Reconstructing Some Key Nodes And Branches Of The Early Tree Of Animals

Darwin (1859) hypothesized an ingenious idea of "Tree of Life" (all life forms originated from one or a few common ancestors) in his "On the Origin of Species". During the last few decades, accumulated morphological and molecular evidence has demonstrated the truth of his conjecture. Since the early 21st century, western countries jointly launched a grand project of "Tree of Life" to construct a complete phylogenetic tree for all extinct and extant organisms on Earth, and to clarify their origin and evolution, as well as the relationships among them. The grand Tree of Life consists manly of the Tree of Monera, Fungi, Plantae, and Animalia, among which, the Tree of Animal (TO A) justifiably draws most public attention.Constructing a complete TOA needs the verification and supplementation of fossil evidence. The exceptional Chengjiang fossil Lagerstatte in Yunnan, South China may serve as a "Rosetta Stone" for deciphering the nature and panorama of the Cambrian explosion of life and for reconstructing key nodes and branches of the early TOA. Metazoans preserved in this deposit include not only basal animals (poriferans, cnidarians, ctenophores, etc.) commencing from the Ediacaran radiation, and protostomes (priapulids, lobopodians, arthropods, brachiopods, etc.) extending from the burst of the Meishucun fauna (small shelly fossils), but predecessors of all major groups of extant deuterostomes (i.e., echinoderms, hemichordates, cephalochordates, urochordates, and vertebrates). Such a high diversification of deuterostomes in the Chengjiang fauna indicates the assemblage of the framework of the early TOA.The current research is on the basis of examination of some 4000 specimens of Chengjiang fossils collected from six sections (Erjie, Sanjiezi, Ercaicun, Mafang, Jianshan, and Maotianshan) in eastern Yunnan. Novel morphologies revealed in some primitive or transitional groups enable the reconstruction of some key nodes and branches of the early TOA as follows.1. Xianguangia sinica, while showing evidence for a close affinity to polypoid cnidarians, proves to be a suspension feeder using a ciliary system. Comparative biology and cladistic analysis both suggest that X. sinica is a stem-group cnidarian and that this polypoid suspension feeder represents a key node (last common ancestor) in the phylogeny of Cnidaria.2. All taxa of Chengjiang ctenophores, including four new forms (Thaumactena ensis gen. et sp. nov., Gemmactena Actinala gen. et sp. nov., Helmetctena hemispherica gen. et sp. nov., and an unnamed taxon) and three described forms(Maotianoascus octonarius, Trigoides aclis, and Batofasciculus ramificans), are demonstrated here to possess a biomineralized skeleton and a radial symmetry. They can be collectively assigned to a new clade (Eoctenophora class nov.), being an early branching stem-group of the phylum Ctenophora.3. Chengjiang lobopodians (Onychodictyon ferox, Aysheaia pedunculata, etc.) shows a pristine head configuration (terminal mouth, proboscis, single-segment head) and homologies to heads of some panarthropod groups, thus uncovering the cephalization pattern in Panarthropoda and further demonstrating the early lobopodians to be stem panarthropods.4. Ontogeny of Eoredlichia intermedia reveals that it is phylogenetically related to Laurentian olenellids and African fallotaspids, probably resulting from heterochrony, and in turn demonstrates that the Order Redlichiida is basal among Trilobita.5. Nailiania transformis gen. et sp. nov. is described and demonstrated to be a primitive lophophorate based on the presence of circumoral tentacles and a potential recurved gut.6. New evidence discovered from examination of ca.500 specimens of verulicolians further supports the presence of primitive pharyngeal gill slits and their function as expelling seawater within the pharynx so as to facilitate filter feeding and breathing. Morphological evidence and a cladistic analysis all strongly suggest the basal placement of Vetulicolia in Deuterostomia.