Paläontologisches Institut und Museum, Universität Zürich, Karl Schmid-Strasse 4, CH-8006 Zürich, Switzerland.
Evolution. 2012 Jun;66(6):1788-806. doi: 10.1111/j.1558-5646.2011.01567.x. Epub 2012 Feb 14.
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 whereas 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 might have played an important role in their subsequent success as well as in their demise.
在泥盆纪的浮游动物革命中，菊石类动物的壳逐渐向螺旋形进化，就像许多其他浮游软体动物群体一样。这些现已灭绝的、具有外部壳的头足类动物起源于早泥盆世的直壳杆菊石类。在泥盆纪，菊石类胚胎壳的螺旋程度和尺寸减小呈现出进化趋势。在至少三个谱系中，具有封闭脐孔的后代从第一旋（脐窗）有开口的形式趋同进化而来。其他具有开口脐孔的谱系在重要的泥盆纪事件中灭绝，而只有那些具有更紧密螺旋形胚胎壳的谱系幸存下来。这种变化伴随着初始室形状的进化趋势，但大小没有明显的趋势。几个菊石类谱系独立地或多或少同时减少并封闭脐孔的事实表明，共同的驱动因素涉及其中。一些谱系中胚胎尺寸减小的趋势以及成体尺寸同时增加，可能反映了在菊石类的进化历史早期，生殖策略向更高繁殖力的根本性转变。这可能在它们随后的成功以及灭绝中发挥了重要作用。
