Brachiopods From The Early Cambrian Chengjiang Lagerst(?)tte, South China

Brachiopods, a group of benthic ciliary suspension-feeding, bivalved invertebrates, were an important component of the Early Palaeozoic marine communities. Following the Early Cambrian Meishucunian "Small Shelly Fossils" distributed worldwide, brachiopods are thought as among the earliest-known skeletalized and mineralized animals, and thus generally referred to as the result of the "Cambrian Explosion" and of the attempt to mineralization of early animals. Brachiopoda and Phoronida, together with Ectoprocta possess a tentacular, ciliated, feeding organ, the lophophore, and thus sometimes have been grouped as Lophophorata. Based on traditional morpho-anatomical characters, brachiopods and phoronids are undoubtedly closely related, and usually regarded as sister groups among deuterstomes. Molecular phylogenetic studies confirm phoronid-brachiopod monophyly and, however, establish their affinities with protostomes. One important reason for this discrepancy between morpho-anatomical studies and molecular phylogenetic results is that only narrow sets of characters, instead of all characters of all ontogenetic stages, were taken into consideration. It should be accounted that all characters are of importance: the only question is at which level do the assumed characters contain phylogenetic information? Another weakness has been that 'advanced' characters, such as coelom and metanephridia, have been used to characterize higher taxa without discussing whether these characters have evolved more than only once as noted by Nielsen (2001).Basically the two opposing viewpoints, the monophyletic or polyphyletic derivation of the brachiopods, try to explain and emphasize the relative importance of similarities interpreted as representing synapomorphies and synplesiomorphies (or convergences). The morphology-based solution to this brachiopod phylogenetic controversy is to make a comprehensive analysis of the morph-anatomical and ontogenetic characteristics of extant brachiopods from the 3 different subphyla, also to make a detailed investigation into the character-states of fossil brachiopods during different geological intervals, so as to determine when their internal soft-tissues originated and diverged, and thus to establish the evolutionary polarity in the recognized diagnostic characters of brachiopods. The Chengjiang Lagerstatte, surpassing the well-known the Middle Cambrian Burgess Shale, has been celebrated for superbly detailed preservation of an abundant variety of soft-bodied and slightly mineralized or scleritized fossils with fine fidelity. The deposits also yield abundant and somewhat diverse brachiopods in some cases with fine preservation of valve interiors, notably the lophophores, digestive tracts and mantle canals, which are of pivotal importance for our understanding the acquisition of morpho-anatomical traits and character-states of brachiopods during the Cambrian explosion interval.hi the present study, all the brachiopods occurring in the celebrated Early Cambrian Chengjiang Lagerstatte, Yunnan, South China, are systematically described. The brachiopods comprise a total of 9 genera and species, of which the two articulate brachiopod Kutorgina Chengjiangensis sp. nov and Obolella. sp are new, and recognized as representatives of Subphylum Rhynhonelliformea in the fauna. The Subphylum Craniiformea was proposed to be represented by the problematic brachiopod Heliomedusa orienta Sun et Hou 1987, but now this classification seems less tenable based on new additional specimens. The other 6 were assigned to lingulid brachiopods (Lingulata, Linguliformea), including the Lingulellotretid Lingulellotreta malongensis Rong 1974;the obolid Lingulella chengjiangensis Jin et al., 1991, Longtancunella chengjiangensis Hou et al., 1999, Xianshanella haikouensis Zhang et Han 2004, Wangyuia Chengjiangensis Jin et al., 2004, and the botsfordiid Diandongia pista Rong 1974. Thus, the diverse Chengjiang brachiopod assemblage presumably reflects the first radiation of Brachiopoda during metazoan macroevolution in South China.Lingulids are the most abundant brachiopod forms in the Chengjiang deposits. They are typical of the fine preservation of soft parts, notably pedicles, and in some cases of setae, lophophores, vascular markings, even digestive canals, thus distinctly distinguished from the fossil lingulid assemblages from the other region, for example, Kazakhstan and Vaastergotland of Sweden. Comparative study of the soft parts of the lingulids from Chengjiang demonstrates or suggests the views as follows: 1. Early Cambrian lingulid pedicles were much like those of Recent Lingulidae in anatomy and structure, but they are much more varied in morphology than those seen in Recent forms. The abundant variety of pedicle fossils affords little support for the view that all fossil inarticulates possessed the type of pedicle characteristic of their few living survivors. 2. The disposition of setae along the mantle edge between the Early Cambrian lingulids and Recent ones is distinguishable: the setae of the fossil lingulids were fringed equidistantly with the mantle edge. By contrast, the setae of living lingulids are disposed in three pseudosiphons at the anterior mantle margin (two inhalant and one exhalant). Hence, it is assumed that the evenly fringed setae along the entire mantle edge could be the ground state of setae of stock lingulids. 3. The vascular system in some Cambrian lingulid forms is more complicated than previously thought. The mantle canals are either peripherally divergent at the mantle margin (e.g. Diandongia), or baculate, arising from the anterior body wall at midlateral positions (e.g. Xianshanella and Lingulella). However, the characteristic feature common to the Early Cambrian lingulids is of presence of dorsal vascula media that are absent in Recent forms. Thus, the presence of dorsal vascula media is the ground state of vascular system in lingulid stocks, and a loss of dorsal vascula media and an increase in the length of the lateral mantle canals, from an 'asymmetrical' positioning of the termination of the canal tips up to termination at the same distal level are presumed to be adaptations to an infaunal lifestyle. 4. The lophophore of Lingulella chengjiangensis, like those of Lingulellotreta malongensis, include trocholophes, schizolophes and the less intricately coiled spirolophes. All this ontogeny of lophophores is mirrored in the ontogeny of the lophophores of Recent lingulids. Lophophore ontogeny in Cambrian lingulids is therefore homologous to that of extant lingulids, and has remained fairly constant over 530 million years. Thus, the early stages of lophophoral ontogeny and development appear to be plesiomorphic characters inherited from an ancestral form. 5. So far, there are at least four genera of lingulid brachiopods from the Chengjiang fauna, in which a U-shaped digestive tract with ananterior anus and a lophophore have been observed. The fossils examined herein give further support to the claim that by the Atdabanian brachiopods already possessed advanced features, and that a lophophore and a U-shaped digestive tract with an anteriorly placed anus are lingulid brachiopod plesiomorphies. 6. The variable preservation of modified shells in the Chengjiang lingulids could indicate the thinness and less mineralization of the integuments, notably in Wangyuia Chengjiangensis Jin et al., 2004. The shell of Wangyuia is commonly preserved as 'arenaceous' mould, but in a few cases with setae fringed with mantle edge and a simple spiral lophophoral imprints inside. Thus, these fossils argue for the view that the mineralization of brachiopod shell (calcareous and phosphatic) is synapomorphy derived by all Recent brachiopod lineages. 7. Since the publication of Darwin's theory of evolution in 1859, lingulids have probably been the most widely quoted examples of arrested evolution. The highly characteristic shape of the steramlined, parallel-sided shell has long been considered to be critical evidence of their evolutionary conservatism. This widespread view, to some degree, may be because few anatomical features are impressed either on or inside valves so these may not adequately reflect the extent of change incurred during lingulid evolution. Comparison between the interior of the fossil lingulids with that of modem lingulids does little to support the widespread notion that the morphology of this lineage has remained remarkably constant since at least the early Palaeozoic. Analysis of the valve interior and soft parts favors the claims that an epifaunal mode of life could be a plesiomorphic state in contrast to the infaunal one of modern lingulids, and that the latter lifestyle has probably evolved as a response to periodic burial of the animals in sandy sediment in highly turbulent, shallow-water environments. The evolutionary changes, for example, the disposition into three' pseudosiphons' of the setae, the reduction of the mantle cavity, the loss of dorsal vascular media and the highly spirally coiled lophophore are all more likely therefore to be an adaptive result of an active burrowing life habit.Heliomedusa orienta Sun and Hou 1987 from the Chengjiang lagerstatten, Yunnan was initially taken for being allied with jellyfish, most recently assigned provisionally to the craniopsid group of brachiopods (Subphylum Craniiformea, Class Craniata, Order Craniopsida) in the revised Treatise (2000). The systematic position of the Lower Cambrian enigmatic brachiopods is reevaluated in the light of new material. The new specimens demonstrate that Heliomedusa has a punctate shell that was perforated by tubes, some of which contain chitinous setae at the surface. The shells in both the juvenile and mature have distinctive pustulose ornamentation, with pustules arranged in radiating rows. The presence of these characters casts doubt on the craniopsid affinity of Heliomedusa. Detailed observation of more than 1500 specimens of H. orienta deposited in ELI, Northwest University, suggests that the brachiopod taxon is comparable with larva of Recent Lingulidae (Lingula and Glottidia) in the configuration of lophophore, setal fringes along mantle edge, the visceral cavity and possible muscular system. It is worth emphasizing in the lophophore of H. orienta there is a double row of filaments (small tentacles ventrally and large tentacles dorsally) in the trocholophe developmental stage. The small tentacle is delicate and short, while the large tentacles radiate outwards from the brachial arms and bend anterolaterally, with a single palisade of cilia imbricated with neighboring rows. Arrangement of lophophore filaments is one of the diagnostic characters in the definition of higher brachiopod taxa, and hence plays a major role in their phylogenetic analysis. In all extant Lingulata, there is a double row of lophophore filaments in the trocholophe stage, whereas in extant Calcareous brachiopods (Craniiformea andRhynhonelliformea), the trocholophe stage has only a single row of filaments. Therefore, the findings of a double row of filaments lend supports for the view that double row of filaments is plesiomorphic state during the development of brachiopod lophophore, and suggest that H. orienta is most likely to represent a stem group of lingulids, probably as an example of paedomorphism of Lingulidae.In addition, Kutorgina chengjiangensis Zhang et Shu sp.nov is described. They are recognized as the earliest-known representatives of Subphylum Rhynhonelliformea derived from the Early Cambrian Chengjiang mudstone deposit. This exceptional brachiopod is characterized by a ventribiconvex to bioconvex shell with strong peripheral lamellae. In our collection of kutorginids, a group of the most primitive types of articulate brachiopods, some soft parts are superbly preserved, notably pedicles, setae, lophophore and vascular markings. The lophophore, as seen in the Chengjiang lingulids, corresponds to an early spirolophe developmental stage. The pedicle retained coelomic cavity and not completely filled with a connective tissue as proposed by Popov, L.E (1992). It follows that the absence of coelomic cavity in Recent articulate relatives represents a loss. The presence of pedicle cavity in the early articulate brachiopods gives a strong support for the monophyly of phosphatic and calcareous shelled brachiopods, in conjunction with the lophophore occurring in the kutorginid fossils, which is distinctly homologous with that seen in the coeval lingulids.