Several possible explanations for the presence of two forms, tusked and tuskless, exist: the fossils could be of different species; different ages within a species; or the same species, opposite sexes.
The researchers were able to eliminate the first two possibilities, and concluded that the tusked animals were the males of the species. [See sidebar]
"If you look at vertebrate history, there's a basic split that occurs, and one line leads to reptiles, dinosaurs, and birds, and the other side to mammals," said Sullivan. "Dicynodonts are on the mammalian side of vertebrate history, like a second or third cousin to mammals."
"What these findings tell us is that sexual dimorphism goes a long way back in the lineage of mammals, and that this very early group had relatively complex social behavior," he said.
Living in a Sexually Dimorphic World
By looking at sexually dimorphic animals living today, scientists can infer certain kinds of social behaviors.
"In species characterized by the male having some form of armament, whether horns or spurs, there is almost always some form of male aggression," said J. Michael Plavcan, a paleontologist at the University of Arkansas.
The males could be competing directly for females or for territory.
"In a situation in which males can exclude other males to gain a reproductive advantage, there's a tremendous selective pressure for the males to develop weaponry and large size because those attributes will give them more opportunities for mating," said Plavcan.
The competition need not be a fight to the death.
"It's very common for antagonistic displays to be the first stage of a confrontation, and it can frequently stop at that point, with one male deciding the other guy is bigger, or has bigger horns and 'I'd better back down,'" said Sullivan.
Why would a group go from monomorphic (no physical differentiation) to sexually dimorphic?
"There's no real way of knowing what might have been going on in this population," said Sullivan. "I can imagine a scenario where they went from having lots of food or territory to a situation where those resources became more limited. Or there may have been a more internal reason for their having evolved a more elaborate mating system. Those are things we just don't know."
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