Oikozetetes from the early Cambrian of South Australia: implications for halkieriid affinities and functional morphology

Shells of Oikozetetes and isolated halkieriid sclerites from a section of the lower Cambrian Mernmerna Formation in the Flinders Ranges, South Australia, are tentatively considered as being derived from the same scleritome. Details of shell morphology and the possible combination of biomineralized shell and sclerites suggest that Oikozetetes , if interpreted correctly, is closely related to Halkieria . A new interpretation of Oikozetetes shell morphology, in addition to the first report of paired muscle scars on the interior surface, sheds new light on the possible functional morphology of halkieriid shells and the means of attaching the shell to the body. The occurrence of Oikozetetes in South Australia extends its biostratigraphic range to the lower Cambrian and biogeographic range to East Gondwana.     

The Early Cambrian Halkieria is a mollusc

Halkieria evangelista was originally interpreted as a stem-group brachiopod. It was later proposed that the brachiopods evolved from a halkieriid ancestor that curved ventrally, lost the foot and radula, developed a lophophore as a new feeding organ, and modified the sclerites to chaetae. Molluscan affinities have been discussed by a number of authors. We have studied some of the original material of Halkieria and found that all the characters that can be observed with certainty are compatible with characters in living molluscs. The combination of characters observed in Halkieria does not fit with any of the living classes of molluscs, so we propose placing it in a new class, Diplacophora. Living polyplacophorans have hairs secreted by a number of epithelial cells and containing nerve cells; they are clearly not homologous with annelid chaetae. Thus, there is no support for theories that regard Halkieria as belonging to the stem group of either the annelids or brachiopods.     

Halkieria系统进化及其化石分类学研究进展

Halkieria是寒武纪一类躯体上披有骨质鳞片的疑难后生动物,前后各有一个大壳板,两侧对称,其钙质骨片在软躯体上排列形成几条规律的纵向带。半个世纪以来,学术界对Halkieria生物系统确切位置的讨论以及Halkieriidae不同属种亲缘关系的探索从未停止。本文总结了Halkieria化石在骨片属种分类、古地理和地层分布、生物系统分类学的研究进展,对当前的生物系统分类归纳出以下几种观点:1) Halkieria为腕足动物的祖先,并与托莫特壳类有一定的亲缘关系;2)Halkieria与Wiwaxia等骨片化石具有单源性,组成一种新的分类单元Halwaxiidae,属于软体动物干群或腕足动物和环节动物的干群;3)Halkieria为软体动物有刺亚门干群,与无板纲、多板纲更为亲近; 4) Halkieria为环节动物的祖先。目前,虽然多数人根据Halkieria与现生多板纲的骨片排列一致将Halkieria归属软体动物有刺亚门的干群,但近年来世界各地新化石材料的陆续发现及壳体发育研究使得Halkieria的系统分类位置出现新的争议。通过对化石分类学的系统厘定和生物分类研究进展的总结梳理,...     

RECONSIDERATION OF THE PHYLOGENETIC AFFINITIES OF ZHIJINITIDS

Isolated sclerites are an important component of the Early Cambrian small shelly fos-sils(SSF) ,accounting for one-third of both the total number of taxa and the total numberof specimens.Estimating the phylogenetic affinities of these sclerites is one of the mostdifficult problems in the analysis of the SSF.Sclerites referred to such taxa as the halkieri-ids,siphogonuchitids,chancelloriids,tommotiids,zhijinitids and cambroclavids generallyhave been interpreted as Problematica.As recently a…     

A Stem Group Brachiopod From The Lower Cambrian: Support For A Micrina (Halkieriid) Ancestry

The shell structure of the Lower Cambrian Mickwitzia , a bilaterally symmetrical bivalve hitherto doubtfully assigned to the Brachiopoda, confirms that the genus shares characters with linguliform brachiopods. The columnar lamination of its organophosphatic shell is homologous with that characterizing acrotretides. The shell, however, is also pervaded by striated apatitic tubes indistinguishable from those permeating the sclerites of the problematic organophosphatic, laminar–shelled Micrina which is close to Halkieria . No crown group brachiopods have such tubes that are presumed to have contained setae. The presence of both these features in the Mickwitzia shell suggests that the stock is a stem group brachiopod with a halkieriid ancestry.     

An aplacophoran postlarva with iterated dorsal groups of spicules and skeletal similarities to Paleozoic fossils

Abstract. A tiny neomenioid postlarva (Neomeniomorpha, or Solenogastres) collected from the water column 3 to 6 m above the east Pacific seamount Fieberling Guyot has 6 iterated, transverse groups of spicules and 7 regions devoid of spicules between the transverse groups and the anterior-and posteriormost spicules. Three pairs of ventral, longitudinal zones with columns of single spicules, each pair with its own distinctive spicule morphology, lack transverse iteration. The 7 regions bare of spicules are compared to shell fields in developing polyplacophorans, and spicule arrangement is compared to sclerite arrangement on the Cambrian fossils Wiwaxia corrugata and Halkieria evangelista and to the spines and shell plates of the Silurian Acaenoplax hayae. The term iteration is used to denote processes that result in both metameric segments and repeated ectodermal skeletal structures. Iterative morphogenesis was probably present in bilateral animals before the Cambrian. Comparisons of iterated ectodermal skeletal structures among fossil and extant forms are suggested to indicate evolutionary relationship.     

A new shell-bearing organism from the Cambrian Spence Shale of Utah

A new shell-bearing organism with preserved soft tissue, Armilimax pauljamisoni n. gen. n. sp., is reported from the middle Cambrian (Miaolingian: Wuliuan) Miners Hollow locality of the Spence Shale of northern Utah. The described organism is known from a single articulated specimen and preserves a prominent shell, a slug-like body, as well as a U-shaped digestive tract. Its overall appearance is similar to halkieriids, but it does not preserve sclerites. The possible affinities of the new taxon and potential reasons for the presence of a U-shaped gut are discussed. Armilimax pauljamisoni is the first shell-bearing animal of its kind from the Great Basin and extends the diversity of body plans in the Spence Shale Fossil-Lagerstätte.     

多毛纲环节动物软背鳞虫的电镜观察与描述及其与Wiwaxia骨片微结构的比较

通过对现生环节动物门（Annelida）多毛纲（Polychaeta）叶须虫目（Phyllodocidae）鳞沙蚕亚目动物（Aphroditiformia）软背鳞虫（Lepidonotus helotypus Grube,1877）的鳞片、刚毛等疣足附属物的超微结构观察及其与Wiwaxia骨片的比较研究,探讨Wiwaxia的系统分类位置。研究结果表明,软背鳞虫的刚毛与Wiwaxia骨片具有以下相似性：1）两者表面均具有浅纵纹;2）均向身体背中线方向倾斜或弯曲;3）着生方式均以刚毛囊或类似根状基的结构插入软组织。此外,文中首次对鳞片上的锥形突起与Wiwaxia骨片的相似性进行比较和探讨。结果显示,Wiwaxia骨片与鳞沙蚕刚毛具有一定相似性,但在形态上仍存在显著差异（如二者的排列方式不同,Wiwaxia骨片上不存在鳞沙蚕刚毛的覆瓦状结构等）。     

云南永善肖滩剖面早寒武世Lapworthella (托莫特壳类)骨片化石

描述了采自云南永善肖滩剖面下寒武统石岩头组的托莫特壳类(tommotiids)骨片化石Lnpworthella rete Yue,1987,其骨片锥状或塔状,横向环脊明显,环脊尖棱状;骨片外表面具多边形网状纹饰,但无齿、瘤或纵脊等装饰;骨片可分为两种类型,即弯曲型和近直立型,前者数量明显多于后者。简要地讨论了托莫特壳类化石的分类及生物亲缘关系,认为目前为止,其仍为亲缘关系不明的化石类别。尚难断定所有的托莫特壳类骨片化石皆属于同一生物门类,因为难以证明将它们归人同类生物所依据的骨片性状是共同衍征(synapomorphy)还是由于趋同演化(convergent evolution)的结果;在缺乏躯干化石记录的情况下,离散骨片的生物亲缘关系研究较为困难。     

Pyritized preservation of chancelloriids from the Cambrian Stage 3 of South China and implications for biomineralization

The enigmatic Cambrian animal chancelloriids were discovered in a wide range of taphonomic settings; however, preservation of biomineralized sclerite microstructure was solely known from secondarily phosphatized skeletal remains. Here, we investigate a uniquely pyritized chancelloriid from the lower Cambrian Guojiaba Formation in southern Shaanxi Province, China, using a combination of advanced analytic techniques. Results of the energy dispersive spectroscopy (EDS), X-Ray Fluorescence (XRF), and Raman spectrum show that the sclerites and scleritomes are preserved as pyritized internal moulds with a calcitic outer layer. The outer layer enveloping the internal moulds likely represents the recrystallized counterpart of the original biomineralized sclerite wall. Distinctive fibrous microstructures are discovered in the sclerites, which echo the features seen in the phosphatized fossils of chancelloriids. The typical microstructure, along with the recrystallized calcite, corroborate the interpretation that chancelloriid sclerites were originally constructed by fibrous aragonite. The stability of the microstructure and mineral composition in both carbonate and siliciclastic backgrounds indicate that chancelloriids were adapted to exploit aragonitic fibres to build their skeletons regardless of the change of their living environments.     

湖北房县下寒武统西蒿坪段钉形骨片化石

本文对采自湖北房县三座庵剖面下寒武统西蒿坪段的钉形骨片(nail-shaped)化石进行了研究,系统厘定和描述了2属2种,分别为Parazhijinites guizhouensis和Cambroclavus angxianensis;其中以C.fangxianensis骨片化石最为丰富,由三种类型的骨片组成,它们可能...     

Early Palaeozoic diversification of chitons (Polyplacophora, Mollusca) based on new data from the Silurian of Gotland, Sweden

Isolated, well-preserved silicified sclerites of an unusually diverse Silurian paleoloricate assemblage from Gotland preserve morphological features that are important in interpreting palaeobiology. The typically granular dorsal ornament is comparable with Recent chitons, and is hence possibly linked functionally with sensory aesthetes. Ventral structures particularly in thickened shells indicate major muscle attachment sites sub-apically or, equivalently, beneath the rim of the apical area, and also marginally. In early chitons such as Cambrian Matthevia, deep ventral cavities represent comparable sub-apical sites. Three Gotland genera with an unusual, holoperipheral shell growth style apparently represent plated aplacophorans (cf. Acaenoplax), which coexisted with paleoloricate chitons in shallow inshore carbonate shelf environments. Sclerite features of all the Gotland genera are discussed together since they share most characteristics. The new family Heloplacidae includes those genera and Acaenoplax, which in a preliminary cladistic analysis form a sister group to other Lower Palaeozoic paleoloricates. Multiplated skeletons in paleoloricates and this group of aplacophorans represent parallel evolution of dorsal armour, which in chitons resulted in overlapping, articulating sclerites. The diversity of the mid-Silurian Gotland assemblage is examined against early evolutionary diversification of polyplacophorans, aplacophorans, and in relation to the overall record of Palaeozoic paleoloricate and neoloricate chitons. Peaks in diversity in early Ordovician, mid-Silurian and early Carboniferous times correspond to periods with widespread development of low latitude carbonate shelves. Neoloricates, apparently with an additional shell layer that contributed articulatory plates, appeared in the Devonian from where the fossil record remains poorly known.     

SKELETAL MICROSTRUCTURE INDICATES CHANCELLORIIDS AND HALKIERIIDS ARE CLOSELY RELATED

Abstract: Chancelloriids are problematic, sac‐like animals whose sclerites are common in Cambrian fossil assemblages. They look like sponges, but were united with the slug‐like halkieriids in the group Coeloscleritophora Bengtson and Missarzhevsky, 1981 based on a unique mode of sclerite construction. Because their body plans are so different, this proposal has never been well accepted, but detailed study of their sclerite microstructure presented here provides additional support for this grouping. Both taxa possess walls composed of a thin, probably organic, sheet overlying a single layer of aragonite fibres orientated parallel to the long axis of the sclerite. In all halkieriids and in the chancelloriid genus Archiasterella Sdzuy, 1969 , bundles of these fibres form inclined projections on the upper surface of the sclerite giving it a scaly appearance. On the lower surface of the sclerite, the projections are absent. This microstructure appears to be unique to chancelloriids, halkieriids, and their relatives, siphogonuchitids and sachitids. (The sclerites of another putative halkieriid relative, Wiwaxia Walcott, 1911 , are unmineralized, making direct comparisons impossible.) Thus, similarity both at the level of sclerite construction and the level of sclerite microstructure suggests that chancelloriid, halkieriid, sachitid, and siphogonuchitid sclerites are homologous. The difference in chancelloriid and halkieriid body plans can be resolved in two ways. Chancelloriids either represent a derived condition exhibiting complete loss of bilaterian characters or they represent the ancestral condition from which the bilaterally symmetric halkieriids, and the Bilateria as a whole, derived. The latter interpretation, proposed by Bengtson (2005) , implies that coeloscleritophoran sclerites (‘coelosclerites’) are a plesiomorphy of the Bilateria, lost or transformed in descendent lineages. Given that mineralized coelosclerites appear in the fossil record no earlier than c. 542 Ma, this in turn implies either that the Ediacaran record of bilaterians has been misinterpreted or that coelosclerite preservability increased at the beginning of the Cambrian Period. The former is difficult to reconcile with Ediacaran trace and body fossil evidence, but the latter may be possible, reflecting either independent mineralization of organic‐walled sclerites in chancelloriids and halkieriids or the opening of a taphonomic window that favours coelosclerite preservation.     

The Early Cambrian tommotiid Micrina, a sessile bivalved stem group brachiopod

The tannuolinid Micrina belongs to the tommotiids-a common and widely distributed, but poorly understood, group of Early Cambrian fossil metazoans with multiple external organophosphatic sclerites. Recent findings of sessile articulated tommotiid scleritomes indicate that previous reconstructions of tommotiids as slug-like bilaterians with a dorsal cover of sclerites require detailed re-evaluation. Comparative ultrastructural work has already indicated that the tommotiids might be a sister group to the Brachiopoda, with Micrina representing the most derived and brachiopod-like bimembrate tommotiid. Here we further develop and strengthen this controversial phylogenetic model with a new reconstruction of Micrina, where the two types of sclerites--mitral and sellate--belong to a near bilaterally symmetrical bivalved sessile organism. This new scleritome configuration was tested by recreating an articulated bivalved Micrina from isolated mitral and sellate sclerites; both sclerites have muscles that would have enabled movement of the sclerites. The mitral and sellate sclerites of Micrina are considered to be homologous with the ventral and dorsal valves, respectively, of organophosphatic linguliform brachiopods, indicating that a simple type of filter-feeding within an enclosed bivalved shell had started to evolve in derived tannuolinids. The new reconstruction also indicates that the phylogenetic range of 'bivalved', sessile lophophorates is larger than previously suspected.     

New Cambrian lobopods and chaetognaths of the Siberian Platform

The association of isolated remains of lobopods with a netlike structure, including plates of Microdictyon rozanovi sp. nov. and sclerites with spinelike process of Onychomicrodictyon spiniferum gen. et sp. nov., as well as saberlike sclerites of Protohertzina yudomica sp. nov. interpreted as grasping spines of chaetognaths, is described from the middle part of the Inikan Formation of the Toyonian Stage of the Lower Cambrian (Lermontovia dzevanovskii Zone). The diagnoses and species composition of the genera Microdictyon and Protohertzina are refined.     

Early Cambrian lobopodian sclerites and associated fossils from Kazakhstan

A low diversity fossil assemblage dominated by Mongolitubulus spines, from the Early Cambrian Geres Member of the Shabakhty Formation at Koksu, Malyi Karatau, Kazakhstan, enables identification of its sclerite variability within the scleritome. No other sclerotised structures could be matched with the spines, indicating that these were the only sclerotised part of the body and were periodically shed. The prominent spines of Mongolitubulus have a scale–like external ornamentation and fibrous internal structure. Possibly they armed the body of a lobopodian similar to Xenusion . The Mongolitubulus spines belong to a series of Cambrian phosphatic fossils, the other end members of which are sclerites of Microdictyon , the second most abundant component of the Koksu assemblage. The latter superficially resemble schizochroal trilobite eyes, but in life covered the dorso–lateral sides of the body segments, as is evident from complete specimens from the Chinese Chengjiang locality. A pair of sclerites of Microdictyon , representing uncompleted exuviation, from Koksu show that each hole originally contained a non–phosphatised lenticular body. This makes a compound eye nature of the Microdictyon sclerites likely, and even their homology with arthropod eyes cannot be excluded.     

A myriapod-like arthropod from the Upper Cambrian of East Siberia

Although the fossil record of biramous arthropods commences in the Lower Cambrian, unequivocal uniramous arthropods do not appear until the Upper Silurian, in association with terrestrial biotas. Here we report an Upper Cambrian marine arthropod from East Siberia that possesses some significant myriapodan features. The new arthropod,Xanthomyria spinosa n. gen., n. sp., closely resembles examples of archipolypodans from the Late Palaeozoic. If this resemblance genuinely represents myriapod affinities, this would be the first convincing myriapod from the Cambrian. Suggestions of an early branching point of the myriapods from other arthropods would be consistent with this. Conversely, an as yet poorly known clade of multi-segmented arthropods may exist in the Cambrian, with no close affinities to the myriapods.      

Chancelloriid sclerites from the lowermost Cambrian of North China and discussion of sclerite taxonomy

Chancelloriid sclerites from the lowermost Cambrian Xinji Formation (Series 2, Stage 3), southern margin of the North China platform, are systematically described. Thousands of isolated sclerites from three sections are assigned to three genera and four species, including Chancelloria cf. eros, Allonnia tripodophora, Archiasterella pentactina, and Ar. tetraspina. To accurately document the taxonomic significance of the sclerite structure, modified formulas (m+nC, m+nA, and m+0) are put forward to represent the full series and variation of sclerite forms. Based on the sclerite construction, statistical analysis on the proportions of different sclerite forms in the rock samples and the composition of sclerites in previously described chancelloriid scleritomes, a new scheme for identification and classification of isolated sclerites reconciled within the framework of the complete scleritome, is proposed.     

Scleritome construction,biofacies, biostratigraphy and systematics of the tommotiid Eccentrotheca helenia sp. nov. from the Early Cambrian of South Australia

Abstract: Large collections of Eccentrotheca helenia sp. nov. from the lower Cambrian Wilkawillina and Ajax limestones in the Arrowie Basin, South Australia, contain abundant low, cap‐shaped and high, laterally compressed isolated sclerites in addition to partially articulated tubular specimens. The scleritome of Eccentrotheca helenia sp. nov. is fully described for the first time and shown to be formed by ontogenetic fusion of sclerites into successively stacked sclerite rings, forming a larger, tubular structure. The apical termination of the tube is highly variable, but is primarily constructed by low, cap‐shaped sclerites and characterised by a central aperture of variable inclination. The adapical portion of the tube is predominantly constructed by high, laterally compressed sclerites, but individual sclerite rings can contain both cap‐shaped and laterally compressed sclerites along with sclerites of intermediate morphology. The apical aperture presumably housed organic structures for attachment to a hard substrate, but the scleritome also occasionally preserves small lateral perforations between fused sclerites, which may have served to stabilise the scleritome by providing additional points of anchorage. In the Arrowie Basin, E. helenia is found in association with archaeocyath‐microbial‐spongiomorph‐dominated bioherms and most likely inhabited pendant or cryptic habitats within these bioherms. Eccentrotheca‐like sclerites form an integral part of the scleritomes of many tommotiids which may confuse taxonomic analysis. Sclerites previously assigned to ‘E.’guano, consistently occur together with sclerites of Kulparina rostrata in stratigraphic intervals consistently older than strata hosting E. helenia. Rare fused specimens indicate that the sclerites of K. rostrata and ‘E.’guano belong to the same scleritome.     

Chancelloriid sclerites from the Lower Cambrian (Meishucunian) of eastern Yunnan,China, and the early history of the group

Chancelloriids are a group of enigmatic sessile animals that are covered with sclerites shaped as rosettes of spines, producing an appearance like that of a barrel cactus. They are known only from Cambrian rocks. Isolated sclerites of chancelloriids are widespread in small shelly faunas, but they have proven difficult to treat taxonomically due to the variation within and between individual animals. We report on large samples of chancelloriid sclerites from the Meishucunian (pre‐trilobitic Lower Cambrian) of eastern Yunnan Province, China, including material from the Dahai Member of the Zhujiaqing Formation (the Zhujiaqing section, Dahai, Huize County) and the Shiyantou Formation (the Xiaotan section, Yongshan County). The material from the Dahai Member appears to come from a single species, Chancelloriella irregularis. Statistical analysis of morphotype co‐occurrences in different samples suggests that several species are represented in the material from the Shiyantou Formation, which we herein tentatively place in four species, Allonnia erromenosa, Allonnia tetrathallis, Archiasterella charma sp. nov. and Archiasterella cf. pentactina. We suggest therefore that careful study of large collections of isolated chancelloriid sclerites permits the identification of different species, opening the possibility of their use in evolutionary or stratigraphical studies. In particular, in South China, it seems that Chancelloriella irregularis is characteristic of rocks of the Paragloborilus subglobosus – Purella squamulosa Assemblage Zone and the Watsonella crosbyi Assemblage Zone, whilst Archiasterella and Allonnia first appear in the overlying Sinosachites flabelliformis – Tannuolina zhangwentangi Assemblage Zone. C. irregularis is thus amongst the oldest well‐characterized chancelloriids, and the irregularity and poor organization of its sclerites relative to those of younger forms support the hypothesis that chancelloriid sclerites are compound structures arising from the fusion of originally separate elements, perhaps homologous with sclerites of halkieriids and other coeloscleritophorans. The similarity between the arrangement of rays in a chancelloriid sclerite and the arrangement of bubbles in small bubble clusters suggests that in many cases, aspects of the form of chancelloriid sclerites are the result of simple physical processes.