SHELL GEOMETRY IN PALEOZOIC AMMONOIDS: A CONFLUENCE OF FUNCTION, PHYLOGENY, EXTERNAL FACTORS AND LUCK

In 1966-67, David Raup elegantly showed that coiled molluscs occupied only a small part of available geometric morphospace and concluded that the distribution of ammonoid shells does not appear to represent the optimization of any single functional factor, but rather is a geometric region which minimizes several problems. Raup’s sample comprised 405 genera drawn from the American Treatise, including 44 Paleozoic genera. Here we report on tracking the geometry of all 588 named Paleozoic ammonoid genera, spanning three mass extinctions and 145 my. The results show that the evolution of ammonoid geometries reflected selection in concert with the following factors: PHYLOGENETICS determined the cast of potential characters, including (1) the Goniatitida, which dominated Paleozoic ammonoids (64% of genera) for 90 my and survived both the F/F and D/M extinctions; (2) the Clymeniida, which radiated in the U. Devonian; (3) the Prolecanitida which provided the ceratitid stock from which were derived all Mesozoic ammonoids. FUNCTIONALITY is reflected in the three primary modal geometries (proscribing 84% of genera) that permitted neutral buoyancy with particular hydrostatic, hydrodynamic and physiologic characteristics. EXTERNAL FACTORS that influenced the course of ammonoid history included: (1) the F/F extinction, which eliminated the ancestral Agoniatitida, leaving generalized Goniatitida the sole post-extinction clade; (2) the D/M extinction which eliminated the Clymeniida (which had exceeded goniatitid diversity) leaving, again, generalized goniatitids as the dominant successor clade; (3) the P/Tr event which ended the 125 my history of the goniatitids, though they were already in decline. Their replacement by the ceratitids in the basal Triassic set a new course for Mesozoic ammonoid history. CONTINGENCY is the most elusive factor to assess, but it had obvious and pervasive influence in the history of life.