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A Greenhouse Timeline: What Plant Fossils Show

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

Answer Key

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?
  • A. Flowering plants appear first, and simpler plants such as mosses appear later.
  • B. New groups appeared at different times, and most still have living members.
  • C. The plant groups first appear in rocks within about 20 million years of each other.
  • D. Each plant group died out soon after the next group in the table appeared.
2.
Use Table 1. Compare the oldest club moss fossils with the oldest flowering plant fossils.

Club mosses appear in the fossil record about 290 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).
A fern leaf in rock about 300 million years oldA conifer needle in rock about 350 million years oldFlowering plant pollen in rock about 100 million years oldA flowering plant leaf in rock about 200 million years oldScale tree bark in rock about 320 million years oldScale tree bark in rock about 50 million years old
Inside the group's spanOutside the group's span
A fern leaf in rock about 300 million years old
Flowering plant pollen in rock about 100 million years old
Scale tree bark in rock about 320 million years old
A conifer needle in rock about 350 million years old
A flowering plant leaf in rock about 200 million years old
Scale tree bark in rock about 50 million years old
4.
Use paragraph 4 and Table 1. What do the scale tree fossils show about the club moss group?
  • A. A branch of tree-sized club mosses died out, while small club mosses lived on to today.
  • B. Each scale tree shrank as it aged and grew into one of today's small club mosses.
  • C. Club mosses grow tall in swamps and stay small in pots, so scale trees are swamp-grown spike mosses.
  • D. Scale trees still live today in coal swamps, but their trunks rarely form fossils.
5.
Match each piece of evidence to the pattern in the fossil record that it shows best.
  1. Scale tree fossils end in rocks about 250 million years old.C
  2. Flowering plant fossils are first found in rocks about 130 million years old.A
  3. Ancient horsetails grew as tall as small trees, but most living ones are under 1.5 m.B
  4. Club moss fossils start about 420 mya, and spike mosses grow in the greenhouse today.D
  1. A. A new group appearing
  2. B. A change in body form within a group
  3. C. A group going extinct
  4. D. A group lasting to the present

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 70% 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?
  • A. Flowering species rose from 0 to 70; conifer and cycad species fell from 30 to 10.
  • B. Fern species fell from 20 to 5 as flowering plant species rose from 0 to 70.
  • C. Conifer and cycad species rose to a peak at 80 mya, then fell as flowering plants spread.
  • D. The three groups gained species at about the same rate, so their shares stayed about the same.
8.
Tavi drafts a sign that says, "Flowering plants pushed out all the older plants." Which revision fits Table 2 best?
  • A. "Flowering plants spread after conifers and cycads had died out in the region."
  • B. "Flowering plants pushed out the ferns, which fell to a few species by 70 million years ago."
  • C. "Flowering plants turned into ferns, conifers and cycads, which is why those groups survived."
  • D. "Flowering plants spread fast as conifers and cycads declined; ferns held about steady."
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?
  • A. No flowering plants, 30 conifers and cycads, and 20 ferns
  • B. About 5 flowering plants, 32 conifers and cycads, and 22 ferns
  • C. About 30 flowering plants, 23 conifers and cycads, and 21 ferns
  • D. About 75 flowering plants, 8 conifers and cycads, and 20 ferns
10.
Circle the two results that are evidence that the diversity of flowering plants grew over time.

The 100 mya sites held 20 flowering plant species, and the 80 mya sites held 45.Fern species stayed between 20 and 22 at every site age.Scale tree fossils are found in rocks from about 380 to about 250 mya.The 120 mya sites held 3 flowering plant species, and the 70 mya sites held 70.The 120 mya sites held 32 conifer and cycad species.

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.

The club member is wrong: plant groups appeared at different times, and plant life changed a lot. Table 1 shows that moss-like spores are about 470 million years old, but flowering plants such as the tomato and marigold first appear only about 130 million years ago, about 340 million years later. Scale trees appear about 380 mya and are not found after about 250 mya, so a whole group died out. Table 2 shows that diversity changed too: flowering plant species rose from 0 at the 140 mya sites to 70 at the 70 mya sites, while conifer and cycad species fell from 30 to 10. Because the fossils of each group start and stop in rocks of different ages, the record shows new groups appearing, some groups spreading or shrinking, and some going extinct, not plant life staying the same.