No 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.

Principal components analysis of Upper Carboniferous (Pennsylvanian) ammonoids (all 117 genera), using 21 variables to measure shell geometry, sculpture and suture complexity, 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 polyphyletically 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 ammonoids. Innovations in suture geometry distinguished at least 17 of 39 (44%) Pennsylvanian ammonoid families. Average suture complexity increased almost threefold; this was achieved by various methods (lobe serration, insertion of umbilical elements, prong subdivision, lobe trifurcation, and secondary bifurcation), which were recurrent and crossed morphotype boundaries. The Pennsylvanian record supports suggestions that Paleozoic ammonoids were confined to a certain suite of basic shell geometries, showing preference for a limited number of sites in the spectrum of available 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.

The ammonoid order Prolecanitida constitutes a relatively small (43 genera, ~250 species) but long-ranging lineage (Lower Carboniferous—Triassic, ~108 m.y.), which narrowly survived 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 dominated by the Goniatitida, and showed many features that set them far apart from their contemporaries, 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 complexity. 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 analogous to Paleozoic goniatites.