Abstract.-Principal components analysis (PCA) of 21 shell parameters (geometry, sculpture, ap-
erture shape, and suture complexity) in 597 L. Devonian to L. Triassic ammonoid genera (spanning
~166 Myr) shows that eight basic morphotypes appeared within ~20 Myr of the first appearance
of ammonoids.  With one exception, these morphotypes persisted throughout the Paleozoic, occur-
ring in ~75% of the ~5-Myr time bins used in this study.  Morphotypes were not exclusive to par-
ticular lineages.  Their persistence was not just a product of phylogenetic constraints or longevity,
and multiple iterations of the same morphotypes occurred at different times and in different
groups.  Although mass extinction events severely condensed the range of morphologic Variation
and taxonomic diversity, the effects were short lived and most extinct morphotypes were usually
iterated within 5 Myr.  The most important effect of mass extinctions on ammonoid evolutionary
history seems to have been their role in large scale taxonomic turnovers; they effectively eliminated
previously dominant orders at the Frasnian/Famennian (F/F) (Agoniatitida), the Devonian/Mis-
sissippian (D/M) (Clymeniida), and the Permian/Triassic (P/T) (Goniatitida and Prolecanitida)
extinctions.  Survivors varied from two (P/T) to four (D/M) and five genera (F/F).  These events
generated sharp reductions in morphologic disparity at the D/M (58%) and at the P/T (59%), but
there was a net increase at the F/F (38%).  There was no obvious survival bias for particular mor-
photypes, but 64% are interpreted to have been Nautilus-like nektobenthic.  The recurrence of par-
ticular combinations of morphology and their strong independence of phylogeny are strong ar-
guments for funetional constraint.  Intervals between mass extinctions seem to have been relatively
static in terms of morphotype numbers, in contrast to numbers of genera.  Significant decreases in
genus diversity (54%) and morphologic disparity (33%) commenced in the mid-Permian (Wordian/
Capitanian boundary), well before the final P/T event.