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Fossilized burrow from Antarctica
Photo by Daniel Dubois |
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Bacteria existed on earth
at least 3.5 billion years ago. Such simple organisms ruled the world until
approximately 600 million years ago, when there was an explosion of complex
animal life in marine environments. Miller finds this rapid development
of life and how organisms continued to evolve into increasingly complex
forms endlessly fascinating.
In the space of less than 100 million years, all the major phyla appeared,
including animals such as sponges, corals, worms, clams, snails, arthropods,
starfish and vertebrates. A variety of life styles developed, including
swimmers, floaters, herbivores and carnivores. Burrowers occupied much
of the marine floor, churning the sediment as they moved and creating dens
to live in. Although fossil traces of the animals themselves are exceedingly
rare, Miller can tell a great deal about their evolution by examining the
subtle signatures that they have left in the rocks.
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A pictorial view of Miller's field research
Courtesy of Molly Miller |
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The question Miller
brought to Antarctica on her second trip in 1995 was how animals developed
differently in fresh and salt-water environments. Five hundred million
years ago, animals had spread throughout the bottom of the world's oceans.
But far less is known about the initial colonization of lakes and rivers.
Freshwater deposits are rare in the geological record because they are
deposited above sea level were they are subject to erosion. In fact, the
four-kilometer thick sequence of rock in the Transantarctic Mountains that
Miller identified have turned out to be the best-preserved freshwater deposits
in the world.
This unique record has enabled Miller
to study the manner in which life first spread from the marine into freshwater
environments millions of years ago. She has done so by analyzing and dating
the extent that these ancient lake and river beds show evidence of disruption
by animals (a process scientists call bioturbation).
In freshwater environments
that date from 250 million years ago, she found that more animals occupied
quiet lake bottoms than stream channels or floodplains. She has determined
that, at this early date, the animals were thinly spread through the freshwater
environment: Half of her observations show absolutely no indication of
life. This low level of habitation contrasts markedly with that in marine
environments at the same time. Marine rocks dating from 500 million years
show high levels of bioturbation, reflecting extensive use of habitable
space. The fact that the much younger freshwater deposits show much lower
levels of bioturbation indicates that freshwater habitats were colonized
much later than marine environments, she says.
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Illustration of one type of mammalian reptile Lystrosaurus that lived
in Antarctica 200 million years ago
Illustration by Jill Baker |
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Perhaps
Miller's most intriguing and surprising find has been the discovery of very
large burrows, two to 20 centimeters in diameter, in 245-million-year-old
sandstone deposited in an ancient floodplain beside a river. These burrows
fall into two distinct size groups. She has determined that the really
big burrows, some of which are over 2.5 meters long, were almost certainly
produced by mammal-like reptiles. Skeletal fragments of these creatures
have been found near by. Similar burrows of the same age in South Africa
contain skeletons of curled up mammal-like reptiles. The identity of the
animals that made the smaller burrows is less clear; crayfish or juvenile
mammal-like reptiles are good candidates.
Although mammals did not make the burrows, Miller
believes they place scientists on the right track for studying the evolution
of warm-blooded animals. "We
are suggesting that Antarctica would have been an ideal place for the evolution
of mammals from mammal-like reptiles because of the fact that the climate
was not hospitable to the dinosaurs of the time," explains Miller. “Mammal-like
reptiles were lumbering and not as quick. If agile carnivorous dinosaurs
had been around, they probably would have preyed on these animals.”
At the
time, Antarctica was located at a high latitude, above 70 degrees south.
The animals would have to adapt to an environment in which it was mostly
dark for half the year and then mostly light for the other part. The environment
would have changed from being very cold during the darkest portion of the
year to fairly temperate in the light. Miller's contention is "the characteristics
of the mammal-like reptiles, who evolved into mammals and who lived in Antarctica
during this time, might have suited them well for life in Antarctica and
the addition of more and more mammal-like characteristics might have been
favored in this kind of environment."
She also hopes that the burrows will lead her to the
early mammals as well. "We
know that mammal-like reptiles knew how to live in burrows," says Miller. "If
you want to find the earliest mammals, a good search tool is that they would
be hanging out a lot of the time in their burrows because it allowed them to
avoid the extremes of climate during the long winter. We can recognize the burrows
very quickly, it gives us a search strategy."
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