Abstract.-Principal components analysis of Upper Carboniferous (Pennsylvanian) ammonoids (all
 117 genera), using 21 variables to measure shell geometry, sculpture and suture cornplexity, shows
 that following a sharp decline (-30%) in generic diversity after the mid-Carboniferous boundary,
 seven morphotypes persisted throughout the Pennsylvanian (ca. 30 m.y.). Six of these were poly-
 phyletically adopted at different times, while the seventh was monopolized by the prolecanitids,
 a group whose evolution accelerated during the Pennsylvanian and later gave rise to Mesozoic am-
 monoids.  Innovations in suture geometry distinguished at least 17 of 39 (44%) Pennsylvanian am-
 monoid families.  Average suture complexity increased almost threefold; this was achieved by var-
 ious methods (lobe serration, insertion of umbilical elements, prong subdivision, lobe trifurcation
 and secondary bifurcation), which were recurrent and crossed morphotype boundaries.  The Penn-
 sylvanian record supports suggestions that Paleozoic ammonoids were confined to a certain suite
 of basic shell geometries, showing preference for a limited nurnber of sites in the spectrum of avail-
 able morphospace.  However, these morphic constraints did not, with one possible exception (the
 prolecanitids), control the emergence of increasing sutural complexity during the Pennsylvanian,
 which occurred among different lineages in all seven morphotypes.