Diversity patterns of ammonoids are analyzed and compared with the
timing of anoxic deposits around the Cenomanian/Turonian (C/T) boundary
in the Vocontian, Anglo-Paris, and Monster basins of Western Europe.
Differing from most previous studies, which concentrate on a narrow time
span bracketing the C/T boundary, the present analysis covers the latest
Albian to Early Turonian interval for which a high resolution,
ammonoid-based biochronology, including 34 Unitary Associations zones,
is now available. During the latest Albian-Middle Cenomanian interval,
species richness of ammonoids reveals a dynamical equilibrium
oscillating around an average of 20 species, whereas the Late
Cenomanian-Early Turonian interval displays an equilibrium centered on
an average value of 6 species. The abrupt transition between these two
successive equilibria lasted no longer than two Unitary Associations.
The onset of the decline of species richness thus largely predates the
spread of oxygen-poor water masses onto the shelves, while minimal
values of species richness coincide with the Cenomanian-Turonian
boundary only. The decline of species richness during the entire Late
Cenomanian seems to result from lower origination percentages rather
than from higher extinction percentages. This result is also supported
by the absence of statistically significant changes in the extinction
probabilities of the poly-cohorts. Separate analyses of species richness
for acanthoceratids and heteromorphs, the two essential components of
the Cenomanian ammonoid community, reveal that heteromorphs declined
sooner than acanthoceratids. Moreover, acanthoceratids showed a later
decline at the genus level than at the species level. Such a decoupling
is accompanied by a significant increase in morphological disparity of
acanthoceratids, which is expressed by the appearance of new genera.
Last, during the Late Cenomanian, paedomorphic processes, juvenile
innovations and reductions of adult size dominated the evolutionary
radiation of acanthoceratids. Hence, the decrease in ammonoid species
richness and their major evolutionary changes significantly predates the
spread of anoxic deposits. Other environmental constraints such as
global flooding of platforms, warmer and more equable climate, as well
as productivity changes better correlate with the timing of diversity
changes and evolutionary patterns of ammonoids and therefore, provide
more likely causative mechanisms than anoxia alone.