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Unformatted text preview: The Fossil Record and Horse Evolution
In this demo section you will learn:
• how the fossil record can be used to establish evolutionary
relatedness of different organisms over time. • how the fossil record can be used to establish life history, as well as
the selective forces acting on different organisms over evolutionary
All living things are related to each other through an enormously complex lineage of descent that stretches back more
than 3.5 billion years. Disentangling this convoluted history and describing the evolutionary changes that have
occurred over this long time period is within the realm of the science of paleontology. Reconstructing evolutionary
history is no easy task. While it is difficult enough for historians to reconstruct events that occurred only hundreds to a
few thousands of years ago, paleontologists are faced with the task of reconstructing events that happened millions
or even billions of years ago.
The most direct source of information about the evolutionary history of organisms comes from the fossil record. By
studying this record, paleontologists have been able to reconstruct how living organisms are related to each other,
how groups of organisms have changed over time, and how changes in organisms are related to changes in past
In this lab, we will use fossils to study evolutionary history in animals. Because the fossil record of horses is very rich,
it provides a good case study that can help elucidate the pattern of past evolutionary change. As you go through the
exercises, remember that natural selection results in adaptive changes over time that reflect the organismsʼ
environment. See if you can determine how horse evolution may have paralleled changes in its environment over
time. Think about how characteristics you observe in other animals may be adaptations for their environments at this
particular time in their evolutionary history. Fossil Formation
Broadly speaking, fossils are any preserved remnant trace of a once-living organism. This includes the cured remains
of early hunter-gatherer humans found in Scottish peat bogs, the jeweled remains of insects trapped in amber, the
intricate impressions of early bird feathers made in German limestone, and the mineralized bones of dinosaurs that
now fill the great halls of natural history museums. Los Angeles contains a great repository of fossils at the La Brea
Despite the variety of fossil types, fossils form only rarely, and usually represent a biased sample of living
communities. Once an organism dies, all traces of its existence generally are obliterated by scavengers,
decomposers, and physical erosion. Preservation only occurs in environments where the normal forces of
decomposition are greatly reduced, such as the bottom of an anoxic lake or the margin of a tar pit. Also, typically only
hard structures such as bone fossilize, although soft tissue is sometimes recorded. Even after preservation, geologic
forces such as erosion and faulting (and, to a lesser extent, volcanic activity) can destroy the preserved remains.
Finally, surviving fossils must exposed on the surface so that paleontologists can find them.
Because of these difficulties, the overall fossil record can be viewed as annoyingly incomplete. Of all the species
existing today, less than 10% can be expected to be preserved as fossils. Still, given all these difficulties the fossil record is remarkably useful, and for some groups (such as horses) the fossils show a clear history of evolutionary
change and relationships. G eologic Time
Perhaps the most remarkable thing about the fossil record is its length. Fossil evidence of life has been found in spme
of the oldest rocks ever discovered on earth, around 3.5 billion years old. The magnitude of “geologic time,” measured
in millions or even billions of years, can be hard to comprehend as we measure our lives in days, weeks, and months.
Nonetheless, all of the life we see around us today is the product of 3.5 billion years of continuous change, the
surviving tips of the enourmous, ancient tree of life.
To organize this overwhelming expanse of time, paleontologists divide geologic time into smaller units based on the
fossils characteristically found during each time period. Geologic time is divided into two major units called eons
( Table 3.1). The Precambrian is the longer and least understood of the two. It lasted some 4 billion years, from the
formation of the planet to the first appearance in the fossil record of hard-bodied, multicellular organisms around 700–
570 million years ago. The Phanerozoic extends to the present, during which most of the major forms of life evolved.
The Phanerozoic is further divided into three e ras. The Paleozoic lasted from 590 million to 248 million years ago
and is the time during which the first vertebrates, such as fish, first appeared. The M esozoic lasted from 248 to 65
million years ago and is the time period during which dinosaurs first appeared and came to dominate the terrestrial
fauna. The C enozoic Era is the present era and is the time period during which the diversity of mammals (including
horses and humans) was generated. Eras are further divided into smaller units called p eriods and e pochs.
Note that the table is not drawn to scale. The period of time during which horses have been present on earth (from
the Eocene to the present) represents less than 2% of the total time that life has existed on the planet! G eneral Pattern of Horse Evolution
The story of horse evolution is part of a broader story of the evolution of mammals. The first mammals were shrewlike insectivores that first appeared during the Mesozoic, 200 million years ago (MYA). Their diversity paled in
comparison with the profusion of dinosaur forms. This changed abruptly 65 MYA when a massive asteroid slammed
into the planet triggering a mass extinction, including the dinosaurs. Smaller animals such as the early mammals had
a better chance of surviving in the aftermath of the impact. Mammals subsequently diversified into the newly vacant
habitats and niches, diversifying from 40 genera in the Late Cretaceous to over 200 in the Early Eocene. This rapid
evolution of many diverse forms from a common ancestor is called a daptive radiation. For instance, when the first
finches arrived in the Galapagos Islands, which had until that time harbored no similar creatures, they diversified and
evolved to fill different niches. A similar explosion of speciation and diversification was seen following the arrival of
fruit flies to the Hawaiian Islands.
The evolutionary history of horses is part of this major adaptive radiation (horses are members of the mammalian
order P erissodactyla). The number of horse forms increased dramatically beginning in the Eocene. From North
America, where all the early fossils of horses are found, forms eventually radiated into Eurasia and Africa. These
exhibited a variety of distinct adaptations to their new environments. At the end of the Pleistocene, much of this
diversity disappeared (including all the horses in North America) leaving only a single extant genus, Equus. This
pattern of evolutionary relationship has made it tempting to view horse evolution as a single line that extends from the
first horse directly to the living modern horse. However, the overall horse phylogeny displayed in Figure 3.1 is a bit
more complicated than this, it does not look like a simple straight line. This is because many fossil forms existed, and
only a single branch of the tree represents all living taxa today.
Despite this complexity, broad patterns of morphological change are still discernible from the fossil record of horses.
In the following exercises, we will examine these general morphological changes, and attempt to relate these
changes to the environments in which each horse species lived. Table 3.1: Geologic time intervals.
Eon H ow many years
Era Period Epoch ago did division
Phanerozoic Cenozoic 10,000
Pleistocene 23 million Oligocene 34 million Eocene 55 million Paleocene
Cretaceous 5 million Miocene First Homo sapiens 1.8 million Pliocene Tertiary N otable Event 65 million
144 million First horses 200 million
Mesozoic First mammals
Jurassic 206 million Triassic 248 million Permian 290 million Carboniferous 354 million Devonian 417 million Silurian 443 million Ordovician 490 million Cambrian 543 million Paleozoic 670 million
Precambrian 3.8 billion
4.5 billion First animals
First life forms P ast Environmental Change
The environment did not remain constant over the 55 million years during which horses were evolving. Large climatic
changes occurred which induced equally large changes in the vegetation of North America, Eurasia, and Africa. As
the environment changed, natural selection produced morphological adaptations that we can observe in the fossil
When the first members of the horse lineage arose in the Eocene of North America, the climate was much warmer
and wetter than it is today. Thick semitropical forests and fern swamps covered much of the continent. Today,
herbivorous animals inhabiting habitats such as these eat a diet consisting of nuts, berries, leaves, seedlings, and the
young tender shoots of shrubs and trees. It is a diet that is relatively easy to digest and high in nutrients. Animals that
use this dietary strategy are called browsers.
F igure 3.1: A horse phylogeny. (Adapted from Prothero, 1995). Table 3.2 : Changes in the environment and associated horses in North America over the past 55 million years. Time (mya) Epoch N orth American Environment H orse Genus Holocene Modern climate Equus Pleistocene Increasing aridity; cooling trend; polar caps advance
and retreat cyclically; large-scale mammalian
extinction at close of epoch Equus
Equus) Pliocene Development of modern prairies; Bering Land Bridge
and Panama Isthmus open at close of epoch Plesihippus
(now just called
Equus) 17–11 Miocene Closed forests reduced; first vast grasslands appear;
height of warming trend; diversity and sizes of
mammals increase Merychippus 37–32 Oligocene Closed forests decrease; woodlands begin to develop;
land bridges open to Old World Mesohippus 55–45 Eocene Closed forest, fern & swamp vegetation; subtropical
climate; no large mammals Hyracotherium 0.01 4-present ~4 By the Oligocene (34 mya), the climate became drier and cooler. The thick forests and swamps were gradually
replaced by more open woodland composed of shrubs and widely spaced trees. The appearance of the hillsides of
southern California could be a fair approximation of the habitats in North America in the Late Oligocene. The drying
and cooling trend continued into the Miocene (10 mya). The climate became more seasonal, with large extremes in
temperature and moisture occurring between winter and summer. Additionally, fire became a regularly occurring
disturbance in many places. In response, forests continued to decline, shrublands continued to expand, and grasses
evolved for the first time. Grasses are adapted to deal with dry seasonal climates affected by frequent fires. In many
places, the grassland replaced the woodlands and forests all together, and vast grassy plains were formed. Animals
that eat grasses are called grazers. Compared to things like nuts and berries, grasses are poor quality food; they are
low in nutrients and high in contents such as lignin, which are hard to chew and digest. Many grasses also contain
silica, the gritty material used in sandpaper and bathroom cleansers. In order to effectively eat and digest grasses,
grazers have evolved suites of specialized adaptations.
By the Pliocene (5 mya), extensive grassland prairies extended throughout much of North America. The cooling trend
reached its peak in the Pleistocene (beginning 1.8 mya) when large continental ice sheets alternately expanded and
retreated in northern North America. The vegetation was much as it still is in the remnant patches of the native prairie
of Canada and the steppes of Mongolia. Table 3.2 outlines the major climatic changes in North America over the time
period of horse evolution. H orse Anatomy: Form and Function
How can we use fossils to learn about the diet and environment of the past? Anatomy can tell us a lot about the
ecology of animals. For example, teeth provide information on diet, as diet will cause selection on their number and
shape. We canʼt measure the muscles of extinct horses directly, as soft parts rarely fossilize, but we can estimate
their size based on where they attached to the body. In order to make sense of the evolutionary changes and
adaptations reflected in the fossil record of horses we must understand some fundamentals of horse anatomy. A. The Skull (Figure 3.2)
A horse skull is divided into the cranium and the mandible . The mandible is the lower jaw. The cranium contains the
brain case and the rostrum. Imagining a vertical plane cut into the skull at the front of the orbits (eye sockets).
Everything in front of this plane is the rostrum, and the cranium is to its rear. Within the rostrum, the premaxillae and
maxillae hold the upper teeth. The masseter is the principal muscle that opens and closes the mouth. It attaches to
the maxilla and to the rear portion of the mandible.
The skull provides characters we can examine to investigate dietary changes.
1. The length of the masseter attachment indicates the size of the muscle. Larger masseters are required to chew
tougher foods. Would you expect browsers or grazers to have larger masseters? 2. The size of the rostrum relative to the cranium. A relatively longer rostrum can hold more teeth. What advantage
would more teeth provide? Would you expect browsers or grazers to have more teeth? All of these measures will be scaled to the body size of the animal. Why is it important to do this? Why canʼt we simply
use the absolute measurement?
F igure 3.2: A t ypical modern horse skull. B. Dentition (Figures 3.3 - 3.5)
As in humans, horses generally have four different types of teeth.
• incisors, used for cropping and pulling
canines, used for fighting
premolars and m olars, both used for grinding food However, the canines are absent or reduced in most horses, whereas the premolars and molars are nearly identical
in form and they function as a single unit called the cheek te e th . So from a practical point of view, we can consider
horses as having two types of teeth, the incisors and the cheek teeth. See Figure 3.3 for a diagram of browser,
grazer, and carnivore teeth and jaws, and Figure 3.4 for typical herbivore and omnivore molars.
F igure 3.3: Teeth and jaws of a grazer, browser and carnivore. F igure 3.4: Typical herbivore (horse) and
o mnivore (human) molars. A horse molar shows two units (Figure 3.5). The lower unit is called the root; it rests inside a socket in either bones of
the mandible or in the maxilla and none of its surfaces are exposed to grinding action. The upper portion of the tooth
is called the crown; it sits above the gum line, exposed to the grinding action of food. Because of this exposure the
crown is covered by a hard layer called enamel. Underneath the enamel is a softer layer called d entine . Below the
surface of the gum, c ement holds the tooth in the bone. The top of the crown is the surface that actually comes in
contact with food and the opposite tooth. This structure is called the occlusal surface. Compare this surface on the
molar of a modern horse with the same surface on one of your teeth.
F igure 3.5: Typical herbivore horse molars. With dentine, enamel, and cement intertwined around each other in complex patterns, the tooth surface in horses is
much more complicated than in humans. Each of these materials wears away at slightly different rates, creating sharp
ridges and valleys. As a horse ages, its teeth are continually pushed up from the roots to provide new crown as the
teeth wear. The teeth of horses are also high crowned, hypsodont. The ratio of tooth height to tooth length is
measured by the hypsodonty index , which provides a way of comparing tooth morphology between species.
While comparing the dentition of the various fossil specimens in lab, consider:
1. How many teeth are adapted for grinding? 2. What are the sizes and shapes of the grinding teeth? 3. How do the grinding surfaces differ between specimens? 4. How would these characters differ between a grazer and a browser? C. Limbs (Figure 3.6)
Humans have fairly generalized hind limbs (the ones we stand on).
They consist of several long bones that articulate to bones called
t arsals, which make up our ankle and heel. These bones articulate
with a series of five smaller bones called m etatarsals, which make
up our foot. Attached to these are even smaller bones called
phalanges , which make up our five toes. A modern horse, by
comparison, has only one relatively long and thick metatarsal. It
attaches to a single phalangeal set (digit). Attached to this digit is
an enlarged sheath called the hoof, which supports the entire
weight of the animal. F igure 3.6: M odern horse hind limb in
c omparison to a human leg skeleton. How a limb is put together reveals much about the way an animal
moves. Animals that exhibit great dexterity, balance, and agility
generally have several functioning digits that form a broad padded
foot. The relative lengths of their limbs tends to be shorter, and their
limbs are heavily muscled. A drawback to this type of limb is that its
many interdigitated parts can easily become injured with extended
use. (Orthopedic surgeons make a fortune from this fact.) Also, the
many muscles required to move the limb become easily fatigued.
Finally, a short limb is inefficient at generating great speed. (What
body shape do you expect the worldʼs top sprinters to have?) In contrast, animals such as the modern horse that spend almost all
their lives running away from predators have long, columnar limbs that end in a small number of functioning digits.
Few muscles attach to the limbs, reducing the total weight of the limb aiding rapid locomotion. Instead, springy
tendons run the length of the limb. This type of limb is restricted to moving only in a single forward plane of motion,
but is highly durable. It is also efficient at transferring muscular energy into forward motion, allowing for incredible
As animals get bigger, their limbs must adjust to support the added weight. This can be accomplished in different
1. The limb bones can get thicker. The strength of a limb bone is proportional to the cross-sectional area of the
bone. All other things being equal, you would expect larger animals to have bones with higher width/length ratios
than smaller animals. 2. The internal structure of the limb bone can be strengthened with internal struts. Again, all other things being equal,
you would expect larger animals to have more internal struts in their bones than smaller animals. Think about a horse living on an open plain as opposed to a dense forest. In which habitat would speed be selected for?
In which would maneuverability be selected for? How will this be reflected in the skeletons youʼll examine? ...
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- Spring '05