Dinosaur Detectives

Objectives

• Students will learn about amphibian evolution by comparing fossils with a modern-day skeleton.
  
  
• Students will experience the scientific thought process of drawing conclusions from limited paleontological data.

Materials

• Photocopies of the printable geologic time scale and the four figures of skeletons
• Scissors

Procedure

1. View background information on Reptiles and Amphibians Through the Ages.
   
   
2. Photocopy the four figures of herp skeletons and geologic time scale so that each student or team has one copy of each.
   
   
3. Have the students study the four figures carefully. Then have each team use its best judgment to order the figures from most ancestral to most recent animal. Remind them that early amphibians evolved from a fish ancestor.
   
   
4. Discuss the correct sequence of evolutionary development among the four organisms. The correct order is as follows:
   
   
   • Icthyostega (B) – One of the earliest fossil skeletons of a four-legged animal ever found. It represents the earliest transition from the fishlike ancestor to a land vertebrate with weight-bearing limbs. This organism lived during the Upper Devonian, 360 million years ago, in what is now Greenland and Russia. Note that this fossil was incomplete, missing its front toes.
     
     
   • Temnospondyls (D) – One of many predecessors to the modern amphibians that lived over the immense span of time between 335 and 210 million years ago.
     
     
   • Triadobratrachus massinoti (A) – This small animal (10 cm [4 in] long) lived in the Triassic period 230 million years ago. It bears the strongest resemblance to modern-day amphibians.
     
     
   • Skeleton of a modern-day frog (C) – Modern frogs do not appear in the fossil record much before 100 million years ago.
   
   
5. Have the students align the figures with the time on the Geologic Time Scale when the animals lived. Discuss some of the key changes that occurred along the evolutionary pathway that led to the form of the modern-day frog.
   
   
   • Loss of the tail
   • Fusion of the head bones
   • Reduction in the number of digits (toes)
   • Reduction in the ribs and vertebrate
   • Elongation of the digits
   • Elongation of the leg bones
   
   
6. Discuss how paleontologists use information from rare fossils collected at different times and from different places around the world. Using these bits of evidence, paleontologists piece together possible pathways in the evolution of amphibians and reptiles. Discuss the uncertainty involved in working with such limited information.