Chapter 14—Life of the Mesozoic

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The Earth Through Time, 10th Edition

by Harold L. Levin

Chapter 14—Life of the Mesozoic


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, a 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 orbital parameters

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 content in atmosphere

f. changes in Earth’s orbital parameters that limit solar input
B. Triassic
1. Relatively cool

a. Pangaea continents still clustered

b. sea level was 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 an arm of proto-Pacific: warm ocean currents flowed through Tethys

D. Cretaceous
1. Relatively warm

a. subtropical flora at ± 70o of equator

b. high and low latitude coals

c. high sea-level stand: maximum inundation of Phanerozoic

2. Continents near today’s position

a. Arctic Canada near the 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 million years ago

d. strong connection to global extensions at 65.5 million years ago

III. Mesozoic Invertebrates
A. Marine Invertebrates: Middle Triassic resurgence
1. Pelecypod bivalves (mainly oysters)

a. Gryphaea: a small oyster

b. Exogyra: a large oyster

c. Rudistids (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, 20o C, algal symbionts

b. ahermatypic (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

d. crinoids: including stemmed types (e.g., Pentacrinus) and stemless forms (e.g., Uintacrinus)

4. Ammonoid cephalopods (Mesozoic also “age of ammonoids” —widespread, abundant index and guide fossils)

a. three groups

(i) goniatites

(ii) ceratites

(iii) ammonites

b. suture pattern (septal fluting where septum joins inner wall): key to taxonomy

c. septal fluting: related to function as pressure-resisting support and/or function as attachment for fleshy parts

d. variety due to environmental adaptation: expansion follows sea level rise and epicontinental sea flooding

e. diversity of forms: planispiral (Early Cretaceous) to open spirals, straight conchs, helicoids (Late Cretaceous)

f. assemblage decline and extinction at End-Cretaceous

g. 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 octopus

6. Gastropods

a. helicoids conchs with cap shells

b. reef, beach dwellers

7. Crustaceans: barnacles, crayfish, lobsters, crabs, shrimp, ostracodes (abundant during Jurassic-Cretaceous)

8. Protozoans: radiolarians and foraminifers

a. prolific in Jurassic-Cretaceous

b. radiolarians: siliceous tests and oozes (chert)

c. foraminifers: calcium carbonate tests (forams); widespread index fossils that are important for correlation; benthic (Jurassic) and planktonic (Cretaceous); contributor to chalk deposition

B. Terrestrial Invertebrates: limited preservation record

1. Air-breathing snails

2. Freshwater clams

3. Freshwater ostracodes

4. Centipedes, spiders, millipedes, scorpions

5. Insects: several groups appeared before end of Jurassic (preserved best in amber)

IV. Mesozoic Vertebrates
A. Amphibians: rise of modern forms
1. Temnospondyls: surviving group of tetrapods from end-Permian extinction

a. Triassic - 17 families survived

b. Jurassic - 2 lineages survived

c. Cretaceous - 1 lineage survived

2. Oldest forms known

a. oldest frog: Triadobatrachus (in Lower Triassic, Madagascar)

b. oldest salamander: Karaurus (in Upper Jurassic, Kazakstan)

c. oldest caecilian: Eocaecilia (in Lower Jurassic, Arizona)

3. Modern forms that survive today

a. frogs and toads (anurians)

b. salamanders and newts (urodelans)

c. limbless amphibians (caecilians)

B. Triassic Transition: continuity among land animals

1. Survivors of Permian extinction

a. amphibians

b. mammal-like reptiles

2. New reptile groups

a. first turtles (toothed turtles)

b. rhynchocephalians (tuataran lizards)

c. archosaurs: crocodiles, flying reptiles, thecodonts, dinosaurs

3. Example “basal archosaur”: Hesperosuchus

a. small, agile, light, long tails, short forelimbs

b. walked erect, bipedal

c. caught prey with forelimbs

d. forelimbs evolved to wings

e. ancestors of dinosaurs and flying reptiles

4. 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

5. Diapsid groups

a. lepidosaurs: snakes, lizards, and their ancestors

b. archosaurs: ornithischian and saurischian dinosaurs, flying reptiles (pterosaurs), and archosaurs

C. Triassic-Jurassic Extinction Event
1. 199.6 million years ago

2. 20% of marine families and many large land animals lost

3. Probable causes: sea-level change and ocean anoxia
D. Dinosaurs
1. Composed of two orders: saurischia (lizard-hipped) and ornithischia (bird-hipped)

a. Saurischia: triradiate pelvic bones like thecodonts

b. Ornithischia: pubis parallel to ischium-like birds

c. earliest dinosaurs: saurischia (Triassic, 225 million years old, Argentina)

2. Saurischians: “first dinosaurs” and small forms

a. Eoraptor and Herrerasaurus: Late Triassic forms called “first dinosaurs” (Argentina)

b. Coelophysis: a younger Late Triassic dinosaur (New Mexico)

c. Ceratosaurus: the saurischian with a nasal horn

3. Carnivorous saurishians or theropods (larger carnivores; hind limbs robust; claws on toes; small forelimbs; serrated teeth)

a. Deinonychus and Velociraptor: Cretaceous predators

b. Family Allosauridae: Allosaurus (U.S.)

c. giant theropods: Tyrannosaurus (13 m, 4 tons, North America); Gigantosaurus (Argentina); Carcharodontosaurus (Africa)

4. Herbivorous saurischians or sauropodomorphs (Jurassic-Cretaceous)

a. subdivided into prosauropods and sauropods

b. long necks, long tails, 4-limbs stance

c. Apatosaurus (formerly ‘Brontosaurus;’ Jurassic, Colorado): 30 tons d. Brachiosaurus: longer forelimbs to reach higher vegetation

e. Supersaurus: 80 to 100 tons

f. sauropodomorphs left extensive foot-print record (“track ways” of Colorado, for example)

g. advantages of size: avoid predators, slow temperature changes due to surface-to-mass ratio (gigantothermism)

5. Ornithischians (Late Triassic-Cretaceous)

a. characteristics: beak for cropping plants, quadrupeds and bipeds (primitive), jaw allows teeth closure at one time

b. Ornithopods: bipedal forms

(i) heterodontosaurs

(ii) hypsilodonts

(iii) iguanodonts

(iv) hadrosaurs

c. Hadrosaurs (trachodonts): web-footed aquatic adaptation; vertically flattened tail for swimming; nesting behavior; broad flat toothless face (“duck bill” dinosaur); abundant chewing teeth

(i) hadrosaurines

(ii) lambeosaurines

6. Thyreophorans

a. Stegosaurs: two-paired, spiked tail; heavy plates protruding from spine along back and tail (defense or temperature regulation)

b. ankylosaurs: bony plates over sides and back; squat format; small head; weak teeth

(i) with bony tail club: ankylosaurs

(ii) without bony tail club: nodosaurs

7. Marginocephalia

a. pachycephalosauria

b. ceratopsia

(i) psittacosauridae (Psittacosaurus)

(ii) neoceratopsia (Protoceratops; Triceratops; Kosmoceratops)

8. Dinosaurs: warm-blooded?

a. ectothermic: rely on outside temperature

b. endothermic: generate body heat (e.g., mammals and birds)

c. Ostrom’s proposal: 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

9. Dinosaurs and birds

a. several Chinese dinosaurs with feathers: Caudipteryx and Protoarchaeopteryx

b. other feathered dinosaurs?

10. Dinosaur parenting behavior

a. Horner’s (1984) Maiasaura (“good mother lizard”) hypothesis about nesting sites in Cretaceous of Montana

b. Horner’s evidence: hadrosaur 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 skin wings

a. Coelurosauravus (Permian-Triassic)

b. Icarosaurus (Triassic)

2. Gliders with long, modified ribs functioning as wings: Longisquama (Triassic of Kyrgyzstan)

3. Active flyers: pterosaurs (Late Triassic-Cretaceous)

a. Sharovipteryx (Traissic contemporary of Longisquama: skin membrane between elbows and knees—rear legs to tail, a “parachute” glider)

b. Sordes pilosus (“hairy devil”): “hair” covered, wing-flapping reptile; hair longer on under side 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-tailed) and pterodactyloids (tail-less)

e. Pteranodon: example of tail-less pterodactyloid; 7 m wingspan

f. Quetzalcoatlus northropi (Late Cretaceous, Texas): pterosaur with wingspan of 12 m

g. Pterodausto (Cretaceous, Argentina): long curved jaws with teeth

E. Marine Reptiles: return to sea
1. Nothosaurs (paddle-shaped limbs)

a. appearance during Triassic

b. ancestors of plesiosaurs

2. Placodonts

a. mollusk-eating reptiles

b. paddle-shaped flippers

c. pavement-type teeth in jaws and palate (shell crushing function)

3. Plesiosaurs

a. first appearance during Jurassic

b. short, broad bodies with flippers, small heads

c. long-neck and short-neck forms

d. slender curved teeth (fish-eater)

e. examples

(i) Elasmosaurus (Cretaceous, 12 m long, long-neck form)

(ii) Kronosaurus (Early Cretaceous, 3 m long, short-neck form)

4. Ichthyosaurs (reptilian counterparts of modern toothed whales)

a. fish-like tail, boneless dorsal fin, paddle limbs

b. large eyes with boney plates to protect them

5. Mosasaurs: giant sea-going lizards; vertically flattened tails; an extra hinged lower jaw; fish and mollusk-eater; out-competed the crocodiles

6. Sea turtles: example Archelon, a 4 m Cretaceous turtle

F. Birds
1. Evidence that ancestors were likely Triassic theropods similar to Velociraptor

a. bipedal stance

b. pelvic structure

c. feathers as modified scales

d. beak as toothless jaw

2. First undisputed bird: Archaeopteryx

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. Non-flying (swimming) birds (Cretaceous): Hesperornis

a. feathers, no wings

b. webbed feet

5. New fossil finds documenting dinosaur-bird connections

a. Liaoning Province, northern China

b. key interval in China: 120 to 131 million year ago

c. Sinosauropteryx

G. Mammals: groups recognized on tooth morphology
1. Most primitive (Late Triassic) mammals

a. has vestigial 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; survived for 100 million years starting during Late Jurassic

a. herbivores

b. chisel-like incisors

c. rodent-like appearance (ground dwellers)

6. Subdivision of mammalian groups (present by Middle Jurassic)

a. prototherians: triconodonts, multituberculates, monotremes

b. therians: marsupials (euherians) and placentals

c. gave rise to marsupials and placentals during Late Cretaceous

d. long, slender toothy jaws

e. earliest known placental mammal: Eomaia (Early Cretaceous, China)

V. Mesozoic Flora: “age of the cycads”
A. 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; spiney and stellate forms; covering called frustule (epitheca = upper half, hypotheca = lower half)
B. Terrestrial Plants
1. Cycads (Phylum Cycadophyta): seed plants in which true flowers have not developed

a. cycadeoids (fossil cycads)

b. cycadales (true cycads, e.g., sago palm)

2. Conifers: 6 families in Jurassic-Cretaceous

3. Angiosperms: seed and flower-bearing

a. Middle 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. co-evolution of insects, birds, reptiles, and mammals

b. production of seeds, nuts, fruits

c. advent of flora color due to competition for pollination

VI. Late Cretaceous Catastrophe: second great biotic crisis of the Earth; 65.5 million years 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: loss of many families

a. echinoids

b. bryozoans

c. planktonic foraminifers

d. calcareous phytoplankton
B. Terrestrial Extinctions: dinosaurs and pterosaurs
C. Reptile survivors: turtles, snakes, lizards, crocodiles, tuataran lizard (Sphenodon)
D. Possible Extraterrestrial Causes
1. Asteroid impact

a. Alvarez’ theory: 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, soot component in clay, 180 km diameter crater in Yucatan, Mexico; Antarctic fish kill; tsunami deposits in the Gulf of Mexico region

2. Massive volcanism Deccan (and other sites) 65.5 million years ago: possible iridium source; antimony and arsenic enrichment in clay suggest volcanism elemental distribution in 30-40 cm of clay suggests lengthy event; impact may have occurred but aided biotic crisis, not caused it

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 million years based on astronomical statistics

E. Possible Terrestrial Causes
1. Loss of epicontinental seas due to rapid sea-level fall at end Cretaceous

2. Change in water chemistry due to sudden release of Arctic Ocean water which was fresh not saline-like Cretaceous ocean

3. Widespread volcanism: dust, aerosols, greenhouse gasses, acid rain

4. Mitigation of effects to collapse food chain

Answers to Discussion Questions

1. Chalk is particularly by abundant in Cretaceous strata. Foraminifers (“forams”) are single-celled protozoans that began to proliferate as plankton during Cretaceous. Their tests make up much of Cretaceous chalk (along with coccolithophores). Chalks were rare during Paleozoic (and any pre-Cretaceous rocks) because foraminifera were not nearly as common in younger and older seas as during Cretaceous.

2. Diatoms are silicoflagellate phytoplankton. They are made of silica, whereas coccolithophores are made of calcium carbonate. Diatoms are built like a box with a lid and base; coccolithophores are spherical with tiny armor plates over their surface. Diatoms are commonly associated with cold sea water whereas coccolithophores are not. Being composed of silica, diatoms are more chemically stable in slightly acidic water than are calcium-carbonate plankton like coccolithophores.
3. Jurassic flora was dominated by cycads, ginkgoes, seed ferns, and conifers. After middle Cretaceous, angiosperms prevailed. The shift to angiosperms coincided with global warming during Cretaceous (abetted by Pangaean breakup and continental movements).
4. Ammonoid cephalopods are mollusks that were pelagic swimming animals during Mesozoic, which attained global distribution and are thus exceptionally useful for as guide and index fossils in correlation. Ammonoid cephalopods are distinguished from nautiloid cephalopods (an extant group) by their complex suture patterns along joints between chambers.
5. Foraminifers are single-celled protozoans, which build durable tests and have an abundant Mesozoic and subsequent fossil record. Because of their small size and strong tests, large numbers of foraminifers can be obtained unbroken from small pieces of rock recovered while drilling for oil and (or) gas. Because most planktonic species were widely distributed in the world’s oceans, they are useful for age determination and temporal correlation. As sensitive indicators of water temperature, salinity, and depth, they are useful for reconstructing ancient environmental conditions.
6. Mesozoic marine invertebrate faunas are dominated by mollusks, especially pelecypods and cephalopods. Mesozoic could be called the “era” of the ammonoids and oysters. In the older, Paleozoic seas such groups as trilobites, brachiopoids, bryozoans, graptolites, and echinoderms dominated. Reef formers of Mesozoic were scleractinian corals; Paleozoic reef-builders were stromatolites, stromatoporoids, primitive corals, and echinoderms. Paleozoic groups not seen at all during Mesozoic are trilobites, graptolites, many brachiopods and corals, archaeocyathids, and stromatoporoids.
7. Endothermy in dinosaurs is suggested by their mammal-like stance, microscopic bone structure of blood vessels, isotopic variability in bones like that seen in living endotherms, apparently low predator/prey ratios, and the challenges of Late Mesozoic climates. Large forms like the giant saurischians would have a smaller surface to volume ratio than small dinosaurs, thus endothermy would not be so critical for them.
8. Cretaceous mammals had an exceptionally reliable system of body temperature control, small size, and effective reproductive anatomy which may have allowed them to have an advantage over many reptiles in surviving the end-Cretaceous catastrophe.

9. World geography at the end of Permian was characterized by a single continent, Pangaea, in an equatorial position. By the end of Cretaceous, the supercontinent had rifted and split into all the major continental pieces that we have seen today. The global geography during Mesozoic and was more like today.

10. The classes Aves and Dinosauria first appear during Mesozoic.
11. Pterosaurs and birds (as well as ichthyosaurs and fish) are two examples of convergent evolution of large groups.
12. (a) The evolutionary importance of “basal archosaurs” is that they were diapsid ancestors of both the dinosaurs and the flying reptiles. (b) The evolutionary importance of Archaeopteryx is that this species is the first undisputed bird fossil and is a close evolutionary link between small, bipedal theropods and modern birds. (c) The evolutionary importance of angiosperms lies in the coevolutionary relationship between many angiosperms and many types of insects.
13. Feathers may function as insulation, camouflage, or display.
14. Convergent evolution is the process of producing similar physical forms in unrelated organisms. Because they are unrelated, the similarity of ornithischian dinosaur hip bones and bird hip bones seems more coincidental than convergent.
15. Reptile groups surviving the end-Cretaceous extinction are: turtles; snakes; lizards; crocodiles; and tuataran lizards.
16. Oceans cover 71% of the earth’s surface and the ocean floor, because it is continually moving and re-melting, is at its oldest Early Mesozoic. This is so because of the rifting following the breakup of Pangaea. As Early Mesozoic, say 230 million years ago, is relatively recent in Earth’s long history, the record of craters will be minimal on the seafloor.

17. Evidence for the terminal-Cretaceous bolide impact includes iridium-enriched clay at many locales, shocked-quartz grains, tektite beds, soot within boundary clay, and 180 km crater in Yucatan, Mexico. Evidence against the hypothesis is antimony and arsenic concentrations in boundary clay favoring a volcanic origin for the unusual chemistry. Deccan and same-age basaltic outpourings of lava data at 65.5 million years suggest a volcanic source. The thickness of boundary clay and the homogenous elemental distribution suggest deposition over long time. Further, not all extinctions seem to be truly simultaneous. Some groups were in decline for several million years.

18. The Deccan Traps were extruded 65.5 million years ago at the same time as many other basaltic outpourings. The aerosol, dust, and greenhouse gas contribution of such eruptions would have been enormous. The CO2 contribution could have poisoned the upper 45 to 100 m of seawater, thus making it lethal for key plankton. Collapse of the marine food chain could have resulted.
19. a
20. e
21. b and d

Chapter Activities
Student activities for in-depth learning.
1. Take a look at this web page on ammonites ( Ammonites.htm) and address the following questions. Why are ammonites so aesthetically pleasing? Of what is an ammonite shell composed? How do ammonites grow and how do they float and move in the water. How has ammonite morphology (shape) changed over time?
2. Using the resources linked to this page ( dinosaurs/dinoclassification/), find simple sketches of the lizard-hipped and bird-hipped pelvic bones and re-draw them for yourself. Label the three main bones, the ilium, ischium, and pubis. What are the main dinosaur groups with each type of pelvic structure?


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