1. Life of the Mesozoic
CHAPTER 12

2. 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.

3. 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

4. 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 Earth’s orbital parameters

5. 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

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

7. Jurassic Climate Relatively mild Continents at latitudes of today
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

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

9. Cretaceous Climate Relatively warm Continents near today’s position
subtropical flora at ± 70o of equator
high and low latitude coals
high sea-level stand: maximum inundation of Phanerozoic
Continents near today’s 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?

10. Figure 11-12 (p. 392) Generalized paleogeographic map for the Cretaceous of North America.

11. 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.

12. 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.

13. 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

14. Mesozoic Vertebrates (cont)
Diapsid groups
lepidosaurs: snakes, lizards, and their ancestors
archosaurs: ornithischian and saurischian dinosaurs, flying reptiles (pterosaurs), and crocodilians.

15. 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.

16. 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

17. Relationships among fossil and living reptiles and birds

18. 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.

19. 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)

20. Dinosaurs Saurischian dinosaurs - lizard-hipped
Ornithischian dinosaurs - bird-hipped

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

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

23. 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.

24. Saurischian Dinosaurs
Two groups:
Theropods - bipedal carnivorous dinosaurs
Sauropods - large quadrupedal herbivorous dinosaurs

25. Coelophysis. One of the earliest known theropods (about 6 feet long)

26. 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)

27. Theropods Bipedal carnivorous dinosaurs Coelophysis Ornithomimus
Giganotosaurus
Allosaurus
Tyrannosaurus
Deinonychus
Velociraptor

28. Artist reconstruction of Velociraptor

29. Large 4-legged herbivorous dinosaurs - “long necks”
Sauropods
Large 4-legged herbivorous dinosaurs - “long necks”
Apatosaurus
Brachiosaurus
Supersaurus
Ultrasaurus
Seismosaurus
Argentinosaurus
Nuoerosaurus
Diplodocus

30. 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.

31. 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

32. Nuoerosaurus

33. Apatosaurus (Brontosaurus)

34. 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).

35. Ornithischian Dinosaurs
Examples:
Ceratopsians
Stegosaurs
Ankylosaurs
Ornithopods

36. Ceratopsians
Triceratops
Styracosaurus
Microceratops

37. 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

38. Ankylosaurs

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

40. Parasaurolophus (hadrosaur)

41. Figure (p. 435) Internal structure of the skull crest of Parasaurolophus cyrtocristatus. (From Hopson, J. A Paleobiology 1:24.)

42. 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.

43. 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.

44. 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.

45. 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.

46. 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.

47. 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.

48. 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:

49. Ecology Cold/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?

50. 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.

51. 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

52. 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

53. 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.

54. 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

55. 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)

56. Archaeopteryx

57. 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.

58. 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

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

60. 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.

61. 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

62. 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.

63. 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.

64. Animals both on land and in the sea were affected
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

65. 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

66. 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

67. 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.

69. Asteroid impact
Alvarez’s 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

70. 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

71. 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.

72. Figure (p. 456) Occurrences of the iridium-rich sediment layer of the Cretaceous-Tertiary boundary. (From Alvarez, W. et al Geol. Soc. Am. Special Paper 190: )

73. K/T Boundary Clay (New Mexico)

74. Closeup of K/T Bopundary Clay

75. 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.

76. 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.

77. 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.

78. 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?

79. Appearance of angiosperms changed food chain on land
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?

80. 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.

81. The Mesozoic Era