|
07/27/2005
Bones Tell a Tale
My 1962 World Book Encyclopedia defines a bird quite simply:
“an animal with feathers.” Some years ago, my wife excitedly
called to me to come see an unusual bird perched on the side of a
tree in our backyard. Consulting our Roger Tory Peterson bird
book, we identified it as a pileated woodpecker. My wife
decided to alert the Audubon Society to the presence of this
striking bird, looked up and dialed the phone number. When a
gentleman answered, she promptly went into great detail
describing the woodpecker. Finally, the fellow interrupted
saying, “Lady, this is all very interesting but this is the Audubon
Garage!” Never did get the Audubon Society.
On the front page of this past Sunday’s New York Times, in an
article by James Gorman, there was a reproduction in color of
Peterson’s pileated woodpecker, together with his rendition of
the ivory-billed woodpecker, the subject of extensive media
coverage recently. The ivory-billed, the largest woodpecker in
America, was thought to be extinct until Bobby Harrison, who
teaches photography at Oakwood College in Alabama, and Tim
Gallagher of the Cornell Laboratory of Ornithology went
canoeing last year in a wildlife refuge in Arkansas trying to
confirm a rumored sighting of the bird.
Harrison and Gallagher did spot a bird that Harrison immediately
identified as an ivory-billed woodpecker. There followed an
intensive, secret search by Cornell Lab workers and others to
confirm the sighting and a paper in the June 3 issue of Science
proclaimed that the ivory-billed had indeed been sighted.
Birders everywhere embraced the discovery. There are some
skeptics, however, and, according to the Times article, a paper is
in the works claiming that proof of the ivory-billed’s existence
has not been demonstrated.
Initially, another claim about birds was greeted skeptically. That
claim is that the birds among us are modern day members of the
dinosaur family. I can’t understand such skepticism. Some
dinosaurs had feathers and a bird is an animal with feathers,
right? OK, early reports of fossils of feathered dinosaurs were
questionable in some cases but today the existence of feathered
and fuzzy dinos is well established. The Liaoning Province in
China, north of Beijing, was the locale for a series of volcanic
eruptions back in the period 130 to 110 million years ago. The
ash from the eruptions killed and buried dinosaurs and other
plants and animals of that era, creating a layered treasure trove of
unusually well preserved fossils. An article, “Jewels in the Ash”
by Cliff Tarpy in the August issue of National Geographic, cites
Liaoning fossils including one of a 125 million-year-old
tyrannosaur sporting downy, hairlike protofeathers, precursors to
later real feathers.
Until birds were identified as the real dinosaurs among us, the
crocodile was generally considered to be the animal with the
characteristics most closely resembling those of the dinosaur. A
new study of the respiratory systems of dinosaurs and birds
concludes that dinos had respiratory systems much like birds’
systems and nothing like the respiratory system of the crocodile.
Patrick O’Connor of Ohio University’s College of Osteopathic
Medicine and Leon Claessens, a newly minted Ph.D. from
Harvard, have examined bones in museums from California to
Berlin and London and points in between. O’Connor has also
looked at a dinosaur he found in 1996 in Madagascar when he
was a graduate student. The two researchers were working
independently but found their results and interests converging
and teamed up for the study, published recently in Nature.
The study compares the pulmonary systems of the ancient dinos
with modern birds, with emphasis on how the soft tissues may
have been structured. Birds have a distinctive type of pulmonary
system in which the air passes through the lungs twice as it
breathes. Birds have their own air-conditioning in which their
lungs can pump cool air into air sacs in their body. The anatomy
of the bird leads to a high rate of metabolism and warm-
bloodedness. O’Connor and Claessens point out that the
dinosaur’s anatomical features, while not identical, are quite
similar to those of the bird and push the dino towards warm-
bloodedness. They feel that the dinos were likely to have been in
between warm- and cold-bloodedness.
A high metabolic rate implies that growth can be pretty fast.
Take the sparrow, which reaches its adult size a mere seven days
after emerging from the egg. What about dinosaurs? How fast
did they grow? An article titled “How Dinosaurs Grew So Large
and So Small” in the July Scientific American answers the
question. The article is by John Horner, Kevin Padian and
Armand de Ricqles. The title page of the article is illustrated
with an eye-catching, huge Brachiosaurus peering down at a tiny
chicken-sized Microraptor.
Horner is a Professor of Paleontology at Montana State
University, while Padian and de Ricqles hail from UC Berkeley
and the College de France in Paris, respectively. They’ve
worked together for a dozen years on bones, which, like trees,
tell a lot about their age and growth when cross-sectioned. With
a tree, just count the rings and you know the age. With a bone,
it’s more complicated. Counting rings in bone doesn’t tell you
the age directly. As new bone grows on the outer part, the inner
core of the bone is being eaten away by a type of cell known as
an osteoclast cell. At the same time, there’s another process that
may build new bone in that same region. A bone is not a static
thing, as we’ve discussed in some distant column. So, it takes
skill and studies on the bones of young and old dinos to come up
with a reliable ages and growth conditions.
We also have two types of bone growth to consider. One is
“fibro-lamellar” tissue growth; the other is “lamellar-zonal”
tissue growth. In a dinosaur, the predominant bone growth is via
the fibro-lamellar tissue growth. Here, the bone is highly
fibrous, sort of “woven” in texture in which the bone forms
around a poorly organized mass of fibers with lots of blood
vessels to supply nutrients. With all those blood vessels, there’s
a good supply of building materials and bone growth is relatively
fast. The crocodile starts out growing fibro-lamellar bone but
quickly switches over to lamellar-zonal growth. The lamellar-
zonal tissue is more compact, highly mineralized and the number
and size of the blood vessels are much smaller than in the fibro-
lamellar case. The bone is more solid and bone growth is slower,
with fewer and smaller blood vessels to supply nutrients.
The dinosaur’s bones, in contrast to the crocodile’s, kept growing
by the fibro-lamellar tissue mechanism until the dino reached
adulthood. This implies that bone growth was rapid until
maturity. From their studies of the bones of dinos, birds,
crocodiles and their extinct relatives, Horner and his colleagues
and others preceding them have come up with a surprising
conclusion. The really big dinosaurs grew at prodigious rates.
For example, Kristina Rogers of the Science Museum of
Minnesota found that the Brontosaurus (now called Apatosaurus)
reached adulthood in only 8 to 10 years, corresponding to a
weight gain of about 5 or 6 tons a year! This upsets the long held
view that the big dinosaurs had to be very old to have grown so
large. They may grow more after reaching maturity but chances
are the they’ve switched to the lamellar-zonal, slower bone
growth mode.
About 230 million years ago, the reptile line split into one that
would produce the dinosaurs and the one that would produce
crocodiles and their ancestors. Most of the slower growing
crocodile relatives went extinct while the fast growing dinos took
over. Eventually, the size of the huge dinos must have proved to
be a handicap. They died out while the smaller dinos, which also
grew fast but stopped growing much sooner to a much smaller
size, took over and survived as the warm-blooded birds we know
today.
Meanwhile, let’s hope it turns out that Bobby Harrison did see an
ivory-billed woodpecker and that he can be recognized by the
proper Audubon entity!
Allen F. Bortrum
|
