Ontogeny in the fossil record: diversification of body plans and the evolution of "aberrant" symmetry in Paleozoic echinoderms
Ontogeny in the fossil record: diversification of body plans and the evolution of "aberrant" symmetry in Paleozoic echinoderms
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DOI：
10.1666/06053.1
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发表时间：
2007-12-01
期刊：
影响因子：
2.7
通讯作者：
Wray, Gregory A.
中科院分区：
文献类型：
--
作者：
Sumrall, Colin D.;Wray, Gregory A.
棘皮动物长期以来的特点是存在步带，表现出五放射对称性和定义五个主要的身体轴。事实上，真正的五辐步带对称性在棘皮动物的进化史上只出现过一次，并且仅限于棘皮动物，这是包含大多数棘皮动物物种的进化支。与此相反，早期棘皮动物有一个双侧对称的2-1-2排列，有三个步带从口部辐射。在个体发育过程中，两侧步带的分支产生五条成年射线。在寒武纪爆发和奥陶纪辐射期间，大约有30个棘皮动物分支进化，其中许多具有异常的步带系统，具有一到四条射线。不幸的是，还没有出现任何基础模型来解释不同形式之间的步带同源性。在这里，我们表明，大多数古生代棘皮动物的特点是独特的可识别的步带，在三个不同的postlarval阶段的发展。几乎所有的“异常”棘皮动物的形态可以解释的幼态步带减少（PAR）模型，通过这些生长阶段的一些组合在个体发育过程中的损失。表面上相似的模式，在远亲分支的步带减少导致在个体发育过程中同源步带的平行损失。在芽虫总科中发现的假五重对称和在五兽门中发现的真五重对称分别是由于个体发育早期2-1-2对称的大量减少和完全丧失。这些步带的变化表明，发展和生态的限制影响新的棘皮动物的身体计划的演变。
Echinoderms have long been characterized by the presence of ambulacra that exhibit pentaradiate symmetry and define five primary body axes. In reality, truly pentaradial ambulacral symmetry is a condition derived only once in the evolutionary history of echinoderms and is restricted to eleutherozoans, the clade that contains most living echinoderm species. In contrast, early echinoderms have a bilaterally symmetrical 2-1-2 arrangement, with three ambulacra radiating from the mouth. Branching of the two side ambulacra during ontogeny produces the five adult rays. During the Cambrian Explosion and Ordovician Radiation, some 30 clades of echinoderms evolved, many of which have aberrant ambulacral systems with one to four rays. Unfortunately, no underlying model has emerged that explains ambulacral homologies among disparate forms. Here we show that most Paleozoic echinoderms are characterized by uniquely identifiable ambulacra that develop in three distinct postlarval stages. Nearly all "aberrant" echinoderm morphologies can be explained by the paedomorphic ambulacra reduction (PAR) model through the loss of some combination of these growth stages during ontogeny. Superficially similar patterns of ambulacral reduction in distantly related clades have resulted from the parallel loss of homologous ambulacra during ontogeny. Pseudo-fivefold symmetry seen in Blastoidea and the true fivefold symmetry seen in Eleutherozoa result from great reduction and total loss, respectively, of the 2-1-2 symmetry early in ontogeny. These ambulacral variations suggest that both developmental and ecological constraints affect the evolution of novel echinoderm body plans.