寒武纪大爆发与动物树的成型

Cambrian Explosion:Formation of Tree of Animals

由"真动物"构成的动物界(不包括海绵)通常被划分为双胚层动物、原口动物和后口动物3个亚界。寒武纪大爆发经历了爆发的前奏—序幕—主幕3个阶段,其中后两个阶段处于早寒武世初期(依次以小壳化石的首次辐射和澄江动物群爆发为代表),另一个则发生在"寒武前夜"(即前寒武纪末期),以伊迪卡拉生物群为代表。这次独特的三幕式大爆发分步完成了动物形态演化谱系树(简写为TOA)的成型。已有化石证据显示,动物树的3大主体或3个亚界的起源及其早期辐射分别发生于寒武纪大爆发这3个主要阶段。澄江化石库中发现的早期后口动物亚界涵盖了该亚界中所有6大分支(棘皮类、半索类、头索类、尾索类、脊椎类和绝灭了的古虫类)的原始类群,澄江动物群时代标志着寒武纪大爆发的顶峰,完成了动物树框架的成型,从而宣告了寒武纪大爆发的基本终结。其学术重要性超越了中寒武世的布尔吉斯页岩,因为前者位于动物树形成的"源头",而后者仅代表此后的"一段流程"。在动物演化历史上4个最具转折意义的重大创新事件(即多细胞动物起源、两侧对称动物起源、后口动物起源和脊椎动物起源)中,第一个发生在伊迪卡拉纪甚至更早的时期,其余3个则可通过梅树村化石群和澄江化石群观察到其完成的主体过程。已灭绝的古虫动物门,是初具鳃裂构造的原始分节动物,作为由原口动物向后口动物过渡的一个珍稀"缺环",很可能代表着后口动物亚界的一个根。古囊类兼有两侧对称的古虫类和一些原始棘皮动物的镶嵌体征,最可能代表棘皮动物的一个根。在早寒武世动物演化谱系中,后口动物的"顶端类群"的代表是被誉为"第一鱼"的昆明鱼目,目前已知包括昆明鱼、海口鱼和钟健鱼,它们具对眼、脊索与串珠状脊椎软骨共存的原始脊椎,代表了真正的脊椎类或有头类始祖。相反,包括云南虫属和海口虫属的云南虫类,既没有成对的眼睛也没有可信的脊椎软骨,而且其背神经索也未扩大成脑,甚至不具备低等脊索动物的肌节,因此与真正的有头类或脊椎动物无关。其实,这类奇特动物与古虫类共享相似的躯体构形,应该代表着低等后口动物中的一个侧枝。自达尔文以来,进化生物学一直在动物世界的悠长演化历程中,努力追寻"人类远古由来"的一些重大形态学创新事件的证据,无疑,古虫动物门原始鳃裂构造的出现以及第一鱼之头颅和原始脊椎软骨的形成,应是这条历史长河中人们期待已久的两大里程碑。

The Kingdom Animalia is here divided into three subkingdoms; Diploblasta, Protostomia and Deuterostomia. Cambrian Explosion, including its first episode represented by the small skeletal fossils（SSFs） at the base of the Cambrian and the succeeding main episode represented by the Chengjiang faunas（CFs）, together with its prelude represented by the Ediacaran faunas, have been recognized to give birth of the whole morphological tree of animals（or metazoans）, for short the TOA. Its ＂seed＂ in Precambrian, some sort of protists from which the complete TOA have grown, remains unknown paleontologically. Ediacaran fauna was dominated by diploblasts （the ＂trunk＂ of the TOA） with a few possible stem-group triploblasts, the Early Cambrian in two phases explosively yielded almost all the major triploblastic crown-branches（Bilateria： the huge ＂crown＂ of the TOA）, which include the other two Subkingdoms： first the extremely diverse protostomes in the Meishucunian Age and followed by a nearly entire lineage of early deuterostomes, i.e. the early or even the oldest representatives of all the six major groups in deuterostomes from the Chengjiang, including echinoderms, hemichordates, urochordates, cephalochordates and the vertebrates. As all the major branches of the Tree of Animals had been formed at the Chengjiang Age, the Cambrian Explosion reached its summit and also principally came to the end by the Early Cambrian. Among the four most significant milestone events of morphological origins and radiations in animal history, the first one（i, e. appearance of metazoans） took place in the Ediacaran Period or deeper times, and the other three can be practically seen from the Chengjiang and the Meishucunian windows. The newly discovered extinct Phylum Vetulicolia, which has primitively segmented body with simple gill slits in its anterior division, most probably represents one of the roots in deuterostome subkingdom. Sharing the mosaic basic features of both the bilateral vetulicolians and some primitive echinoderms homalozoans, the softbodied vetulocystids are best regarded as one of the roots of the extant pentamerous radial echinoderms. Standing on the ＂top＂ of the deuterostome super-branch in the early Cambrian TOA are the ＂the first fish＂ Myllokunmingia and Haikouichthys, which bear the definite paired eyes and the salient proto-vertebrae and then should represent the real root of the whole vertebrates or craniates. On the contrary, yunnanozoans, including Yunnanozoon and Haikouella, possess neither eyes and reliable vertebrae, nor myomeres, may have nothing to do with craniates or vertebrates. On the other hand, sharing a similar body-plan with vetulicolians, those enigmatic creatures might be best treated as a side-branch in the lower deuterostomes. The appearance of gill slits in Vetulicolians and the formation of the real brain and the most primitive vertebrae in ＂the First Fish＂ have been recognized as two of the most significant milestones in the evolution from protostomes to the vertebrates.