Nielsen Claus
Zoological Museum, The Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark.
Biol Bull. 2009 Jun;216(3):203-15. doi: 10.1086/BBLv216n3p203.
The two main types of theories for the evolution of the biphasic life cycles in marine invertebrates are discussed. The "intercalation" theories propose that the larval stages (planktotrophic or lecithotrophic) have evolved as specializations from the ancestral, direct life cycle. The opposing "terminal addition" theories propose that the ancestor was holopelagic and that the adult stage was added to the life cycle with the pelagic stage retained as a planktotrophic larva. It is emphasized that theories based on hypothetical ancestors that were unable to feed must be rejected. This applies to planula theories based on a compact planula. Various arguments against the theories that consider the feeding larvae as ancestral in the major eumetazoan lineages and in particular against the trochaea theory are discussed and found untenable. It is suggested that the "Cambrian explosion" was actually a rapid Ediacaran radiation of the eubilaterians that was made possible by the evolution of a tubular gut with all the resulting possibilities for new body plans.
本文讨论了海洋无脊椎动物双相生命周期进化的两种主要理论类型。“插入”理论认为，幼虫阶段（浮游营养型或卵黄营养型）是从祖先的直接生命周期进化而来的特化阶段。相反的“终端添加”理论则认为，祖先为全浮游生物，成体阶段被添加到生命周期中，而浮游阶段则保留为浮游营养型幼虫。需要强调的是，基于无法进食的假设祖先的理论必须被摒弃。这适用于基于紧密浮浪幼虫的浮浪幼虫理论。本文讨论了各种反对将摄食性幼虫视为主要真后生动物谱系祖先的理论的观点，特别是反对担轮幼虫理论的观点，并发现这些观点站不住脚。有人认为，“寒武纪大爆发”实际上是真双侧对称动物在埃迪卡拉纪的快速辐射，这是由管状肠道的进化以及由此产生的所有新身体结构的可能性所促成的。
