The Deep Root of Modern Animal Diversity  

Beautifully preserved organisms from the Lower Cambrian Maotianshan Shale in central Yunnan, southern China, document the sudden appearance of diverse metazoan body plans at phylum or subphylum levels, which were either short-lived or have continued to the present day.  These 530-million-year-old fossil representatives of living animal groups provide us with unique insight into the foundations of living animal groups at their evolutionary roots.  Among these diverse animal groups, many – including ctenophores, priapulids, sipunculans, arrow worms, tunicates, and linguloids – are conservative, changing very little since the Early Cambrian.  Others, especially the Panarthropoda superphylum, evolved rapidly, with origination of novel body plans representing different evolutionary stages, one after another, in a very short geological period of Early Cambrian time.  These nested body plans portray a novel ‘big picture’ of panarthropod evolution as a progression of step-wise changes both in the head and the appendages.  During the evolution of the panarthropods, the head/trunk boundary progressively shifted to the posterior, and the simple annulated soft uniramous appendages progressively changed into stalked eyes in the first head appendages; into whip-like sensorial and grasping organs in the second appendage; and into jointed and biramous bipartite limbs in the post-antennal appendages.  The two-segment head in the original arthropod body plan may be related to the otd/ems-Hox boundary, which would explain why this boundary is buried within the head.  It also challenges the deeply-rooted hypothesis of the tripartite-brain.  Haikouella is one of most remarkable fossils representing the origin body plan of Cristozoa, or “crest animals” (procraniates+craniates). The anatomy of Early Cambrian crest animals, including Haikouella and Yunnanozoon, contributes to new understanding and discussion of the origins of the vertebrate brain, neural crest cells, branchial system, and vertebrae.  Lophophore-bearing organisms include a group of sessile, stalked organisms known as Phlogites (=Chenungkongella) and a group of medusoid organisms (including Eldonia and Rotadiscus), which have been interpreted as tunicate and problematic, respectively.  Restudy of their body plans reveal that they are likely the fossil representatives of extinct basal deuterostomes, in a close affinity with echinoderms.  Vetulicolians remain one of the Cambrian’s biggest controversies, being recently interpreted as a basal deuterostome group.  Restudy of their body plan suggests that they were a distinct panarthropod group.