Megatoothed Sharks

Another exercise explores the evolution of the megatoothed shark lineage leading to Carcharocles megalodon, the largest predatory shark in history with teeth up to 17 cm long (Figure 7). Megatoothed shark teeth have an excellent fossil record and show continuous transitions in morphology from the Eocene to Pliocene (Figure 8).	fig 7
Figure 8. Megatoothed Shark Lineage. From left to right: Otodus obliquus (Paleocene to Early Eocene), Carcharocles aksuaticus (Early Eocene to Middle Eocene), C. auriculatus (Middle Eocene to Early Oligocene), C. angustidens (Early Oligocene to Late Oligocene), C. chubutensis (Late Oligocene to Early Miocene), C. megalodon (Early Miocene to Pliocene)

Objectives

Upon completing this activity students should be able to:
- Sort fossil shark teeth into biologically reasonable species
- Form hypotheses about evolutionary relationships among fossil shark teeth
- Describe and interpret evolutionary trends in the fossil megatoothed lineage

Materials

- One set of shark tooth cards for each group of students (Figure 9)
- Photographs of teeth from the different megatooth sharks
- Geologic timescale with ranges of sharks

Procedure

- Put the students into groups of two or three and give each group a set of shuffled shark tooth cards. Instruct the
students to group the cards into what they believe are separate species of sharks (Figures 10 and 11).
- Have the groups of students consider the evolutionary relationships among their species and ask them to arrange
their species chronologically according to their first appearance in the fossil record. Have each group place the
older appearing species below the younger appearing species. Engage the groups in a class discussion of their
predictions and explain why some predictions may be more reasonable than others.
- Hand out the figures showing the ages and different species of megatooth sharks in their cards. Have students
describe the evolutionary trends in the fossil megatooth lineage. Then ask the students to formulate several
hypotheses that could explain the observed evolutionary trends.
- Conclude this exercise with a discussion of the environmental and biotic events occurring between the Eocene and
Miocene epochs that may have caused the evolutionary charges in the megatooth shark’s teeth. Fig. 10 Fig. 11

fig 10

fig 11

Resources

- http://www.elasmo.com/frameMe.html?file=genera/cenozoic/sharks/carcharocles.html&menu=bin/menu_genera-alt.
- http://www.elasmo-research.org/education/evolution/origin_megalodon.htm
- http://www.elasmo.com/genera/slides/o_lineage/o_segue.html