Title: A warm and dangerous time.
Subject(s): DINOSAURS
Source: Fantasy & Science Fiction, Jul96, Vol. 91 Issue 1, p59, 10p
Author(s): Asimov, Janet
Abstract: Focuses on dinosaurs. Domination of dinosaurs during the
Mesozoic era; Characteristics; Examination of fossils; Behaviors;
Theories on their extinction.
AN: 9605234959
ISSN: 1095-8258
Note: .
Database: MasterFILE Elite

Section: SCIENCE

A WARM AND DANGEROUS TIME

Shakespeare, if he'd known about it, might have admired and even
envied the drama of a certain small period-- only 80 million years --
of Earth's 4.5 billion year old history. The Cretaceous had
everything. Consider the setting: a mild climate, green decor and lots
of newfangled things called flowers. The cast of mobile characters
featured huge monsters, while furry bit players scurried under the
scenery. And, of course, there was that smasheroo of an ending.

The Cretaceous period was only the last act of the mighty Mesozoic
era. The Mesozoic began 248 million years ago, after the Permian
period ended with the Great Dying, when about 96 percent of life
became extinct. The Permian extinctions are somewhat mysterious, but
probably caused by the travels and breakup of the supercontinent
Pangea. Ocean circulation was altered, the planet's temperature
fluctuated, and there were volcanic eruptions, sun-obscuring dust, and
acid rain.

The first period of the Mesozoic was the Triassic, during which the
surviving 4 percent of life proliferated and evolved. Dinosaurs
developed from thecodont reptiles, and some therapsid reptiles became
rather mammalian. The Triassic ended with many extinctions, perhaps
from an "impact event," perhaps not.

The next period of the Mesozoic era-- the Jurassic--began 213 million
years ago and was even more spectacular than the movie. Swimming
reptiles filled the waters and flying reptiles filled the air.
Dinosaurs dominated a land covered with cycads, ferns, gingkos and
conifers. Our mammalian ancestors were small and probably scared most
of the time because they were preyed on not only by reptiles and the
remaining amphibians, but also by the first birds.

Then the supercontinents of Laurasia and Gondwana (into which Pangea
had split) began to come apart. Sea level and climate changes caused
many extinctions, including dinosaurs like Stegosaurus, Allosaurus and
(in North America), the big sauropods like Apatosaurus.

The curtain then rose on the Cretaceous period, 144 million years ago.
Earth was still changing {it always does but our lives are too short
for us to notice} and a seaway began to split North America. In the
southern continent of Gondwana a rift developed between Australia and
Antarctica.

Exactly how and when the continents separated has been subject to
debate. South American dinosaurs differed from North American, but it
was thought that they would be similar to those of Africa. After all,
Africa and South America had been joined, their undersea continental
shelves fitting together.

Two years ago in the Sahara desert, Paul C. Sereno (University of
Chicago) and his colleagues discovered 130 million-year-old fossils
from the early Cretaceous. The 30-foot carnivore named Afrovenator
abattensis was related to the fierce predator Allosaurus that had
lived 30 million years before in the Jurassic of North America.
Broad-toothed sauropods were similar to the Jurassic North American
Camarasaurus.

These African dinosaurs were not at all like those from the Cretaceous
of South America, but seemed to be holdovers from the Jurassic. To
Sereno, this indicates that the African part of Gondwana's connection
to North America lasted longer than the one to South America.

The feathered variety of dinosaur made a strong showing in the early
Cretaceous. When !35 mil-lion-year-old Sinornis was found to have a
jaw containing teeth, paleontologists assumed that birds had not
changed much since the late Jurassic's Archaeopteryx, 12 million years
older.

Evolved from reptilian scales, beaks are light-weight structures made
of keratin, replacing teeth and lips. For years it was thought that
the first bird with a true beak was Gobiteryx, from 70 million years
ago in the late Cretaceous.

Recently a new bird from the early Cretaceous holds the title of first
beak, according to a Chinese and American team of paleontologists
--Lian-hai Hou, Zhonghe Zhou, Larry D. Martin and Alan Feduccia. Given
the charming name of Confuciusornis sanctus, the 140 million-year-old
species consists so far of three partial skeletons found in
Northeastern China.

Pigeon-sized Confuciusornis probably perched on tree limbs, for the
fossils were found in what had once been a fresh-water lake ringed by
forest. Furthermore, Confuciusornis had the typical avian three
anterior toes with re-curved claws on the long middle toe. Although it
had the primitive long bony tail and claws on leathered wings, its jaw
had been replaced by a bill.

Since Confuciusornis lived in China and only 7 million years later
than the European Archaeopteryx, it seems likely that the first birds
spread quickly and diversified early. Why did they lose their teeth?

Beaks are an asset to fliers because they weigh less than toothy jaws.
Another theory is that beaks go along with the loss of efficient
hands: teeth inevitably get in the way when you are trying -- without
hands --to turn captured prey around so it will go down the gullet
head first.

I don't know why, if you are a predatory bird, your food has to be
swallowed head first. Perhaps it's because much of your food also has
teeth, and your gullet will constrict around its head while its body
is still wriggling outside your mouth (in case, like the Klingons, you
didn't bother to kill it first).

Confuciusornis was not the ancestor.of modern birds for it was
enantiornithine, with foot bones partially fused from the top down,
and all enantiornithine birds became extinct at the end of the
Cretaceous. Modern birds have foot bones partially fused from the
bottom up.

Some scientists believe that modern birds diversified during the
Cretaceous, but Feduccia thinks that the only modern bird living then
was like the present-day shore-bird, the stone curlew.

After the cataclysm at the end of the Cretaceous, birds like the
curlew might have been able to go on eating shoreline animals while
the dominant enantiornithine birds died along with the dinosaurs. Then
these shorebirds diversified to fill all the avian biological niches,
much the way the finches that were blown to the Galapagos Islands have
-within historical times -- developed many kinds of beaks to eat
different types of food.

During the Cretaceous the early birds had to compete with flying
reptiles. These pterosaurs evolved from reptiles that had learned to
glide during the Triassic. Their wings were made of skin stretched
along the elongated arm and the upper leg.

Pterosaurs were the largest vertebrates ever to fly, the biggest
called Quetzalcoatlus, with a wingspan up to 39 feet. Some pterosaurs
were more light-weight because they no longer had bony tails, and some
had unusual heads.

In the early Cretaceous a pterosaur called Pterodaustro had a mouth
full of what looked like bristles that presumably helped it accomplish
filter-feeding. By studying sections of the bristles with an electron
microscope, Luis M. Chiappe (New York's American Museum of Natural
History) and A. Chinsamy (South African Museum) found that these
bristles were not made of material derived from the epidermis, like
whale baleen or the bills of modern filter-feeding birds. The
bristles, however strange, were genuine teeth.

Many pterosaurs fed on the proliferating teleost fish of the
Cretaceous, but there were also plenty of other water-dwelling
reptiles like huge turtles and crocodilians. A 120 million-year-old
crocodile found recently was not even carnivorous. Its teeth showed
that it ate plants!

The dominant sea reptiles of the previous period had been live-bearing
icthyosaurs. In the Cretaceous they had been reduced to a single
species, Platypterygius, and the seas were dominated by another
reptile, the egg-laying plesiosaurs that used flippers to "fly"
through water (the way penguins do today).

The reptilian and avian creatures of the Cretaceous were certainly
more eye-catching than the mammals. Some of the most primitive mammals
were the little-known multituberculates, who may have been ancestral
to the monotremes or could have arisen independently. Named for the
rows of uniform cusps on their teeth, the multituberculates lived past
the end of the Cretaceous before becoming extinct after 150 million
years of existence.

During the Cretaceous, the well-known early mammals were monotremes
--furry and warm-blooded, small (mostly mouse-sized), not very bright,
and egg-laying. Paleontologists in Australia have recently discovered
the jawbone of a Cretaceous monotreme called Kollikodon ritchiei, 120
million years old.

Kollikodon was so different from a previously discovered monotreme,
Steropodon, that it's probable that monotremes had taken on many forms
long before the Cretaceous. Cat-sized, Kollikodon was 10 times bigger
than any other mammal alive then. It may have lived mostly in water
because -- like some modern fish, crabs, and sea otters-- it had teeth
adapted for chomping on shellfish.

Magnificent dinosaur bones do tend to distract from the little bones
of Cretaceous mammals, but given Kollikodon, it's possible that our
distant ancestors were more interesting and colorful than we've
thought.

Our self-image may be improved by a treasure trove of mammalian
fossils recently found at a late Cretaceous Mongolian site named
Flaming Cliffs.

Examining these fossils, Michael J. Novacek (American Museum of
Natural History) says that the mammals were more diverse than had been
suspected -"the Cretaceous gives us the first inkling of the roots of
our ancestry."

Many more rocks in that Gobi desert site are still being investigated.
After that, according to Jay Lillegraven (University of Wyoming),
we'll be clearer on mammalian diversity that had produced "leaf and
seed eaters as well as animals that fed on grubs."

Which brings me to what the land herbivores were eating. In the
Cretaceous, vegetation was changing. The angiosperms -- flowering
plants-- were gradually taking land away from the gymnosperms.

According to Michael J. Sanderson (University of Nevada) and Michael
J. Donoghue (Harvard University), the ancestors of angiosperms may
have split off from their closest anthophyte relatives in the late
Triassic, 210 million years ago.

During the Jurassic, angiosperms had spread very slowly, but after the
extinctions at the end of that period, there was time for flowering
plants to get going before the next batch of hungry herbivores took
over.

In the Cretaceous, angiosperms soon diversified into five major
clades. Sanderson and Donoghue think that the spurt in angiosperm
diversity may have been due to the evolution of plants with
"accelerated life cycles."

This evolution toward faster botanical living may have been helped by
climate alterations that increased remarkably during the
mid-Cretaceous, thanks to dramatic geological changes.

Roger Larson (University of Rhode Island) showed that ocean crust
formed faster during the mid-Cretaceous, about 125 million years ago.
Huge flows of hot melted rock poured out over the floors of the
Pacific and Indian oceans, raising sea levels, while outgassing of
carbon dioxide created a greenhouse effect.

One theory is that this mid-Cretaceous event was caused by the
eruption of a gigantic "superplume" of lava from the mantle of Earth.
Don Anderson (Cal Tech), however, thinks that the outflow of hot
basalt was due to the long duration of Gondwana's existence. A
supercontinent tends to promote the building up of heat within the
mantle. When Gondwana split, heat was released by volcanos and
earthquakes, not by a superplume.

Whatever the explanation, mid-Cretaceous Earth soon had shallower,
warmer seas, a hotter climate, and different animals. Angiosperms
flourished.

We know that many herbivores continued to eat non-flowering plants
because the remains of conifers were found -- by Karen Chin
(University of California at Santa Barbara) -- in 100 million-year-old
fossilized dino dung from what had been the burrows of Cretaceous dung
beetles.

Gymnosperms like conifers do not grow as fast and are not as adaptable
as angiosperms. A herd of enormous medium to high browsing herbivores
could easily overgraze conifers and cycads, leaving open spaces in
which the fast-growing seeds of flowering plants could prosper.

After the mid-Cretaceous change, there were many new herbivores like
Triceratops and Iguanadon whose bodies and teeth were adapted for
low-feeding, so they were efficient munchers of the early angiosperms.
Some angiosperms retaliated by growing quickly to heights above these
lowbrowsers.

According to paleontologist Robert T. Bakker, the lowfeeding dinosaurs
helped promote the success of angiosperms even while they ate them.
Dinosaurs like Triceratops were big and could devour a lot of
flowering plants, but they could not do it as fast as some angiosperms
could flower, reproduce, and root again. Those faster angiosperms
prospered.

Our modern dandelions are speedy angiosperms. Their yellow flowers are
followed quickly by downy-sailed seeds before you remember that you
were supposed to mow the lawn. Even when you do mow it, the dandelion
roots are still there and ready to do the whole thing all over again
--examples of the kind of angiosperm that evolved to survive heavy low
feeding.

Some Cretaceous birds may have pollinated the new flowers, but the
main work was done by insects, whose ability to fly was now exploited
by angiosperms.

Insects have been flying for 300 million years. The latest theory
about how they achieved this is that the gill flaps of some insects
increased in size. This provided lift for the bodies to elevate on the
legs, allowing these insects to skim the water's surface tension like
skaters on ice.

Leaving the water altogether and rising into the air was a cinch once
the flaps became actual wings. Insects with wings were attracted to
the new flowering plants, and soon insects and flowers were evolving
together, surviving handily in a world full of plant-eaters.

Among these plant-eaters was a late Cretaceous hadrosaur, a
duck-billed dinosaur named Maiasaura --good mother lizard -- by
paleontologist John B. Horner. It's thought that Maiasaura may have
been the species that left behind those fossilized feces, but its real
fame is for making nests.

A baby Maiasaura used a sharp projection on the end of its nose to
break out of its shell. Then it apparently stayed in the nest with its
brothers and sisters, which means that its mother (and even its
father) had to tend the nest and bring it food. Modern baby turtles
have no such luck -- they hatch out untended by any adult -- but some
modern crocodiles do tend their babies.

Other dinosaurs also made nests, some in large colonies. Last year
Spanish paleontologists reported that one colony of dinosaur nests
found in the southern Pyrenees contained the remains of almost 300,000
eight-inch-wide spherical eggs, in an area that had once been a beach.

Apparently herds of as yet unknown dinosaurs repeatedly used this
beach for nesting, crowded as close together as the nesting sites of
many modern birds.

But did dinosaurs ever brood their offspring the way most birds do,
sitting on the nest until the eggs are hatched and even afterwards?
It's certainly hard to imagine one of the more immense dinosaurs
sitting on its eggs, but at least one small dinosaur did.

In one of the most marvelous dinosaur discoveries ever made, an 80
million-year-old fossil was recently found in the Gobi desert of
Mongolia by a joint expedition of the American Museum of Natural
History and the Mongolian Academy of Sciences.

Painstaking excavation revealed a carnivorous dinosaur named
Oviraptor, a therapod relative of the Cretaceous Tyrannosaurus rex and
the Jurassic predator Allosaurus, but about the size of a small
ostrich. This dinosaur had died on its nest, where the eggs were
arranged in a circular pattern with their broad ends toward the nest's
center.

The Oviraptor was obviously brooding those eggs, for it (I hesitate
saying "she" because many modern male birds also brood) had positioned
its long arms and legs protectively around the eggs the way birds do.
The photograph of its skeleton on the eggs is curiously moving.

Brooding behavior occurs, surprisingly enough, in the python, a
cold-blooded reptile. In the tropics, where the eggs will stay warm
anyway, the python seems to be merely guarding them. In the cooler
areas of Australia, the mother snake basks in the sun to heat her body
and, presumably, the eggs she coils around. The Indian python
rhythmically contracts her muscles to raise her own body temperature
while she's brooding.

Since pythons are cold-blooded, egg-brooding is not an absolute
guarantee that Oviraptor was warm-blooded. Mark A. Norell and
colleagues, who found the fossil, remind us that brooding could also
be for protection and to shade the eggs.

Which brings me to the fact that--from all sorts of evidence -many
people now believe that dinosaurs were warm-blooded. Others say that
dino bone structure is intermediate between reptiles and warm-blooded
birds and mammals.

I agree with those who believe that dinos were "in between," and
unique. It's a waste of time trying to jam everything into neat
categories.

Before I leave the dinosaurs of the Cretaceous, I must add a few words
about T. rex, who wasn't even the biggest carnivore (that's 9-ton
Gigantosaurus carolinii of Argentina) and is known from only 12
fossils, so far. But T. rex fits into a "dangerous but beloved"
category all his own.

[I'm sorry about using the word "his." We tend to associate larger
size, aggression and mayhem with males, but that's illogical. Remember
the female praying mantis? And the many other animal species whose
females are not only bigger but deadlier? One of my favorite cartoons
is the (bigger) male lion saying to the (smaller but deadlier) female,
"Aren't you supposed to be out getting me my dinner?"]

The American Museum has now positioned T. rex with his body
horizontal, running full out with his head toward the humans coming
into the exhibit hall. James Farlow (Indiana-Purdue University) thinks
T. rex would have had to run at less than 35 kilometers per hour in
order to be safe from disastrous falls (fatal if you're going fast and
weigh up to 6 tons).

That's all right. Even going slowly, with his neck stretched out and
his teeth shining, a Tyrannosaurus would have frightened off other
scavengers from a carcass -unless a scavenger was another T. rex.

Amateur paleontologist Steven Sacrison found that according to the
evidence of their bones, T. rex often battled to the death with those
of his own kind. There are vertebrae bitten in half, and only another
T. rex had the jaws for this. One skull contained a hole just the size
of a T. rex tooth. An actual piece of T. rex tooth was imbedded in
another's broken rib.

The warm and dangerous Cretaceous ended 65 million years ago with the
extinction of many animals, including all the dinosaurs except a few
birds. When geophysicists David M. Raup and David Jablonski
(University of Chicago) analyzed fossil bivalve mollusks from the
Cretaceous, their data indicate that the extinction was world-wide,
including many ocean species.

This "K-T" boundary disaster was due to an object or objects from
space crashing on Earth. The primary site was the enormous Chicxulub
crater in Mexico's Yucatan peninsula, caused by something as large as
10 kilometers.

There is also a 35-kilometer wide crater in Iowa from the same time
period. Evidence for a third site in the Pacific ocean was found by
the Deep Sea Drilling Program that brought up K-T deposits containing
large amounts of iridium, characteristic of melted meteoric debris.

Some people think the three sites were caused by three comets hitting
Earth at once, but it's more likely that a large asteroid broke up as
it zoomed in on Earth. The disaster filled the air with particles that
shut out sunlight, causing several years of deep gloom that must have
been like the dreaded "nuclear winter" we humans could inflict upon
ourselves.

There was no radioactive fallout 65 million years ago, but in addition
to the dust-filled atmosphere, the impact or impacts set off fires
that burned much of Earth's vegetation and contributed to the deadly
air pollution.

In "what if" scenarios, it's fun to speculate on what would have
happened if there'd been no K-T disaster, if the dinosaurs had gone on
living -- as they already had been, for over 180 million years.

Would the dinosaurs have become intelligent eventually? Would they
have created their own civilizations, complete with space ships and
ambitions to colonize not only the Solar System but beyond?

Probably, although it seems to me that egg-laying is a handicap to
successful evolution of high intelligence. Primates that were becoming
hairless hominids could carry their big-brained, helpless young, but
eggs?

Medium-sized, bipedal, bright dinosaurs with hands might have
succeeded if they'd become live-bearing. And maybe they'd have grown a
little fur, at least on the tops of their heads, to shield them from
the sun.

Imagining Earth's history without that K-T boundary seems to result in
a fantastic image of a live-bearing, handy, slightly furry creature
perhaps too bright for its own good, who could create civilization,
kill off many other animal species, destroy the environment, and
produce a rise in global temperature.

Which brings to mind the fact that the "T" in K-T boundary stands for
the Tertiary period, when the surviving mammals dominated. Eventually
those surviving mammals evolved the most dominating and dangerous
predator of them all.

Who, during its increasingly warm and dangerous time, had better keep
scanning space for incoming large objects.

~~~~~~~~

By JANET ASIMOV
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Source: Fantasy & Science Fiction, Jul96, Vol. 91 Issue 1, p59, 10p.
Item Number: 9605234959