Abstract.-The ammonoid order Prolecanitida constitutes a relatively small (43 genera, -250 spe-
 cies) but long-ranging lineage (Lower Carboniferous-Triassic, -108 m.y.), which narrowly sur-
 vived the P/Tr extinctions and provided the stock from which were derived all later Mesozoic
 ammonoids.  Prolecanitids were a minority among Late Paleozoic ammonoids, which were domi-
 nated by the Goniatitida, and showed many features that set them far apart from their contem-
 poraries, including (1) long-term, gradual changes in shell geometry (W-D-S); (2) the most strongly
 constrained morphospace of any Paleozoic ammonids examined to date; (3) an eight-fold increase
 in mean suture complexity (three times that of Pennsylvanian goniatitids); (4) high correlations
 between shell geometry, shell and septal thickness, and suture complexity; (5) short body chambers
 and, as a consequence, high aperture orientations; (6) indications that cameral liquid may have been
 used for buoyancy control; and (7) a genus longevity that averaged 14.7 m.y. compared with 5.7
 m.y. in Upper Carboniferous goniatitids, and that appears to have been unrelated to suture com-
 plexity. Prolecanitids showed a pervasive   tendency to increase suture complexity (in the clade as
 a whole as well as within subclades and in more than 90 percent of ancestor-descendant genera),
 thus arguing a case for a driven complexity trend.  The uniqueness of the prolecanitids calls into
 question whether they and their Mesozoic descendants, ceratites and ammonites, were strictly anal-
 ogous to Paleozoic goniatites.