May 18, 2005 | David F. Coppedge

Stegosaur Plates Were for Decoration

Berkeley scientists are disputing the notion that the rows of plates on the backs of stegosaurs served as heat exchangers.  Instead, they were for show.  EurekAlert and Science Daily explain that this was probably true of other dinosaur decorations: “The team’s analysis of stegosaur plates lends support to a growing consensus among paleontologists that the weird adornments of many dinosaurs – the horns of triceratops, the helmet-like domes of the pachycephalosaurs, and the crests of the duck-billed hadrosaurs – likely served no function other than to differentiate species, akin to birds’ colorful feather ornamentation.”  If they evolved as decorations, maybe heat exchange was an “exaptation” – i.e., an incidental benefit.  (Some stegosaur-like species have little or no plates.)
    Sexual selection is not a likely explanation, though.  Kevin Padian said, “we don’t see a clear distinction between male and female stegosaurs.  Without sexual dimorphism [physical distinctions between male and female], you have no evidence for sexual selection, so you can’t invoke sexual display as an explanation.”  Neither does defense make sense.  The structures were too flimsy to provide protection; the munch from an allosaur would be “like biting through a sandwich.”  Padian argued for the only explanation left: that the structures were for “elaborate displays for social group recognition,” like bird calls, underscoring the “importance of behavior to evolution.”

The structures would have to be pretty large and elaborate to function for social group recognition.  How many lucky mutations did that take?  A mole or nub on one stegosaur’s back would probably not be enough to get the ball rolling, to make all the others think that it was so attractive, he or she would be the only one getting a mate.  Maybe some things in nature are just for looks and contribute little or nothing to survival of the fittest.  Structures might be amplified by microevolution into extreme forms, but Darwinian theory would have a hard time explaining how they got there in the first place.

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