I. Overview of Mesozoic Era: "Age of Reptiles"
A. Diverse Reptiles including Dinosaurs
B. Beginning of Evolution for Birds and Mammals
C. Expansion of Grasses and Flowering Plants
D. Climate was strong influence
1. locations of continents
2. major sea level changes
3. mountain building
II. Mesozoic Climates
A. Primary Control: Balance of incoming and outgoing solar radiation
1. Factors affecting balance
a. configuration and dimension of oceans and continents
b. development and location of mountain systems and land bridges
c. changes in snow, cloud, or vegetative cover
d. carbon dioxide content of atmosphere
e. location of poles (no ice caps)
f. amount of radiation-aerosols contributed by volcanoes
g. astronomic factors: Changes in Earth’s axis and orbit
2. Factors favoring cooler climates (opposite favors warmer climates)
a. vast continental areas
b. low sea level
c. uplifting of mountains
d. glacial development and polar position of continents
e. lower CO2 in atmosphere
f. changes in Earth’s axis or orbit that limit solar input
B. Triassic
1. Relatively cool
a. Pangaea continents still clustered
b. sea level lower
c. mountain building, many highlands
2. Paleo-equator: Central Mexico to northern Africa
3. Wind shadow deserts (aridity) in continental interiors
a. red beds
b. evaporites
C. Jurassic
1. Relatively mild
a. no glacial deposits
b. evidence of monsoons and aridity
c. coals in many spots including Antarctica
d. tropical conditions in wide belts
2. Continents at latitudes of today
a. Atlantic opening
b. Tethys was arm of proto-Pacific: Warm ocean currents flowered through Tethys.
D. Cretaceous
1. Relatively warm
a. subtropical flora at 70 degrees of equator
b. high and low latitude coals
c. high sea level stand: Maximum inundation
2. Continents near today’s position
a. Arctic Canada near north pole
b. Antarctica at South Pole
3. End-Cretaceous change
a. rapid cooling and temporary vast chilling
b. Vast regression
c. major mountain building
d. plankton produced CO2 shortage
e. volcanic activity
4. Terminal Cretaceous Climatic Event
a. tropical cycads sharply reduced
b. hardy conifers and angiosperms expanded
c. oxygen isotope studies of shells show ocean temperature decline began
80 m.y.a
d. strong connection to global extensions at 65 m.y.a.
III. Mesozoic Flora: Base of the Food Pyramid
A. Plant Animal Connection
1. Plants provide
a. starches, oils, sugars
b. oxygen by products (needed for respiration)
2. Animals provide
a. carbon dioxide by products (needed for photosynthesis)
b. waste production
B. Atmosphere Plant Interaction: McLean’s 1984 hypothesis
1. Massive outpouring of CO2 during eruption of Deccan Traps and other same
age lavas loaded atmosphere with greenhouse gas
2. Atmosphere contributed excess CO2 to upper 45 to 100 m of ocean waters
3. Excess CO2 in upper ocean poisoned key plankton (foraminifera and
coccolithophorids) which thus became extinct
4. Loss of such a flora base of marine food chain led to mass extinction’s of
higher marine fauna is Late Cretaceous, a "biotic crisis"
C. Marine Plants (Phylum Chrysophyta)
1. Characteristics
a. suspended in water
b. no vascular or support system
c. typically unicellular or colonial
2. Geologic Record of Phytoplankton (floating plants)
a. Pre-Mesozoic: Not mineralized (cyanobacteria, green algae,
architarchs)
b. Mesozoic: Mineralized coverings (coccolithophorids, dinoflagellates,
silicoflagellates, diatoms)
3. Most Important Mesozoic Phytoplankton (microfossils -0.1 mm and smaller)
a. Dinoflagellates (Jurassic-present): Sporopollenin walls; cysts preserved
as microfossils
b. Coccolithophorids (Jurassic-Cenozoic): Calcium carbonate plates
(coccoliths) cover cell; accumulation of coccoliths called marine calcareous
ooze
c. Silicoflagellates (Cretaceous-present): Diatoms having siliceous cell
coverings; spiny and stellate forms; covering frustule (epitheca = upper
half, hypotheca = lower half)
D. Terrestrial Plants (Mesozoic was also: "Age of Cycads")
1. cycads (Phylum Cycadophyta): Seed plants in which true flowers have not
developed
a. Cycaeloids (fossil cycads)
b. Cycadales (true cycads, e.g. sajo palm)
2. Conifers: 6 families in Jurassic-Cretaceous (note 39 living examples of
Mesozoic conifers were recently found, the "Wollemi pines" of Australia)
3. Angiosperms: Seed and flower bearing
a. Mid-Cretaceous - present
b. Birch, maple, walnut, beech sassafras, poplar, willow
c. abundance and diversity surpassed non-flowering plants
4. Impact of Mesozoic Floral Revolution
a. coevolution of insects, birds, reptiles, and mammals
b. production of seeds, nuts, fruits
c. advent of flora color due to competition for pollination
IV. Mesozoic Invertebrates
A. Terrestrial Invertebrates: Limited to preservation record
1. Pulmonate (air breathing) snails
2. Fresh-water clams
3. Fresh-water ostracoes
4. Centipedes, spiders, millipedes, scorpions
5. Insects: Flies, mosquitoes, caddis flies, earwigs, wasps, beetles, bees, ants
(Jurassic - present; preserved in amber)
B. Marine Invertebrates: Mid Triassic resurgence
1. Pelecypod bivalves (mainly oysters)
a. Gryphaea: Small oyster
b. Exogyra: Large oyster
c. Rudists (horn-like valve, left valve as "lid"): reef former of Jurassic
Cretaceous
2. Scleractinian Corals (proliferated in equatorial Tethys)
a. hermatypic (reef builders): normal salinity, less than 50 m water 20
degree C, algal symbionts
b. a hermatypic (non-reef corals): Not so restrictive
3. Echinoderms
a. starfish, sea urchins, crinoids, ophiuroids
b. regular echinoids: 5 fold symmetry, spherical shell (test), numerous in
European chalks (Early Cretaceous)
c. irregular echinoids: Displaced regular echinoids during Cretaceous,
flattened forms, bilateral symmetry
4. Ammonoid Cephalopods (Mesozoic also "Age of Ammonoids" -
widespread, abundant index and guide fossils)
a. Suture pattern (septal fluting where septum joins inner wall): key to
taxonomy
b. Septal fluting: Related to function as pressure-resisting support
c. Variety due to environmental adaptation: Expansion follows sea level
rise and epicontinental sea flooding
d. Diversity of forms: Planispiral (Early Cretaceous) to open spirals,
straight conchs, helicods (Late Cretaceous)
e. Assemblage decline and extinction at End-Cretaceous
f. Nautiloids: Sole survivor of extinction
5. Belemnite Cephalopods
a. highly successful: Jurassic Cretaceous
b. straight, pointed conch shell
c. moved through water by jetting propulsion
d. related to squid and octopods
6. Gastropods
a. helicoid conchs with cap shells
b. reef, beach dwellers
7. Crustaceans: Barnacles, crayfish, lobsters, crabs, shrimp, ostracodes
(abundant in Jurassic-Cretaceous)
8. Protozoans: Radiolarians and foraminifers
a. prolific in Jurassic-Cretaceous
b. radiolarians: Siliceous tests and oozes (chert)
c. foraminifers: Calcium carbonate tests (forarms); widespread index
fossils that are important for correlation; benthic (Jurassic) and plankonic
(Cretaceous); contributor to chalk deposition
V. Mesozoic Vertebrates
A. Amphibians: Rise of modern forms
1. Temnosponyls: Labryrinthodont’s surviving group from end-Permian
extinction
a. Triassic - 17 families
b. Jurassic - 2 families
c. Cretaceous - 1 family
2. Lissamphibia: Modern form
a. Order Anuria: frogs and toads
b. Order Urodela: salamanders and newts
c. Order Gymnophiona: Caecilians
3. Lissamphibia: Oldest forms known
a. Order Anuria: Triadobatrachus (Early Triassic Madagascar)
b. Order Urodela: Karaurus (Late Jurassic, Kazakstan)
c. Order Gymnophiona: Eocaecilia (Early Jurassic, Arizona)
B. Reptile Clans: Classification based on number and location of openings in
skull behind the eye orbits
1. Diapsida (two openings): Dinosaurs, pterosaurs, living reptiles except
turtles
2. Synapsida (one opening low on skull)
3. Euryapsida (one opening high on skull)
4. Anapsida (no opening): Turtles
C. Triassic Transition: Continuity among land animals
1. Survivors of Permian Extinction (245 m.y.0
a. labyrinthodont amphibians
b. mammal like reptiles (including ictidosaurs, mammal ancestors)
2. New Reptile Groups
a. first turtles (toothed)
b. rhynchocephalians (tuataran lizards)
c. archosaurs: crocodiles, flying reptiles, dinosaurs, thecodonts
3. Thecodonts (e.g. Hesperosuchus)
a. small, agile, light, longtails, short forelimbs
b. walked erect, bipedal
c. caught prey with forelimbs
d. forelimbs evolved to wings
e. ancestors of dinosaurs and flying reptiles
f. Phytosaurs: Reverted to 4 legged stance; some armored (crocodile like
forms); an example of convergence in evolution; phytosaur nostrils just
ahead of eyes on snout
D. Dinosaurs
1. Composed of Two Orders: Saurischia (lizard-hipped) and Ornithischia (bird-
hipped)
a. Saurischia: Tritadiate pelvic bones like thecodonts
b. Ornithischia: Pubis parallel to ischium like birds
c. Earliest dinosaurs: Saurischia (Triassic, 225 m.y. old Argentina)
2. Saurischians: "First dinosaurs" and small forms
a. Euraptor and Herrerasaurus: Late Triassic forms called "first
dinosaurs" (Argentina)
b. Coelophysis: Younger Late Triassic dinosaur (New Mexico)
c. Ornithomimus: A coelurosaur like Coelphysis; "bird mimic"; presumed
egg eater (toothless)
3. Carnivorous saurishians or Theropods (larger carnivores; hind limbs robust;
claws on toes; small fore limbs; serrated teeth)
a. Deinonchus - Cretaceous predator
b. Family Allosauridae: Allosaurus (U.S.); Piatnizkysaurus (Argentina);
Szechuanosaurus (China)
c. Giant theropods: Tyrannosaurs (13m, 4T, North America);
Gigantosaurus (Argentina); Carcharodontosaurus (Africa)
4. Herbivorous Saurischians (Jurassic-Cretaceous)
a. evolved from Late Triassic protocerapod (Plateosaurus)
b. long necks, long tails, 4 legged stance
c. Apatosaurus (formerly Brontosaurus; Jurassic, Colorado): 20 m, 3 T
(T = metric tons)
d. Supersaurus and Ultrasarus (80 T)
e. Seismosaurus (35 m long)
f. Diplodocus (Wyoming, 10 T)
g. left extensive footprint record ("trackways" of Colorado)
h. advantages of size: Avoid predators, slow temperature changes
(surface: volume ratio)
5. Ornithischians (Late Triassic-Cretaceous)
a. characteristics: beak for cropping plants, quadrupeds and bipeds
(primitive), jaw allows teethe closure at one time
b. Ornithopods: Bipedal forms (e.g. Camptosasurus: medium size, short
fore limbs, long hind limbs)
c. Larger Cretaceous Ornithopods (e.g. Iguanodon, the "thumbs up"
dinosaur; herd animal)
d. Hadrosaurs (trachodonts): Web-footed aquatic adaptation; vertically
flattened tail for swimming, nesting behavior, broad flat toothless face ("duckbill" dinosaur), abundant chewing teeth, boney skull crests with
nasal passages, crestless forms called "bone heads" (e.g.
Pachycephalosaurus)
6. Quadrupedal Ornithischians
a. Stegosaurus: Two paired, spiked tail; heavy plates protruding from
spine along back and tail (defense or temperature regulation
b. Ankylosaurs: Boney plates over sides and back; squat format; small
head; weak teeth; Nodosaurus was toothless; Edmontonia
c. Ceratopsians: Horn on face from forehead outward; boney trill on back
of head; parrot’s beak like jaws; Late Cretaceous migration from Asia to
N. America (Triceratops)
7. Dinosaurs: Warm Blooded?
a. Ectothermic - rely on outside temperature
b. Endothermic - generate body heat
c. Bakker’s proposal (1969): Include dinosaurs and birds in a new class
Dinosauria
d. Bakker’s evidence for warm bloodedness: Stance like mammals;
microscopic bone structure like mammals; isotopic variability in bones like
warm blooded vertebrates; predator/prey ratios; Mesozoic climates
8. Dinosaur parenting behavior
a. Horner’s 1984 Maiasaura ("good mother lizard") hypothesis about
nesting sites in Cretaceous of Montana
b. Horner’s evidence: Hadrosasur nests with clutches of 20 eggs, nests
7.5 m apart, babies stayed in nests during 3 years growth; fed in nests;
decaying vegetation placed over eggs by adults
c. Norell’s 1993 work in Gobi Desert, Mongolian: Oviraptor and
Protoceratops nests with eggs and embryos; evidence of incubator
behavior
D. Aerial Reptiles: Adaptive radiation of Permian forms
1. Gliders with rib supported wings
a. Coelurosauravis (Permian Triassic)
b. Icarosaurus (Triassic; like modern Draco lizard)
2. Gliders with long, modified scales as wings
a. Longisquama (Triassic): Scales along forward edge of forelimb
b. Scales may have evolved to bird’s feathers
3. Active flyers: Pterosaurs (Late Triassic-Cretaceous)
a. Sharovipteryx (Triassic contemporary of Longisquama: skin membrane
between elbows and knees (rear legs to tail)
b. Sordes pilosus ("hairy devil"): Hair covered, wing flapping reptile; hair
longer on undersides for egg incubation
c. Common characteristics: Large heads and eyes; long jaws; thin slanted
teeth; fourth finger bones long to support wing; skin sail as wings
d. Two general groups evolved: Rhamphorphynchoids (primitive long
tails) and pterodactyloids (tail-less
e. Eudimorphodon: Example of rhamphorphynchoid
f. Pteranodon: Example of pterodactyloid; 7 m wingspan
g. Quetzalcoatlus northropi (Late Cretaceous, Texas): Pterosaur with
wingspan of 15.5 m
h. Pterodaustra (Cretaceous, Argentina): Long curved jaws with teeth
E. Marine Reptiles: Return to Sea
1. Archosaurs: Only sea crocodiles invaded Mesozoic oceans
2. Teleosts (modern boney fish): Cretaceous appearance
3. Nothosaurs (paddle shaped limbs)
a. Triassic appearance
b. ancestors of Pleiosaurs
4. Placodonts
a. mollusk eating reptiles
b. paddle shaped flippers
c. pavement type teeth in jaws and palate (shell crushing function)
5. Plesiosaurs ("swan lizards")
a. Jurassic appearance
b. short, broad bodies with flippers, small heads
c. long neck and short neck forms
d. slender curved teeth (fish-eater)
e. Elasmosaursrus (Cretaceous, 12 m long, long neck form)
f. Kronosaurus (Early Cretaceous, 3 m long short neck from)
6. Ichtyosaurs (reptilian counterparts of modern toothed whales)
a. fish like tail boneless dorsal fin, paddle limbs
b. large eyes with boney plates to protect them
c. Mesasaurs: Giant seagoing lizards; vertically flattened tails; extra
hinged lower jaw; fish and mollusk eater; outcompeted the crocodiles
7. Sea turtles: Example; Arachelon (4 m) a Cretaceous form
F. Birds
1. Ancestors were likely Triassic theropods
a. bipedal stance
b. pelvic structure
c. feathers as modified scales
d. beak as toothless jaw
2. First undisputed bird: Archaeopteryx (147 million years old)
a. primitive wings
b. feathers
c. jaws with teeth
d. lizard like tail
e. claw bearing free fingers (climbing and grasping)
3. Earliest bird with toothless beak: Confuciusornis
a. long tail and claws on wings
b. several million years older than Archaeopteryx
4. Disputed first bird: Protoavis
a. 75 m.y. older than Archaeopteryx
b. has keeled sternum (lacking in Archaeopteryx) to support flight muscles
c. size of modern crow
5. Non-flying (swimming) birds (Cretaceous): Hesperornis
a. feathers, no wings
b. webbed feet
G. Mammals: Groups recognized on tooth morphology
1. Most primitive (Late Triassic) mammals
a. has vestigal reptilian structures
b. have two new inner ear bones
c. mammalian jaw structure and teeth (incisors, molars, canines,
premolars)
d. Morganucodon (Late Triassic, Wales); Megazostrodon; Eozostrodon
2. Docodonts: Multicusped teeth
a. monotreme ancestors
b. fed on insects
3. Triconodonts: Cheek teeth with 3 cusps in a row
a. size of modern cat
b. fed on smaller vertebrates
4. Symmetrodonts: Triangular plan molars
5. Multituberculates: Many cusped teeth
a. herbivores
b. chisel like incisors
c. rodent like appearance
6. Eupantotheres: Asymmetrical triangular pattern of cusps on teeth
a. tooth cusps inherited from panotheres
b. ate insects, small vertebrates
c. gave rise to marsupials and placentals in Late Cretaceous
d. long, slender toothy jaws
VI. End Mesozoic Extinctions: Second Great Biotic Crisis of the Earth (65 m.y. ago);
25% of all known families lost
A. Oceanic Realm Extinctions
1. Vertebrates
a. ichthyosaurs
b. pleisosaurs
c. mosasaurs
2. Invertebrate Groups Completely Lost
a. ammonoid cephalopods
b. belemnites
c. rudistid pelecypods
3. Invertebrate Groups Losing many Families
a. echinoids
b. bryozoans
c. planktonic foraminifers
d. calcareous phytoplankton
B. Terrestrial Extinctions
1. Reptile clans: Dinosaurs and Pterosaurs
2. Reptile survivors: Turtles, snakes lizards crocodiles, tuatara lizard
(Sphenodon)
C. Possible Extraterrestrial Causes
1. Asteroid Impact
a. Alvarez 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
b. Support more likely for Alvarez theory: Iridium clay found at same
level in Denmark, Spain New Zealand, N. America, Austria, Haiti, Russia,
and in Atlantic and Pacific sediment cores
c. Other support: Shocked quartz, tektites, soot component in clay, 180
km diameter crater in Yucatan, Mexico; Antarctic fish kill
d. Arguments against Alvarez theory: Massive volcanism at Deccan and
other sites 65 m.y.a. could have provided iridium; antimony and arsenic
enrichment in clay suggests volcanism; elemental distribution in 30 to 40
cm of clay suggests lengthy event; impact may have occurred but aided
biotic crisis, not caused it
2. Periodic Extinctions and Tenth Planet Hypothesis
a. Extinctions (less dramatic than 65 m.y.a.) seem to occur at 26 m.y.
periods
b. Periodicity may be due to half orbit perturbation of comet paths as yet
unknown tenth planet (having orbital period of 56 m.y.)
c. In this idea, extinctions 65 m.y.a. are part of a repeating pattern but
were exacubated by other global changes at same time
3. Cosmic Ray Bombardment
a. Magnetic Reversal induced event: Reversal of field causes lowering of
electro magnetic shield against normal cosmic rays; rays cause genetic
damage and lethal mutations; does not explain water shield effects or some
lack of age correlation of extinction with times of known reversals
b. Supernova-induced: Cosmic ray bursts may occur with periods of 70
m.y. based on astronomical statistics
D. Possible Terrestrial Causes:
a. loss of epicontinental seas due to rapid sea level fall at end of
Cretaceous
b. change in water chemistry due to sudden release of Arctic Ocean water
which was fresh not saline like Cretaceous ocean
c. widespread volcanism: Dust, aerosols, greenhouse gasses, acid rain
d. mitigation of effects to collapsed food chain
thanks to Mr. Brofass for his time