Dinosaurs
And Birds
from an article of Dr Paul Willis
Here's how the joke goes: Have you eaten a dinosaur lately?
Answer? Ya! I love chicken!
 Chances are dinosaurs are part of many people's regular diet
. . .
. . . because, if birds evolved from
dinosaurs, every time they eat chicken, they're eating dinosaur.
But how do we know birds evolve from
dinosaurs?
According to Dr. Paul Willis, it all
depends on how you work out relationships.
Working out Relationships -- A
Scientific Side Trip
Systematics is the science of evolutionary relationships, and it has
undergone a major change over the last couple of decades.
In the
mid 1960's the German entomologist Willy Hennig claimed that the only features
that carry any useful information about pedigree are the evolutionary novelties shared between organisms.
Here's
an example of what he was thinking:
Suppose
we're trying to work out the relationships between a mouse, a lizard, and a
fish. They've all got backbones so the feature "backbone" is useless;
it's a "primitive" character that tells you nothing.
But
the feature "four legs" is useful because it's an evolutionary
novelty shared only between the lizard and the mouse.
This
implies that the lizard and mouse are more closely related to each other than
either is to the fish.
Put
another way, the lizard and the mouse share a common ancestor that had four
legs.
So,
the more evolutionary novelties we can find that support a particular
relationship, the greater our confidence that the relationship is correct.
Think
of this: "Air breathing", "neck" and "amniotic
egg" are another three evolutionary novelties that tie the lizard and the
mouse together and leave the fish as a more distant relative.
This
new approach to systematics is called Cladistics or Phylogenetic
Systematics. Cladists identify useful characters then measure
their effect on the relationships of the organisms they're studying.
So,
with this in mind, let's look for relationships -- or common features between
dinosaurs and birds.
Are Birds Dinosaurs?
Here's a great story -- and a true one
at that:
Thomas Huxley, a famous biologist, was
eating quail one evening while thinking about a puzzle poised by a strange bone
back in the lab.
He knew it was the lower leg bone
(tibia) of a meat-eating dinosaur but fused across the bottom of it was an
unidentified extra bone.
He happened to suck the flesh off the
bottom of the quail leg and there, fused across the bottom of the quail tibia,
was the same bone.
Because he was holding a complete bird
leg, Huxley realized that the strange bone was the anklebone (astragalus).
But, Huxley also realized that the form
of the anklebone -- in both the dinosaur and the bird -- were so similar that
they must be closely related.
He saw the common feature, the common
relationship.
Unfortunately, and typical to science,
Huxley's dino-bird theory didn't last long and fell into disfavor. A new theory
postulated that birds evolved from some pre-dinosaurian reptile came along.
Here's how: In 1916 the Danish medical
doctor Gerhard Heilmann published The Origin Of Birds, in which he
commented on the many similarities between the skeletons of meat-eating
dinosaurs (Theropods) and birds.
But Heilmann also noted that theropods
lacked collarbones (clavicles) which fuse together to become the wishbone
(furcula) in birds. Heilmann argued that such a feature could not be lost and
then re-evolved at a later date, so theropods couldn't be the ancestors of
birds. Thus the theropods were banished from the bird's family tree for the next
fifty years. . . .
Well, the story picks up in the late
1960's.
John Ostrom from Yale University noted
22 features in the skeletons of meat-eating dinosaurs that were also found in
birds and nowhere else.
This reset the thinking on bird
ancestry -- and took the focus way from Heilmann's ideas. Birds and Dinos were
back.
Other researchers found up to 85
features or characters that tie Theropods and birds together. Although some of
them may be questionable, so many characteristics shared between Theropods and
birds was a pretty convincing argument in favor of the relationship.
But what of Heilmann's missing
collarbones?
Well, it was kind of a scientific joke.
It turned out that theropods not only had clavicles but that they were fused
together into a furcula. Unfortunately for Heilmann, the fossil evidence was
sparse in his day and the few theropod furculae that had been found were
misidentified, as belly ribs.
A cosmic belly laugh.
Here's a comic illustration.
Objections to Dinobirds
Two vocal opponents of the "Birds
Are Dinosaurs" theory are Alan Feduccia, of the University of North
Carolina and Larry Martin, from the University of Kansas.
They contend that birds evolved from
some unknown reptile from a time before the dinosaurs. Rather than argue from a cladistic perspective, Feduccia and Martin advance a series of other
arguments against the dino-bird hypothesis.
- Feduccia and Martin claim that flight
is most likely to have started from a tree-climbing (arboreal) ancestor but
that all the proposed dinosaurian ancestors were ground-dwellers (cursorial).
This
is the "trees-down" versus the "ground-up" debate and the
trees-downers do have a point.
There
is a variety of modern animals that are gliders, possibly representing a bridge
between flying and non-flying animals, and all gliders are arboreal (including
frogs, snakes, lizards and a variety of mammals).
But
the Birds-Are-Dinosaurs crowd argue that an unknown dinosaurian bird-ancestor
could have been arboreal or that birds evolved flight from the ground up by
chasing and leaping after insects (there are plenty of little theropods thought
to have made a living by doing just that).
- Most of the arguments raised by these
guys against the Birds-Are-Dinosaurs hypothesis are based on differences between
birds and dinosaurs.
For
example, they argue that the theropod ribcage is compressed from side to side
while in birds it's compressed more from back to belly. Cladists argue that differences between organisms don't matter, it's the similarities that
count.
Evolution dictates that organisms will
change through time so it's only the features that remain the same that will
carry useful information about their origins.
The Fossil Evidence
"In the beginning there was Archaeopteryx..."
Most combatants in this debate agree
that Archaeopteryx is the first bird.
Recovered from limestone quarries in
southern Germany, Archaeopteryx is a 145 million-year-old, crow-sized skeleton
covered in feathers.
There is no disputing that
Archaeopteryx had feathers, they are clearly preserved along with two of the
seven known specimens, and feathers are a distinctly birdie feature.
But the skeleton of Archaeopteryx is
distinctly non-bird-like with a long bony tail, teeth instead of a beak and
claws on the wings.
Here's another view:
The Birds-Are-Dinosaurs group contend
that, if feathers had not been found with Archaeopteryx, it would have been
identified as a small dinosaur (in fact the five specimens without feathers had
previously been identified as the small dinosaur Compsognathus).
The skeleton does have some bird-like
features such as a wishbone (furcula) and bird-like feet that suggest to the
Birds-Are-Not-Dinosaurs camp that Archaeopteryx is too bird-like to be
considered a dinosaur.
The
problem becomes one of linking an ancestor to Archaeopteryx then linking
Archaeopteryx to modern birds.
This is where things get really
interesting because in the last decade several significant dinosaurs with
bird-like features and primitive birds with dinosaur-like features have been
found around the world.
Bird-like dinosaurs
China has given its name to what may be
a feathered dinosaur. Sinosauropteryx comes from rocks thought to
be around 130 million years old in northeast China.
Sinosauropteryx Fossil
It's the skeleton of a dinosaur but
it's surrounded by a halo of fuzz.
No one's quite sure what the fuzz is.
It appears to be hair-like structures that could have helped insulate the beast
and some authorities think that these are proto-feathers.
Whatever the fuzz turns out to be, it's
significant that at least one member of the group of dinosaurs thought to be
ancestral to the birds was experimenting with a body covering more complicated
than bog-standard reptilian scales.
Further, the fact that this body
covering consists of stiff rods projecting away from the body is at least part
of the way to creating a structure something like feathers.
Here's
another specimen of interest: Scipionyx.
An exceptionally preserved baby
theropod happens to also be the only dinosaur ever found in Italy. Scipionyx lived
during the Cretaceous Period, 110 million years ago.
Details of preservation of Scipionyx
include the intestines, liver and chest musculature. And, nestled at the top of
the chest is a beautifully preserved furcula.
Fossil of a juvenile individual of Scipionyx samniticus. The fossil (to date the first of this species ever) preserves in an exceptional way clear traces of soft tissues - a rather rare event
Picture by Ghedo, August 6, 2006
While not the first theropod furcula,
it does underline the fact that this bird-bone was close to the heart of
meat-eating dinosaurs.
There
are other dinosaurs with bird-like features.
Unenlagia, from 88 million year old beds of Patagonia folded its arms
in the same way that birds do.
How Unenlagia may have looked, copyright Anness Publishing / NHMPL
Oviraptor from Mongolia, once thought to be an egg thief, is now
known from several specimens crouching over nests of their own eggs in exactly
the same pose as brooding emus.
Illustration of Oviraptor by John Agnew
For
more about John Agnew and his art, go here.
www.johnnagnew.com/murals
And a reassessment of other theropods
reveals such bird-like features as hollow bones and a foot with three functional
toes, bird-like features that appeared over 50 million years before the first
feeble flying flaps flung Archaeopteryx into the air.
So structurally, the fossils are
offering a pretty consistent picture that the Birds-Are-Dinosaurs hypothesis is
correct.
But there's a hitch.
The closest dinosaurian relatives to
the birds occur in the fossil record after Archaeopteryx.
Unless Velociraptor and kin perfected
time travel, there's no way they can be the ancestors of a bird that lived
sixty million years earlier. Some recent finds suggest that bird-like dinosaurs
did exist earlier than previously thought, but the fossils are scrappy and
inconclusive.
Given the improbability of
fossilisation, it's quite possible that pre-Archaeopteryx dino-birds were simply
not preserved.
Red
herring or real ancestor?
There's a highly controversial fossil
in the bird ancestry story.
Protoavis is known from a handful of bony fragments collected from
200 million-year-old rocks in Texas. At this time the dinosaurs were just
getting started and it's 50 million years before Archaeopteryx.
Protoavis does have some bird-like
features, but nothing that an early theropod dinosaur shouldn't have. Further,
there seems to be a strong possibility that the fossils described as a single
skeleton may actually belong to two or more individuals and possibly two or
more species! The material is too fragmentary and ambiguous to work with so,
until better material is found, most palaeontologists have set Protoavis aside.
Dinosaur-like birds
Rahonavis is a primitive bird from 80 million-year-old rocks of
Madagascar.
Rahonavis ostromi, a dromaeosaur from the Late Cretaceous of Madagascar, pencil drawing by Arthur Weasley.
A GNU Free Documentation License.
Despite being more bird-like than
Archaeopteryx, raven-sized Rahonavis retains some very distinctive theropod
features including the distinctive slashing claw used to murderous effect by
Velociraptor in the film Jurassic Park.
Velociraptor is thought to be about as close as a dinosaur gets to being
a bird without actually being one.
Velociraptor Fossil Skull
Besides these Dino-Birds, a host of
small primitive birds have been found elsewhere around the world.
Here's some of them.
From
Mongolia comes a large flightless bird, Mononykus, with wings
replaced by a lethal-looking pair of single-digit hands that stuck out in front
like a pair of combat chop sticks.
Another flightless bird from Patagonia
confirms that not all early birds thought the sky was a great place to be.
Other primitive birds lived along side
Rahonavis in Madagascar. A sparrow-sized bird from Spain had a more modern
shoulder joint than Archaeopteryx and a perching foot but it still had teeth.
The first known beak adorned a Chinese
bird 130 million years ago, along with the oldest pygostyle (the
"parsons-nose" in birds that is all that remains of the reptilian
tail). 110 million-year-old feathers and bird bones have even been recovered
from Victoria and Queensland.
It would appear that once true birds
had evolved they didn't waste much time in spreading around the world!
Back
to the Chicken Dinner . . .
In a nutshell, the majority of
palaeontologists working on the ancestry of birds agree that dinosaurs,
particularly small theropods, are the grandparents of present-day parrots,
partridges and pigeons.
There are some detractors to this
emerging orthodoxy but the dino-bird theory is supported by both the most
widely used methodology (cladistics) and a rapidly growing collection of
primitive birds and advanced meat-eating dinosaurs.
A reasonable assessment of the debate
would have to conclude that it's all over, including the shouting, in favour of
dino-birds.
Perhaps this may cause you to think
twice next time you bite into a roasted chicken. Would you relish your meal
quite as much if your humble chicken's relatives (such as Tyrannosaurus) were
still around to defend them?
But Here's More . . .
Paleontologists in China recently found
fossils that provide the best evidence yet for a link between dinos and birds.
Protarchaeopteryx lived
in the Early Cretaceous, 124.6 million years ago. It is probably more primitive than Archaeopteryx, making it a non-avian theropod dinosaur rather than a true
avian bird.
Protarchaeopteryx© Photographed by O. Louos Mazzatenta1998 National Geographic Society. All rights reserved.
Protarchaeopteryx has
dinosaur features along with recognizable feathers on its tail. The animal has
more primitive feathers than the earliest known bird, Archaeopteryx.
Caudipteryx (which means "tail feather") is a genus of
peacock-sized theropod dinosaurs that lived in the early Cretaceous Period (about 124.6 million years ago). They were feathered
and remarkably birdlike in their overall appearance.
Caudipteryx has an
interesting mix of reptile- and bird-like anatomical features.
Caudipteryx zoui fossil replica displayed in Hong Kong Science Museum
A speedy runner, Caudipteryx looked like a dinosaur but was covered with feathers
that lacked the aerodynamic quality necessary for flight.
Model of Caudipteryx zoui
Here's another view of this dinosaur,
showing its feathers.
Caudipteryx.
Photograph by O. Louis Mazzatenta© 1998 National Geographic Society. All rights reserved.
Caudipteryx was
an animal with features of a theropod dinosaur but with feathers over its body.
Feathers are especially visible on the animal's arms (center).
Confuciusornis is a genus of crow-sized, primitive, birds from the Early Cretaceous, dating to 124.6 million years ago.
Confuchisornis snactus skeleton displayed in the Hong Kong Science Museum
Like modern birds, Confuciusornis had a toothless beak, but close relatives of modern
birds such as Hesperornis and Ichthyornis were toothed, indicating that the loss of teeth occurred
convergently in Confuciusornis and living
birds. It is the oldest known bird to have a beak.
Features linking birds and dinosaurs
Over a hundred distinct anatomical
features are shared by birds and theropod
dinosaurs. Some of the more interesting similarities are discussed here. This
comes from Wikipedia. (Refer to the complete discussion, linked below.)
Feathers
 |
Archaeopteryx, the first good example
of a "feathered dinosaur," was discovered in 1861. The initial
specimen was found in the solnhofen limestone in southern Germany, which is a
lagerstätte, a rare and remarkable geological formation known for its superbly
detailed fossils.
Archaeopteryx is a transitional
fossil, with features clearly intermediate
between those of modern reptiles and birds.
Discovered two years after Darwin's Origin
of Species, its discovery spurred the debate
between proponents of evolutionary
biology and creationism. This early bird is so dinosaur-like that, without a clear
impression of feathers in the surrounding rock, at least one specimen was
mistaken for Compsognathus.
Since the 1990s, a number of additional feathered
dinosaurs have been found, providing even
stronger evidence of the close relationship between dinosaurs and modern birds.
Most of these specimens were unearthed
in Liaoning province, northeastern China,
which was part of an island continent during the Cretaceous period. Though
feathers have been found only in the lagerstätte of the Yixian Formation and a few other places, it is possible that non-avian
dinosaurs elsewhere in the world were also feathered. |
| From Owen, Richard. "On the Archeopteryx of von Meyer, with a decription of the Fossil Remains of a Long-tailed species, from the Lithographic Stone of Solenhofen," in: Philosophical Transactions of the Royal Society of London, vol. 153 (1863), pp. 33-47 |
The lack of widespread fossil evidence
for feathered non-avian dinosaurs may be due to the fact that delicate features
like skin and feathers are not often preserved by fossilization and thus are absent from the fossil record.
A recent development in the debate
centers around the discovery of impressions of "protofeathers"
surrounding many dinosaur fossils. These protofeathers suggest that the tyrannosauroids may have been feathered.
Cast of the fossil dromaeosaur specimen NGMC 91 (nicknamed "Dave", cf. Sinornithosaurus), at the American Museum of Natural History in New York.
The feathered dinosaurs discovered so
far include:
- Beipiaosaurus,
- Caudipteryx,
- Dilong,
- Microraptor,
- Protarchaeopteryx,
- Shuvuuia,
- Sinornithosaurus,
- Sinosauropteryx, and
- Jinfengopteryx, along with dinosaur-like birds, such as
- Confuciusornis, which are anatomically closer to modern avians. All of
them have been found in the same area and formation, in northern China.
The Dromaeosauridae family, in particular, seems to have been heavily feathered
and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.
Skeleton
Because feathers are often associated
with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs.
However, the multiple skeletal features
also shared by the two groups represent the more important link for paleontologists. Furthermore, it is increasingly clear that the
relationship between birds and dinosaurs, and the evolution of flight, are more
complex topics than previously realized.
For example, while it was once believed
that birds evolved from dinosaurs in one linear progression, some scientists,
most notably Gregory
S. Paul, conclude that dinosaurs such as the dromaeosaurs may have evolved from birds, losing the power of flight
while keeping their feathers in a manner similar to the modern ostrich and
other ratites.
Comparisons of bird and dinosaur
skeletons, as well as cladistic
analysis, strengthens the case for the link,
particularly for a branch of theropods called maniraptors.
Skeletal similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, shoulder
blade, clavicle, and breast bone.
Lungs
Large meat-eating dinosaurs had a
complex system of air sacs similar to those found in modern birds, according to
an investigation which was led by Patrick O'Connor of Ohio University.
The lungs of theropod dinosaurs
(carnivores that walked on two legs and had birdlike feet) likely pumped air
into hollow sacs in their skeletons, as is the case in birds. "What was once formally
considered unique to birds was present in some form in the ancestors of
birds," O'Connor said.
Heart and sleeping posture
Modern computed
tomography (CT) scans of a dinosaur chest cavity
(conducted in 2000) found the apparent remnants of complex four-chambered
hearts, much like those found in today's mammals and birds.
The idea is controversial within the
scientific community, coming under fire for bad anatomical science or simply
wishful thinking. A recently discovered troodont fossil demonstrates that the dinosaurs slept like certain modern
birds, with their heads tucked under their arms.
This behavior, which may have helped to
keep the head warm, is also characteristic of modern birds.
Reproductive biology
A discovery of features in a Tyrannosaurus rex skeleton recently provided even more evidence that dinosaurs and
birds evolved from a common ancestor and, for the first time, allowed
paleontologists to establish the sex of a dinosaur.
When laying eggs, female birds grow a
special type of bone in their limbs. This medullary bone, which is rich in calcium, forms a layer inside the
hard outer bone that is used to make eggshells. The presence of
endosteally-derived bone tissues lining the interior marrow cavities of
portions of the Tyrannosaurus rex specimen's hind limb suggested that T.
rex used similar reproductive strategies, and revealed the specimen to be
female.
Further research has found medullary
bone in the theropod Allosaurus and ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the
evolution of dinosaurs, this suggests that dinosaurs in general produced
medullary tissue
Brooding and care of young
Photograph of a nesting oviraptorid dinosaur Citipati osmolskae, specimen IGM 100/979 (nicknamed"Big Mamma"), at the American Museum of Natural History in New York.
Several Citipati specimens have been found resting over the eggs in its nest
in a position most reminiscent of brooding.
Numerous dinosaur species, for example Maiasaura, have been found in herds mixing both very young and adult
individuals, suggesting rich interactions between them.
A dinosaur embryo was found without
teeth, which suggests some parental care was required to feed the young
dinosaur, possibly the adult dinosaur regurgitated food into the young
dinosaur's mouth (see altricial). This behaviour is seen in numerous bird species; parent
birds regurgitate food into the hatchling's mouth.
Gizzard stones
Another piece of evidence that birds
and dinosaurs are closely related is the use of gizzard stones.
These stones are swallowed by animals
to aid digestion and break down food and hard fibres once they enter the
stomach. When found in association with fossils, gizzard stones are called gastroliths. Gizzard stones are also found in some fish (mullets, mud
shad, and the gilaroo, a type of trout) and in crocodiles.
Molecular evidence and soft tissue
The National history museum in Milan, Italy. Detail from the fossil of a juvenile individual of Scipionyx samniticus. The fossil (to date the first of this species ever) preserves in an exceptional way clear traces of soft tissues - a rather rare event.
Picture by Giovanni Dall'Orto 22-4-2007
One of the best examples of soft tissue
impressions in a fossil dinosaur was discovered in Petraroia, Italy.
The discovery was reported in 1998, and described the specimen of a small, very
young coelurosaur, Scipionyx samniticus. The fossil includes portions of the intestines, colon,
liver, muscles, and windpipe of this immature dinosaur.
In the March 2005 issue of Science, Dr. Mary Higby Schweitzer and her team announced the
discovery of flexible material resembling actual soft tissue inside a
68-million-year-old Tyrannosaurus
rex leg bone from the Hell
Creek Formation in Montana.
After recovery, the tissue was
rehydrated by the science team. The seven collagen types obtained from the bone fragments, compared to
collagen data from living birds (specifically, a chicken), reveal that older theropods and birds are closely
related.
When the fossilized bone was treated
over several weeks to remove mineral content from the fossilized bone marrow
cavity (a process called demineralization), Schweitzer found evidence of intact
structures such as blood
vessels, bone matrix, and connective tissue
(bone fibers).
Scrutiny under the microscope further
revealed that the putative dinosaur soft tissue had retained fine structures
(microstructures) even at the cellular level. The exact nature and composition
of this material, and the implications of Dr. Schweitzer's discovery, are not
yet clear; study and interpretation of the specimens is ongoing.
The successful extraction of ancient
DNA from dinosaur fossils has been reported on two separate occasions, but upon
further inspection and peer
review, neither of these reports could be
confirmed.
However, several proteins have putatively been detected in dinosaur fossils,
including hemoglobin.
So
What Does This All Mean?
To date, most researchers support the
view that birds are a group of theropod dinosaurs that evolved during the Mesozoic Era.
Birds share hundreds of unique skeletal
features with dinosaurs, especially with theropods like the dromaeosaurids, which most analyses show to be their closest relatives.
Although harder to identify in the
fossil record, similarities in the digestive and cardiovascular
systems, as well as behavioral similarities and the shared presence of feathers, also link birds with dinosaurs.
The ground-breaking discovery of
fossilized Tyrannosaurus
rex soft tissue allowed comparison of cellular anatomy and protein sequencing of collagen tissue, both of which provided additional evidence
corroborating the dinosaur-bird relationship
However, the origin of bird flight is a separate but related question for which there are
several proposed answers.
And so the study and debate
continue.... As it always does with science.
Some
Reading . . .
Here's an extinsive article from the on-line encyclopedia, Wikipedia, on the Origin
of Birds:
http://en.wikipedia.org/wiki/Origin_of_birds#cite_note-Trex-0
|
