SCIENCE SAMPLER
Fossil patterns in time
by Larry Flammer
Close your eyes and visualize the world 70 million
years ago. What do you see? As a science teacher
familiar with geologic time and the likely scener y
of different geologic eras, you may envision wideranging herds of many kinds of dinosaurs and few,
if any, small mammals hiding in the shadows. Included in this vision of the age of dinosaurs might
be the sail-backed Dimetrodon, plate-backed stegosaurs, huge long-necked sauropods (such as Brachiosaurus), swimming ichthyosaurs and plesiosaurs, flying pterosaurs, three-horned Triceratops,
and T. rex.
However, many may be surprised to learn that
only four of these are actually considered dinosaurs
(stegosaurs, Brachiosaurus, Triceratops, and T. rex).
Furthermore, only Triceratops and T. rex were abundant 70 million years ago (mya), while the sauropod
dinosaurs, pterosaurs, plesiosaurs, and ichthyosaurs
were all nearly extinct, and the stegosaurs and
premammalian Dimetrodon had completely died
out much earlier (Munsart 1993;
Scotchmoor et al. 2002).
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Why do so many people have this vague and muddled
sense of past life? This is due in part to our lack of real
experience with deep time. The various efforts by textbooks and teachers using timescales to provide some
sense of the vastness of geologic time never connect
with time periods familiar to most people. In addition,
most textbook treatments of geologic time and ancient
life-forms fail to clearly show the patterns of change
in life over the ages—the first appearance of different
groups, how their populations changed over time, and
when they went extinct (Figure 1). As a result, most of us
tend to lump “prehistoric life” into a composite collection
of life-forms, most of which we don’t see today, with little
sense of how or when these changed over time. And this
has led to some widespread misconceptions.
It’s no wonder, then, that when people read about the
“Cambrian explosion,” they get the impression that all
the major groups of animals “suddenly” appeared, and
included the first appearances of fishes, amphibians,
reptiles, birds, and mammals. Both assumptions are
false. These false impressions of that early time are due
primarily to the incompleteness of our knowledge—until
recently. With the latest studies of many more fossils of
that time, and more precise age-dating strategies, we can
now see that the earliest animals in most major animal
groups began to diversify through several series of
transitional stages (the phyla of today, not classes) from
the beginning of the Cambrian (about 542 mya) through
a 17-million-year period before the
supposed “explosion”—about 525
mya (Maloof et al. 2010). When
considered along with the following “explosion” period (the next
5 to 6 million years), it turns out
SCIENCE SAMPLER
FIGURE 1
Animals of the past: Patterns in the fossil record
Note the staggered emergence of the different classes of vertebrates over time.
that this emergence of most animal phyla was actually a
gradual process, spanning some 20 to 30 million years—
nothing close to an explosion. Furthermore, the major
groups (classes) of vertebrates that we all know—fishes,
amphibians, reptiles, birds, and mammals—made their
first appearances over the next 350 million years!
Purpose of the lesson
After students have completed this activity, they
should be able to do the following:
• show how a linear timescale works—one that
ultimately relates the unfamiliar to the familiar.
Specifically, students should know that if we
equate one year of their lives to the thickness of
a $1 bill, when this scale is extended, a million
years would equal the length of a football field,
and 500 million years would equal 500 football
fields (about 30 miles), which they have likely
traveled in their region.
• recognize, based on the fossil record, the gradual,
stepwise pattern of the major groups of vertebrate
animals making their first appearance several tens
of millions of years apart over 350 million years.
This lesson works well as an introduction to Earth
history, when students are asked to consider time
periods of millions and billions of years. It would be a
most helpful experience when introducing fossils, as
part of your overview of the diversity of life, or before
your introduction to evolution in life science. With this
preparation, you can then easily introduce evolution as
a testable explanation for what we consistently find in
the fossil record.
To be most effective, the lesson is best presented
by roughly following the 5E instructional model. Furthermore, the material described here is merely an
overview, intended to give you the flavor of the lesson.
Detailed dialogues, graphics, handouts, and assessments can be found in the complete lesson Patterns in
Time on the ENSI website www.indiana.edu/~ensiweb/
lessons/pat.in.time.html.
Engage
First, share with students a few real fossils and their
approximate ages. Pictures or models of dinosaurs
and other extinct animals are always of interest to students, but not as engaging as holding real fossils. If
you don’t have real fossils, try to borrow some from
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the paleontology, geology, biolTime map showing use of the time-marker scale
ogy, or zoology department at a
FIGURE 2 (easy locator of time/distances from school)
nearby university. You could also
check with a local museum or
even a local high school biology
teacher. You can also find fossils
for sale in most science supply
catalogs, however students are
especially engaged by fossils that
were found in their area. Be sure
to include fossils that are 500 to
300 million years old. Two fossils
to include are a trilobite (the last
ones died out about 350 mya, before there were any dinosaurs)
and an ammonite (the last one
died out about 65 mya, when the
last dinosaurs went extinct, and
before there were any mammals
familiar to us today).
When you pass the fossils
around, ask students to respect
each fossil as the precious, irreplaceable artifact that it is. Make
row or table monitors responsible
for handing back each fossil.
Meanwhile, openly share the
thrill and wonder of holding a
real fossil that is known to be hundreds of millions of years old.
After students have had a
chance to examine the fossils,
arrange them on a counter in
a sequence by age. Point out to
students the dif ferent ages of
the fossils and ask them if they
can envision the amount of time
Map adapted from Google Maps for San Francisco Bay region.
between the present and the
oldest fossil in the sequence. Do
they have any idea what even just one million years
you could also have them vote as a class on the diflooks like? What about when compared with one year?
ferent guesses. Ask why certain guesses were made.
Hold up a dollar bill and ask students the following
Estimates will vary widely (and that’s the point), so
questions: If the dollar represents one year, how high
you might ask students why they are so different.
would a stack of one million $1 bills be? Up to the
Hopefully, some students will point out (or imply)
ceiling? A two-story building? A football field? A few
that these deep-time dimensions are nearly unimagimiles? The distance to [the next town or nearby city]
nable, because no one has experienced them. So how
or more? List these estimates on the board, and ask
can students get a realistic sense of the vastness of
students to write down their best guesses privately,
“deep” geologic time? By looking at a familiar “time
or in groups. Then have students share their guesses;
machine”—a calendar!
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SCIENCE SAMPLER
Explore
FIGURE 3
Vertebrates over time: Completed chart
A regular month-by-month
picture calendar can be
used to move students forward—and backward—in
time. If a calendar includes
notes for appointments,
things to do, places to go,
and holidays, then its user
can be transported back
to those events—or ahead
to anticipated events—in
a kind of mental time machine. A calendar can also
be seen as a timescale, with
one day equal to the width
of a square on the calendar.
On a desk calendar, one day
may equal about 1 inch, and
on that scale, students can
easily calculate that a week
(seven days) would equal
7 inches, and a year would
equal 365 inches (about 30
feet or 10 yards), so 10 years
would equal 10 × 10 yards
(100 yards)—the length of
a football field—and a million years would be 10,000 ×
100 yards, or 10,000 football
fields, equal to 600 miles.
(Details for helping students
track those equivalents can
be found in the ENSI lesson
online.) Most students will
probably have a difficult
time visualizing 600 miles,
much less the 60,000 miles
that would be equivalent to 100 million years, or
the 300,000 miles for 500 million years.
Therefore, the scale needs to use a much smaller
number, something that will help students visualize (from experience) the hundreds of millions of
years since animals lived and became fossilized.
The smallest dimension that can be easily seen is
about the thickness of a hair or a $1 bill—1/10th
of a millimeter. If the thickness of a dollar bill is
equated with one year—a time dimension students
have experienced—one million years equates to
the length of a football field, and 500 million years
equates to 500 football fields, or about 30 miles,
distances students have also experienced. (See the
online ENSI lesson for instructions to help your
students easily work through the scaling process
using multiples of 10.)
Explain
To see the practical value of this deep-time sense,
students should connect real events in deep time
with appropriate time markers. A good way to do
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this is to associate the earliest known fossils of each
familiar class of vertebrates (fishes, amphibians,
reptiles, mammals, and birds) with their appropriate distances on the new timescale. Students can
plot these (or see them plotted by you) on a map of
their region (Figure 2), using a handy time-marker
scale (to save time). If you want students to have
more experience using map scales, have them make
these calculations themselves. Students can quickly
see that, contrary to traditional thinking, these familiar animals did not all appear at the same time;
rather they first appeared many tens of millions of
years apart. It needs to be emphasized that the ages
and timing of these earliest fossils have been repeatedly tested, and their reliability has been thoroughly confirmed. To reinforce this pattern of fossils in
time, a large classroom chart showing this can be
posted for easy reference during the year (Figure
1). You could also show pictures of modern members of each class, as well as the earliest members
depicted based on their fossils. (Examples of these
earliest fossils are available in the Classroom Cladogram lesson on the ENSI site.) For further clarification and reinforcement, have students plot the
pattern of emergence and survival of the major vertebrate classes on their own Vertebrates Over Time
chart (Figure 3). (Student instructions and chart
layout, with key, are provided online in the Patterns
in Time lesson.)
If you are doing this lesson as part of your overview of the major groups of organisms, just continue
sampling living examples of those groups in whatever
classification hierarchy you are using, giving your
students a better sense of the great diversity of contemporary life, built upon the early connectedness
as revealed by the fossil record and modern DNA
analyses. Be sure to emphasize that we can trace,
using fossils, the likely beginnings of each group of
organisms backward in time, much as we are doing
for the vertebrates.
Elaborate and evaluate
This stair-step pattern suggests the hypothesis that
each successive group may have emerged from a
previous group. To test this idea, provide students
with the Vertebrate Traits table showing the key
traits of vertebrates (available online in the Patterns in Time lesson) and how they have changed
over time, forming the key additional traits asso-
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ciated with each class (subgroup) of vertebrates.
Or, divide students into several teams (nine students maximum) and assign a vertebrate class to
each team. Have each team research its assigned
class and produce a large picture of a typical member of that class to hold up for viewing by other
students. Students can collectively use an internet
search to obtain information and illustrations for
their group, and prepare a brief presentation to
the entire class about each vertebrate class. Using
the Vertebrate Traits chart, students can ask for
clarification or confirmation about each key trait
as that group is presented.
This hypothesis also leads to the testable prediction that there should be transitional fossils with intermediate features. You can either ask selected (or
volunteer) groups to find and share a few examples
of this, or (in order to save time) you can show that
such sequences do exist and present a few examples
(see the PowerPoint provided with the Patterns in
Time lesson). These include graphics demonstrating jaw changes in the premammal series, and
patterns of whale evolution, human evolution, and
horse evolution.
Finally, ask each group of students to show how
and where their group most likely branched off the
preceding group, collectively producing a provisional phylogenetic tree. Even though nobody actually
saw this happen (this was long before people were
around), the many clues are undeniable. Students
can also do this on their own Ver tebrates Over
Time chart.
As mentioned earlier, this lesson is posted on the
ENSI website as Patterns in Time (see Resources).
There you will find detailed instructions with sample
interactive worksheets to provide the structure for
students to develop the key concepts of this lesson.
There is also a PowerPoint to give you an over view
of one way to present the lesson. If students or
parents should question the reliability of geologic
time measurements, the lesson provides a link to
articles that clearly explain the compelling evidence
for this. If you want students to do the math for applying the timescales to a map, those instructions
are on the ENSI website, too. Finally, a short quiz
of six key questions is also provided. You can use
this as an assessment tool for pre-/posttesting to
see how effective the lesson is with your students.
Sample testing by the author in seventh-grade life
SCIENCE SAMPLER
science classes has shown an average of about 90%
improvement in this short, six-item quiz. Here is a
sample of the questions:
1. How high would a stack of one million $1 bills be?
(Select choice closest to your estimate.)
a. as high as this room
b. as high as a two-story building
c. as high as the length of a football field
d. about three miles high
e. about 30 miles high
2. When did the different kinds of vertebrates (animals with backbones such as fishes and mammals)
first appear? a. all at the same time
b. all within a short period of time (days, months,
or years)
c. at very different times, spread out over millions of years
3. Were there any true mammals before the time of
the earliest true mammal fossils? (yes or no)
Note that in this lesson, the scaling is focused on
the past 500 million years, because that’s about how
far back the most familiar fossil record goes. If you
(or your students) want to go even further back in
time, say to the time when our solar system formed,
that’s OK, too, and the online lesson offers details for
doing so. In any case, keep reminding your students
(while waving a dollar bill in the air), that, in the scale
described here, one year of their lives is only the thickness of a $1 bill, and one million years is a football field.
Hopefully, they will forever think of this scale when
they encounter such large numbers (in time, dollars,
or objects) in the future.
Follow-up
In science, the strength and reliability of any concept
is increased with multiple lines of evidence. It’s important for students to know that these evolutionary relationships of major groups (based on fossil analyses)
are consistently confirmed by comparative DNA and
protein analyses of living members of all vertebrate
classes. If your students are quick learners, it would
be most helpful to provide them with some interactive experiences with that confirmation, by actually
having them compare some DNA or protein samples
from animals that have been considered either closely
related or more distantly related based on anatomy or
fossils. There are several lessons in the evolution section of the ENSI website that do just that, for example,
the Whale Ankles and DNA, Molecular Biology and
Phylogeny, and Molecular Sequences and Primate
Evolution lessons (see Resources).
Conclusion
This lesson in its entirety provides an easy-to-remember scale for deep time that relates directly to familiar
time, and gives students the opportunity to apply this
scale to a clear pattern in the fossil record. This, in
turn, provides a useful background experience with
observations and their logical inferences, for which
evolution can be introduced as the best scientific
(testable) explanation. However, there’s no reason
why each part by itself (the timescale or the fossil pattern of vertebrate origins) couldn’t be used alone. n
References
Maloof, A.C., S.M. Porter, J.L. Moore, F.O. Dudás, S.A. Bowring, J.A. Higgins, D.A. Fike, and M.P. Eddy. 2010. The earliest Cambrian record of animals and ocean geochemical change. Geological Society of America Bulletin 122
(11/12): 1731–74.
Munsart, C.A. 1993. Investigating science with dinosaurs.
Englewood, CO: Teacher Ideas Press.
Scotchmoor, J.G., D.A. Springer, B.H. Breithaupt, and A.R.
Fiorillo. 2002. Dinosaurs: The science behind the stories.
Alexandria, VA: American Geological Institute.
Resources
29+ evidences for macroevolution: Part 1: The unique universal phylogenetic tree—www.talkorigins.org/faqs/com
desc/section1.html.
The Cambrian explosion—www.pbs.org/wgbh/evolution/
library/03/4/l_034_02.html.
Classroom cladogram of vertebrate/human evolution—www.
indiana.edu/~ensiweb/lessons/c.bigcla.html
Index of evolution lessons—www.indiana.edu/~ensiweb/
evol.fs.html
Patterns in time—www.indiana.edu/~ensiweb/lessons/pat.
in.time.html
Larry Flammer ([email protected]) is webmaster
of ENSIweb in San Jose, California.
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