澄江动物群的核心价值:动物界成型和人类基础器官诞生

澄江动物群以富含多门类精美软躯体构造化石享誉学界,是寒武纪大爆发主幕的见证者;它亲眼目睹了地球动物树上几乎所有主要门类祖先的首次大聚会。对该动物群30多年的大规模调查研究大体上可以划分为3个阶段。始自1984年的第一个10年(1984—1994年),早期动物树上的基础动物亚界和原口动物亚界的大量门类被发现,然而对后口动物亚界全然无知。第二个10年研究(1995—2005年),基础动物亚界和原口动物亚界中的更多门类被发现;而最重要的进展,是西北大学团队主导的早期后口动物大发现使后口动物亚界的基本框架被构建,由此地球三分动物树首次成型。第三个10年及其以后时期(2005年之后),3个动物亚界中进一步添加了一些新的门类认知;综合20世纪40年代以来对埃迪卡拉生物群、寒武纪第一世和第二世的珍稀动物群研究成果,学界开始思考三分动物树分步成型与多幕式寒武纪大爆发事件的内在关联,提出了三幕式寒武纪大爆发新假说。澄江动物群研究具有多方面的科学价值,包括动物群结构研究,生态环境研究等,然其核心学术价值主要集中在如下两个方面:(1)作为寒武纪大爆发的主幕见证者,澄江动物群首次创建了地球上三分动物树的完整谱系框架。三幕式寒武纪大爆发假说揭示了地球上动物门类由低等到高等、分阶段爆发创造的本质内涵,即历时约0.4亿年的三幕式寒武纪大爆发与动物树阶段性快速成型的内在耦合性。第一幕发生于埃迪卡拉纪晚期,它诞生了众多基础动物亚界门类(包括大量绝灭门类),也可能出现了少量原口动物的先驱分子;第二幕发生于寒武纪的第一世,除了延续基础动物门类的繁盛外,更创生了原口动物亚界里的主要门类(包括蜕皮类和触手担轮类);第三幕发生于寒武纪第二世(以澄江动物群为代表),不仅延续了基础动物和原口动物门类的兴盛,更重要的是诞生了后口动物亚界里的所有门类。至此,整个三分动物树的框架成型,寒武纪大爆发创新事件宣告基本结束。文章还对几个重要动物类群的生物学属性进行了评述。第一鱼昆明鱼目是已知唯一的最古老脊椎动物,而云南虫、海口虫既不是脊椎动物(即高等脊索动物),也不属于低等脊索动物,而是一类特殊的低等后口动物;长江海鞘是可信的始祖尾索动物,它支持了经典的尾索动物起源假说;古虫动物门首创鳃裂构造,对后口动物亚界的起源探索提供了关键信息。(2)澄江动物群具有深刻的人文哲学意义,所发现的古虫动物门创造的“第一鳃裂”,昆明鱼目创造的“第一头脑”“第一脊椎”“第一心脏”,为达尔文在《人类的由来》中关于人类的主要基础器官起源悬案的破解提供了关键证据。澄江动物群承上启下,其各种形态解剖学信息对解读埃迪卡拉生物群以及寒武纪相关化石库中绝大多数生物类型都具有重要的参考价值。

Fossil Association from the Lower Cambrian Yanjiahe Formation in the Yangtze Gorges Area,Hubei,South China

Apart from previously reported Small Shelly Fossils （SSFs）, a macroscopic fossil assemblage, comprising abundant algae, cone-shaped tubular fossil forms, and probable impressions of a megascopic metazoan, comes from the Lower Cambrian Yanjiahe Formation in the Yangtze Gorges area of western Hubei Province, south China. The visible fossils are preserved in thinlaminated siltstone or muddy siltstone intercalated between 8-15 ram-thick carbonate deposits, probably representing sedimentary settings of a constrained local depression in the shallow water carbonate platform during the Early Cambrian Meishucunian Stage. The macroscopic fossil association provides significant fossil evidence about the evolution of life from the late Precambrian to the ＇Cambrian explosion＇ interval.

Core value of the Chengjiang fauna:formation of the animal kingdom and the birth of basic human organs

Well known for its abundant and extraordinary soft-tissue fossils,the Chengjiang fauna has witnessed the main phase of the Cambrian explosion and the first great congress of the ancestors of nearly all major phyla of animals on Earth.The large-scale survey and exploration of the fauna by Chinese paleontologists,partly in collaboration with international scientists,over the past 30 years can be broadly divided into three stages.In the first decade since 1984(19841994),a large number of invertebrates,including basal animals and the protostomes of the early animal tree were discovered,but the subkingdom Deuterostomia was completely unknown.Then in the second decade(19952005),the great discoveries of various deuterostomes led to the construction of the basic framework of the Deuterostomia and then to the formation of the tripartite phylogenetic trees of early animals(TPTEA,including basal animals,protostomes and deuterostomes),for the first time.In the third decade(after 2005),the academic community began to think about the internal relationships between the formation of TPTEA and the multiepisode Cambrian explosion,leading to the new hypothesis of the three-episode Cambrian explosion.The Chengjiang fauna is important for deciphering the fauna structure,paleoecological environment,and so on.However,its core academic values mainly rest on two aspects.Firstly,The Chengjiang fauna,as the main phase witness of the Cambrian explosion,has created a nearly complete phylogenetic framework of the TPTEA on Earth for the first time.The three-phase radiation hypothesis reveals the essential connotation of the Cambrian explosion:a step-wise divergent evolution of animals,from basal to highly advanced groups,lasting about 40 million years.In the first phase,it gave birth to a bulk of basal animals(including some now extinct“animal”groups)in the latest Ediacaran,probably including some pioneer protostomes.The second phase took place in the first epoch of the Cambrian period(Terreneuvian),giving rise to the main invertebrate protostomes with a persistent prosperity of basal animals.The third phase proceeded in Cambrian Epoch 2(represented by the Chengjiang fauna),which not only maintained the prosperity of basal animals and protostomes,but also,more importantly,gave birth to all the main phyla of the subkingdom Deuterostomia.Thus,the rudimental framework of the whole TPTEA has been shaped,with the termination of the major innovation events of the Cambrian explosion.Here,we discussed the evolutionary properties of Ediacaran biota,small shelly fossils and the Chengjiang fauna in the Cambrian explosion with emphasis on the biological properties of several important animal groups.The order Myllokunmingiida is the only known oldest vertebrate,while Yunnanozoon and Haikouella are neither vertebrates nor stem-group chordates but a special group of basal deuterostomes;Cheungkongella is a credible ancestor of the urochordate and it supports the classical hypothesis on the origin of the urochordates;and the gill slits were first invented in the members of the phylum Vetulicolia to provide key information on the origin of the deuterostomes.The second core value of the Chengjiang fauna is of profound humanistic and philosophical significance:the discoveries of the‘first gill openings’,‘first brain’,‘first vertebrae’and‘first heart’provide the pivotal evidence for solving the unsolved mystery of the origin of the main basic human organs as described in Darwin’s“The Descent of Man”.In addition,the morphological and anatomical information of the Chengjiang fauna can provide important clues for a better understanding of most components of Ediacaran and Cambrian metazoans.

Dimorphism of Bivalved Arthropod Branchiocaris？ Yunnanensis from the Early Cambrian Chengjiang Biota, South China

The bivalved arthropod Branchiocaris？ yunnanensis Hou, 1987 is redescribed on the basis of new fossil material from the early Cambrian Chengjiang biota, South China. In total, 81 specimens have been examined. The carapace comprises two identical valves, each valve are sub-circular in outline, ranging from 24 mm to 58 mm in length, and from 15 mm to 46 mm in height. The dorsal margin is straight and bears two short cardinal spines. The valve surface is either smooth or ornamented with polygonal reticular structures, which may result from different preservation conditions. Two morphotypes have been recognized based on the presence or absence of a subdorsal swelling of the valve, which is convex dorsolaterally and extends beyond the dorsal margin when it appears. Statistic and ontogenetic analyses show both morphotypes grow isometrically and have the same growth trajectory. Therefore, the two morphotypes are interpreted as dimorphism within the same species. Our results are helpful for investigating the taxonomy and ontogeny of arthropod fossils and suggest that dimorphism might be fairly common in these early arthropods.

Message from the Editor-in-Chief

Dear Authors and Readers:First of all,on behalf of the new Editorial Board,I would like to express our sincere appreciation for your continuing support of our journal.This journal,founded in 1922,is the oldest geological journal in China.As the flagship journal of the Geological Society of China,it has become more and more influential in earth sciences in China and

Causes and consequences of the Cambrian explosion

The Cambrian explosion has long been a basic research frontier that concerns many scientific fields.Here we discuss the cause-effect links of the Cambrian explosion on the basis of first appearances of animal phyla in the fossil record,divergence time,environmental changes,Gene Regulatory Networks,and ecological feedbacks.The first appearances of phyla in the fossil record are obviously diachronous but relatively abrupt,concentrated in the first three stages of the Cambrian period(541–514 Ma).The actual divergence time may be deep or shallow.Since the gene regulatory networks(GRNs)that control the development of metazoans were in place before the divergence,the establishment of GRNs is necessary but insufficient for the Cambrian explosion.Thus the Cambrian explosion required environmental triggers.Nutrient availability,oxygenation,and change of seawater composition were potential environmental triggers.The nutrient input,e.g.,the phosphorus enrichment in the environment,would cause excess primary production,but it is not directly linked with diversity or disparity.Further increase of oxygen level and change of seawater composition during the Ediacaran-Cambrian transition were probably crucial environmental factors that caused the Cambrian explosion,but more detailed geochemical data are required.Many researchers prefer that the Cambrian explosion is an ecological phenomenon,that is,the unprecedented ecological success of metazoans during the Early Cambrian,but ecological effects need diverse and abundant animals.Therefore,the establishment of the ecological complexity among animals,and between animals and environments,is a consequence rather than a cause of the Cambrian explosion.It is no doubt that positive ecological feedbacks could facilitate the increase of biodiversity.In a word,the Cambrian explosion happened when environmental changes crossed critical thresholds,led to the initial formation of the metazoan-dominated ecosystem through a series of knock-on ecological processes,i.e.,"ecological snowball"effects.

On the Phylum Vetulicolia

The origins and radiations of metazoans, bilat- erians, deuterostomes and vertebrates have been recognized as the four most important milestone events in animal his- tory and, conversely four of the toughest and puzzling ques- tions in evolutionary biology. Among them, the first two oc- curred in the deep time of the Precambrian and have left little information in the fossil records. Fortunately, intensive investigation into the Early Cambrian Chengjiang faunas in the past two decades have revealed numerous significant data on the main episode of the Cambrian Explosion, par- ticularly information on the various earliest-known deu- terostomes, leading to a much better understanding of the last two events. The “first fish” Haikouichthys and Myllo- kunmingia have been proved the oldest-known vertebrates, and to be among the most primitive ones, representing a key transitional group from acraniates to craniates. Indeed, “Pa- leontological work of Shu and collaborators is revising our understanding of the early evolution of chordates”. Moreover, the discovery of the Phylum Vetulicolia may also throw new light on the origin of deuterostomes. This extinct group with simple gill slits might represent one of the roots in the deu- terostome lineage, as the evidence of molecular biology and developmental biology has predicted. Here the first discovery of tiny vetulicolians from the Chengjiang Lagerst?tte is re- ported. The evolutionary trend from the giant to the small might represent a special adaptation in early animal history. Since all vetulicolians share similar characters in their ante- rior section (pharynx), and on the basis of differences in their posterior section, a new classification system is proposed, with a new class, the Heteromorphida erected. As both the “root” group vetulicolians and the “top” group vertebrates in the early deuterostome lineage, as well as those members in between, such as vetulocystids, yunnanozoans, cephalo- chordates and urochordates, have been recovered, a rela- tively complete early deuterostome phylogeny, which is compatible with phylogenies of extant deuterostomes, is nearly mature.

The earliest-known ancestors of Recent Priapulomorpha from the Early Cambrian Chengjiang Lagerstatte

The taxonomy of an early ancestor of Recent Priapulidae, Xiaoheiqingella peculiaris (= Yunnanpriapulus halteroformis Huang et al., 2004) from the Early Cambrian Chengjiang fossil Lagerst?tte, is revised. Morphological characters comprise a pair of caudal appendages rather than a single appendage flanking the trunk end and a possible urogenital duct found inside the preanal region. An addi- tional extremely rare fossil priapulid worm, Paratubiluchus bicaudatus gen. nov., sp. nov. is also described herein. Its diagnostic characters are: an introvert bearing 25 longitudi- nal rows of scalids, a distinct neck region, no annulus on the oval trunk, and a pair of caudal appendages. The proportion of body parts is similar in size to that of loricate larvae of Recent priapulids and larva-formed Palaeopriapulitidae. Taking account of the features of Xiaoheiqingella, bicaudal appendages are considered to be a synapomorphy of Pri- apulidae and Tubiluchidae. Paratubiluchus gen. nov. is most likely a candidate for the ancestor of the Tubiluchidae; it probably originated from a larva-formed priapulid with 25 rows of scalids, thus representing an intermediate link be- tween the priapulids in mature-form and the priapulids with lorica.

Preliminary notes on soft-bodied fossil concentrations from the Early Cambrian Chengjiang deposits

The efforts of labor-intensive collecting in the Early Cambrian Chengjiang deposits in eastern Yunnan Province, China led to the discovery of many horizons containing exceptionally well preserved soft-bodied fossil concentrations, many of which can be assigned to either monospecific concentrations or paucispecific concentrations. The features of these fossil concentrations support the hypothesis that frequent storm events producing tempestites mainly contributed to the preservation of abundant soft- bodied fossils in the Chengjiang deposits, and indi- cate that the balance of the ecological web in this region was probably frequently destroyed or upset by such storm events during that geological time. Ani- mals in a fossil assemblage in such fossil concentra- tions probably occupied similar ecological biotopes.

New observations of the lobopod-like worm Facivermis from the Early Cambrian Chengjiang Lagerstatte

Facivermis yunnanicus (Hou & Chen, 1989), from the Early Cambrian Chengjiang Lager- st?tte, a worm-like fossil with 5 pairs of tentacles and a perceived shrunken end, has been regarded as related to polychaetes, later it has been variously interpreted as lobopods, Pentastoma and lopho- phorates. Newly discovered complete specimens by the ELI field team show that the taxon has, in addition to the 5 pairs of appendages, a pear-shaped trunk end bearing two or three circles of hooks. Accordingly, based on these important morphological characters, reconsideration of its affinities is provided and the taxonomy is remedied herein. Because the five pairs of appendages of Facivermis yunnanicus resemble the appendages of the fore-trunk of lobopod Mi- raluolishania (Liu & Shu, 2004), it seems that they are homologous structures. Therefore, the affinities of Facivermis are regarded here as being close to the lobopods, in addition, the lobe-like appendages of Facivermis are very crucial to exploring the origin of the appendages of lobopods and arthropods.

Comparative study of Cambrian lobopods Miraluolishania and Luolishania

The rare fossil Miraluolishania described by Liu et al. from the Lower Cambrian Chengjiang Lagersttte in 2004 is regarded as an arthropod sphinx because it bears mosaic features of both lobopods and arthropods. The discovery of this rare transitional form offers direct fossil evidence for exploring the relationship between lobopods and arthropods. However, some scientists consider Miraluolishania to be a junior synonym of Luolishania because the former superficially resembles the latter in general appearance. Considering the significant differences between the two taxa, a thorough comparative study of Miraluolishania and Luolishania leads to the conclusion that there are definitely two different genera. Nevertheless, the "Luolishania" of the Haikou area is indeed "Miraluolishania", whereas Luol-ishania is most likely the typical genus of the Maotianshan area of Chengjiang County.