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Reading the Layers: How an Ancient Lake's Populations Changed

Grade 7 life science: use fossil counts from ancient lake layers to argue how volcanic ash, predators and drying changed populations.

Grade 7ScienceInteractive · Printable

Aligned toMS-LS2-4

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What’s included

Single worksheet
Auto-graded online
1 NGSS standard
Printable answer key
10 questions
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Details

Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Matching, Sorting, Short answer

About this worksheet

What students practice

Students analyze fossil count data to identify changes in populations over time.
Students argue from evidence that changes to physical or living parts of an ecosystem affect its populations.

How to use

Read the passage aloud and have students label each table row with what the rock clues say about the lake before answering the questions.

Curriculum

NGSS standard covered

MS-LS2-4

Argue from evidence that changes to physical or living parts of an ecosystem affect its populations.

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Reading the Layers: How an Ancient Lake's Populations Changed

Use fossil counts and rock clues to argue what changed the lake ecosystem.

The fossil wall

Read the passage and study the table. Then answer the questions.

A Lake Written in Stone

1The fossil hall at the Ridgeline Museum of Natural History has a wall of rock cut from the bed of an ancient lake. Over a very long time, mud and other material settled to the bottom of the lake in layers, burying the remains of plants and animals. The oldest layer is at the bottom of the wall, and the youngest is at the top. A label says the five layers took roughly the same amount of time to form.

2The rock itself holds clues about the lake. Dark, fine mud usually settles in deep, calm water. Cracks in mud form when a muddy lake bottom dries out in the sun. Tiny cube-shaped salt crystals form when lake water evaporates and the salt left behind becomes concentrated. A gray ash layer forms when a volcano erupts and its ash falls over the land and water.

3The museum's paleontologist, Dr. Imelda Ruiz, described the four kinds of fossils to a visiting seventh-grade class. Small fish, about the length of a finger, fed on tiny drifting organisms. Predator fish, up to a meter long, ate the small fish. Snails crawled on the lake bottom, scraping algae off rocks and mud. Leaves blew or washed into the lake from trees growing along the shore. Close living relatives of these fish and snails live in fresh water today, and most of them do not survive in very salty water.

4To compare the layers, Dr. Ruiz's team counted every fossil they could identify in a 1 m² patch of each layer, leaving out pieces too broken to identify. When a layer holds more fossils of one kind, more of those organisms usually lived in the lake while that layer formed. Seventh grader Kofi Mensah copied their counts into the table below, with the top layer first, just as the layers appear on the wall.