Long John A, Gordon Malcolm S
Museum Victoria, Melbourne, Victoria 3001, Australia.
Physiol Biochem Zool. 2004 Sep-Oct;77(5):700-19. doi: 10.1086/425183.
Recent discoveries of previously unknown fossil forms have dramatically transformed understanding of many aspects of the fish-tetrapod transition. Newer paleobiological approaches have also contributed to changed views of which animals were involved and when, where, and how the transition occurred. This review summarizes major advances made and reevaluates alternative interpretations of important parts of the evidence. We begin with general issues and concepts, including limitations of the Paleozoic fossil record. We summarize important features of paleoclimates, paleoenvironments, paleobiogeography, and taphonomy. We then review the history of Devonian tetrapods and their closest stem group ancestors within the sarcopterygian fishes. It is now widely accepted that the first tetrapods arose from advanced tetrapodomorph stock (the elpistostegalids) in the Late Devonian, probably in Euramerica. However, truly terrestrial forms did not emerge until much later, in geographically far-flung regions, in the Lower Carboniferous. The complete transition occurred over about 25 million years; definitive emergences onto land took place during the most recent 5 million years. The sequence of character acquisition during the transition can be seen as a five-step process involving: (1) higher osteichthyan (tetrapodomorph) diversification in the Middle Devonian (beginning about 380 million years ago [mya]), (2) the emergence of "prototetrapods" (e.g., Elginerpeton) in the Frasnian stage (about 372 mya), (3) the appearance of aquatic tetrapods (e.g., Acanthostega) sometime in the early to mid-Famennian (about 360 mya), (4) the appearance of "eutetrapods" (e.g., Tulerpeton) at the very end of the Devonian period (about 358 mya), and (5) the first truly terrestrial tetrapods (e.g., Pederpes) in the Lower Carboniferous (about 340 mya). We discuss each of these steps with respect to inferred functional utility of acquired character sets. Dissociated heterochrony is seen as the most likely process for the evolutionarily rapid morphological transformations required. Developmental biological processes, including paedomorphosis, played important roles. We conclude with a discussion of phylogenetic interpretations of the evidence.
近期对先前未知化石形态的发现极大地改变了人们对鱼类向四足动物过渡诸多方面的理解。更新的古生物学方法也促使人们对哪些动物参与了这一过渡、过渡发生的时间、地点以及方式的观点发生了变化。本综述总结了已取得的主要进展，并重新评估了证据重要部分的不同解释。我们从一般问题和概念开始，包括古生代化石记录的局限性。我们总结了古气候、古环境、古生物地理学和埋藏学的重要特征。然后我们回顾泥盆纪四足动物及其在肉鳍鱼类中最接近的干群祖先的历史。现在人们普遍认为，第一批四足动物起源于泥盆纪晚期先进的四足形类（扇鳍鱼类），可能在欧美地区。然而，真正的陆生形态直到很久以后，在下石炭纪，在地理上分布广泛的地区才出现。整个过渡过程持续了约2500万年；明确的登陆事件发生在最近的500万年里。过渡期间特征获得的顺序可被视为一个五步过程，包括：（1）中泥盆世（约3.8亿年前开始）硬骨鱼类（四足形类）的多样化增加，（2）弗拉斯阶（约3.72亿年前）“原始四足动物”（如埃尔金螈）的出现，（3）法门阶早期至中期（约3.6亿年前）某个时候水生四足动物（如棘螈）的出现，（4）泥盆纪末期（约3.58亿年前）“真四足动物”（如土列螈）的出现，以及（5）下石炭纪（约3.4亿年前）第一批真正的陆生四足动物（如佩德鱼）的出现。我们根据所获得特征集的推断功能效用讨论了每一步。解离异时性被视为进化上快速形态转变所需的最可能过程。发育生物学过程，包括幼态持续，发挥了重要作用。我们最后讨论了对证据的系统发育解释。
