurs
• All of these marine predators
• were thoroughly aquatic,
• but other than all being reptiles
• they are not particularly closely related

84
Ichthyosaurs

• The streamlined, rather porpoise-like
  ichthyosaurs
• varied from species measuring only 0.7 m long
• to 15-m-long giants
• Although details of their ancestry
• are still not fully known,
• they evolved from small animals
• whose fossils remind one researcher of
• "a lizard with flippers that still retained
  some key features of their land-dwelling
  ancestors1
• 1Motani, R. 2000 Rulers of the Jurassic Seas,
  Scientific American, v 283, no 6, p 55

85
Ichthyosaurs

• Restoration showing ichthyosaurs
• fully aquatic animals
• that evolved from land-dwelling ancestors

86
Ichthyosaurs

• They had numerous sharp teeth
• Preserved stomach contents reveal a diet of
• fish, cephalopods, and other marine organisms
• It is doubtful that ichthyosaurs
• could come onto land,
• so females must have retained eggs
• within their bodies
• and gave birth to live young
• A few fossils with small ichthyosaurs
• in the appropriate part of the body cavity
• support this interpretation

87
Mary Anning

• An interesting side note in the history of
  paleontology
• is the story of Mary Anning (1799-1847),
• who when only about 11 years old discovered
• and directed the excavation
• of a nearly complete ichthyosaur
• in southern England
• This and subsequent discoveries
• made her a well-known fossil collector

88
Plesiosaurs

• The plesiosaurs,
• another well-known group of Mesozoic marine
  reptiles,
• belonged to one of two subgroups
• short necked and long-necked
• Most were modest sized animals 3.6 to 6 m long,
• but one species
• found in Antarctica
• measures 15 m

89
Plesiosaurs

• Although the marine reptiles
• plesiosaurs
• were aquatic animals,
• their fipperlike forelimbs
• probably allowed them to come out onto land

90
Mosasaurs

• Mosasaurs were Late Cretaceous marine lizards
• related to the present-day
• Komodo dragon or monitor lizard
• Some species measured no more than 2.5 m long,
• but a few such as Tylosaurus were large,
• measuring up to 9 m
• Mosasaur limbs resemble paddles
• and were used mostly for maneuvering
• whereas the long tail provided propulsion

91
Tylosaurus

• Tylosaurus was
• a large,
• Late Cretaceous
• mosasaur
• It measured up to 9 m long

92
Mosasaur Skull

• Mosasaur skull on display
• in the Museum of Geology and Paleontology,
• University of Florence, Italy

93
Mosasaurs Were Predators

• All mosasaurs were predators,
• and preserved stomach contents indicate
• that they ate fish, birds, smaller mosasaurs,
• and a variety of invertebrates
• including ammonoids

94
Crocodiles

• By Jurassic time,
• crocodiles had become
• the most common freshwater predators
• All crocodiles are amphibious,
• spending much of their time in water,
• but they are well equipped for walking on land
• Crocodile evolution has been conservative,
• involving changes mostly in size
• from a meter or so in Jurassic forms
• to 15 m in some Cretaceous species

95
Turtles

• Turtles, too, have been evolutionarily
  conservative
• since their appearance during the Triassic
• The most remarkable feature of turtles
• is their heavy, bony armor
• turtles are more thoroughly armored
• than any other vertebrate animal, living or
  fossil
• Turtle ancestry is uncertain
• One Permian animal
• had eight broadly expanded ribs,
• which may represent the first stages
• in the development of turtle armor

96
Lizards, and Snakes

• Lizards and snakes are closely related,
• and lizards were in fact ancestral to snakes
• The limbless condition in snakes
• some lizards are limbless, too
• and skull modifications
• that allow snakes to open their mouths very wide
• are the main differences between these two groups
• Lizards are known from Upper Permian strata,
• but they were not abundant until the Late
  Cretaceous

97
From Reptiles to Birds

• Scientists were aware of a number of
  characteristics
• shared by reptiles and birds
• even before they found fossil evidence
• showing a relationship between the two groups
• For example, birds and reptiles
• lay shelled, yolked eggs,
• and both share a number of skeletal features
• such as the way the jaw articulates with the skull

98
Birds Do Not Closely Resemble Living Reptiles

• But of course birds have feathers
• whereas reptiles have scales
• or a tough, beaded skin
• Furthermore, birds do not closely resemble
• any living reptile,
• so why do scientists think they are justified
• when they claim that birds evolved from reptiles?

99
Archaeopteryx

• Fossil bird, Archaeopteryx
• from the Jurassic Solenhofen Limestone in Germany
• has feather impressions in the wings and long
  tail
• A wishbone and feathers made it a bird

100
Archaeopteryx

• In most anatomical details Archaeopteryx more
  closely resembled a small theropod
• It had claws on its wing and reptilian teeth
• but was a bird

101
Archaeopteryx

• These animals, known as Archaeopteryx
• are birds by definition,
• but their numerous reptilian features
• convince many scientists
• that they evolved from
• some kind of small theropod
• Even fused clavicles are found in several
  theropods,
• and recent discoveries in China of theropods
• with some kind of feathery covering
• provide more evidence of this relationship

102
Ancestor/Descendant Gap

• Opponents of the theropod-bird scenario
• point out that theropods
• are typically found in Cretaceous-aged rocks,
• whereas Archaeopteryx is Jurassic
• However, some of the fossils
• coming from China
• are about the same age as Archaeopteryx,
• thus narrowing the gap
• between presumed ancestor and descendant
• Nevertheless, opponents also note
• that some of the details
• of the theropod and bird hand differ

103
Bird Evolution

• A Mesozoic fossil, from China,
• is slightly younger than Archaeopteryx
• and possesses both primitive and advanced
  features
• It retains abdominal ribs
• similar to those of Archaeopteryx and theropods,
• but it has a reduced tail
• more typical of present-day birds
• Another Mesozoic bird, from Spain
• is also a mix of primitive and advanced
  characteristics,
• but it appears to lack abdominal ribs

104
Chinese Bird Dinosaurs!
Note the 4 wings!!...the preserved imprints of
feathers (Xu, Xing et al 2003. Four winged
Dinosaurs from China, Nature, vol 421, p. 335-340)
105
Bird-Reptile Relationship

• Another possible candidate for bird ancestry
• is a small lizardlike reptile
• known as Longisquama
• that was discovered during the 1960s in Kyrgystan
• Longisquama is probably an archosaur,
• and it too appears to have feathers,
• but some paleontologists
• think these structures are actually long scales
• In any event, the bird-reptile relationship is
  firmly established,
• but disagreement exists on the exact bird ancestor

106
Protoavis

• Archaeopteryx's fossil record is not good enough
• to resolve whether it is the actual ancestor of
  today's birds
• or an animal that died out without leaving
  descendants
• Of course, that in no way diminishes the fact
• that it had both reptile and bird characteristics
• However, some claim that fossils of two
  crow-sized individuals
• known as Protoavis
• represent an even earlier bird than Archaeopteryx

107
Cretaceous Climates

• Cretaceous climates were more strongly zoned by
  latitude,
• but they remained warm and equable
• until the close of that period
• Climates then became more seasonal and cooler,
• a trend that persisted into the Cenozoic
• Dinosaur and mammal fossils
• demonstrate that interchange was still possible,
• especially between the various components of
  Laurasia

108
Mass ExtinctionsA Crisis in the History of Life

• The greatest mass extinction took place
• at the end of the Paleozoic Era,
• but the one at the close of the Mesozoic
• has attracted more attention
• because among its casualties were
• dinosaurs, flying reptiles, and marine reptiles
• Several kinds of marine invertebrates
• also went extinct, including ammonites,
• which had been so abundant through the Mesozoic,
• rudistid bivalves, and some planktonic organisms

109
Mesozoic Extinctions

• Numerous hypotheses proposed
• to explain Mesozoic extinctions
• have been dismissed as improbable,
• untestable, or inconsistent with the available
  data,
• A proposal that has become popular since 1980
• is based on a discovery
• at the Cretaceous-Tertiary boundary in Italy
• a 2.5-cm-thick clay layer
• with a remarkably high concentration
• of the platinum group element indium
• High iridium concentrations
• have now been identified
• at many other Cretaceous-Tertiary boundary sites

110
Cretaceous-Tertiary Boundary

• At this Cretaceous-Tertiary boundary site
• in Italy,
• a 2.5-cm-thick clay layer (1 inch!!)
• shows a concentration
• much higher than expected
• of the platinum-group element iridium

111

• Closeup view of the boundary clay in the Raton
  Basin, New Mexico

112
Iridium Anomaly

• The significance of this iridium anomaly
• is that iridium is rare in crustal rocks
• but is found in much higher concentrations
• in some meteorites
• Accordingly, some investigators propose
• a meteorite impact to explain the anomaly,
• and further postulate that the meteorite,
• perhaps 10 km in diameter,
• set in motion a chain of events leading to
  extinctions

113
Impact Consequences

• According to the impact hypothesis,
• about 60 times the mass of the meteorite
• was blasted from the crust high
• into the atmosphere,
• and the heat generated at impact
• started raging forest fires
• that added more particulate matter
• to the atmosphere
• Sunlight was blocked for several months,
• causing a temporary cessation of photosynthesis,
• food chains collapsed,
• and extinctions followed

114
Boundary Sites

• Some Cretaceous-Tertiary boundary sites
• also contain soot
• and shock-metamorphosed quartz grains,
• both of which are cited
• as additional evidence of an impact

DSDP Site 718 US E Coast
115
Acid Rain

• Furthermore, with sunlight greatly diminished
• Earth's surface temperatures
• were drastically reduced,
• adding to the biologic stress
• Another proposed consequence of an impact
• is that sulfuric acid (H2SO4) and nitric acid
  (HNO3)
• resulted from vaporized rock and atmospheric
  gases
• Both would have contributed
• to strongly acid rain that might have had
• devastating effects on vegetation
• and marine organisms

116
Impact Site

• Some now claim that a probable impact site
• centered on the town of Chicxulub
• on the Yucatan Peninsula of Mexico
• has been found
• The 180-km diameter structure
• lies beneath layers of sedimentary rock
• and appears to be the right age

117
Meteorite Impact Crater

• Proposed meteorite impact crater
• centered on Chicxulub
• on the Yucatán Peninsula
• of Mexico

118
Evidence at Chicxulub

• Evidence supporting the conclusion
• that the Chicxulub structure
• is an impact crater includes shocked quartz,
• what appear to be the deposits of huge waves,
• and tektites,
• small pieces of rock
• that were melted
• during the proposed impact
• and hurled into the atmosphere

119
Terrestrial and Marine Extinctions

• Even if a meteorite did hit Earth,
• did it lead to these extinctions?
• If so, both terrestrial and marine extinctions
• must have occurred at the same time
• To date, strict time equivalence
• between terrestrial and marine extinctions
• has not been demonstrated
• The selective nature of the extinctions
• is also a problem

120
Impact Hastened Extinction?

• In the terrestrial realm,
• large animals were the most drastically affected,
  
• but not all dinosaurs were large,
• and crocodiles, close relatives of dinosaurs
• survived although some species died out
• Some paleontologists think that dinosaurs,
• some marine invertebrates,
• and many plants were already on the decline
• and headed for extinction
• before the end of the Cretaceous
• A meteorite impact
• may have simply hastened the process

121
Satisfaction?

• In the final analysis,
• Mesozoic extinctions
• have not been explained to everyones
  satisfaction
• Most geologists now concede
• that a large meteorite impact occurred
• but we also know that vast outpourings of lava
• were taking place in what is now India
• Perhaps these brought about
• detrimental atmospheric changes

122
Shallow Seas Withdrew

• Furthermore, the vast shallow seas
• that covered large parts of the continents
• had mostly withdrawn by the end of the
  Cretaceous,
• and the mild equable Mesozoic climates
• became harsher and more seasonal
• by the end of that era
• But the fact remains
• that these extinctions were very selective,
• and no single explanation accounts
• for all aspects of this crisis in life history

123
Cynodonts and the Origin of Mammals

• The transition from cynodonts to mammals
• is so well documented by fossils
• that classification of some
• as either reptile or mammal
• is difficult
• We can easily recognize living mammals
• as warm-blooded animals with hair or fur
• that have mammary glands and,
• except for the platypus and spiny anteater,
• give birth to live young

124
Skeletal Modifications

• Obviously these criteria are inadequate
• for classifying fossils
• for them, we must use skeletal structure only
• Several skeletal modifications
• characterize the transition
• from mammal-like reptiles to mammals
• but distinctions between the two groups
• are based largely on details of the middle ear,
• the lower jaw,
• and the teeth

125
Reptile and Mammal Jaws

• Reptiles have only one small bone
• in the middle ear the stapes
• while mammals have three
• the incus, the malleus, and the stapes
• Also, the lower jaw of a mammal
• is composed of a single bone called the dentary,
• but a reptile's jaw is composed of several bones
• In addition, a reptile's jaw
• is hinged to the skull at a contact
• between the articular and quadrate bones,
• while in mammals the dentary
• contacts the squamosal bone of the skull

126
Transition From Cynodonts to Mammals

• During the transition from cynodonts to mammals,
• the quadrate and articular bones
• that had formed the joint
• between the jaw and skull in reptiles
• were modified into the incus and malleus
• of the mammalian middle ear
• Fossils document the progressive enlargement
• of the dentary
• until it became the only element
• in the mammalian jaw

127
Mammal and Reptile Skulls
128
Transitional Cynodonts

• Likewise, a progressive change
• from the reptile to mammal jaw joint
• is documented by fossil evidence
• In fact, some of the most advanced cynodonts
• were truly transitional because they had a
  compound jaw joint consisting of
• (1) the articular and quadrate bones typical of
  reptiles and
• (2) the dentary and squamosal bones as in mammals

129
Differentiated Teeth

• Several other aspects of cynodonts
• also indicate that they were ancestors of mammals
• Their teeth were somewhat differentiated
• into distinct types
• in order to perform specific functions
• In mammals the teeth are fully differentiated
• into incisors, canines, and chewing teeth,
• but typical reptiles do not have differentiated
  teeth

130
Sets of Teeth

• In addition, mammals have
• only two sets of teeth during their lifetimes
• a set of baby teeth and the permanent adult teeth
• Typical reptiles have teeth
• replaced continuously throughout their lives,
• the notable exception being in some cynodonts
• who in mammal fashion had only two sets of teeth

131
Tooth Occlusion

• Another important feature of mammal teeth is
  occlusion
• that is, the chewing teeth meet surface to
  surface
• to allow grinding
• Thus, mammals chew their food,
• but typical reptiles, amphibians, and fish do not
• However, tooth occlusion
• is known in some advanced cynodonts

132
Secondary Palate

• Another mammalian feature,
• the secondary palate,
• was partially developed in advanced cynodonts
• This bony shelf
• separating the nasal passages from the mouth
  cavity,
• is an adaptation for eating and breathing at the
  same time,
• a necessary requirement for endotherms
• with their high demands for oxygen

133
Secondary Palate

• Views of the bottoms of skulls
• showing the progressive development
• of the bony secondary palate

• an early therapsid

• an early mammal

• a cynodont

134
Mosaic Evolution

• In short,
• some mammalian features
• evolved more rapidly than others
• Recall the concept of mosaic evolution major
  evolutionary changes tend to take place in
  stages, not all at once
• from earlier in the term

135
Mammals Diverged

• The early mammals diverged
• into two distinct branches
• One branch includes the triconodonts
• and their probable evolutionary descendants,
• the monotremes,
• or egg-laying mammals,
• which includes the platypus
• and spiny anteater of the Australian region

136
Early Mammals and Descendants

• Mammal evolution proceeded along two branches

• egg-laying mammals

• one leading to todays monotremes

• and the other led to

• marsupials and placental mammals

137
Second Branch

• The second evolutionary branch
• included the marsupial mammals,
• pouched mammals
• and the placental mammals and their ancestors,
• the euphantotheres
• All living mammals
• except monotremes
• have ancestries that can be traced
• back through this branch

138
Insectivora

• Placental mammals of the order Insectivora, in a
  scene from the Late Cretaceous

• None of these animals measured more than a few
  centi-meters long

139
Placental Mammals

• Placentals,
• other than bats and a few rodents,
• never reached Australia,
• thus explaining why marsupials
• continue to dominate the continent's fauna
• even today

140
Mesozoic Climates and Paleogeography

• Fragmentation of the supercontinent Pangaea
• began by the Late Triassic,
• but during much of the Mesozoic,
• close connections existed
• between the various landmasses
• The proximity of these land-masses alone,
• however, is not sufficient
• to explain Mesozoic bio-geographic distributions,
  
• because climates are also effective barriers
• to wide dispersal

141
Extensive Geographic Ranges

• During much of the Mesozoic,
• climates were more equable
• and lacked the strong north and south zonation
• characteristic of the present
• In short, Mesozoic plants and animals
• had greater opportunities
• to occupy much more extensive geographic ranges

142
Triassic Climate

• Pangaea persisted as a single unit
• through most of the Triassic
• The Triassic climate
• was warm-temperate to tropical,
• although some areas,
• such as the present southwestern United States,
• were arid
• Mild temperatures
• extended 50 north and south of the equator,
• and even the polar regions
• may have been temperate

143
Paleogeography of the World

• During the Triassic Period

144
Mild Jurassic Climate

• The mild Triassic climate persisted into the
  Jurassic
• Ferns, whose living relatives
• are now restricted to the tropics of southeast
  Asia,
• lived as far as 63 south latitude
• and 75 north latitude

145
Late Jurassic

• By the Late Jurassic,
• Laurasia had become partly fragmented
• by the opening North Atlantic,
• but a connection still existed
• The South Atlantic had begun to open
• so that a long, narrow sea
• separated the southern parts of Africa and South
  America,
• otherwise the southern continents were still
  close together

146
Dinosaurs Roamed Widely

• Dinosaurs roamed widely across Laurasia and
  Gondwana
• For example, the giant sauropod Brachiosaurus
• is found in western North America and eastern
  Africa
• Stegosaurs
• and some families of carnivorous dinosaurs
• lived throughout Laurasia and in Africa

147
Late Cretaceous

• By the Late Cretaceous,
• the North Atlantic had opened further,
• and Africa and South America
• were completely separated
• South America remained an island continent
• until late in the Cenozoic,
• and its fauna became increasingly different
• from faunas of the other continents
• Marsupial mammals reached Australia
• from South America via Antarctica,
• but the South American connection
• was eventually severed

148
Paleogeography of the World

• During the Late Cretaceous Period

149
Summary

• Among the invertebrates,
• survivors of the Paleozoic extinctions
• diversified and gave rise
• to increasingly diverse marine communities
• Some of the most abundant invertebrates
• were cephalopods, especially
• ammonoids, foraminifera,
• and the reef-building rudists
• Land-plant communities
• of the Triassic and Jurassic
• consisted of seedless vascular plants and
  gymnosperms

150
Summary

• The angiosperms, or flowering plants,
• evolved during the Early Cretaceous,
• diversified rapidly,
• and were soon the most abundant land-plants
• Dinosaurs evolved from small,
• bipedal archosaurs during the Late Triassic
• but they were most common
• during the Jurassic and Cretaceous periods

151
Summary

• All dinosaurs evolved from a common ancestor,
• but differ enough so that two distinct orders are
  recognized, the Saurischia and the Ornithischia
  
• Bone structure, predator-prey relationships,
• and other features have been cited
• a