The Cretaceous Period

144 - 65 Ma

A. Global setting

High sea levels worldwide.
Continents covered by shallow seas. (Epicontinental seas).
Interior of U.S. flooded by shallow seas.
Continents began to move toward their present configuration. Atlantic Ocean widened.
Gondwanaland breaks up.
Temperatures on earth were much warmer. No glaciers.
Black shale deposits.
Accumulation of organic-rich, black muds in deep water indicates oceans were anoxic on bottom.
Anoxic oceans may be related to warm global temperatures.
No ice caps to supply cold (dense, heavy) oxygen-rich water to ocean bottoms.
Volcanism + mountain building in western U.S.; subduction. (Cordilleran region)
Eastern US Coastal Plain sediments deposited.

B. Life in the Cretaceous

Fossils display a mixture of ancient and modern features.

Includes many extinct taxa, plus many important modern taxa which had just appeared.

Life in the seas
1. Diversification of the planktonic foraminifera (ex. Globigerina)
2. Evolution + appearance of diatoms
3. Coccolithophores became abundant.
  Coccoliths (shells) accumulated in large numbers on the sea floor forming CHALK.
  (The word Cretaceous means "chalk bearing") White Cliffs of Dover (England) are Cretaceous chalk.
4. Ammonoids + belemnoids = major swimming carnivores (predators) (Ammonoids are valuable index fossils in Cret. rocks)
  Most Cretaceous top carnivores were not "modern".
5. Teleost fish appeared. New predator!
  symmetrical tails, round scales, specialized fins (ray finned)
  (dominant group of marine + fresh water fish today)
  
  Xiphactinus audax, a ray-finned fish, the largest bony fish of the Cretaceous.
  Typically 18-20 feet long. Niobrara Chalk, Lane County, Kansas.
6. Reptiles were largest marine carnivores
  Plesiosaurs (up to 35' long)
  
  Plesiosaur. Elasmosaurus platyurus, Pierre Shale, NW Kansas.
  
  (Ichthyosaurs & large marine crocodiles had become rare.)
  Mosasaurs (up to 50' long) probably top predators; attacked ammonoids.
  
  Mosasaur, Tylosaurus poriger.
  78 m.y. old, 26 ft long.
  On display at Georgia Southern University, Statesboro, GA.
  
  Mosasaur fossil skeleton from Greene County, Alabama
  Red Mountain Museum, Birmingham, Alabama
  
  Mosasaur chasing turtle. Clidastes propython
  Niobrara, Kansas
  Smallest mosasaur in North American seas
  
  Ammonite showing bite marks from a mosasaur.
Life on the sea floor
1. Decline of the brachiopods (may be related to diversification of predators)
2. Scleractinian corals diversified
3. Pelecypods (bivalves) and gastropods diversified
  Unique group of large pelecypods appeared (the rudists).
  Rudists were important reef formers; up to 1 m tall; (extinct at end of Cret.)
  
  Fossil rudists. On display at the Smithsonian Institution, Museum of Natural History, Washington, D.C.
4. Sea urchins diversified
5. Foraminifera (benthic) diversified
6. Encrusting bryozoans diversified
7. Modern types of crabs appeared (new predator/ scavenger)
8. Predatory gastropods appeared (able to drill holes in shells).
  New mode of predation not seen before.
  
  Modern pelecypods with holes drilled by predatory gastropods.
9. Decline of crinoids (may be related to diversification of predators).
10. Overall decline in sessile benthos. Ability to swim or burrow may have been best defense against increasingly diverse predators.
Life on land
1. Flowering plants (angiosperms) appeared.
  
  Unidentified flower fossil.
2. Conifers became dominant gymnosperms.
  Today there are 200,000 flowering plant species & 550 modern conifer species
  
  Cycadeoidea marylandica, a gymnosperm that produced both pollen and seeds in a flowerlike structure. Diamond-shaped leaf scars are present on its trunk, arranged in spirals. Small whorls mark the position of the flower-like structures. May be a precursor to the evolution of the angiosperm. Cretaceous. 105 m.y. old, Prince Georges County, Maryland. On display in the U.S. National Museum, Smithsonian Institution.
3. Angiosperms diversified and gymnosperms declined during late Cret.
  
  Angiosperm leaf, similar to modern sweet gum. Late Cretaceous Dakota Formation, central Kansas.
4. Rapid rate of insect diversification paralleled and accompanied the diversification of flowering plants. Symbiotic relationship.
5. Cretaceous Dinosaurs continued dominance; new types appeared and became very diverse
  1. duck-billed dinosaurs = hadrosaurs fast-running herbivores
  2. large carnivorous dinosaurs (Tyrannosaurus)
    More species & individuals of herbivorous dinosaurs. Few carnivorous dinosaurs (top of food chain/web).
    Trend toward evolution of large body size.
6. Flying vertebrates
  1. birds
  2. flying reptiles (pterosaurs)
7. Smaller vertebrates
  1. amphibians (frogs, salamanders)
  2. reptiles (snakes, lizards, turtles)
  3. mammals
Mass extinction at the end of the Cretaceous totally wiped out:
1. Dinosaurs
2. Ammonoids
3. Large marine reptiles (plesiosaurs & mosasaurs)
4. Rudists and many other invertebrate taxa
Drastic reductions with few survivors:
1. Coccolithophores
2. Planktonic foraminifera
3. Radiolarians
4. Belemnoids (cephalopoda)
Animals both on land and in the sea were affected.
On land, only SMALL (less than 50 lb) animals survived.
Many groups that died out declined gradually, and others disappeared suddenly.
Many hypotheses about these extinctions:
sea level change?
climatic change? hot? cold?
meteorite impact? comet impact? (iridium)
volcanic eruptions; dust in atmosphere
change in CO₂ levels?
change in O₂ levels?
appearance of angiosperms changed food chain/web
disease?
spillover of freshwater cap onto world oceans from Arctic Ocean melting?

Return to Online Historical Geology Course Outline

Return to Georgia Geoscience Online

This page copyright Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

November 15, 1995
Last modified November 10, 1997
Updated October 19, 1999
Images added March 21, 2003 and April 9, 2003
Images added February 7, 2006 |
Links updated August 15, 2009