Dinosaur Paper

Shannon Francis [email protected] 1

Ferocious Tyrannosaurs to Clucking Hens: How Chickens Evolved from Dinosaurs

Chickens are not exactly the most ferocious of creatures. They seem far more suited to clucking

and pecking about the farmyard and brooding over their clutch of eggs than to using razor sharp claws

and wits to turn some of the biggest creatures to have walked the earth into helpless prey. However,

chickens have a much grander and more vicious past than their benevolent appearances let on; one of

America's favorite barnyard friends is in fact a living dinosaur. While their bigger and badder ancestors

died off approximately 65 million years ago, there is substantial evidence that suggests that chickens, as

well as other birds, are the evolutionary offspring of a group of dinosaurs known as theropods.

Anatomical and behavioral similarities link birds to their reptilian relatives, and prove that chickens

and other birds evolved not alongside but from dinosaurs. By studying the fossils and other evidence

left behind by dinosaurs, scientists can learn more about the evolutionary process that turned creatures

such as the fearsome Tyrannosaurus Rex into the modern day chicken. Furthermore, the similarities

between living chickens and extinct dinosaurs allows paleontologists to use modern birds to make

assumptions and draw conclusions about what dinosaurs were like in the past. However, in order to

understand this particular evolutionary line, one must first understand the concept of evolution.

Charles Darwin first put forth the Theory of Evolution in his groundbreaking book The Origin

of Species. To state it simply, evolution is “the change in the gene pool of a population from generation

to generation by such process as mutation, natural selection, and genetic drift” (Dictionary.com). Even

within members of the same species, there is a great deal of genetic diversity caused both by mutation

and the recombination of genes from previous generations. This means that the genetic traits of the

species are ever-changing; this is due to the ever-changing nature of the Earth’s environments. Those

species that can adapt, be it to better obtain resources or better take advantage of their ecosystem, are

the creatures who survive to breed and create the next generation, while those who cannot simply die

off; this is the process of natural selection, one of the basic driving forces of evolution. It is through


this process of natural selection, adaptation, and genetic mutation that a creature such as a theropod can

evolve into something as seemingly unrelated as a modern chicken.

Chickens' ancestors come from a group of dinosaurs known as coelurosaurs. These dinosaurs

were far from the popular image of giant, reptilian monsters, however; coelurosaurs were “probably

feathered, some could fly, and new evidence shows that certain coelurosaurs grew, reproduced, and

even slept like birds” (Brusatte). Following the principals of evolution explained earlier, one can

assume that the traits that chickens and other birds have in common with their coelurosaurs are not

simply coincidence, but rather positive traits that were passed down from generation to generation and

along the evolutionary line. However, coelurosaurs are not the only dinosaurs that chickens are similar

to. Coelurosaurs were part of a larger group of dinosaurs known as theropods, a group which modern

birds are placed in as well. This means that we can not only compare chickens to coelurosaurs, but also

to their evolutionary cousins such as Velociraptor and Tyrannosaurus Rex. In fact, when comparing the

anatomy of chickens and various species of theropod, one can draw many comparisons between

everything from the structure of the skeleton to the intricacies of the cardiovascular system.

When looking for similarities between theropods and chickens, one must first turn to their

bones. Not only do these two creatures share light, hollow bones, but many structural similarities as

well. One of the most prominent skeletal features that chickens and dinosaurs share is the structure of

the hips. Theropods belonged to a group of dinosaurs known as “saurichians,” or “lizard-hipped”

dinosaurs (as opposed to “ornithiscians,” or “bird-hipped” dinosaurs). Contrary to the name, birds are

not descended from ornithiscians; the hips of ornithischians only superficially resembled those of birds,

and modern birds are in fact classified as saurischians. This is due to similarities in many of the

structures in the hip, most notably the ilium. The ilium is “the uppermost and widest of the three bones

that fuse together to form each of the hipbones” (The American Heritage Science Dictionary). In most

reptiles this bone is extremely short, which means that it is unable to support anything more than


narrow thigh muscles. Chickens and theropods, on the other hand, have much longer ilia, which allows

for larger, stronger thigh muscles that can support bipedal movement (Paul). This also results in similar

hip positioning, where the bodies of both chickens and theropods “hang down from a vertebral column

that balances, more or less horizontally like a teeter-totter, across the hip joint” (Kaiser).

In addition to similarities in the hip, theropods and chickens share many other skeletal traits,

especially in the hindlimb. Unlike sauropods and stegosaurs, which evolved straight-jointed limbs

similar to those of an elephant, theropods had large hips and slender, flexed legs that allowed them to

run at speeds similar to those of modern ground birds. This is because their knees moved in a motion

similar to that of modern birds, where the knee had to be flexed to be fully articulated (Paul).

Furthermore, theropods and chickens also share four-toed feet, where three main toes support the

animal's weight and the first toe is reversed, as well as elongated bones between the ankles and toes.

Even the structure of theropod arms and hands share similarities to the wings of a chicken;

while somewhat hard to believe, the skeletal structure of a chicken's wings does contain the remnants

of three fingers on each wing. Furthermore, paleontological evidence “indicates that three digits in the

hand of birds and maniraptorian theropods are 'I-II-III' of the ancestral five digits” (Zhou). This means

that modern chickens and the small coelurosaurs who are their closest non-avian evolutionary relatives

shared the same three digits derived from the original five digits of an ancestor further up the

evolutionary chain. Furthermore, these same, small theropods and birds share a “flattened, half-moon-

shaped bone in the wrist that limited movement of the hand, much like a similar element of the bird

wrist” (Brouchu). This means that theropods had wrists that were semi-fixated, an intermediary

between fully-flexible wrist and the fixed wrists that make up the structure of a chicken's wings.

In the past, the relationship between birds, such as chickens, and dinosaurs was heavily debated

because of the supposed lack of a crucial avian skeletal feature in theropods; “birds have a very large

set of collarbones, or clavicles, that are fused to form together the wishbone. No dinosaurs then known


had a collarbone” (Brochu). This led scientists of the time to

believe that, instead of being direct descendants of dinosaurs,

birds had evolved alongside the beasts and were descended

from some other kind of archosaur. Although dinosaurs with

clavicles had been discovered as early as the 1920s, their

discoveries went largely unnoticed until over 40 years later. It

was not until 1964, when John Ostrom of Yale University noted the staggering amount of similarities

between theropod skeletons and bird skeletons, that the idea that birds evolved from dinosaurs was

brought back into scientific light; furthermore, the discovery of Sue (figure 1), and exquisitely

preserved Tyrannosaurus Rex, showed clear evidence of a wishbone, “the thin, slightly curved

horizontal bone between the two massive shoulder bones” (Wild Birds Unlimited).

The anatomical similarities are not limited to hard tissue, however. Under extraordinarily rare

conditions, the soft-tissue of a dinosaur will be preserved – and it is in the soft tissue that some of the

most compelling evidence for the evolutionary relationship between chickens and theropods can be

found. The first and possibly most striking example is the similarities between the respiratory systems

of birds and theropods. Birds have quite possibly the most efficient and complex breathing system of

all vertebrates. Unlike other animals such as reptiles and mammals, birds do not have dead-end lungs;

“instead, they are connected to a large complex of air sacs whose flexibility and especially volume

greatly exceed those of the lungs” (Paul). These sacs can sometimes extend as far as the pelvis and

allow most of the fresh air inhaled by the animal to bypass the lungs on its way in. After going through

the air sacs, the air is “injected through the lungs in one direction on its way out. Because this

unidirectional airflow eliminates the stale air that remains in dead-end lungs at the end of each breath

and allows the blood and airflow to work in opposite, countercurrent directions and maximize gas

exchange, the system is very efficient” (Paul). While early theropods show no evidence of air sacs,

Figure 1


later specimens have pneumatic vertebrae and increased hinge jointing of the ribs, “indicating that they

were probably helping to ventilate the lungs by inflating and deflating air sacs” (Paul). As theropods

further evolved, their air sacs lengthened and their lungs became shorter and stiffer, as seen in modern

birds. The soft-tissue similarities of theropods and birds are not limited to the respiratory system,

either.

Most modern reptiles such as snakes, lizards, and turtles, have three-chambered hearts,

incapable of producing high blood pressure. Crocodilians have four-chambered hearts, but, like lizards,

are incapable of producing high blood pressure. Birds, on the other hand, have “fully developed, four-

chambered, double-pump hearts able to propel blood in large volumes at high pressures” (Paul). In the

past, all scientists could do was assume that dinosaur hearts were similar to those of birds, as soft tissue

very, very rarely was preserved. However, the earth-shattering discovery of a small dinosaur called

Thescelosaurus soon gave scientists more solid evidence. Its heart was preserved in an ironstone

nodule within the animal’s chest. This was the result of a very rare process where iron-bound oxygen

in the heart muscle mineralizes while in contact with groundwaters – in layman’s terms, iron in the

heart caused it to fossilize. This meant that scientists could study the structure of a dinosaur heart and

confirm that it was in fact a four-chambered heart, similar to that of birds. Furthermore, the preserved

heart gave scientists evidence that some dinosaurs (namely smaller theropods) had high metabolic rates

and were therefore warm-blooded, just as modern birds are. This would also tie into the presence of

feathers on some theropods, which would act as insulation only for warm-blooded creatures.

While many people today picture dinosaurs as great scaled beasts, the truth is that many of them

had feathers – or at least some form of precursor. Recent archaeological discoveries in China have

shown that, before Archaeopteryx, many theropods as early as the beginning of the Cretaceous period

had basic, feather-like filaments covering their bodies. The discovery of the small theropod

Sinosauropteryx was among the first; when it was discovered in 1996, its fossil clearly showed a coat


of fine filaments covering its body. While these filaments lacked the complexity of a feather, they were

still far more advanced than reptilian scales. This rare preservation of soft filaments led scientists to

speculate that many other dinosaurs hat protofeathers that were not preserved. Another theropod,

Sinornithosaurus (a relative of Velociraptors) was also found to be covered in protofethers, and,

“although unable to fly, [it] might have been able to 'flap' its arms to catch small prey out of the air”

(Brochu). Later species, such as Caudipteryx and dinosaurs of the genus Beipiaosaurus, had more

complex, quill-like and even barbed feathers, probably used for display and insulation.

The coloring of modern chickens is even

something that was shared by their dinosaur

ancestors. Groundbreaking new research has

allowed scientists to determine the coloring

of dinosaur protofeathers by studying the

shape and density of fossilized pigmentation

organelles underneath an electron

microscope. Like modern birds, the shape of

the different organelles, called “melanosomes,” dtermines which colors they are able to produce.

Rodlike eumelanosomes produce black and grey; round phaeomelanosomes produce reddish-brown to

yellow pigments; and a lack of melanosomes produces the color white. When this technique was

applied to a fossil of the chicken-sized dinosaur Anchiornis huxleyi (figure 2), a pattern very similar to

that of a modern silver-spangled Hamburg chicken emerges on its leg feathers (Sloan). This data

concerning the coloring of a non-bird dinosaur (as many scientists classify modern birds as dinosaurs)

lead to groundbreaking revelations concerning the purpose of pre-flight feathers, as well as further

evidence of the similarities between modern chickens and their dinosaur ancestors. Dinosaur feathers

had many uses before flight was achieved, “including sexual display, territorality, et cetera.... It could

Figure 2


also have been like modern redstarts, which use their bright wing and tail patches to scare up insects,

which [the birds] then seize in flight” (Sloan). The similarities between these dinosaur behaviors and

those of modern birds lends credence to the theory that these uses for feathers other than areal

locomotion were passed down through evolution from theropods to modern birds.

However, as strong as the similarities between chickens, and other birds, and theropods are,

their evolutionary relationship cannot be proven without a link between the two. Without this missing

link, the stunning similarities between the two were assumed to simply be a case of covergant

evolution, where similar traits evolved in two different evolutionary trees by coincidence. However, in

the 19th century, when Dinosaurs were still very poorly understood and Darwin had just published the

Origin of Species, many people still believed that both dinosaurs and birds had been created at the same

time by God. The theory of evolution was still struggling to gain footing in both public opinion and the

scientific community. By coincidence, however, 2 years after Darwin had published his theories, a

very strange fossil, seeming to be a hybrid of both bird and lizard, was discovered near Eichstätt,

Germany. It had a very dinosaurian skeletal structure, complete with a “long, bony tail and teeth [that]

were similar to those of reptiles, but the stunningly preserved feathers and light, hollow bones were

trademark features of birds” (Brusatte). It also had arms with wing-like proportions and wrists that

were partially fixed in a similar fashion to the wrists in bird wings. But perhaps most stunning of

all,the feathers of this creature were arranged in the exactly the same pattern as the feathers on modern

birds. The strange hybrid was dubbed “Archaeopteryx,” and was dubbed one of the earliest examples

of birds. However, it wasn't until Darwin's theory of evolution came to be more widely accepted that

the scientific community began to realize that the fossil of Archaeopteryx was not just a record of one

of the earliest birds, but rather a snapshot of evolution in process. The combination of bird-like and

theropod-like anatomical structures and features made Archaeopteryx not only one of the earliest

discovered examples of evolution, but also the perfect missing link between birds and dinosaurs. In


more recent times, even more missing links have been found that further fill in the gaps between

dinosaurs, Archeopteryx, and birds. With such evidence creating a visual reference of the evolutionary

process between theropods and birds, their ancestral relationship is all but cemented in scientific fact.

While chickens are certainly not creatures that are expected to be seen soaring through the skies,

they are capable of flying in short bursts. They take to the air to briefly explore their surroundings,

clear obstacles, and find places to roost for the night. However, even this basic and rather primitive

form of flight is something that chickens and dinosaurs seem to lack. It is difficult to imagine

dinosaurs being capable of short bursts of flights similar to chickens, let alone true flight that many

other birds are capable of. The transition from ground-based theropods into soaring, flying birds (and

even less skilled, frantically flapping and hopping chickens) is an important part of the evolution from

dinosaurs into birds. The answer to the question as to just how flight evolved is up to some debate; the

two theories are the Arboreal Theory (“from the trees down”) and the Cursorial Theory (“from the

ground up”). The Arboreal Theory states that tree-dwelling reptiles developed a form of gliding (and

eventually flight) that allowed them to jump from tree to tree without facing the dangerous predators

that lurked on the ground, much as a modern flying squirrel does today. On the other hand, the

Cursorial Theory states that flying started with leaps and jumps in attempts to escape prey and outrun

predators, that eventually evolved into more efficient gliding and finally flight. However, the Cursorial

theory has faced much criticism due to “the problems of drag and needing to work against gravity. It is

biomechanically easier to evolve flight from gliding than from the ground…” (Kate). Furthermore,

Archaeopteryx is thought to have lacked a supracoracoideus system – the tendon that powers the

upstroke of a bird’s flight. Experiments conducted on pigeons have shown that the lack of a

functioning supracoracoideus tendon could not take off from ground level. However, there is no

evidence to suggest that Archaeopteryx had the ability to climb trees, and there is evidence of an

absence of large trees near the areas inhabited by Archaeopteryx. Due to the overall weak evidence to


support the Arboreal Theory, most scientists believe that a modified version of the Cursorial Theory is

the likely origin of flight.

The closest sister groups of birds, such as Deinonychus and other dromaeosaurs, had a

sideways-flexing wrist joint used to seize prey. However, in birds, this same type of wrist joint is

essential to the production of thrust; “it would have only taken a slight adjustment of the angle of attack

of this predatory stroke to create a suitable vortex wake. By running, leaping, and a few such strokes,

extension of the time in the air, and eventually flight from the ground up, could have evolved” (Kate).

Running leaps to catch airborne prey such as insects was aided by feathered arms or wings outstretched

for balance, and the wings evolved to expand at the distal ends to increase stability when in the air.

Short flapping motions mimicking the already-present sideways-thrust provided even more airtime, and

these leaps were further extended by jumping from small heights. Eventually, through evolving more

physical and behavioral adaptations to extend time in the air, short leaps and glides evolved into full-

fledged flight.

While fossil evidence can tell us much about many aspects of Dinosaurian anatomy,

appearance, and even the development of flight, it is extremely difficult to determine how dinosaurs

behaved using physical evidence alone. This is where their relationship with chickens comes in handy.

Because the natural selection process favors positive behavioral traits as well as anatomical ones, so it

can be assumed that chickens share both anatomical and behavioral similarities with dinosaurs. By

looking to chickens as well as other birds, scientists can make educated guesses about how chickens'

reptilian ancestors behaved hundreds of millions of years ago.

One of the most widespread views of chickens in popular culture is that of the brooding hen.

Chickens are often depicted as loving mothers attentively caring for their chicks. Dinosaurs, though

related to chickens, are afforded a much different image in popular culture; they are viewed as simple-

minded, violent creatures who, like many modern reptiles, abandon their offspring as eggs and lack all


maternal instincts. Even though fossils of adult dinosaurs had been found on nests of fossilized eggs, it

was assumed for many years that these dinosaurs were simply stealing other species' eggs to eat. This

even resulted in some dinosaurs being labeled as egg-stealers, such as Oviraptor, which was discovered

in Mongolia on top of a nest of eggs that were believed to belong to a herbivore known as

Protoceratops. However, once an embryo that had been preserved in one of the eggs was examined, it

was determined that the eggs belonged not to Protoceratops but to Oviraptor; “poor Oviraptor,

maligned as an egg stealer, was apparently brooding its own nest as subsequent discoveries by

American Museum and Sino-Canadian expeditions have confirmed” (Meier). Subsequent fossils were

even found “huddled over its fossilized eggs in a posture identical to that of a modern bird” (Brochu).

In light of this evidence as well as the similarities to modern birds, it can be deduced that at least some

species of dinosaurs actively cared for their eggs. However, evidence has shown that, like chickens,

dinosaurs were rather attentive mothers as well.

When examining the nest of a duck-billed dinosaur called Maiasaura, John R. Horner's team of

scientist discovered two groups of young Maisaura fossils, the first group being newborn hatchings and

the second group being substantially older. Upon looking at the fossils under the found that “the ends

of the limb bones had been cartilaginous at the time of death. Such immature limbs were too weak for

the young animals to have run about on their own. Similar growth patterns occur in the nest-bound

young of some birds” (Meier). By comparing similar anatomical aspects of the young dinosaurs and

young birds, scientists can assume that, like chickens, dinosaurs had to care for their vulnerable young

until they were mature enough to fend for themselves. This brooding and maternal behavior further

links dinosaurs and birds such as chickens together.

Despite their less than ferocious image in popular culture, chickens are in fact very close

evolutionary relatives of theropod dinosaurs that lived millions of years ago. The anatomical and even

behavioral similarities between the two offer very strong evidence for this relationship, and the


discovery of missing links such as Archaeopteryx has shown that chickens, as well as other birds, did in

fact evolve from dinosaurs. Because of this close relationship, modern chickens can be used to learn

more about dinosaurs by comparing behaviors, anatomy, and even appearance. Especially when one

takes the latest scientific research into account and realizes that, instead of cold-hearted, scaled, simple-

minded beasts, some types of dinosaurs were found to be feathered, fluffy, hen-like creatures with

clutches of needy chicks nipping at their heels, their relationship with chickens becomes even more

apparent. With that in mind, chickens are not simply fluffy, egg-producing birds found in farmyards

across America; they are something much more vicious and amazing than many people realize. They

are dinosaurs.


Works Cited

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to Dinosaurs. Ed. Michael K. Brett-Surman. San Francisco: Fog City, 2002. Print.

Brusatte, Stephen, and Michael Benton. Dinosaurs. London: Quercus, 2008. Print.

Darwin, Charles. The Origin of Species. Oxford: Oxford UP, 1996. Print.

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<http://www.helium.com/items/1410877-similarities-between-dinosaurs-and-birds>.

"Dinosaurs and Birds." Wild Birds Unlimited | Bird Food, Bird Seed, Bird Feeder, Birdhouse,

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<http://dictionary.reference.com/browse/ilium>.

Kaiser, Gary. "Bird Ancestors and Feathered Dinosaurs." Gary Kaiser's The Inner Bird. Web. 02

Nov. 2011. <http://www.innerbird.com/ancestors_feathered_dinos/bird_ancestors.html>.

Katie. "The Origin and Early Evolution of Birds | GeologyRocks." GeologyRocks. 08 Feb. 2007.

Web. 01 Nov. 2011.

<http://www.geologyrocks.co.uk/tutorials/origin_and_early_evolution_birds>.

Meier, John J. Dinosaurs. New York: H.W. Wilson, 2011. Print.

Paul, Gregory S. The Princeton Field Guide to Dinosaurs. Princeton, NJ: Princeton UP, 2010.

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Mayr, Ernst. What Evolution Is. New York: Basic, 2001. Print.

Sloan, Chris. "True-Color Dinosaur Revealed: First Full-Body Rendering." National

Geographic. 4 Feb. 2010. Web. 01 Nov. 2011.

<http://news.nationalgeographic.com/news/2010/01/100127-dinosaurs-color-feathers-science/

o/>.

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Perspectives from Fossil Evidence." Naturwissenschaften (2004). SpringerLink. Web. 31 Oct.

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