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.

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.

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. 
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. 

Advantages of Using Horse 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 modern-day 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.

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 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 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.

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)LocationSpeciesCatalog no.Crown Height (mm)APL (mm)HI (Height/APL)
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    
Love Site, Alachua County, Florida Calippus elachistus UF 53577    
Calippus cerasinus UF 60323    
Neohipparion trampasense UF 62299    
Bone Valley, Polk County, Florida 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    
0.1 Waccasassa River Site 9, Levy County, Florida Equus ferus UF/TRO 2149    
Age (mya)LocationSpeciesCatalog no.Crown Height (mm)APL (mm)HI (Height/APL)
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    
Love Site, Alachua County, Florida Calippus elachistus UF 53577    
Calippus cerasinus UF 60323    
Neohipparion trampasense UF 62299    
Bone Valley, Polk County, Florida 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    
0.1 Waccasassa River Site 9, Levy County, Florida Equus ferus UF/TRO 2149    

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.

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).

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).

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).

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.

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 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

References

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