| The earliest representatives of many extant phyla,including chordates,occurred in the Cambrian Lagerst?tte such as Chengjiang biota,which dates to Cambrian Epoch 2,Age 3.As the oldest Burgess Shale-type biota,it represents the first palaeontological record of the main burst of the Cambrian Explosion.The problematic fossils from the Chengjiang,normally with unique body-plan,are thus thought to be of importance in understanding the radiation of phyla if their phylogenetic positions can be resolved accurately in the tree of life.However,studies on these problematic fossils have long been impeded due to lacking resultful methodology.Vetulicolians are a group of the typical enigmatic fossils in the Cambrian Lagerst?tte.Vetulicolians are a group of exclusively Cambrian animals characterized by an anterior section with lateral pouches and a posterior section that appears segmented.The precise phylogenetic affinity of vetulicolians is debated because there is a lack of consensus regarding the interpretation of their anatomical features.Their disparate morphology might even question whether this is a monophyletic taxon.As currently understood,vetulicolians are a group of exclusively marine,enigmatic and extinct Cambrian animals,represented by three families: the Vetulicolidae,the Didazoonidae and the Banffidae.Soft-part anatomy has been reported in some vetulicolians,including structures interpreted to represent muscle fibres.The vetulicolian body plan and the limited information about soft-bodied anatomical structures within the anterior section make the interpretation of these animals difficult.As a result,the phylogenetic position of vetulicolians is unclear,with proposed affinities much debated and varying from their interpretation as unusual arthropods,kinorhynchs,or stem-group deuterostomes or chordates.Vetulicolians,as currently understood,are represented by 15 species,which occur worldwide,but only in Cambrian Lagerst?tten,including: Chengjiang,Guanshan,and the Shipai Formation of South China;the Burgess Shale,and the Mural Formation,both in Canada;the Sirius Passet biota of Greenland;the Emu Bay Shale of Australia;and the Spence Shale in Utah,USA.They are known almost exclusively from their characteristic anterior section that possesses lateral pouches,and segmented posterior section.Despite being widespread and common animals throughout Cambrian Lagerst?tten,the affinity of vetulicolians remains controversial.Accordingly,specimen-based approaches have been carried out in this thesis as follows:We have attempted to develop a unified terminology for the description of vetulicolians that avoids inferring animal relationships or functional morphology.Therefore,terms such as 'carapace'-suggesting an arthropod affinity-or 'oral end'-suggesting functionality for feeding-have been replaced with purely descriptive terms.We believe that much of the existing terminology applied to vetulicolians cannot be sustained.There is no convincing evidence to show the homology of the anterior part of vetulicolians with the carapace or head shield of arthropods.Furthermore,there is no evidence in vetulicolians to sustain that the anterior opening was ‘oral',or even that it functioned in food collection.In deconstructing this taxonomically 'loaded' terminology we now propose a ‘ground-up' approach to interpreting these animals that examines their full range of morphological characteristics,without preconception.New analyses should determine: 1)whether any features are homologous with extant groups of organisms;2)whether any features can be accorded a true function,based on basic observations of their likely adaptation for different functions.From this deconstructed database we hope that a more realistic picture of the interspecific variation of vetulicolians can be assessed,that will lead to a greater understanding of their wider animal affinities.By targeting the key issues of taphanomical artefacts and homological comparison in interpreting problematic fossils,a decay sequence is revised and successfully applied to the interpretation of the vetulicolians.We conduct a comparative study on the most common members of Genus Vetulicola that bear a quadrate carapace from these two biotas.Two morphotypes are recognized both within Vetulicola rectangulata from the Chengjiang Biota and within a new undetermined species,Vetulicola sp.from the Guanshan Biota.In both localities morphotypes differ from each other in the presence/absence of a posteroventral projection in the anterior body,and this is interpreted to possibly represent sexual dimorphism.These two Vetulicola species from the Chengjiang and Guanshan biotas also exhibit other morphological differences.Most notable is the position of the junction between the anterior and posterior parts of the body which is further towards the anterior in the specimens from the Guanshan Biota than those from the Chengjiang Biota.In addition,the expansion of the distal portion of the posterior body occurs from the third segment in the Chengjiang specimens but from the fourth segment in the Guanshan specimens.The most likely interpretation of these differences is that they reflect evolutionary adaption of the swimming ability of Vetulicola with a quadrate carapace from the Chengjiang to the Guanshan Biota.We have described new material of Yuyuanozoon magnificissimi,provide a new interpretation for the opening at the presumed anterior end of the animal,and reassign this taxon to the vetulicolian family Didazoonidae.We made detailed morphological observations of the new material from Chengjiang Xiaolantian,trying to understand the taphonomical features and the ecological habitats of vetulicolians,providing some new evidence for the future work.This thesis aims to systematically investigate the vetulicolian taxa from the Chengjiang biota and Guanshan biota,primarily focusing on their morphology,taxonomy,autecology and taphonomy.The thesis has significant impacts in understanding the early radiation of animal phyla and the complexity of marine ecosystem in the early Cambrian. |
PDF Full Text Request