title: Evolution of Complexity in Paleozoic Ammonoid Sutures.
abstract: The septal sutures of 588 genera of Paleozoic ammonoids showed a 1600 percent increase in mean complexity over 140 million years. Within 475 ancestor/descendant pairs, descendants were more than twice as likely to be more complex than their ancestors. Twelve subclades (373 genera) averaged 34 percent increased complexity. These patterns are compatible with an active or driven system of long-term bias for increased complexity. Mass extinctions acted in opposition to this long-term trend, tending to eliminate more-complex forms and resetting the trend with each extinction event.
authors.affiliation: Department of Geology, Bryn Mawr College, Bryn Mawr, PA 19010, USA. Cincinnati Museum Center, Geier Collections and Research Center, Cincinnati, OH 45202, USA. Paleontological Research Institute, Russian Academy of Sciences, Profsoyuznaya 123, 117647 Moscow, Russia.
authors.initials: WB
authors.lastname: Saunders
authors.initials: DM
authors.lastname: Work
authors.initials: SV
authors.lastname: Nikolaeva