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A Greenhouse Timeline: What Plant Fossils Show

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A Greenhouse Timeline: What Plant Fossils Show

Use the fossil record of plant groups to plan a timeline path.

Part 1: When plant groups first appear

Read the garden club notes and Table 1. Then answer questions 1 to 5. "mya" means million years ago.

Planning the Timeline Path

1The garden club at Oak Hollow Middle School grows vegetables in raised beds outside and keeps a small greenhouse full of potted plants. This year the club wants to build a "Timeline Path" through the greenhouse, with plants set out in the order their groups first show up in the fossil record. Club president Tavi Nakashima asked the club's adviser, Ms. Hedda Larkspur, how anyone could know which plants came first.

2Ms. Larkspur explained that the fossil record is all the fossils that have been found, together with the ages of the rocks they were found in. Lower rock layers usually formed before the layers above them, and volcanic ash beds between layers can be dated. Plants rarely become fossils, because most rot before they are buried. Pollen, spores, wood and tough leaves are preserved most often. So a group may have appeared somewhat earlier than the oldest fossil of it found so far.

3She gave the club a chart from a university museum (Table 1). Most of the groups still have living members in the greenhouse: a mat of moss, spike moss, a scouring rush, a fern, a potted pine, a sago cycad and many flowering plants, from tomatoes to marigolds. Spike mosses belong to the club moss group, and scouring rushes are horsetails.

4One group in the chart has no living members. Scale trees were club mosses that grew taller than most trees in today's forests, with trunks covered in a diamond pattern of leaf scars. Their remains formed much of the coal mined in parts of North America and Europe. Today's club mosses are small, usually shorter than 30 cm. Some ancient horsetails also grew as tall as small trees, while most living horsetails are shorter than 1.5 m.

5The museum also counted the plant species found at fossil sites of five ages in one region, and sorted them into three groups (Table 2). Every site was searched for the same number of days, so the counts can be compared from one age to the next. Tavi wants the sign at the end of the path to tell visitors how plant life changed during that time.

Table 1: Plant Groups in the Fossil Record
Plant groupGreenhouse exampleOldest fossilsYoungest fossils
Mosses and relativesSheet mossabout 470 mya (spores)Still living
Club mossesSpike mossabout 420 myaStill living
Scale treesNoneabout 380 myaabout 250 mya
FernsBoston fernabout 360 myaStill living
ConifersPotted pineabout 310 myaStill living
CycadsSago cycadabout 280 myaStill living
Flowering plantsTomato, marigoldabout 130 myaStill living
1.
Which statement best describes the pattern in Table 1?
Question 1 options
2.
Use Table 1. Compare the oldest club moss fossils with the oldest flowering plant fossils.

Club mosses appear in the fossil record about million years before flowering plants.

3.
Use Table 1. Suppose each fossil below were found. Sort it by whether its age falls inside the time span Table 1 gives for its group, from its oldest fossils to its youngest fossils (up to today for a group that is still living).
4.
Use paragraph 4 and Table 1. What do the scale tree fossils show about the club moss group?
Question 4 options
5.
Match each piece of evidence to the pattern in the fossil record that it shows best.

Part 2: Counting species at fossil sites

Use paragraph 5 and Table 2 to answer questions 6 to 11.

Table 2: Plant Species Found at Fossil Sites in One Region
Site ageFlowering plantsConifers and cycadsFerns
140 mya03020
120 mya33222
100 mya202721
80 mya451921
70 mya701020
6.
Use Table 2. Add up all the species found at the 70 mya sites first.

Flowering plants made up % of the species found at the 70 mya sites.

7.
Which statement best describes the pattern in Table 2 from 140 mya to 70 mya?
Question 7 options
8.
Tavi drafts a sign that says, "Flowering plants pushed out all the older plants." Which revision fits Table 2 best?
Question 8 options
9.
A new fossil site in the same region is about 90 million years old. Based on Table 2, which counts would you most expect there?
Question 9 options
10.
Circle the two results that are evidence that the diversity of flowering plants grew over time.

11.
A club member says, "The plants in our greenhouse have all been around since the first plants grew on land, and plant life has not really changed." Write a claim that answers the club member. Give at least two pieces of evidence from Table 1, Table 2 or the passage, and explain your reasoning.