Data from: Early evolutionary trends in ammonoid embryonic development

During the Devonian Nekton Revolution, ammonoids show a progressive
coiling of their shell just like many other pelagic mollusk groups. These
now extinct, externally shelled cephalopods derived from bactritoid
cephalopods with a straight shell in the Early Devonian. During the
Devonian, evolutionary trends toward tighter coiling and a size reduction
occurred in ammonoid embryonic shells. In at least three lineages,
descendants with a closed umbilicus evolved convergently from forms with
an opening in the first whorl (umbilical window). Other lineages having
representatives with open umbilici became extinct around important
Devonian events while only those with more tightly coiled embryonic shells
survived. This change was accompanied by an evolutionary trend in shape of
the initial chamber, but no clear trend in its size. The fact that several
ammonoid lineages independently reduced and closed the umbilical window
more or less synchronously indicates that common driving factors were
involved. A trend in size decrease of the embryos as well as the
concurrent increase in adult size in some lineages likely reflects a
fundamental change in reproductive strategies toward a higher fecundity
early in the evolutionary history of ammonoids. This must have played an
important role in their subsequent success as well as in their demise.