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ORIGINAL SCIENTIFIC ARTICLE
Downsized Dinosaurs: The Evolutionary Transition
to Modern Birds
Luis M. Chiappe
Published online: 16 April 2009
#Springer Science + Business Media, LLC 2009
Abstract Living birds are the most diverse land vertebrates
and the heirs of a rich chapter in the evolution of life. The
origin of modern birds from animals similar to Tyranno-
saurus rex is among the most remarkable examples of an
evolutionary transition. A wealth of recently discovered
fossils has finally settled the century-old controversy about
the origin of birds and it has made the evolutionary saga
toward modern birds one of the best documented transitions
in the history of life. This paper reviews the evidence in
support of the origin of birds from meat-eating dinosaurs,
and it highlights the array of fossils that connect these
fearsome animals with those that fly all around us.
Keywords Dinosaurs .Birds .Origin .Evolution .Mesozoic
With nearly 10,000 living species, birds are the most
diverse land vertebrates and are the product of a long and
fascinating chapter in the evolution of life. The origin of
modern birds is undoubtedly one of the most dramatic
examples of an evolutionary transition—one connecting
animals akin to the fearsome Tyrannosaurus rex with the
feathered marvels we now see all around us—a transfor-
mation documented by a wealth of intermediate fossils that
date back to the Mesozoic Era (Chiappe 2007), the geologic
period that spanned between 245 and 65 million years ago.
The importance of the fossil record in providing evidence
of intermediate stages in an evolutionary transition has long
been recognized (Sues and Anderson 2007). Fossils provide
chronological information about milestones within a tran-
sition, they help us visualize the sequence of physical
transformations involved in it, and they document a series
of intermediate characteristics that are no longer present (or
that are highly modified) in extant organisms. Fossils also
document that the origin of any major group is accompa-
nied by a wide range of evolutionary experimentation in
which closely related lineages—whether contemporaneous
or not—approach to a greater or lesser degree the
characteristic trademarks of the new group. A wealth of
intermediate fossils has made the evolutionary saga toward
modern birds one of the best documented transitions in the
history of life (Fig. 1).
Birds have an ancient and enormously rich history. The
common ancestor of all living groups of birds can be traced
to at least the Late Cretaceous period, more than 75 million
years ago, and the earliest records of fossils widely
accepted as birds—those of the famed Archaeopteryx from
southern Germany—date back twice as far. Deciphering the
origin of birds, namely, identifying the closest relatives to
the most recent common ancestor of Archaeopteryx and
modern birds, has been a matter of scientific debate and
scrutiny throughout the history of evolutionary biology
(Chiappe 2007; Witmer 1991; Chatterjee 1997; Shipman
1998; Feduccia 1999). As early as the eighteenth century,
birds were generally placed immediately ahead of flying
fishes in the “chains of being”postulated by the naturalists
of that time. With the nineteenth century's advent of
evolutionary thinking, especially after Darwin's theory of
evolution by natural selection, more explicit hypotheses
of relationships were formulated. Post-Darwinian times
witnessed a diversity of hypotheses in which birds were
considered to be most closely related to a variety of extinct
and extant lineages of reptiles. These hypotheses related
birds to groups of animals such as turtles, crocodiles, and
their relatives, various primitive Triassic fossils (245 to 208
Evo Edu Outreach (2009) 2:248–256
DOI 10.1007/s12052-009-0133-4
L. M. Chiappe (*)
The Dinosaur Institute,
Natural History Museum of Los Angeles County,
900 Exposition Boulevard,
Los Angeles, CA 90007, USA
e-mail: chiappe@nhm.org
million years ago), pterodactyls, and their kin, and the
plant-eating ornithischians and the meat-eating theropod
dinosaurs. For decades, the origin of birds remained
obscure and controversial—the fossil record was too
fragmentary to provide a clear picture. Today, however,
most of the other hypothetical relationships have been
abandoned and the theropod hypothesis has received nearly
universal acceptance (Shipman 1998; Rowe et al. 1998;
Sereno 1999; Chiappe and Witmer 2002). In fact, because
birds are overwhelmingly interpreted as the descendants of
a group of carnivorous dinosaurs, most scientists argue that
they be considered living dinosaurs. Therefore, birds are
today interpreted as avian dinosaurs—Velociraptor,Tyran-
nosaurus,Brachiosaurus, and all other traditional “dino-
saurs”that coexisted with a variety of primitive Mesozoic
birds are referred to as nonavian dinosaurs.
Birds as Living Dinosaurs
The idea that the ancestry of birds can be traced back to a
group of carnivorous dinosaurs called theropods is not new
(Chiappe 2007). Nearly 150 years ago, soon after the
publication of Darwin's Origin of Species, German embry-
ologist C. Gegenbaur used similarities in the structure of
the ankle to place the small, 150-million-year-old theropod
Compsognathus in an intermediate position between birds
and other reptiles. At about the same time, American
paleontologist E.D. Cope compared the ankle of the
Jurassic theropod Megalosaurus to that of an ostrich, and
on the basis of this and other skeletal similarities, argued
for a close relationship of theropods and birds. Despite
these initial considerations, it was British anatomist T.H.
Huxley (Huxley 1868) who first popularized the idea that
birds had originated within theropod dinosaurs. In the
ensuing years, a myriad of other skeletal features support-
ing the dinosaurian origin of birds has been discovered in
the fossils of large and small theropods. Since the 1960s, a
greater understanding of small predatory dinosaurs of the
Cretaceous age, such as the dromaeosaurid Deinonychus
(Ostrom 1969,1976), has led to the idea that birds had
originated from within a group of bird-like theropods called
maniraptorans (Gauthier 1986). Today, the skeletons of
such maniraptoran theropods such as the sickle-clawed
dromaeosaurids (Deinonychus,Velociraptor, and their kin;
Fig. 1), the lightly built troodontids (Troodon,Mei, and
their kin), the parrot-headed oviraptorids (Oviraptor and
relatives), and the short-armed alvarezsaurids (Mononykus
and its kin) are recognized as sharing a great deal of similarity
with birds (Chiappe and Witmer 2002; Weishampel et al.
2004). Not only have birds retained the bipedalism,
hollowed bones, and the three fully developed toes of their
theropod predecessors, but these animals also share a series
of air spaces connected to the ear region, unique structures
of their vertebral column and rib cage, elongate forelimbs
with wrist bones allowing swivel-like movements of the
hand and similar structures in the pelvis and hindlimbs, as
well as many other characteristics distributed over the entire
skeleton (Rowe et al. 1998; Sereno 1999; Chiappe and
Witmer 2002; Weishampel et al. 2004; Novas and Puerta
1997; Holtz 1998). Indeed, many skeletal features previously
thought to be exclusively avian—such as wishbones, laterally
facing wingpits, and large breastbones—have now been
discovered among nonavian maniraptorans (Padian and
Chiappe 1998).
In recent years, a wealth of evidence taken from
comparisons between the skeletons of these dinosaurs and
those of birds has been supplemented by diverse lines of
evidence in support of the same evolutionary relationship.
Paleontologists have determined that the shape and struc-
ture of nonavian maniraptoran eggs were similar to those of
living birds (Mikhailov 1992; Zelenitsky 2006; Varricchio
and Jackson 2004; Grellet-Tinner et al. 2006). Some of
these features involve the presence of more than one
Fig. 1 The skeletons of the nonavian maniraptoran Velociraptor, the
Jurassic bird Archaeopteryx, the Early Cretaceous short-tailed bird
Sapeornis and enantiornithine Longipteryx, the Late Cretaceous
Ichthyornis, and the living Gallus (chicken). In recent years, a wealth
of bird-like nonavian maniraptorans and primitive (“dinosaur”-like)
birds have been unearthed from Mesozoic rocks worldwide—these
discoveries have consolidated the notion that birds evolved from
maniraptoran theropod dinosaurs. Drawings not to scale
Evo Edu Outreach (2009) 2:248–256 249
distinct crystalline layer in the eggshell (distinguished by a
differential disposition of eggshell crystals), reduction in
the number of airholes perforating the eggshell, a relative
increase in the volume of the egg (with respect to the adult's
size), and the development of asymmetrical eggs in which
one pole is narrower than the other (Fig. 2). Snapshots of
ancient behavior revealed by a handful of exceptional
fossils have also provided support to the hypothesis that
birds evolved from maniraptoran dinosaurs. The discovery
of a “gravid”oviraptorid female containing a pair of shelled
eggs inside her pelvic canal (Sato et al. 2005)has
confirmed previous interpretations based on the spatial
arrangement of eggs within clutches of nonavian manir-
aptorans. These clutches—particularly well known among
oviraptorids—show that the eggs were arranged in pairs, as
opposed to typical reptilian clutches (turtles, crocodiles, and
other dinosaurs), in which the eggs lack any spatial
arrangement (Grellet-Tinner et al. 2006) (Fig. 2). This
evidence indicates that, as with birds, nonavian maniraptor-
ans laid their eggs sequentially, at discrete time intervals. It
probably took several days for a nonavian maniraptoran
female to lay its egg clutch (Varricchio and Jackson 2004;
Grellet-Tinner et al. 2006), a condition shared with birds.
Other extraordinary discoveries have shed light on the
nesting behavior of these dinosaurs. Skeletons of oviraptor-
ids (Norell et al. 1995; Clark et al. 1999) and troodontids
(Varricchio and Jackson 2004) have been discovered on top
of their clutches of eggs. The fossils show evidence that
these animals adopted a posture similar to that of brooding
birds. In oviraptorids, the adult tucked its legs inside an
open space at the center of the egg-clutch and hugged the
periphery of the clutch with its long forelimbs; in the more
lightly built troodontids, the adult sat on top of the
vertically buried eggs. These discoveries suggest that,
regardless of its specific role (protection, incubation),
typical avian nesting behaviors (adults sitting on top of
their nests) were widespread among nonavian maniraptor-
ans. Additional evidence further documents behavioral
similarities with birds. Fossils of troodontids with their
skeleton arranged such that the hindlimbs are flexed
Fig. 2 Characteristics of the
eggs and clutches of several
nonavian maniraptorans support
the inclusion of birds within
these theropod dinosaurs. For
example, the presence of at least
two distinct crystalline layers in
the eggshell and the existence of
an asymmetric egg (less asym-
metric among oviraptorids) can
be traced back to as far as the
maniraptoran divergence. The
distribution of the eggs within a
clutch in oviraptorids indicates
that these dinosaurs laid their
eggs sequentially (other evi-
dence also indicates that, as in
the case of birds, they also
brooded their clutch)
250 Evo Edu Outreach (2009) 2:248–256
beneath the belly, the neck is turned backwards, and the
head is tucked between the wing and the body have
documented that at least some of the maniraptoran
precursors of birds had already evolved stereotypical
resting poses familiar to many birds (Xu and Norell 2004).
More specific fields of research have made their own
empirical contributions in support of the dinosaurian legacy
of birds. Studies of dinosaurian growth rates, based on
details preserved in the fossilized tissue of their bones, have
documented that these animals, once believed to be slow-
growing, actually grew at speeds comparable to many
living birds (Erickson et al. 2001), and special bone tissues,
such as the medullary bone characteristic of ovulating birds,
have been documented in a female T. re x (Schweitzer et al.
2005). Evidence in support of the evolutionary transition
between nonavian dinosaurs and birds has also been
uncovered from disciplines as far-off from classic paleon-
tology as genetics. Studies correlating the sizes of bone
cells and genomes (the entire genetic material of an
organism) have revealed that the mighty T. rex and its
fearsome kin had the small genomes typical of modern
birds (Organ et al. 2007), and putative protein sequences
from soft tissues of this dinosaur have also highlighted its
evolutionary closeness to birds (Organ et al. 2008, although
for a different interpretation of this evidence, see Dalton
2008).
Yet, despite the multiplicity of this extensive body of
evidence, nothing has cemented the dinosaurian pedigree of
birds more than the realization that true feathers—the
quintessential avian feature—may have covered the bodies
of a variety of nonavian dinosaurs (Norell and Xu 2005).
The enormous significance of these fossils notwithstanding,
the documented existence of feathers in nonavian dinosaurs
has, thus far, been limited to a dozen or so species, all of
them circumscribed to the Cretaceous deposits of East Asia.
Some of these dinosaurs exhibit feathers that are filament-
like, with a minimal degree of branching, but a number of
others display pennaceous feathers with distinct shafts and
vanes. In certain nonavian maniraptorans, long pennaceous
feathers attach to the distal part of the tail, either in a fan-
like fashion or giving the tail the frond-like appearance
common to primitive birds such as Archaeopteryx (Fig. 1a).
Long pennaceous feathers also attach to the tip of the
forelimbs of some of these maniraptorans, and in the case
of the peculiar dromaeosaurid Microraptor (Norell and Xu
2005), they form a wing of essentially modern design.
Despite the evidence of plumage being restricted to a
handful of nonavian dinosaurs, the fact that these fossils
span a large portion of the family tree of theropods and
display a great diversity of sizes, appearances, and life-
styles, hints at a much larger and yet undocumented
diversity (Fig. 3)—even the colossal T. re x may have been
covered with a cloak of feathers at some early stage of its
life. It is an amazing experience to gaze at the entirely
modern feathers of animals, whereas their skeletal charac-
teristics are so unquestionable dinosaurian.
Fig. 3 Genealogical relation-
ships of feathered nonavian
theropods. Current evidence
supports the hypothesis that fil-
amentous and vaned feathers
evolved with the divergence of
coelurosaurs and maniraptorans,
respectively
Evo Edu Outreach (2009) 2:248–256 251
An important corollary of these discoveries is that
feathers did not evolve in the context of flight. With the
sole exception of Microraptor, it is certain that none of
these feathered dinosaurs were able to take to the air. The
forelimbs and their feathers are both much shorter than in
flying birds and their bodies are larger. The evolutionary
transition toward birds and the origin of their flight
involved a dramatic reduction in body size. These feathered
dinosaurs indicate that, at their onset, feathers must have
had a different function, perhaps insulating the bodies of
animals that had metabolically diverged from their cold-
blooded, reptilian ancestors. My research has suggested that
vaned feathers may have originated in the context of thrust,
evolving in running nonavian theropods that by flapping
their feathered arms were able to increase their running
speed (Burgers and Chiappe 1999). In the end, however, we
simply do not have an answer for what was the original
function of feathers; nonetheless, we have been able to
eliminate flight as an option.
Today, the century-old debate on bird ancestry has
largely been resolved. The uncertainties that led to this
long controversy—both empirical and methodological—
have been clarified and there is an overwhelming consensus
in support of the idea that birds evolved from maniraptoran
theropods. Current evidence highlights the fact that many
features previously thought to be exclusively avian—from
feathers to a wishbone—have now been discovered in the
immediate dinosaur predecessor of birds. The origin of birds
was also preceded by a substantial reduction in body size—
the most primitive members of groups such as troodontids
and dromaeosaurids are smaller than one meter long (Turner
et al. 2007). This notable reduction in the size of the
forebears of birds was an important prerequisite of flight;
even this most characteristic avian attribute is likely to have
been inherited by birds from their dinosaurian predecessors.
The comparative studies that have been the building
blocks of these important evolutionary conclusions have
been greatly assisted by many newly discovered Mesozoic-
aged birds (Chiappe 2007), which by possessing many
skeletal features that are only slightly modified from the
ancestral maniraptoran condition, fill a critical gap in the
evolutionary transition toward modern birds (Figs. 1,4, and
5). This newly-discovered fossil menagerie has unveiled an
unexpected diversity of archaic birds that would take
birding to another dimension. These new discoveries are
reviewed next.
Fig. 4 Cladogram or diagram
depicting the genealogical rela-
tionships among the main line-
ages of premodern birds and
some lineages of nonavian
maniraptoran dinosaurs. The
known fossil record of these
groups is also highlighted. The
concept of a dove as a living
dinosaur—because they share a
common descent—may seem
bizarre, but, in reality, it is just
as logical as the argument that
humans are primates because we
evolved from primates
252 Evo Edu Outreach (2009) 2:248–256
The Long March Toward Modern Birds
Research on the early history of birds and the development
of flight has been at the forefront of paleontology since the
advent of evolutionary thought. For most of this time,
however, the available evidence was limited to a small
number of fossils largely restricted to near-shore and marine
environments and was greatly separated both anatomically
and in time. In the last few decades, however, our
understanding of the origin and ancient divergences of
birds has advanced at an unparalleled rate. This rapid
increase in discoveries has not only filled much of the
anatomical and temporal gaps that existed previously, but
has also made the study of early birds one of the most
dynamic fields of vertebrate paleontology.
New information highlights the fact that the enormous
diversity of living birds is just a remnant of an archaic
evolutionary radiation that can be traced back to Archae-
opteryx (Mayr et al. 2005) (Figs. 1and 4). Few physical
features set this most ancient bird apart from its theropod
dinosaur predecessors. However, Archaeopteryx gives us
paramount clues to the beginning of one of the most
dramatic evolutionary events in the history of vertebrates—
the development of powered flight in birds. This 150-
million-year-old jay-sized bird with toothed jaws, clawed
wings, and a long bony tail stands alone in the fossil record
of birds of the end of the Jurassic period. Yet, in the last
decade, a large number and variety of birds have been
found in early Cretaceous rocks ranging from 130 to 115
million years ago (Chiappe 2007; Zhou 2004). These fossils
reveal that a great diversity of birds with long bony tails
preceded the evolution of birds with an abbreviated bony
tail (Forster et al. 1998; Zhou and Zhang 2003), one
composed of fewer vertebrae ending in a bony stump called
apygostyle (the structure that supports the “parson's nose”).
Characteristics of the plumage, the large wing size, and
specific features of their brain all suggest that Archaeop-
teryx and the remaining long-tailed birds were fliers, even if
these birds probably required a take-off run to become
airborne (Burgers and Chiappe 1999).
A rich diversity of more advanced birds is also recorded
in these early Cretaceous rocks. In fact, the differing design
of skulls, teeth, wings, and feet indicate that, even at this
early phase of their evolutionary history, birds had
specialized into a variety of ecological niches, including
seed-feeders, insect-feeders, fish-eaters, and meat-eaters
(Chiappe 2007). At the same time, a host of novel features
of the wings, shoulders, and tails suggests that, soon after
Archaeopteryx, birds evolved flying abilities not very
different from the ones that amaze us today, a feat that
was most likely the recipe for their dramatic diversification
during the Cretaceous. Paramount among these transforma-
tions is the abbreviation of the tail and the consequent
development of a pygostyle. Yet, the details of this
evolutionary transition are far from clear. One recent fossil
that has shed some light onto this transition is the tiny, 125-
million-year-old Zhongornis (Gao et al. 2008) from
northeastern China. Zhongornis is the first bird discovered
that has a short tail and a corresponding reduced number of
tail vertebrae, yet lacks the pygostyle that is present in all
other short-tailed birds. Therefore, Zhongornis represents
Fig. 5 Photographs of the Berlin specimen of the Late Jurassic
Archaeopteryx (a), the Early Cretaceous short-tailed bird Confuciu-
sornis (b), long-tailed bird Jeholornis (c), enantiornithine Eoenantior-
nis (d), and primitive ornithuromorph Yanornis (e). Photographs not to
scale
Evo Edu Outreach (2009) 2:248–256 253
an intermediate stage between the primitive long-tailed
birds and those with a bony stump at the end of the tail.
Evidence from the skeleton of Zhongornis suggests that a
short tail with a reduced number of vertebrae evolved
earlier in birds than did the pygostyle.
Very early in their evolutionary history, short bony-tailed
birds blossomed in a range of shapes and sizes. Hundreds
of specimens of the stout-beaked Confuciusornis, many
surrounded by a halo of dark feathers, have been unearthed
from the 125-million-year-old deposits of northeastern
China (Chiappe et al. 1999) (Fig. 5). This crow-sized bird
sported long hands with enormous claws and long and
tapering wings. Growth series of Confuciusornis spanning a
large spectrum of sizes suggest that, unlike modern birds,
this and other archaic birds required multiple years to reach
adult size (Chiappe 2007). The contemporaneous and much
larger Sapeornis had longer and narrower wings, superfi-
cially resembling those of albatrosses (Zhou and Zhang
2002) (Fig. 1). Albeit bearing stout teeth and a very
primitive shoulder, the anatomy of this bird suggests a
closer relationship to modern birds than Confuciusornis.
Combined, however, these fossils best illustrate the anato-
my and appearance of the most primitive short-tailed birds,
which, by virtue of their proportionally larger wings, were
likely better fliers than their long-tailed predecessors.
Fossils of more advanced birds are also first recorded at
around 130 million years ago. Among these are the
enantiornithines (Chiappe 2007; Chiappe and Witmer
2002), a group that constitutes the most important evolu-
tionary radiation of premodern birds. Like most early birds,
the majority of enantiornithines had toothed jaws and
partially clawed wings (Figs. 1and 5). Yet their skeletons
show a series of key transformations that approach those of
today's birds. Some of these include the shortening of the
hand and fingers as well as changes in the proportions of
the wing bones and the anatomy of the shoulder. Further-
more, these birds evolved important innovations in their
plumage, namely, a safety device called the alula (a small
tuft of feathers also known as the “bastard wing”), which
assists modern birds during their take-off and landing (Sanz
et al. 1996). The significant transformations of the skeleton
and plumage of these birds suggest that, even at the onset of
their evolutionary history, enantiornithines were able to
take-off from a standstill position and maneuver in ways
similar to those seen among living birds. It is most likely
that the evolution of these enhanced flying capabilities
played a key role in the evolutionary success of the
enantiornithines, which by about 120 million years ago
seem to have risen to dominance.
Rocks from the early Cretaceous also record a number of
transitional fossils that herald the evolution of the closest
relatives of modern birds (Fig. 1). In some respects, these
primitive ornithuromorphs (Chiappe 2007; Chiappe and
Witmer 2002; Zhou 2004; Zhou and Zhang 2005) resemble
the enantiornithines, but their skeletons show, for the first
time, clear trademarks of their living counterparts. The
majority of these primitive ornithuromorphs were lightly
built, flying birds, whose sizes tend to be larger than those
of their contemporaneous enantiornithines. Like the latter,
both their skeletons and plumage show clear evidence of
enhanced aerodynamic capabilities. It is within these birds
that we witness the origin of the extremely fast rates of
body maturation characteristic of modern birds (Chiappe
and Witmer 2002), which reach their full body size within a
year after hatching.
As the rocks of the Cretaceous period become younger, a
series of other lineages of ornithuromorphs make their
debut. The hesperornithiforms—large, flightless, foot-
propelled divers—first appear around 100 million years
ago (Chiappe 2007; Chiappe and Witmer 2002). Albeit
entirely restricted to the aquatic realm, the hesperornithi-
forms exhibit a rich and diverse evolutionary history
spanning over 35 million years—their last representatives
may have disappeared with the latest Cretaceous mass
extinction that wiped out the last of the nonavian dinosaurs.
Despite the fact that their earliest records represent birds the
size of a loon, millions of years later, these supreme fish-
eaters would be crowned kings of the aquatic birds with a
number of large forms such as the tiny-winged, four-foot
long Hesperornis and Asiahesperornis. The hesperornithi-
forms swam the waters of tropical seas that, during the late
Cretaceous, divided in half both North America and
Eurasia. On the shore of these shallow seas, over herds of
duck-billed and other kinds of dinosaurs, soared the tern-
sized Ichthyornis (Clarke 2004) (Fig. 1). In most respects,
this bird represents a step closer to modern birds, yet it had
sharply toothed jaws designed to catch fish. Ichthyornis is
perhaps the best-known, closest relative of modern birds;
other late Cretaceous fossils seemingly close to the latter
are known by much more fragmentary remains.
Not all the birds that lived during the Mesozoic may
have looked as unfamiliar as Archaeopteryx,Confuciusor-
nis, and Hesperornis. The early representatives of today's
lineages of birds can also be traced back to this remote era
of our geological past. In several continents, rocks from the
last part of the Cretaceous period—75 to 65 million years
ago—reveal the remains of early shorebirds, ducks, and
other familiar birds (Kurochkin et al. 2002; Clarke et al.
2005). These discoveries indicate that a number of modern
lineages had their origins prior to the end of the Mesozoic.
It is unclear how these early representatives of modern
birds managed to survive the devastating mass extinction of
the end of the Cretaceous, but these survivors diversified
soon after into a myriad of forms, which today carry the
legacy of the magnificent dinosaurs that ruled the earth tens
of millions of years ago.
254 Evo Edu Outreach (2009) 2:248–256
The Dinosaur in your Backyard
In the last few decades, our understanding of the origin and
subsequent evolutionary diversification of birds has ad-
vanced at an unparalleled pace. These fossil discoveries
have documented the stepwise nature of one of the most
fascinating evolutionary transitions, and they have filled the
large gap that separated living birds from their dinosaurian
predecessors. This new evidence has shown that many of
the features previously considered to be avian trademarks
first evolved within theropod dinosaurs.
The strength of the hypothesis that birds evolved within
maniraptoran theropod dinosaurs is manifested by the
convergent results of a diversity of studies within a
multitude of scientific disciplines. Today, the theropod
origin of birds is supported by a wealth of evidence ranging
from skeletal anatomy to molecular data. This evolutionary
conclusion indicates that the diverse modern birds are a
branch of a much larger avian tree that diverged during the
Mesozoic era and that, in turn, all these birds are but a
shoot of the majestic tree of dinosaurs. This evidence has
led to the realization that the jays, finches, and humming-
birds that so peacefully frequent your backyard are indeed
living dinosaurs—a surviving lineage of vicious predators
that ruled the terrestrial ecosystems of the Mesozoic.
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