WASHINGTON?—?An asteroid may have wiped out the
dinosaurs 65 million years ago not simply by changing
the world's climate and causing years of dark skies
but also by causing too many of them to be born
male, U.S. and British researchers said this week.
If dinosaurs were like modern-day reptiles such
as crocodiles, they change sex based on temperature,
said David Miller of the University of Leeds in
Britain and colleagues. And even a small skewing
of populations toward males would have led to eventual
extinction.
Most experts agree that one or more asteroid impacts
probably triggered a series of global changes that
killed off the dinosaurs and many other species
of life on Earth. The impacts would have kicked
up dust that cooled the air and also triggered
volcanic activity that would have created even
more dust and ash.
No one really knows if dinosaurs were more like
reptiles or something closer to mammals. Reptiles
have very different metabolisms than mammals and
also have various ways of determining the sex of
offspring.
In mammals, if a baby gets an X and a Y chromosome,
it will be male, and if it gets two X chromosomes
it will be female, with a few very rare exceptions.
Similar mechanisms work for birds, snakes, and
some reptiles such as lizards.
But in crocodilians, turtles, and some fish, the
temperature at which eggs are incubated can affect
the sex of the developing babies.
Miller's team ran an analysis that showed a temperature
shift could theoretically have led to a preponderance
of males. Other studies have shown that when there
are too few females, eventually the population
dies out.
"The Earth did not become so toxic that life died
out 65 million years ago; the temperature just
changed, and these great beasts had not evolved
a genetic mechanism (like our Y chromosome) to
cope with that," said Dr. Sherman Silber, an infertility
expert in St. Louis who worked on the study.
But crocodiles and turtles had already evolved
at the time of the great extinction 65 million
years ago. How did they survive?
"These animals live at the intersection of aquatic
and terrestrial environments, in estuarine waters
and river beds, which might have afforded some
protection against the more extreme effects of
environmental change, hence giving them more time
to adapt," the researchers wrote.