Abstract.  Tracking the geometry of all 597 ammonoid genera from the Lower Devonian into the
  Lower Triassic, a 145-Myr period that spans three mass extinctions, shows that Paleozoic ammo-
  noid shell geometries were strongly biased for a few combinations of whorl expansion (W), whorl
  overlap (D), and whorl shape (S). Just three modal combinations accommodated approximately 432
  genera (72% of total) and just one combination accommodated 239 genera (40%).  All three primary
  modal forms have similar low expansion rates (W ~ 1.75) and differ only in coiling tightness (D).
  These geometries resulted in long body chambers (~400°) with Nautilus-like static in-life aperture
  orientations (~30°) for the great majority (>80%) of Paleozoic ammonoids.  The ancestral clade
  Agoniatitida included a unique spectrum of openly coiled geometries that went extinct at the Fras-
  nian/Famennian boundary (and were not seen again until the Triassic).  The Devonian/Mississip-
  pian extinction terminated the brief, explosive radiation of the Clymeniida (64 genera).  The dem-
  inant Paleozoic clade, the Goniatitida (ca. 130 Myr, 374 genera [64% of total], survived both the
  F/F and D/M extinctions, but began declining well before the Permian/Triassic crisis.  The long
  lived Prolecanitida (40 genera [7%]) appeared shortly after the D/M extinction, persisted as a low-
  diversity clade through the Carboniferous, and gave rise to the Ceratitida in the mid-Permian, from
  which were derived all Mesozoic ammonoids.  After each major extinction event the phylogenetic
  composition of ammonoid stocks was fundamentally reordered and geometries were recanalized.
  Without external disturbances, as the relatively uninterrupted Mississippian through Permian rec-
  ord shows, the history of ammonoid shell geometry would probably have been a record of much
  greater constancy, perhaps tied much more closely to the Lower Devonian geometric landscape.