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Early Sexual Dimorphism, Diictodon 250+ mya
interrupting the bird news for a moment:
Jan. 23, 2003 -- The large tusks of an animal that roamed Earth before
the dinosaurs may provide the earliest evidence yet of male-female
distinctions in land animals that existed millions of years ago, say U of
Robert Reisz, a biology professor at the University of Toronto at
Mississauga, and his team have found convincing evidence of sexual
dimorphism - different physical traits between the sexes of the same
species - in their study of fossils from between 252 to 260 million years
ago. They believe that the male Diictodon, a herbivorous barrel-shaped
creature, had two large tusks extending down from the upper jaw. The
tusks, Reisz says, were used as weapons, possibly for ritualistic or
"Our findings give very clear evidence of complex social behaviour,"
Reisz says. "To see this kind of behaviour [physical combat] early in the
history of the group that eventually gave rise to mammals is really quite
Reisz's study, which is featured on the cover of the January issue o
the Proceedings of the Royal Society of London B, was based on detailed
studies of nearly a hundred skeletons unearthed in South Africa over the
last two decades.
Diictodon appeared during the Late Permian Period of the Paleozoic Era, at
least 30 million years before dinosaurs existed. It was part of a group of
animals described as mammal-like reptiles and was an
evolutionary relative of the animals that evolved into mammals.
Diictodon, which was covered in scales and measured about one metre in
length, was a burrowing herbivore with a beaked skull and short tail.
In its investigation, the team was able to rule out other uses for the
tusks, Reisz says. The tusks were not used for feeding because the females
did not have them nor were they used for digging because the ends did not
show signs of wear. It appears the tusks became longer, wider and thicker
as the animals aged and extended well below the jaw line; those lost,
possibly in combat, were never replaced, Reisz says. "All these factors
are very strong indicators of armament."
Reisz says these findings go beyond the standard skeletal descriptions
that accompany research on fossils. "This is a wonderful opportunity to
study the biology of animals that lived so long ago. Rather than just
simply looking at them and describing them, we can do more with their
lifestyle, with their feeding habits, and with their general biology than
just looking at their skeletons would suggest."