The increase in biological complexity over evolutionary time has been the subject of extensive study and many hypotheses, but there are few data to document the increase or how organisms may have benefited by becoming more complex. By measuring the fractal dimension of the sutures of >600 ammonoid genera, we document the increase in anatomical complexity of the Ammonoidea and show that there is no differential survival between simple and complex forms. Simple sutures are present throughout the entire history of the Ammonoidea, but the range of variation of suture complexity increased over time as more complex sutures evolved.
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