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.在泥盆纪游泳生物革命（Devonian Nekton Revolution）期间，菊石类（ammonoids）与诸多其他浮游软体动物类群一样，其壳体呈现出逐渐旋卷的演化趋势。这类现已灭绝的具外骨骼头足类，起源于早泥盆世壳体呈直形的杆石类头足类（bactritoid cephalopods）。泥盆纪时期，菊石胚胎壳演化出旋卷更紧致、壳体尺寸缩小的趋势。至少有三个演化支系中，具有封闭脐部的后裔类群，从最初螺层带有开口（脐窗（umbilical window））的祖先类群趋同演化而来。其他保留开放脐部类群的演化支系，在泥盆纪重大地质事件前后相继灭绝，仅胚胎壳旋卷更紧致的类群得以存续。这一转变伴随初房（initial chamber）形态的演化趋势，但未出现显著的尺寸变化规律。多个菊石演化支系近乎同步地独立缩减并封闭脐窗，表明其背后存在共同的驱动因素。部分演化支系中胚胎尺寸减小、同时成体尺寸增大的趋势，或反映菊石演化早期生殖策略向更高繁殖力的根本性转变。这一转变在菊石后续的繁盛乃至灭绝过程中，均发挥了重要作用。