This paper describes the influence of high environmental stress on
evolutionary trends in some selected Mesozoic ammonite lineages and some
protists. During extinction periods, many ammonoids are affected by
drastic simplifications of their shell geometry, ornamentation and
suture line. We observe that relatively tightly coiled ammonites can
give rise to highly evolute forms or uncoiled heteromorphs with simple
ornamentation and almost ceratitic suture line-a phenomenon called
"proteromorphosis". Such simplifications often correspond to a
reappearance of ancestral geometries (primitive ornamentation, evolute
coiling or uncoiling) which suggest that the evolutionary clock of these
organisms can be reinitialized by extreme, sublethal, environmental
stress such as giant volcanism (including its consequences on diverse
pollutions and on climatic changes) and major regressive events. (c)
2006 Elsevier B.V. All rights reserved.