TIPS, TRICKS &
TECHNIQUES
Using Fossil Teeth to Study the
Evolution of Horses in Response to
a Changing Climate
•
JULIE BOKOR, JENNIFER BROO, JESSICA MAHONEY
ABSTRACT
Advantages of Using Horse Evolution
Students measure and sketch physical characteristics of 15 fossilized horse
teeth. Each student group creates a graph that summarizes the trend
between age of the fossil and length of the tooth. Plant information cards
summarizing the flora of each epoch and guided analysis questions allow
students to develop an explanation for the change in horse teeth in response
to plant evolution due to a changing climate.
Key Words: Evolution; fossils; 3D printing; climate change.
Despite the importance of evolution as the unifying concept in
biology, many students have difficulty with this topic and come
to the classroom with negative perceptions of the theory of evolution, particularly macroevolution. Because macroevolution generally takes place over long periods of time, it is often hard for
students to conceptualize the process and see its relevance to their
lives. Here, using an organism familiar to high school students,
the horse, we present an activity in which students engage in
authentic practices of science as articulated in the Next Generation
Science Standards (NGSS Lead States, 2013), including the opportunity to examine fossils, take measurements, and make claims
based on scientific evidence. Specifically,
the objectives for this lesson align with
NGSS Disciplinary Core Ideas in HS-LS4
Biological Evolution: Unity and Diversity
(Table 1). Through this inquiry lesson, students begin to appreciate the elegance and
predictive power of the theory of biological
evolution. The activities in this lesson
require no prerequisite understanding of
evolution, nor do they task the learner
with difficult vocabulary. By focusing on
concepts and big ideas through active
learning, students construct a deeper understanding and acceptance of evolution.
The 55-million-year fossil record of horses is one of the most
complete records of macroevolution. During that time, horses
have diverged into many species, several of which coexisted
(MacFadden, 2005). Other educators have developed activities
and museum exhibits highlighting the change in hoof structure
and, to a lesser extent, tooth crown length associated with
changes in species (Sloan, 1972; American Museum of Natural
History, 2008; DeSantis, 2009). However, these lessons often
highlight morphological features of only one species of horse
per epoch and could foster a misconception of orthogenesis, or
linear evolution, as is often incorrectly depicted in museum
exhibits (MacFadden et al., 2012). The familiar nature of horses,
in contrast to other extinct organisms, provides relevance in that
many students have interacted with horses and have a personal
connection with them.
Overview of the Lesson
This lesson on horse evolution was implemented at the beginning
of the school year with Honors Biology students
during two 45-minute class periods. Utilizing
fossilized horse teeth, students measured the
crown height and width to determine the hypsodonty index (HI) of each specimen, recorded
their data in a table, and produced a graph that
depicted the trend between fossil age and HI.
Information cards summarizing the climate and
flora and fauna of each epoch from Eocene to
Pleistocene, along with specimens of modernday representative plant species, were provided
to each group. Guiding analysis questions
allowed students to develop an explanation for the change in horse
teeth in response to plant evolution.
The 55-million-year
fossil record of
horses is one of the
most complete
records of
macroevolution.
The American Biology Teacher, Vol. 78, No 2, pages. 166–170, ISSN 0002-7685, electronic ISSN 1938-4211. © 2016 by the Regents of the University of California. All rights
reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Reprints and Permissions web page,
www.ucpress.edu/journals.php?p=reprints. DOI: 10.1525/abt.2016.78.2.166.
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THE AMERICAN BIOLOGY TEACHER
VOLUME. 78, NO. 2, FEBRUARY 2016
Table 1. Next Generation Science Standards disciplinary core ideas.
HS-LS4-1
Communicate scientific information that common ancestry and biological evolution are supported by
multiple lines of empirical evidence.
HS-LS4-4
Construct an explanation based on evidence for how natural selection leads to adaptation of populations.
HS-LS4-5
Evaluate the evidence supporting claims that changes in environmental conditions may result in (1)
increases in the number of individuals of some species, (2) the emergence of new species over time, and
(3) the extinction of other species.
Table 2. Horse tooth data table. Students used calipers to measure the crown height and the
anterior posterior length (APL) for each specimen and then calculated the hypsodonty index (HI).
Age (mya)
Location
Species
Catalog no.
55
Wyoming
Sifrhippus grangeri
UF 252687
33
Nebraska
Mesohippus bairdi
UF 137889
18
Thomas Farms, Gilchrist
County, Florida
Anchitherium
clarencei
UF 2211401
Parahippus
leonensis
UF 6597
Archaeohippus
blackbergi
UF/FGS
11166
Parahippus barbouri
UF 270648
Calippus elachistus
UF 53577
Calippus cerasinus
UF 60323
Neohipparion
trampasense
UF 62299
Dinohippus
mexicanus
UF 124196
Neohipparion
eurystyle
UF 208601
Nannippus aztecus
UF 302417
Santa Fe River Bed,
Columbia County, Florida
Nannippus
peninsulatus
UF 22614
Haile Site 15A, Alachua
County, Florida
Equus (plesippus)
simplicidens
UF/TRO
32072
Waccasassa River Site 9,
Levy County, Florida
Equus ferus
UF/TRO 2149
9
Love Site, Alachua County,
Florida
5
Bone Valley, Polk County,
Florida
2
0.1
Procedure
To engage students at the outset, a series of images representing the
Eocene, Oligocene, Miocene, Pliocene, and Pleistocene epochs were
projected. Students shared their observations in a whole-class discussion and were guided to compare and contrast the vegetation and
abiotic factors for each time episode. After this introduction, which situated the activity in geological time, student groups of two or three
were provided fossil teeth from 15 North American species of horses.
These species existed between 55 mya (Hyracotherium, recently
renamed Sifrhippus) and 1.8 mya (Equus simplicidens). Students
THE AMERICAN BIOLOGY TEACHER
Crown
Height (mm)
APL (mm)
HI (Height/
APL)
examined the physical characteristics of the fossils, recorded their
observations, and took measurements, engaging in authentic practices
of science as paleontologists. Students calculated the HI for each fossilized tooth by dividing the crown height (mm) by the anterior posterior length (mm), as described by MacFadden (1988), and recorded
their data in Table 2. The HI is significant because it is used as a standard measure to determine the crown height of cheek teeth and serves
as a proxy for the diet of an organism. Hypsodont animals, such as
modern-day grazers, have a large HI (>1.0) due to longer crown
height adapted for a diet of coarse materials. By contrast, brachyodont
animals have a small HI (<1.0) and corresponding short crown height
EVOLUTION OF HORSES
167
adapted to a browsing diet (MacFadden, 1988, 1992; Strömberg,
2006). Students used their data to create a graph, similar to graphs generated by research scientists, showing how the tooth size (based on HI)
changed over time (MacFadden, 1988, 1992; Strömberg, 2006). Using
their completed graphs, students observed trends in their data and
inferred that tooth size has increased over time. Importantly, student
graphs clearly showed that multiple species coexisted, with varying
HI indicating that horse evolution occurred in a phylogenetic manner.
Students were next challenged to determine why the size of
the teeth increased over time. The epoch images presented at the
beginning of the lesson were distributed to student groups as cards
accompanied by written descriptions of climate and vegetation, building on the earlier observations and discussions. To facilitate understanding of the physical differences in the types of plants found
many millions of years ago, each lab group compared a pair of vegetation samples collected from the school campus: tree leaves (C3 plant)
and grass (C4 plant). Students learned that C4 plants are better adapted
for hotter, drier climates and provided a food source for grazing animals. The increased coarseness of the C4 grass sample was due to the
increased amount of silica in the plant, which was also illustrated
through comparison micrograph images of C3 and C4 plants (included
in the lesson). Building on the previously completed graphs illustrating
change in tooth size over time, students added color-coding to indicate
the type of vegetation most commonly present in each epoch (i.e., forests; forests and grasslands; grasslands and savannas) (Figure 1). Later
in the school year, students explored photosynthesis in depth and
drew upon this activity to enhance their understanding of different
photosynthetic pathways.
Assessment Strategy
Formative assessment was used to ensure that students made accurate
measurements, recorded data correctly, and generated correct graphical depictions. Additionally, as a distal assessment, students were
given 15 horse species cards, each with information about the size
of the horse and when it existed. Drawing from this information combined with their data table, students created a museum exhibit (as discussed by MacFadden et al., 2012) that accurately depicted the
evolutionary history of the 15 species, from 55-million-year-old Hyracotherium to modern-day Equus. Students presented their displays
orally to their peers and were assessed by the instructor using a rubric.
Moving from Physical to Virtual Artifacts
This lesson utilized physical specimens (a combination of casts and
actual fossils) from the Florida Museum of Natural History. One
classroom set for six groups is currently available for loan from
the University of Florida. However, we recognize the limitation of
having only one classroom set of the casts and are currently working to make the specimens “virtually” accessible through two methods. The fossil teeth in this collection can be 3D printed, allowing
teachers to produce as many study sets as desired for students to
work with physical artifacts (available at http://www.cpet.ufl.edu/
resources/created-by-fellows/evolution/ included in Resources below).
Additionally, digital images of the specimens are available for download and can be used in the same way as the physical specimens to
collect data as a paleontologist (see Resources).
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THE AMERICAN BIOLOGY TEACHER
Figure 1. Student graph of sizes of fossil horse teeth. The
hypsodonty index (HI) is represented on the y-axis and time (mya)
on the x-axis. Each of the epochs from the Eocene to the Pleistocene
is also shown on the x-axis. The prominent vegetation of each
epoch is indicated by color: trees = green (55–34 mya), mixed = red
(34–24 mya), and grasses = yellow (24 mya to the present).
Conclusion
Through the clues on the epoch cards, the tactile experience with
the plants, the observations of fossil teeth, and guiding questions,
students formulated a conclusion regarding the changes they
observed. They argued from evidence that changes in climate led
to changes in available vegetation and that, as a result, horses with
longer teeth had improved fitness and a selective advantage. The
grazing habit of horses corresponds with increased tooth length
and the abundance of grasses. The browsers that fed on the tender
leaves of trees were at a selective disadvantage as the climate continued to warm and the grasslands spread. Through the use of fossil
horse teeth and the authentic practices of paleontology, students
explored the role of the environment in the evolution of all species.
Understanding and accepting evolution gives students the tools
to address many other concepts, such as antibiotic resistance,
genetically modified organisms, and impacts of climate change on
biodiversity. By developing student understanding of macroevolution using a familiar organism, students can place biological and
ecological concepts in the context of deep time and better appreciate the complexity of life and the elegance of the theory of biological evolution and its explanatory power.
Acknowledgments
Lesson development was funded by the Smallwood Foundation.
Production of the study set casts and implementation in the classroom were funded by National Science Foundation grant no.
0966884: “PIRE – Ancient biodiversity and global change in the
VOLUME. 78, NO. 2, FEBRUARY 2016
New World Tropics: A once-in-a-century opportunity along the
Panama Canal” and the University of Florida Center for Precollegiate Education and Training. Huge thanks to Dr. Bruce MacFadden for providing the spark, and access to the fossil horse teeth
in the Vertebrate Paleontology Collection at the Florida Museum
of Natural History; Dr. Cheryl McLaughlin for review of an earlier
draft and lesson support; Sean Moran for lesson edits, being our
paleontologist in the classroom, and digitizing the collection; and
our anonymous reviewers for their thoughtful comments.
Resources
• American Museum of Natural History Evolution of Horses Exhibit
http://www.amnh.org/exhibitions/past-exhibitions/horse/theevolution-of-horses
• Complete curriculum, files for 3D printing, and accompanying
instructional video
http://www.cpet.ufl.edu/resources/created-by-fellows/evolution/
• Florida Museum of Natural History
http://www.flmnh.ufl.edu/vertpaleo/
References
American Museum of Natural History (2008). The Evolution of Horses
[compare horse hoof and tooth fossils]. Available online at http://www.
amnh.org/exhibitions/past-exhibitions/horse/the-evolution-of-horses.
DeSantis, L.R.G. (2010). Straight from the mouths of horses and tapirs: using
fossil teeth to clarify how ancient environments have changed over
time. Science Scope, 32(5), 18–24.
MacFadden, B.J. (1988). Fossil horses from “Eohippus” (Hyracotherium) to
Equus, 2: rates of dental evolution revisited. Biological Journal of the
Linnean Society, 35, 37–48.
MacFadden, B.J. (1992). Fossil Horses: Systematics, Paleobiology, and Evolution
of the Family Equidae. New York, NY: Cambridge University Press.
MacFadden, B.J. (2005). Fossil horses – evidence for evolution. Science,
307, 1728–1730.
MacFadden, B.J., Oviedo, L.H., Seymour, G.M. & Ellis, S. (2012). Fossil horses,
orthogenesis, and communicating evolution in museums. Evolution:
Education and Outreach, 5, 29–37.
NGSS Lead States (2013). Next Generation Science Standards: For States, By
States. Washington, DC: National Academies Press.
Novick, L.R., Schreiber, E.G. & Catley, K.M. (2014). Deconstructing evolution
education: the relationship between micro‐and macroevolution. Journal
of Research in Science Teaching, 51, 759–788.
Sloan, R.E. (1972) Evolution of horses. In Historical Geology Investigations
(Exercise 8.1–8.11). Minneapolis, MN: Burgess. Available online at
http://www.cpet.ufl.edu/wp-content/uploads/2014/06/Sloan-1972Horse-evolution.pdf.
Strömberg, C.A.E. (2006). Evolution of hypsodonty in equids: testing a
hypothesis of adaptation. Paleobiology, 32, 236–258.
JULIE BOKOR ([email protected]) is Assistant Director of the Center for
Precollegiate Education and Training and a doctoral candidate in the
School of Teaching and Learning, College of Education, at the University of
Florida, Gainesville, FL 32611. JENNIFER BROO ([email protected]) is a
high school biology teacher at St. Ursula Academy, Cincinnati, OH 45206.
JESSICA MAHONEY ([email protected]) is a high school biology
teacher at Edgewater High School, Orlando, FL 32804.
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