Life of the Mesozoic

1
Life of the Mesozoic

• CHAPTER 12

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Introduction

• The Mesozoic Era is referred to as the "Age of
  Reptiles". During the Mesozoic, reptiles
  inhabited the land, the seas, and the air. The
  Mesozoic is the time in which the dinosaurs
  lived.
• The Mesozoic is also the time in which mammals
  and birds first appeared on Earth.

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Overview of Mesozoic Era age of reptiles

• Diverse Reptiles, including Dinosaurs
• Beginning of Evolution for Birds and Mammals
• Expansion of Grasses and Flowering Plants
• Climate, a Strong Influence
• Locations of continents
• Major sea-level changes
• Mountain building

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Mesozoic Climates

• Primary Control balance of incoming and outgoing
  solar radiation
• Factors affecting balance
• configuration and dimension of oceans and
  continents
• development and location of mountain systems and
  land bridges
• changes in snow, cloud, or vegetative cover
• carbon dioxide content of atmosphere
• location of poles (no ice caps)
• amount of radiation-aerosols contributed by
  volcanoes
• astronomic factors changes in Earths orbital
  parameters

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Triassic Climate

• Relatively cool
• Pangaea continents still clustered
• lower sea level
• mountain building, many highlands
• Paleo-equator central Mexico to northern Africa
• Wind shadow deserts (aridity) in continental
  interiors
• red beds
• evaporites

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Figure 11-3 (p. 384) Generalized
paleogeographic map for the Triassic of North
America.
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Jurassic Climate

• Relatively mild
• no glacial deposits
• evidence of monsoons and aridity
• coals in many spots, including Antarctica
• tropical conditions in wide belts
• Continents at latitudes of today
• Atlantic opening
• Tethys was an arm of proto-Pacific warm ocean
  currents flowed through Tethys

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Figure 11-7 (p. 387) Generalized
paleogeographic map for the Jurassic of North
America.
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Cretaceous Climate

• Relatively warm
• subtropical flora at 70o of equator
• high and low latitude coals
• high sea-level stand maximum inundation of
  Phanerozoic
• Continents near todays position
• Arctic Canada near north pole
• Antarctica at South Pole
• End-Cretaceous change
• rapid cooling and temporary vast chilling
• vast regression
• major mountain building
• plankton-produced CO2 shortage
• volcanic activity
• Terminal Cretaceous climatic event shown by
• tropical cycads sharply reduced
• hardy conifers and angiosperms expanded
• oxygen isotope studies of shells show ocean
  temperature decline began 80 m.y. ago
• strong connection to global extinctions at 65
  m.y. ago?

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Figure 11-12 (p. 392) Generalized
paleogeographic map for the Cretaceous of North
America.
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The Diversity of Life in the Mesozoic

• At the beginning of the Mesozoic Era, diversity
  (as indicated by the number of genera) was low,
  following the Permian extinctions. Recovery from
  the Permian extinctions was slow for many groups.
  
• In the oceans, the molluscs re-expanded to become
  much more diverse than in the Paleozoic, and
  modern reef-building corals, swimming reptiles,
  and new kinds of fishes appreared.
• A mass extinction occurred at the end of the
  Triassic Period. The Triassic extinction affected
  life on the land and in the sea, causing about
  20 of all marine animal families to become
  extinct.

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The Diversity of Life in the Mesozoic (cont)

• Diversity increased in the Jurassic, and rose
  quickly during the Cretaceous to higher levels
  than had existed previously.
• Much of this expansion in diversity was related
  to the appearance of new types of marine
  predators, including advanced teleost fishes,
  crabs, and carnivorous gastropods.
• Life in the Cretaceous consisted of a mixture of
  both modern and ancient forms.
• A major extinction event occurred at the end of
  the Cretaceous Period, affecting both vertebrates
  and invertebrates, on land and in the sea.

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Mesozoic Vertebrates

• Amphibians rise of modern forms
• Triassic Transition continuity among land
  animals
• Survivors of Permian extinction (245 m.y. ago)
• temnospondyl amphibians
• mammal-like reptiles (including therapsids,
  mammal ancestors)
• New reptile groups
• first turtles (toothed turtles)
• tuataran lizards
• archosaurs crocodiles, flying reptiles,
  thecodonts, dinosaurs
• Example basal archosaur Hesperosuchus

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Mesozoic Vertebrates (cont)

• Diapsid groups
• lepidosaurs snakes, lizards, and their ancestors
• archosaurs ornithischian and saurischian
  dinosaurs, flying reptiles (pterosaurs), and
  crocodilians.

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Basal Archosaurs

• Several groups of archosaurs were present during
  the Triassic, and they are referred to as "basal
  archosaurs" because they are at the starting
  point (or base) of archosaur evolution.

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Basal Archosaurs

• Basal archosaurs (formerly called thecodonts)
  were small, agile reptiles with long tails and
  short fore-limbs.
• Many were bipedal (walked on 2 legs). This freed
  their fore-limbs for other tasks such as catching
  prey, and later, flight.

Hesperosuchus
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Relationships among fossil and living reptiles
and birds
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Dinosaurs

• The name "dinosaur" comes from the Greek deinos
  "terrifying" and sauros "lizard".
• Dinosaurs appeared in the Late Triassic, about
  225 m.y. ago.
• The earliest dinosaurs were small. Many were less
  than 3 ft long.
• By the end of the Triassic, dinosaurs were up to
  20 feet long.
• They became much larger later in the Jurassic and
  Cretaceous.

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Dinosaurs

• Basal Archosaurs (Thecodonts) were the ancestors
  of the dinosaurs
• Dinosaurs were composed of two orders saurischia
  (lizard-hipped) and ornithischia (bird-hipped)
• Saurischia pelvic bones like thecodonts
• Ornithischia pubis parallel to ischium like
  birds
• Earliest dinosaurs saurischia (Traissic, 225
  m.y. old, Argentina)

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Dinosaurs

• Saurischian dinosaurs - lizard-hipped
• Ornithischian dinosaurs - bird-hipped

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Dinosaurs

• Comparison of the skulls and teeth of
  saurischian dinosaurs (A) and ornithischian
  dinosaurs (B).

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Figure 12-18 (p. 428) Major groups of dinosaurs.
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Saurischian Dinosaurs

• "Lizard-hipped". Pelvic structure like lizards.
• Both two-legged and four-legged types.
• Both herbivores and carnivores.
• Teeth extended around entire margin of jaws, or
  were limited to the front.
• Teeth adapted to cutting and tearing, but not
  chewing.
• Food was ground up in the gizzard, probably aided
  by stones the dinosaurs swallowed, called
  gastroliths.
• The earliest dinosaurs and their basal archosaur
  ancestors were saurischians.

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Saurischian Dinosaurs

• Two groups
• Theropods - bipedal carnivorous dinosaurs
• Sauropods - large quadrupedal herbivorous
  dinosaurs

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Coelophysis. One of the earliest known theropods
(about 6 feet long)
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Carnivorous saurischians (Theropods)

• Larger carnivores hind limbs robust claws on
  toes small fore limbs serrated teeth
• Deinonychus and Velociraptor Cretaceous
  predators
• Family Allosauridae Allosaurus (U.S.)
• Giant theropods Tyrannosaurus (13 m, 4 tons,
  North America) Gigantosaurus (Argentina)
  Carcharodontosaurus (Africa)

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Theropods

• Bipedal carnivorous dinosaurs
• Coelophysis
• Ornithomimus
• Giganotosaurus
• Allosaurus
• Tyrannosaurus
• Deinonychus
• Velociraptor

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Artist reconstruction of Velociraptor
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Sauropods

• Large 4-legged herbivorous dinosaurs - long
  necks

• Seismosaurus
• Argentinosaurus
• Nuoerosaurus
• Diplodocus

• Apatosaurus
• Brachiosaurus
• Supersaurus
• Ultrasaurus

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Sauropods and Prosauropods

• The prosauropods were the likely ancestors of the
  sauropods, and lived from Late Triassic to Early
  Jurassic.
• The front legs of the prosauropods were shorter
  than the hind legs, although they walked on four
  legs.
• The prosauropods were replaced by the giant
  sauropods in the Early Jurassic.

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Herbivorous saurischians or sauropodomorphs
(Jurassic-Cretaceous)

• Evolved from Late Triassic protosaurapod
  (Plateosaurus)
• Long necks, long tails, four-legged stance
• Apatosaurus (formerly Brontosaurus Jurassic,
  Colorado) 30 tons
• Brachiosaurus longer fore limbs to reach higher
  vegetation
• Supersaurus 80 to 100 tons
• Sauropodomorphs left extensive footprint record
  (track ways of Colorado, for example)
• Advantages of size avoid predators, slow
  temperature changes due to surface-to-mass ratio
  (gigantothermism)
• Expansion during Early Jurassic lasted until
  Early Cretaceous southern hemisphere
  sauropodomorphs contined into Late Cretaceous

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Nuoerosaurus
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Apatosaurus (Brontosaurus)
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Ornithischians (Late Triassic-Cretaceous)

• Evolved near the end of the Triassic
• "Bird-hipped"
• Pelvic structure resembles that of birds.
• All herbivores.
• Front teeth replaced by a beak for cropping
  vegetation
• Includes both two-legged (bipedal) and
  four-legged (quadrupedal) types.
• Front legs shorter indicating descent from
  two-legged forms (in four-legged forms).

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Ornithischian Dinosaurs

• Examples
• Ceratopsians
• Stegosaurs
• Ankylosaurs
• Ornithopods

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Ceratopsians
Triceratops
Styracosaurus
Microceratops
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Stegosaurs
The plates on the backs of stegosaurs may have
served as body temperature-regulating devices.
They may have been used as "radiators" to
dissipate body heat, or as "solar panels" to
catch the sun's rays.

• Stegosaurus

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Ankylosaurs
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Ornithopods

• Bipedal and quadrupedal herbivores
• Camptosaurus
• Iguanodon
• Pachycephalosaurus
• Hadrosaurs (or duck-billed dinosaurs) in the
  Cretaceous, such as Parasaurolophus,
  Edmontosaurus, Bactrosaurus, and Maiasaura

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Parasaurolophus (hadrosaur)
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Figure 12-32 (p. 435) Internal structure of the
skull crest of Parasaurolophus cyrtocristatus.
(From Hopson, J. A. 1975. Paleobiology 124.)
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Interesting Facts about Dinosaurs

• The oldest dinosaurs were discovered in Late
  Triassic beds in Argentina by paleontologist Paul
  Sereno. They are about 225 million years old.
  Eoraptor, one of the oldest dinosaurs, was only
  about 1 m long its teeth indicate it was
  carnivorous.
• Some dinosaurs apparently roamed in herds.

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Interesting Facts about Dinosaurs

• Dinosaurs showed sexual dimorphism. Skeletons of
  females may be distinguished from skeletons of
  males.
• Fossil dinosaur eggs with embryos inside have
  been found in the Gobi Desert of Mongolia.

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Interesting Facts about Dinosaurs

• The jaws of Tyrannosaurus could exert more than
  3000 pounds of biting force (compared with the
  lion, at "only" 937 pounds of biting force).
• Its tail was held out horizontally to the back,
  serving as a counterbalance to the forward part
  of the body.

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Interesting Facts about Dinosaurs

• Sauropods, with their long necks, apparently fed
  on vegetation high in the treetops. Their heads
  were relatively small, which avoided a heavy
  burden on the long necks.
• The large size of the sauropods provided an
  advantage in dealing with predators, and served
  to prevent body heat loss. (Large animals lose
  body heat slower than small animals.) Animals
  which preserve body heat as a result of their
  large size are called homeotherms.

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Interesting Facts about Dinosaurs

• Their footprints suggest that they walked on four
  legs--able to support weight on land.
• The rear feet rested on large "pads" like those
  of elephants.

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Interesting Facts about Dinosaurs

• Nests of dinosaur eggs suggest that some groups
  of dinosaurs cared for their young. The Maiasaura
  were apparently one group of dinosaurs which
  nurtured their young, as their babies stayed in
  the nests and grew after hatching.

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Interesting Facts about Dinosaurs

• Were dinosaurs warm blooded? Paleontologist
  Robert Bakker has argued since 1968 that
  dinosaurs were warm blooded like birds. If so,
  they would no longer be classified as reptiles.
• Lines of evidence for warm bloodedness include

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EcologyCold/Warm Blooded Debate

• All living reptiles are ectotherms. That is they
  are cold-blooded animals whose body temperature
  varies with the outside temperatures. Mammals and
  birds are endotherms. That is they are
  warm-blooded and maintain a constant body
  temperature, regardless of the outside
  temperature.
• Some dinosaurs may have been warm-blooded. What
  is the evidence for this?

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Warm-Blooded Evidence

• Brain size - endothermy necessary for having a
  large brain because a complex nervous system
  requires constant body temperature. Small
  carnivorous dinosaurs had relatively large
  brains, and are also clearly related to birds.
• Predator-prey ratios - Endotherms have a higher
  metabolism so they need to eat more than
  ectotherms. Thus in endothermic populations, the
  predator/prey ratio is lower than in ectothermic
  populations. Fossil evidence suggests that
  predator/prey ratios among dinosaurs are similar
  to that seen in present day mammal populations.

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Warm-Blooded Evidence (cont)

• Bone histology - Bones of some dinosaurs have
  numerous passageways that once contained blood
  vessels. This is more typical of endotherms than
  ectotherms.
• Apparent activity rates - High activity rates
  require endothermy. Dinosaurs are now known to
  have been a lot more active than previously
  thought.
• Stance

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Warm-Blooded Evidence (cont)

• Presence of feathers and hair Feathers and hair
  insulate and help endotherms maintain body
  temperature. Several dinosaur fossils show
  evidence of feathers or a down-like material. At
  least one species of pterosaur had hair or
  hair-like feathers.
• Isotopic analysis of bones

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Birds

• Birds are warm-blooded (endothermic), have wings
  and feathers, have a toothless beak, and lay
  eggs. Most can fly, but some are flightless.
• Bird bones are hollow, and not easily preserved.
• Birds may have evolved from basal archosaurs, or
  from small Triassic theropod dinosaurs. Both
  groups were bipedal and birdlike in the structure
  of their limbs, shoulder girdles, and skulls.
• Several theropod dinosaurs had feathers, hollow
  bones, and keeled breastbones.

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Birds

• Feathers evolved from reptilian scales. The
  earliest feathers may have been used for
  insulation, camouflage, or display, rather than
  flight.
• The best known bird fossil is Archaeopteryx

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Archaeopteryx

• Bird-like features of Archaeopteryx
• Feathers
• Wings
• Reptile-like features of Archaeopteryx
• Dinosaur-like skeleton
• Teeth
• Large tail
• Forelimbs with claws
• No breast bone (meaning that it would have been a
  weak flier)

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Archaeopteryx
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Origin of Birds

• Bird-like features are found in some dinosaurs,
  including feathers or protofeathers, in
  Sinosauropteryx prima, more than 120 million
  years old, and Caudipteryx zoui, a dinosaur with
  a feathered tail.
• The line between dinosaurs and birds has blurred
  with the new discoveries, so it is difficult to
  say when the first bird appeared.
• Birds probably appeared near the end of the
  Jurassic.
• Many different types of birds lived during the
  Cretaceous Period.

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Mammals

• Mammals evolved from mammal-like reptiles in the
  Late Triassic. Early mammals were rodent-like,
  and remained small throughout Mesozoic (smaller
  than housecats).
• Among the earliest mammals were Megazostrodon,
  Eozostrodon, and Morganucodon

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Figure 12-50 (p. 450) Restoration of
Morganucodon, an early mammal from the Late
Triassic of Wales.
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Mammal Characteristics

• Mammals are warm-blooded (endotherms), and are
  distinctive because they
• Have hair or fur
• Females have mammary glands that secrete milk to
  feed their young
• Fossils of early mammals, like the cynodonts
  (mammal-like reptiles), show evidence of "whisker
  pits" on the snout region of the skull,
  indicating that they were covered with hair or
  fur.

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Geographic Distribution Of Life Through Mesozoic

• Pangea existed as a large landmass through
  Triassic. Climate was relatively similar over a
  wide latitude range. These factors allowed for
  wide distributions of species over many different
  continents
• South America/Antarctica/Australia became island
  continents in Late Mesozoic
• Faunas on these continents began to develop
  independently of other continents
• Marsupials remained the dominant mammals in both
  South America and Australia
• Laurasian continents continued to have strong
  faunal interchanges until Cenozoic

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• A mass extinction occurred at the end of the
  Cretaceous Period that caused the disappearance
  of about 1000 genera of marine animals, and about
  25 of all known families of animals.

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• Many groups died out gradually, and others
  disappeared suddenly.
• The extinctions did not all happen
  simultaneously.
• On land, only small (less than 50 lb) animals
  survived.
• Of the reptiles, only turtles, snakes, lizards,
  crocodiles, and the tuatara (a reptile from New
  Zealand) survived the extinction.
• More than 75 of the marine plankton species
  disappeared at the end of the Cretaceous.

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• Animals both on land and in the sea were
  affected. The extinction at the end of the
  Cretaceous totally wiped out these groups
• Dinosaurs
• Pterosaurs (flying reptiles)
• Ammonoids (cephalopod molluscs)
• Large marine reptiles (ichthyosaurs, plesiosaurs
  mosasaurs)
• Rudists (bivalve molluscs)
• and many other invertebrate taxa

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• There were drastic reductions of these groups,
  wiping out entire families. Some of these groups
  had very few survivors
• Coccolithophores (calcareous phytoplankton)
• Planktonic foraminifera
• Radiolarians
• Belemnoids (cephalopod molluscs)
• Echinoids
• Bryozoans

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What caused the extinctions?

• There are many hypotheses to attempt to explain
  the cause of these extinctions. They can be
  divided into two groups
• Catastrophic external or extraterrestrial
  triggers for the event (comet, asteroid)
• Events occurring on the Earth, without outside
  influences

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Evidence for extraterrestrial causes?

• A thin layer of clay with a concentration of
  iridium is found at the boundary at the end of
  the Cretaceous Period (the boundary clay).
• Since iridium is more abundant in meteorites than
  in normal Earth's surface rocks, it was proposed
  that a large impact of an extra terrestrial
  object with the Earth at the end of the
  Cretaceous might have spread iridium around the
  globe.
• Other things may also have been responsible for
  the presence of the iridium, and all
  possibilities must be considered.

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Asteroid impact

• Alvarezs theory (1977) elemental iridium
  enrichment 30 times normal in terminal Cretaceous
  clay indicates vaporization of asteroid on impact
  with Earth iridium enrichment too great for
  terrestrial-source explanation bolide (comet,
  asteroid, meteorite) impact

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Other evidence for extraterrestrial causes?

• Shocked quartz (from an impact?)
• Tiny glass spherules or tektites (cooled droplets
  of molten rock from an impact?)
• Carbon soot (remnants of forests burned in a
  firestorm caused by an impact?)
• Antarctic fish kill
• 180 km diameter crater in Yucatan, Mexico

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If a bolide (large extraterrestrial object)
collided with the Earth, where is the impact
crater?

• The most likely location of an impact structure
  of the proper age is the Chicxulub structure, a
  buried circular crater-like structure on the
  Yucatan Peninsula of Mexico.

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Figure 12-63 (p. 456) Occurrences of the
iridium-rich sediment layer of the
Cretaceous-Tertiary boundary. (From Alvarez, W.
et al. 1990. Geol. Soc. Am. Special Paper 190
305-315.)
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K/T Boundary Clay (New Mexico)
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Closeup of K/T Bopundary Clay
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Possible Terrestrial Causes

• Volcanic eruptions causing ash and aerosols in
  atmosphere leading to a drop in temperature.
  Volcanism was widespread toward the end of the
  Cretaceous, and volcanic ash can be a source of
  iridium.
• Other elements in the boundary clay like antimony
  and arsenic are common in volcanic ash but not in
  meteorites.

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• Volcanic eruptions releasing sulfur dioxide,
  leading to sulfuric acid in the atmosphere and
  acid rain, changing the alkalinity of the oceans,
  and placing lethal stress on plankton at the base
  of the food chain, and indirectly affecting the
  organisms that depended on them for food.

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• Decrease in rate of seafloor spreading, leading
  to a sea level drop, which eliminated the
  epicontinental seas.There is evidence for a
  global lowering of sea level at the end of the
  Mesozoic. Disappearance of the epicontinental
  seas would have meant a habitat loss for many
  shallow water species.

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• Climatic change as a result of the lowering of
  sea level and disappearance of the epicontinental
  seas?Would have caused a harsher climate and
  more extreme seasonality.
• Change in atmospheric CO2 levels and O2 levels,
  as a result of the appearance of new types of
  plants, or the proliferation of photosynthetic
  plankton that formed the Cretaceous chalk
  deposits?

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• Appearance of angiosperms changed food chain on
  land? (Many of the dinosaurs ate gymnosperms.)
• Disease? Viruses?
• Melting of Arctic Ocean and spillover of
  freshwater cap onto world oceans, killing marine
  plankton?
• Magnetic reversal?
• Other?

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Extinct!

• Whatever the cause, changing environmental
  conditions at the end of the Mesozoic Era led to
  the disappearance of many kinds of organisms in
  what may have been a domino effect, as organisms
  at the base of the food chain were killed,
  sending waves of extinctions through species
  higher on the food chain that depended on them.

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The Mesozoic Era