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.

在泥盆纪游泳动物革命（Devonian Nekton Revolution）期间，菊石类（ammonoid）与诸多其他浮游软体动物类群一样，其壳体呈现出逐渐卷曲的演化趋势。这些现已灭绝的外壳外露的头足类动物，起源于早泥盆世具有直壳的杆石类头足动物（bactritoid cephalopod）。泥盆纪时期，菊石类的胚胎壳体呈现出卷曲程度愈发紧密以及壳体尺寸缩小的演化趋势。至少有三个演化支系中，具有封闭脐部的后裔类群从第一旋环带有开口（脐窗，umbilical window）的类群趋同演化而来。其他保留有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，唯有胚胎壳体卷曲更紧密的类群得以存续。这一演化转变伴随有初始壳室形状的演化趋势，但壳体尺寸并未出现明确的变化规律。多个菊石演化支系近乎同步地独立实现脐窗的缩小与闭合，这一现象表明背后存在共同的驱动因素。部分演化支系中出现的胚胎尺寸减小、同时成体尺寸增大的趋势，很可能反映了菊石演化早期生殖策略向更高繁殖力方向发生的根本性转变。这一转变想必对菊石后续的繁盛乃至最终灭绝都起到了重要作用。