Directions for teachers: After your students read the print version of the Science News article “Here come the (bigger) mammals” (Readability: 9.8), ask them to answer the following questions.
1. What event altered life on Earth 66 million years ago? What may have been a major consequence of the event?
An asteroid struck Earth, which is believed to have killed off all of the dinosaurs except those that evolved into birds.
2. What have scientists found in rocks in Colorado? How are these discoveries helping scientists understand the life-altering event?
Scientists found a cache of plant and mammal fossils from after the asteroid strike. The fossils are helping scientists understand how plants and mammals recovered from the asteroid strike and flourished over time.
3. According to the article, how did mammals change in the hundreds of thousands of years after the event? What factors may have contributed to that change?
Mammals evolved bigger maximum body sizes after the asteroid strike. Scientists think that a lack of large predators and an explosion of plant diversity may have contributed to the change.
4. On the graph “How fast mammals grew in maximum body mass after the dino-killing asteroid,” what information is displayed on the x-axis? What about the y-axis? Don’t forget to include units!
The x-axis shows maximum mammalian body mass in kilograms. The y-axis shows the number of years before and after an asteroid struck Earth 66 million years ago.
5. What does the horizontal red line represent?
The red line represents the asteroid impact that wiped out nonavian dinosaurs 66 million years ago.
6. What does the thick beige line show? What about the numbers on the line?
The beige line represents how mammals evolved in size, indicated by body mass, from before the asteroid impact to 1 million years after the strike. The numbers are examples of mammal species along that evolutionary path, and the colors of the numbers indicate the location where each fossil was found.
7. What do the horizontal dotted lines represent? Explain each line shown. Why do you think the lines are included on the graph?
The dotted lines represent major developments in plant life. Soon after the asteroid impact, ferns were the dominant plant. By about 300,000 years after the impact, palms dominated. And nearly 700,000 years after the impact, legumes appeared. Plants might be included on the graph to give added context. Changes in plant diversity after the strike may be related to the evolution of mammals.
8. Describe fossil No. 1 on the graph. Where was it found? What mammal does it represent? How big was this mammal? And how does it differ from the other mammals shown on the graph?
Fossil 1 is Didelphodon and was found in North Dakota.It had a body mass of a little less than 10 kilograms. Didelphodon is the only mammal from before the asteroid strike shown on the graph and the only one shown that was not found in Colorado.
9. How does the body mass of Baioconodon compare with the body mass of Didelphodon? According to the beige line, how do these mammals’ body masses compare with the body masses of mammals that survived the strike?
Baioconodon (which lived roughly 100,000 years after the strike) and Didelphodon have similar body masses. The biggest mammals from just after the strike had a much smaller body mass.
10. Based on the comparison you made in the previous question, come up with a hypothesis about mammals’ recovery from the asteroid strike.
It took about 100,000 years for mammals’ body mass to rebound from the strike.
11. According to the graph, what were the body masses of the mammals found in Corral Bluffs, Colo.? How do these mammals differ from the mammal found in South Table Mountain, Colo.?
By about 500,000 years, Carsioptychus had a body mass of about 25 kilograms. By about 700,000 years, Eoconodon had a body mass of nearly 50 kilograms. These mammals lived hundreds of thousands of years after and had much larger body masses than Baioconodon.
12. What is one conclusion that you can draw from the graph? Explain.
Mammals’ body mass increased over hundreds of thousands of years after the asteroid strike. About 700,000 years after the strike, the biggest mammals’ body mass was nearly five times that of the biggest mammals’ body mass prior to the strike. The asteroid strike changed the environment in ways that made it possible for mammals to grow.