-Abstract.-Clymeniid ammonoids appeared during the Late Devonian (Mid-Famennian) and quick-
ly radiated before becoming extinct at the Devonian/Carboniferous boundary.  Outwardly indis-
tinguishable from other ammonoids, clymeniids are distinguished internally by their dorsal si-
phuncle, contrasted to a ventral siphuncle in almost all other ammonoids.  Comparisons of a sample
of Clymeniida (n = 22 genera), Goniatitida (n = 33), Prolecanitida (n = 12), and Anarcestida (n =
13) indicate that clymeniids fall within the range of other ammonoids in terms of shell geometry
and suture complexity, but their siphuncles average two to three times larger and clymeniid shells
are approximately 33% thicker than those of other ammonoids.  Although a dorsal siphuncle would
be about 50% smaller in surface area and volume than if located in a ventral position, the enlarged
clymeniid siphuncle partially and, in some cases, fully compensated for this loss.  Hydrostatic sim-
ulations of 15 clymeniid genera indicate that their thicker (therefore heavier) shells would have
resulted in relatively short body chambers (-280') and high aperture orientations (-90').  In static
life-position, these orientations would have placed the dorsal clymeniid siphuncle at or near the
bottom of the most recently formed chambers, seemingly an ideal location for draining liquid from
the chamber.  Migration of the siphuncle to the dorsal side of the shell occurs suddenly during early
ontogeny (within the first two or three chambers), and mutation of homeotic gene expression is
offered as a possible explanation for the sudden shift.  A dorsal siphuncle may have resulted in
selection for enlarged siphuncles, but this may have incurred loss of strength against hydrostatic
pressure (thereby reducing depth limits) and thus rendered the clade more susceptible to the mul-
tiple eustatic and anoxic events that marked the end of the Devonian.