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Re: Haversian systems in lizards (seriously folks)



The actual reference in the forum appears to have been in January, 1998 but I
can't get into the archive.  There is a glancing reference in a Newsweek
article I'm about to paste in.  The picture had to be attached as a gif file.
For finding stuff on the web, may I recommend Copernic 98, free and multi-
engine.






   MYSTERIES OF SCIENCE


Were dinosaurs cold blooded?

In museums, coloring books, and "Jurassic Park," warm-blooded dinosaurs are
the rage. But physics and the laws of large objects may have the final word.
 Mystified? Find answers to your questions on the Web: Click here to get extra
information and "Ask the Expert" links..

BY BRENDAN I. KOERNER

The dinosaurs most of us over the age of 20 grew up with were plodding beasts
with pea-size brains. In textbooks and schlocky B films, they were portrayed
as little more than souped-up crocodiles, lurching lethargically about on
splayed-out legs, hunched over like Quasimodo. Like the modern-day reptiles
they were thought to resemble, dinosaurs were cold blooded: unable to self-
regulate their body temperatures and dependent on the sun alone for warmth.

The budding paleontologists of today's kindergarten set are being raised on a
very different crop of "terrible lizards." Bipedal carnivores, clever and
fleet-footed, zip around children's literature in voracious packs. Ninety-
foot-long sauropods gracefully rear up on their hind legs in coloring books.
And the fierce velociraptors of Jurassic Park are able to fog up a window with
their steamy breath--a sure-fire sign of a warm-blooded animal's ability to
regulate its internal thermostat under almost any condition.

It is that last revisionist detail that has divided the paleontological world
into rival camps. For some, endothermy, the scientific name for warm
bloodedness, is the only way to explain the dinosaurs' evolutionary success.
Without the ability to keep their bodies at optimum temperatures regardless of
their surroundings, they argue, dinosaurs could never have dominated the globe
for 160 million years.

Skeptics counter that ectothermy, the proper label for cold bloodedness, was
the logical strategy for dinosaurs living in the Mesozoic Era's generally
sweltering heat--and, this group claims, the only option that is supported by
physiological, rather than circumstantial, evidence.

The revisionist view that has so captured the public imagination has long been
led by Robert Bakker, a former evangelical preacher who has defended dinosaur
warm bloodedness with sermonlike intensity. As a Yale undergraduate in the
late 1960s, he assisted the legendary paleontologist John Ostrom in his
landmark research on Deinonychus, an agile carnivore whose sleek skeleton
seemed built for a life of speed more befitting a warm-blooded bird than a
cold-blooded reptile. Bakker went on to become paleontology's enfant terrible,
a crusader against slow-moving, dimwitted, crocodilian dinosaurs. He proposed
such self-described "heretical" ideas as a 10-ton triceratops that could
gallop past a charging rhino, and brontosaurs that gave birth to live,
500-pound young.

Above all, he painted a picture of dinosaurs that were every bit as
endothermic as humans, who manage to keep their body temperature around 98.6
degrees Fahrenheit night and day, winter and summer. Instead of spending their
days lazily basking in the sun and occasionally trudging along at a torpid
pace, Bakker's dinosaurs--which he wryly termed "nature's special
effects"--moved at constant speeds, their postures fully erect in the manner
of birds and mammals. "Meat-eating dinosaurs related to Tyrannosaurus rex
cruised at 3 to 4 miles an hour," claims Bakker, who bases his conclusion on
fossilized footprints. "No turtle anywhere cruises at 3 to 4 miles an hour."

Bakker and his acolytes also point to dinosaurs' relatively fast growth as
evidence of endothermy. Mammals and birds, which develop quickly compared with
ectothermic reptiles, have bones characterized by microscopic channels that
appear complex and crystal-like under the microscope. These elegant patterns
form when growing bone meets and meshes with connective tissue, capturing
blood vessels in dense, woven structures called Haversian canals. Armand de
Ricqlès, a University of Paris anatomist, found that dinosaur bones exhibited
those same intricate channels rather than the simpler, less dense structures
common to reptiles. "We see the same well-vascularized bone in mammals but not
in turtles and crocodiles," says Kevin Padian, a paleontologist at the
University of California--Berkeley. "The way the bones grew, dinosaurs seem to
have been active all the time." That pace of activity, argue Bakker and his
cohorts, is the telltale sign of warm bloodedness.

With Bakker's charisma and de Ricqlès's bone histology work, endothermic
dinosaurs quickly became the rage. Books were revised, natural-history museums
scrambled to accommodate the shift, and Bakker became a dinosaur superstar,
commanding speaking fees of up to $10,000.

Feed me. Although the public fell head over heels for the warm-blooded
dinosaurs, many within the scientific community remain wary of Bakker's
claims. Since measurements show that endotherms require up to 20 times more
food than ectotherms, some question how the gigantic dinosaurs could possibly
have eaten enough if they were warm blooded. "Can you imagine if a herd of
brontosaurs were endothermic?" asks Frank Paladino, a physiologist at Indiana-
Purdue University. "They would have eaten through North America in a couple of
weeks." The problem would have been worse for endothermic carnivores, for, as
James Farlow of Indiana-Purdue notes, "there's a lot less meat on the hoof
than plant on the stem."

Bakker has tried to explain away this apparent shortcoming by asserting that
predators were very rare and thus able to feast on ample prey. But, as Paul
Sereno of the University of Chicago notes, an incomplete fossil record has
made it "very, very difficult to reconstruct the number of predators and
prey."

The evidence based on bone structures has come under fire, too. Tomasz
Owerkowicz, a young Harvard University researcher, has asserted that the dense
canals that de Ricqlès detected could have resulted from physical exertion
rather than endothermy. In an ingenious experiment, Owerkowicz gave cold-
blooded monitor lizards regular treadmill workouts and then compared their
bones with those of nonaerobicized contemporaries. The well-exercised group
showed the same kind of complex channels characteristic of mammals, birds, and
de Ricqlès's dinosaurs, suggesting that Haversian canals are causally linked
to an active lifestyle rather than warm bloodedness. South African histologist
Anusuya Chinsamy has also countered some of the bone structure argument,
contending that dinosaur bones exhibit bands called lines of arrested growth.
These are characteristic of modern-day ectotherms, whose growth rate speeds up
and slows down according to seasonal temperature fluctuations. Chinsamy
concluded that dinosaurs grew at a more reptilian pace than envisioned by the
Bakkerites.

Rather than just playing spoilsport, the ectothermic side has sought to boost
its case with hard physiological evidence. John Ruben, a physiologist at
Oregon State University, believes he may have found the answer in turbinates,
tiny whisps of bone or cartilage deep inside the nasal cavities of mammals and
birds. These structures make warm bloodedness possible by limiting water loss.
When warm, moist air is exhaled, the water condenses on the turbinates; the
next breath brings water vapor back into the lungs. "If [endotherms] didn't
have respiratory turbinates, there is no way they could lose that much water"
and survive, says Terry Jones, one of Ruben's assistants. Turbinates have
never been found in living ectotherms--nor in dinosaurs.

Bet on the croc. Although Ruben's team believes they finally have the proof to
cool down dinosaurs for good, they deny that they're trying to drag the
animals back into lethargy. "Cold blooded doesn't necessarily mean slow and
sluggish," says Jones. The Komodo dragon, the world's largest living lizard,
hunts deer. "And deer are pretty active," he says. Paladino agrees:
"Ectotherms can do some pretty amazing things," he says. "If I put you on a
beach with a 15-foot crocodile and you try to get away, I'll put my 10 bucks
on the crocodile."

Many on the cold-blooded side now use the term "gigantothermy" to describe the
unique energetics of large dinosaurs. Being huge is one way to maintain a
relatively constant body temperature despite cold bloodedness: Large
things--which have a lot of bulk in relation to their skin area--lose heat to
the outside world much more slowly than do small things. Had they been
endothermic, argues James Spotila, a biologist at Drexel University, the large
dinosaurs would have experienced a "meltdown," as they would be unable to
dissipate internally generated heat at a fast enough rate. However, if they
were indeed cold blooded, the slow heat loss associated with gigantothermy
would allow them to stay relatively warm--and thus avoid a reptilian
torpor--when confronted by the night or an overcast day.

In the generally tropical climate of the Mesozoic, ectothermy may have given
dinosaurs an edge over warm-blooded mammals, which had to spend a great deal
of energy thermoregulating themselves. Since ectotherms require so much less
energy than do birds and mammals, "it's a very, very nice way to make a living
if you're in an equitable climate," says Ruben. Contrary to the popular belief
that warm bloodedness is always the superior strategy, ectothermy might have
been key to the dinosaurs' long reign. Saying that endothermy is superior,
says Peter Dodson, a paleontologist at the University of Pennsylvania, is just
evolutionary "chauvinism."

The warm-blooded camp, however, is unconvinced by the new set of evidence.
Bakker says that Ruben's turbinate research doesn't take into account the
possibility that dinosaurs could have utilized an alternative, as-yet-unknown
structure to limit water loss. "Ruben's argument is like an expert on piston-
driven airplanes looking at a jet and saying you don't have a propeller," he
says. Berkeley's Padian, who notes that "behavior precedes hardware in
evolution," says dinosaurs may have managed warm bloodedness using mechanisms
far different from those found in contemporary animals. Bakker believes that
chambers found in Tyrannosaurus skulls may have acted as water-loss regulators
in place of nasal turbinates.

Bakker also points to fossils that have been found in Alaska and
Australia--two of the very few Mesozoic locales where the mercury occasionally
dipped below freezing--as chinks in the seemingly ironclad case for
ectothermy. "You don't have Komodo dragons in Seattle, walking into
Starbucks," he says. Adverse weather would have particularly affected the
smallest of dinosaurs--some of which ranged down to chicken size--who couldn't
limit their heat loss through gigantothermy. Cold-blooded advocates have
contended that hibernation or migration would have been viable alternatives,
but those explanations remain in the realm of conjecture.

Unless time machines or Jurassic Park's DNA cloning technique miraculously
become realities, the controversy can never be definitively resolved. "I would
never say we know for sure, because we can't," admits Ruben. But although the
debate will probably never end, there is little doubt as to which side has
more ominous implications for our own species: If dinosaurs were indeed
endothermic, then their sudden disappearance 65 million years ago may bode ill
for a human race that seems to consider itself invincible. "Maybe we have to
rethink our nonvulnerability to global change," explains William Showers, a
geochemist at North Carolina State University. "We can't take comfort in being
warm blooded if the dinosaurs were warm blooded, too."









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