Diverse radial symmetry among the Cambrian Fortunian fossil embryos from northern Sichuan and southern Shaanxi provinces,South China

The Cambrian Fortunian fossil embryos exhibit embryonic development of ancient animals and hence have important bearings on evolutionary developmental biology. They have radial symmetry, and may be early representatives of cnidarians. Here we report new material of three-dimensionally phosphatized fossil embryos from the Fortunian Kuanchuanpu Formation and coeval strata in northern Sichuan and southern Shaanxi provinces, South China. The new material includes previously reported fossil embryos assignable to Pseudooides prima with biradial symmetry or pseudo-hexaradial symmetry, Quadrapyrgites quadratacris with tetraradial symmetry, and Olivooides multisulcatus with pentaradial symmetry. Additionally, we recovered two new types of fossil embryos, i.e., Embryo I with hexaradial symmetry and Embryo II with octaradial symmetry, and they are tentatively suggested to represent new cnidarians. In contrast to the diverse radial symmetry of the Fortunian cnidarians, modern cnidarians exhibit stable tetraradial symmetry in medusozoans, biradial symmetry in anthozoans, and bilateral symmetry in siphonophores (Hydrozoa). The current study supports the view that the tetraradial symmetry of modern medusozoans is a surviving remnant of their Fortunian relatives.     

Response of grazing snails to phosphorus enrichment of modern stromatolitic microbial communities

1. The effects of added phosphorus (P) on the growth, P and RNA : DNA contents, and survivorship of snails grazing on laminated microbial mats (living ‘stromatolites’) were examined in the Rio Mesquites at Cuatro Ciénegas, Mexico (total P, c. 0.60 μmol L^?1) to test the hypothesis that strong P‐limitation of microautotroph growth produces a stoichiometric constraint on herbivores because of mineral P‐limitation. 2. In a 3‐week experiment performed in summer 2001, addition of phosphorus (+15 μmol L^?1) resulted in a strong decline in stromatolite biomass C : P ratio from very high levels (c. 2300 : 1 by atoms) to moderate levels (c. 550 : 1). The endemic hydrobiid snail Mexithauma quadripaludium responded to P‐enrichment with elevated body P content and higher RNA : DNA ratios, especially for small animals likely to be actively growing. This positive response is consistent with the existence of a stoichiometric constraint on snail growth. 3. In a longer experiment (8 weeks) involving a more moderate P enrichment (+5 μmol L^?1) in summer 2002, P enrichment reduced stromatolite C : P ratio from moderate values in control treatments (c. 750) to very low values (<100 : 1). Snails responded to stromatolite P‐enrichment with increased body P content but, in contrast to the first experiment, with lower RNA : DNA ratio, lower growth rates, and higher mortality. 4. These contrasting results suggest that both very high and very low biomass C : P ratios in stromatolites are detrimental to M. quadripaludium performance, leading us to hypothesise that these herbivores live on a ‘stoichiometric knife edge’.     

SYSTEMATICS OF THE CAMBRIAN TRILOBITE FAMILY NEPEIDAE, WITH REVISION OF AUSTRALIAN SPECIES

Abstract:  Taxonomy of the Cambrian trilobite family Nepeidae is revised. Morphometric analyses of the genera Nepea and Penarosa demonstrate that use of proportions of linear dimensions of the cranidium to differentiate species is invalid, and that infrageneric variation is continuous. The taxonomy and biostratigraphy of all Australian species of Nepeidae is revised. Species considered valid herein include: Nepea narinosa (type species), N. tonsillata , N. nans , Penarosa retifera (type species), P. elaticeps , P. rhinodelphis , P. netenta , Loxonepea loxophrys (type species) and Ferenepea hispida (type species). Folliceps is considered to be a junior subjective synonym of Nepea , Trinepea is regarded as a junior subjective synonym of Penarosa , and Ascionepea is considered to be a junior subjective synonym of Ferenepea .     

昆明、武定地区早寒武世关山动物群的双瓣壳节肢动物

报道采自昆明、武定地区下寒武统沧浪铺阶乌龙箐组下部黄绿色页岩层中的双瓣壳节肢动物Tuzoia和Isoxys,讨论Tuzoia和Isoxys两属的特征和种群、地质、地理分布、软体附肢、分类位置、古生态及其演化趋势。详细描述关山动物群中Tuzoia和Isoxys的3种(其中2新种):TuzoiasinensisP’an,1957,T.tylodesaLuoetHusp.nov.,IsoxyswudingensisLuoetHusp.nov.,和I.sp.。     

皖南休宁新元古界-下寒武统沉积岩的有机碳同位素记录

皖南休宁县蓝田地区震旦系皮园村组和下寒武统荷塘组主要由硅质岩和黑色页岩组成。皮园村组硅质岩的分析结果表明:δ13Corg分析值落在-36.39‰—-31.45‰范围之内,而且从该剖面的底部到顶部呈现出逐步下降的趋势,可能归之于大气和海洋中O2/CO2比值上升,生物氧化作用增强所致;有机碳(TOC)含量<1%,有机氮(TON)含量<1‰,两者均处于蓝田地区埃迪卡拉系—下寒武统地层的最低值,证明晚震旦世时期该地区海洋生物产量比前期(蓝田期)海洋产量显著下降,可能归之于海水加深,不利于海洋生物的繁殖。荷塘组黑色页岩的δ13Corg分析值落在-33‰—-31‰范围之内,TOC和TON含量与皮园村组比较有大幅度提高,尤其黑色页岩下段TOC含量达到13.85%,证明早寒武世海洋生物得到了大发展。由于新元古代冰期后海洋藻类的大量涌现和早寒武世海洋生物得到大发展,生物光合作用产生大量的O2,导致大气和海水中O2/CO2比值进一步升高,可能是引发“寒武纪生物大爆发”的重要因素。     

The biostratigraphic and palaeoenvironmental significance of Lower Cambrian (Botomian) trilobites from the Ak-Kaya section of the Altai Mountains (southern Siberia,Russia)

The presence of one of the oldest records of polycystine Radiolaria in the Lower Cambrian sedimentary sequence of the Ak-Kaya section (Gorny Altai) requires a biostratigraphic dating. The trilobites found recently a few tens of meters below the radiolarian-bearing level belong to Calodiscus resimus Repina, Serrodiscus fossuliferus Repina and Alacephalus aff. contortus Repina; they suggest that this part of the Shashkunar Formation can be correlated with the lower part of the Botomian stage. The absence of eyes in the two former species suggests a mode of life buried in the fine pelagic sediments. Indications of the presence of a strongly developed musculature on the third species point to a palaeoenvironment characterized by a relatively high hydrodynamic regime.     

The evolution of molluscs

Molluscs are extremely diverse invertebrate animals with a rich fossil record, highly divergent life cycles, and considerable economical and ecological importance. Key representatives include worm‐like aplacophorans, armoured groups (e.g. polyplacophorans, gastropods, bivalves) and the highly complex cephalopods. Molluscan origins and evolution of their different phenotypes have largely remained unresolved, but significant progress has been made over recent years. Phylogenomic studies revealed a dichotomy of the phylum, resulting in Aculifera (shell‐less aplacophorans and multi‐shelled polyplacophorans) and Conchifera (all other, primarily uni‐shelled groups). This challenged traditional hypotheses that proposed that molluscs gradually evolved complex phenotypes from simple, worm‐like animals, a view that is corroborated by developmental studies that showed that aplacophorans are secondarily simplified. Gene expression data indicate that key regulators involved in anterior–posterior patterning (the homeobox‐containing Hox genes) lost this function and were co‐opted into the evolution of taxon‐specific novelties in conchiferans. While the bone morphogenetic protein (BMP)/decapentaplegic (Dpp) signalling pathway, that mediates dorso‐ventral axis formation, and molecular components that establish chirality appear to be more conserved between molluscs and other metazoans, variations from the common scheme occur within molluscan sublineages. The deviation of various molluscs from developmental pathways that otherwise appear widely conserved among metazoans provides novel hypotheses on molluscan evolution that can be tested with genome editing tools such as the CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats‐associated protein9) system.     

Homologous shell microstructures in Cambrian hyoliths and molluscs

Hyoliths were among the earliest biomineralizing metazoans in Palaeozoic marine environments. They have been known for two centuries and widely assigned to lophotrochozoans. However, their origin and relationships with modern lophotrochozoan clades have been a longstanding palaeontological controversy. Here, we provide broad microstructural data from hyolith conchs and opercula from the lower Cambrian Xinji Formation of North China, including two hyolithid genera and four orthothecid genera as well as unidentified opercula. Results show that most hyolith conchs contain a distinct aragonitic lamellar layer that is composed of foliated aragonite, except in the orthothecid New taxon 1 that has a crossed foliated lamellar microstructure. Opercula are mostly composed of foliated aragonite and occasionally foliated calcite. These blade or lath‐like microstructural fabrics coincide well with biomineralization of Cambrian molluscs rather than lophophorates, as exemplified by the Cambrian members of the tommotiid‐brachiopod linage. Accordingly, we propose that hyoliths and molluscs might have inherited their biomineralized skeletons from a non‐mineralized or weakly mineralized common ancestor rather than as a result of convergence. Consequently, from the view of biomineralization, the homologous shell microstructures in Cambrian hyoliths and molluscs strongly strengthen the phylogenetic links between the two groups.     

Three new naraoiid species from the Burgess Shale,with a morphometric and phylogenetic reinvestigation of Naraoiidae

Naraoiids are non‐biomineralized euarthropods characterized by the complete fusion of post‐cephalic tergo‐pleurae into a single shield, as well as an extensively ramified digestive tract. Ranging from the early Cambrian to the late Silurian (Pridoli), these arthropods of simple appearance have traditionally been associated with the early diversification of trilobites and their close relatives, but the interrelationships and affinities of naraoiids within Artiopoda remain poorly characterized. Three new species from the Burgess Shale (middle Cambrian, Stage 5) of British Columbia, Canada, are described here: Misszhouia canadensis sp. nov., from Marble Canyon (Kootenay National Park), the first species belonging to the genus Misszhouia outside of China; Naraoia magna sp. nov., from Marble Canyon and also from the Raymond Quarry (Yoho National Park), the largest species of Naraoia described thus far, reaching up to 9 cm in length; and Naraoia arcana sp. nov., from two sublocalities on Mount Stephen (Yoho National Park), defined by its unusual combination of spines. This new material shows that gut morphology is no longer a reliable character to distinguish Misszhouia from Naraoia. We demonstrate that Naraoia and Misszhouia can instead be discriminated morphometrically, based on simple metrics of the dorsal exoskeleton. Our quantitative results also help with inter‐specific discrimination and illustrate possible cases of sexual dimorphism. Phylogenetically, the inclusion of morphometric data adds resolution to our cladogram, although parsimony and likelihood treatments provide somewhat different evolutionary scenarios. In all cases, liwiines are nested within Naraoiidae, resolved as the most derived clade of trilobitomorph arthropods.     

Growth and development of agnostids Pentagnostus proanabarensis Fedoseev, 1999

Juvenile development of the Middle Cambrian agnostid trilobite Pentagnostus proanabarensis Fedoseev, 1999 is studied. A sequence of eight juvenile stages is defined based on changes in morphology and measurements. Three of them belong to meraspid degree 0, another three refer to meraspid degree 1, and two represent early holaspid stages. Neither the frequency distribution of the length of cephalons and pygidia nor the length-width scatter diagrams give clear clusters of molts; the number of juvenile molts can be estimated through the analysis of the posteroaxis length distribution. Thus, the measurements and morphology of some juvenile pygidia with vestiges of the pleural and postaxial furrows allow reconstruction of the number of segments during individual growth. At the initial stage of development, the pygidial axis of P. proanabarensis probably had four segments, the pygidial axis increased up to no less than six segments in adults.