Student Activities

Reef Builders and Limestone Formation

Overall Rating

Add your review
Grade Level:
Middle School: Sixth Grade through Eighth Grade
State Standards:
ESS1.C, ESS2.A, ESS2.B, LS2.A, LS2.B, LS4.A, PS1.A,


Background: During the Mississippian Period, most of the United States was under large, shallow sea (Rheic Ocean). Since North America was in a very different location at this time, the sea was also tropical as the equator ran through this area.

On this reef lived a variety of sea dwelling organisms. Swimming amongst the reef was a number of vertebrates including sharks and various fish. But most of the organisms living on the reef were invertebrates.

The shells of many of these invertebrates formed the limestone that Wind Cave is in. 

Probably the most plentiful fossils in the cave are brachiopods. Brachiopods are small shelled creatures that resemble modern day clams but are actually unrelated. These were one of the most numerous reef-building organisms during the Mississippian Period. 

Bryozoans were plentiful at this point. These are aquatic, colonial organisms that were frequently calcareous, composed of calcium carbonate. Bryozoans are found in tidal areas and most commonly encrust surfaces such as coral, seashells, or kelp. Along with calcium carbonate producing algae and some corals, they form a large part of the limestone layer that Wind Cave exists in.

Mollusks and specifically gastropods (snails) are found as fossils in Wind Cave. Gastropods would glide along on a wide flat foot and feed of the abundant algae during the Mississippian Period. Cephalopods are a class of animals in the mollusk phylum as well. They include octopus, squids, and nautilus. During the Mississippian Period this class was represented by ammonites.

Echinodermata is a diverse phylum of organisms including starfish, sea urchins and sand dollars. But in the reef that formed the limestone the crinoids were the most common. These creatures are sometimes called “sea lilies” for the appearance (somewhat similar to flowers). Their stem parts are occassionally found as fossils but the rest of the skeletal remains tended to disintegrate and scatter upon death. But they were so numerous they formed a significant amount of limestone.


  1. Introduce the fact that limestone is made from the shells and skeletal remains of ancient animals.
  2. Give attached PowerPoint presentation introducing the variety of fossils that are in Wind Cave.
  3. Play activity
    1. Tell students that they are going to use their knowledge of reef-building organisms from the Mississippian Period in a game. Have students form into groups of three or four.
    2. Hand out a complete set of the Reef Builder cards, face down, to each group of students and instruct them not to look at the cards before being told to do so.
    3. Explain that each card lists four characteristics of a reef-building organism from the Mississippian Period. They will try to guess what the organism is while being given the clues.
    4. The person initially given the cards will read a clue, one at a time in order, until someone in the group guesses the organism or until all clues are used without a successful guess.
    5. Students will read the clues while others in the group attempt to guess the name of the organism. Points are given for successful answers as shown below. The student who successfully answers will be the reader for the next card.
  • One clue - 4 points
  • Two clues - 3 points
  • Three clues - 2 points
  •  Four clues - 1 point
  1. The game continues until all of the cards have been read. For those students with the highest score, or who score more than a pre-determined number of points, provide a prize or reward.
  2. Discussion Questions
    1. Discuss what might happen to the reef if any of the environmental conditions changed (change in water temperature, sea level, available oxygen, etc.). Have students theorize about what might have brought about the eventual end of growth on the reef.
    2. Discuss current reefs, the Great Barrier Reef is an example, and any conditions that may be threatening their growth (ocean warming, acidification, sea level changes, other organisms that feed on reef builders, etc.). Have students describe how studying ancient environments can help us better understand our Earth today.


Cards the students will using during the activity.

Download Reef Builder Cards

Fossils that can be found in Wind Cave

Download Fossil pictures

Last updated: September 21, 2016