BASAL CAMBRIAN MICROFOSSILS FROM THE YURTUS AND XISHANBLAQ FORMATIONS (TARIM, NORTH-WEST CHINA): SYSTEMATIC REVISION AND BIOSTRATIGRAPHIC CORRELATION OF MICRHYSTRIDIUM-LIKE ACRITARCHS

Abstract:  Micrhystridium -like acritarchs are widely distributed in basal Cambrian cherts and phosphorites in South China. This paper describes similar acritarchs from the basal Cambrian Yurtus and Xishanblaq formations in Tarim, north-west China. The taxonomy of these acritarchs is revised. The basal Cambrian acritarch assemblage in Tarim and South China is characterized by three genera: Asteridium Moczydłowska, Heliosphaeridium Moczydłowska and Comasphaeridium Staplin, Jansonius and Pocock. This assemblage is named the Asteridium - Heliosphaeridium - Comasphaeridium (AHC) acritarch assemblage. In both South China and Tarim, the AHC acritarch assemblage is associated with the tubular microfossil Megathrix longus Yin L. and the small shelly fossil Kaiyangites novilis Qian and Yin G. This assemblage also occurs in the Lower Tal Formation in the Lesser Himalaya. Correlation with small shelly fossil (SSF) assemblages indicates that the AHC assemblage is restricted to the Meishucunian Stage, and possibly to the lower Meishucunian ( Anabarites trisulcatus - Protohertzina anabarica and Siphogonuchites triangularis - Paragloborilus subglobosus SSF assemblages). The AHC assemblage is broadly similar to the Asteridium tornatum - Comasphaeridium velvetum (acritarch) Zone in the East European Platform, which is considered to be Nemakit-Daldynian (and possibly Tommotian) in age.     

Unique lowermost Cambrian small shelly fossils (SSFs) from eastern Yunnan,South China: implications for the early diversification history of animals

With a particular focus on the earliest Cambrian diversification of small shelly fossils (SSFs), stratigraphic analysis was conducted on the lower Cambrian Zhongyicun Member at the Hongjiachong section in the Chengjiang area, Yunnan, China. From ca. 3-m-thick bedded phosphorites (Unit A) in the lower part of the member, we recovered unique SSFs. This interval with 4 SSF-bearing horizons is characterized by the dominance in blade-shaped SSFs, including Halkieria spp., Brushenodus prionodes, Sinosachites delicatus, and Pteronus sp., and the absence of typical tube/cap-shape SSFs common in the Fortunian. This interval tentatively named “Halkieria-dominant interval” is stratigraphically positioned ca. 3 m below the previously known level of the oldest mollusks in the continuous bedded phosphorite sequence between the Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone and the overlying Paragloborilus subglobosa-Purella squamulosa Assemblage Zone of the Fortunian (the earliest Cambrian). The “Halkieria-dominant interval” yields some SSFs likely affiliated with ostracods in view of size and morphology, suggesting that arthropod body fossils appeared in the Fortunian, considerably earlier than previously believed.     

陕南寒武纪西乡生物群中Anabarites对称性演化研究

阿纳巴管(Anabarites)是寒武纪早期一类形态独特的管栖生物,其横切面的轮廓多样,但所有形态都是"3"的倍数。陕南梅树村期西乡生物群产出大量磷质Anabarites,根据管体上纵槽出现的数目和级别,确认Anabarites可分辨出三种对称类型:A型为仅具有三条一级纵槽的三辐射对称类型、B型具有三条一级纵槽和一条二级纵槽类型、C型具有三条一级纵槽和两条二级纵槽。其中B型、C型是从三辐射到六辐射对称之间的过渡类型,为新发现的对称形式。本文以上述三种类型Anabarites为研究材料,对Anabarites对称性演化途径进行了探索性研究,对寒武纪大爆发之初动物的对称性起源演化研究提供了重要证据。     

寒武纪早期疑难生物类群阿纳巴管(Anabarites)亲缘性的探讨

阿纳巴管为生活于寒武纪早期、具三射对称管栖疑难生物。文中通过对采自陕西南部宁强下寒武统宽川铺组阿纳巴管壳体SEM和岩石薄片的观察,并以对称性和矿物组成作为线索对阿纳巴管的系统学位置和生物属性开展了探索。通过对称性对比研究和骨骼矿物组成的研究,将阿纳巴管解释为珊瑚虫纲古老干支类群的化石代表,为现生六射珊瑚的祖先类群。根据阿纳巴管个体发育过程对称性的演变所提供的线索进一步认为以六射及其倍数对称为特征的现代六射珊瑚可能经过三射对称中间阶段,起源于圆形辐射对称的祖先。阿纳巴管为由圆形经三射向六射转变这一中间阶段的化石代表。此外,还对阿纳巴管的成岩模式以及生活方式开展了探索性研究。     

陕南早寒武世早期水螅营养体及管状疑难化石

陕南宁强宽川铺生物群发现具触手的水螅营养体化石,同层位同时发现大量磷质管状疑难化石,其管体上可分辨出可能的出芽特征,推测该类具出芽特征管体可能和现代群体生活的薮枝螅(Obelia)有关。     

陕西南部早寒武世原始锥石类carinachitids两新种记述

对陕西宁强宽川铺地区灯影组宽川铺段原始锥石类carinachitids两个新种Emeiconularia amplicanalis sp.nov.和Carinachites platus sp.nov.进行了记述。外表形态多样、个体大小悬殊的两辐对称、三辐对称和四辐对称的carinachitids系列标本的发现,揭示了陕南地区早寒武世丰富多样的carinachitids生命世界。三辐射对称的Emeiconularia在四川峨眉及陕西南部的震旦-寒武系界线附近都有分布,因而对前寒武纪—寒武纪跨界地层具有划分对比意义。两新种的发现无疑增加了我们对早期锥石类多样性的认识,为证实多样性的刺细胞动物在早寒武世早期已经存在的假说提供了新线索。     

Small shelly fossils and trace fossils near the Precambrian-Cambrian boundary in the Yukon Territory, Canada

In the past an ‘explosion’ in diversity and abundance of small shelly fossils and of trace fossils has served to mark the base of the Cambrian. However, no evidence has been presented to prove that the ‘explosions’ of the two groups were synchronous. We describe small shelly fossils and trace fossils from the same phosphatic limestone beds that indicate that the two events were separate in time. The small shelly fossils are Anabarites trisulcatus, Hyolithellus cf. H. isiticus, Microcornus? sp., Protohertzina anabarica, P. unguliformis, P. sp. A, Pseudorthotheca sp. A, Rushtonia? sp. A, four types of tuberculate plates and one type of reticulate plate. These fossils represent a restricted, ‘pre-explosion’ fauna and are assigned to the Anabarites-Circotheca-Protohertzina Assemblage Zone, an uppermost Precambrian zone in the Meishucun Stage, Yunnan Province, China. A point at the top of this zone has received strong international endorsement for future designation as the base of the Cambrian. Associated with the small shelly fossils are the trace fossils Cruziana sp. A, Cruziana? sp. B, Rusophycus sp. A, Palaeophycus rubdark and arthropod scratch marks. If found in isolation, this trace fossil assemblage would be considered as post-Precambrian because it includes large, highly organized arthropod traces that are traditionally accepted as occurring above the trace fossil ‘explosion’. We therefore conclude that the trace fossil ‘explosion’ predates the small shelly fossil ‘explosion’. If the proposed location of the base of the Cambrian in Yunnan is accepted, the small shelly fossil ‘explosion’ concept and its relationship to the boundary would not be greatly modified. The trace fossil ‘explosion’, however, would no longer indicate the base of the Cambrian and the ranges of some trace fossils would be extended into the Precambrian.     

浙西早寒武世梅树村期棒状化石

该文记述了浙江省西部江山县新塘坞地区灯影组(梅树村期)中新发现的矿化骨骼化石——浙江棒(新属)Zhejiangorhabdion gen.nov.。该化石的形态与以往化石类群均不相同,为个体微小的棒状,表面具较复杂的纹饰,不同个体间的纹饰都有一定变化,推测是动物体内不同部位的骨棒所具有的特征,代表了一个新的化石类群。新属的发现丰富了梅树村期动物群的内容,对了解具骨骼动物群体在这一时期的演化具有一定的意义。     

华中西南区一条国际前寒武系与寒武系界线层型补充剖面

1992年,全球前寒武系与寒武系界线层型剖面和层型点确定在加拿大纽芬兰东南幸运角剖面,并以遗迹化石带Trichophycus pedium的底作为前寒武系与寒武系的分界点。但是,幸运角剖面主要是以硅质碎屑岩相为主,难以与含有丰富小壳化石和具有可对比的稳定同位素资料的碳酸盐相界线剖面进行对比。为此,提出我国云南会泽大海附近的一条以碳酸盐相为主的剖面作为全球前寒武系与寒武系界线层型剖面的补充剖面。理由是（1）在大海剖面上,震旦系灯影组白岩哨段、待补段和下寒武统牛家菁组的中谊村段,大海段之间为整合接触;（2）早寒武世早期地层单元含有丰富的小壳化石并且可划分出4个小壳化石组合带;（3）利用碳同位素资料可以将大海剖面与许多其它剖面相当地层对比;（4）著名的梅树村剖面离大海剖面不远,它们均含有丰富的小壳化石和遗迹化石,两者易于对比。文章进一步认为寒武系的下界放在第I小壳化石组合带（Anabarites trisulcatus-Protohertzina anabarica）的底,相当于大海剖面11层的底。     

陕南早寒武世ANABARITES壳体内部结构及亲缘关系探讨

陕南灯影组宽川铺段的小壳化石具完好的三维保存方式。Anabarites是三辐射对称、单管底栖群居生物,亲缘关系不明。作者通过对陕南宁强下寒武统宽川铺段进行数次系统采样,经过对样品的醋酸溶蚀处理和电子扫描电镜照像后,发现Anabarites壳体内部具螺旋状微结构;经测试分析后认为该结构可能与中槽的深浅成正相关,两者的形成可能与微环境中水体的变化有关。探讨其可能的亲缘关系及其演化序列。     

寒武纪早期四方塔型壳属的胚胎发育及其系统学意义

寒武纪大爆发是生命演化史上重要的创新事件.我国澄江动物群(距今约5.18亿年)代表了寒武纪大爆发的高潮,而产自寒武系最底部,时代上早于澄江动物群的宽川铺生物群(距今约5.35亿年)则为研究寒武纪大爆发的第一幕提供了重要的埋藏窗口.宽川铺生物群产自陕南宁强、西乡等地寒武系底部宽川铺组钙质磷块岩和磷质碎屑灰岩中,以保存了大...     

陕西宁强原始锥石类五辐对称新属种的发现

陕西宁强宽川铺地区灯影组宽川铺段的原始锥石类Carinachitids中,新发现了五辐对称新属种--五分宁强锥石(新属、新种)Pentaconularia ningqiangensis gen.et sp.nov..本文对这一新属种进行了描记.结合前人发现的两辐对称、三辐对称、四辐对称的Carinachitids系列标...     

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.     

Giving the early fossil record of sponges a squeeze

Twenty candidate fossils with claim to be the oldest representative of the Phylum Porifera have been re‐analysed. Three criteria are used to assess each candidate: (i) the diagnostic criteria needed to categorize sponges in the fossil record; (ii) the presence, or absence, of such diagnostic features in the putative poriferan fossils; and (iii) the age constraints for the candidate fossils. All three criteria are critical to the correct interpretation of any fossil and its placement within an evolutionary context. Our analysis shows that no Precambrian fossil candidate yet satisfies all three of these criteria to be a reliable sponge fossil. The oldest widely accepted candidate, Mongolian silica hexacts from c. 545 million years ago (Ma), are here shown to be cruciform arsenopyrite crystals. The oldest reliable sponge remains are siliceous spicules from the basal Cambrian (Protohertzina anabarica Zone) Soltanieh Formation, Iran, which are described and analysed here in detail for the first time. Extensive archaeocyathan sponge reefs emerge and radiate as late as the middle of the Fortunian Stage of the Cambrian and demonstrate a gradual assembly of their skeletal structure through this time coincident with the evolution of other metazoan groups. Since the Porifera are basal in the Metazoa, their presence within the late Proterozoic has been widely anticipated. Molecular clock calibration for the earliest Porifera and Metazoa should now be based on the Iranian hexactinellid material dated to c. 535 Ma. The earliest convincing fossil sponge remains appeared at around the time of the Precambrian‐Cambrian boundary, associated with the great radiation events of that interval.     

Evolution of C isotopes in the Cambrian of China: implications for Cambrian subdivision and trilobite mass extinctions

Carbon and oxygen isotopes were studied in fossiliferous Cambrian carbonates in northwestern Hunan Province (South China) and in northern Anhui and southern Shandong provinces (North China). Two major C isotope excursions related to biological events occur in the Wangcun section (Yongshun County, northwestern Hunan), which consists of a slope carbonate sequence (510 m thick) containing abundant trilobites. The first C isotope excursion (δ¹³C value shifts from -2.3‰ to 2‰) occurs near the boundary between the Qingxudong and Aoxi formations, close to the traditional Lower-Middle Cambrian boundary. The second excursion (δ¹³C value shifts from 0‰ to 3‰) occurs in the interval between the Linguagnostus reconditus Zone and the Glyptagnostus reticulatus Zone. The base of the G. reticulatus Zone define the base of the Paibi Stage and Furongian Series. Similar C isotope excursions also occur in shallow - water carbonate sections in North China. In Jiagou section near Huainan (Anhui Province), recently considered an important interval for defining the lower-middle Cambrian boundary because of dramatic changes in the trilobite fauna (extinction of redlichiids and appearances of ptychopariids), a negative C isotope excursion (δ¹³C value shifts from +1.21‰ to -1.93‰) occurs at the top of the lower member of the Mantou Formation. In the Gushan section (Changqing County, Shandong Province), a C isotope excursion (δ¹³C value shifts from -0.04‰ to 2.23‰) occurs at the base of the Changshan Formation and is coincident with the base of the Chuangia Zone. This excursion can be correlated with the excursion in the lower part of Glyptagnostus reticulatus Zone in the Wangcun section. The above two distinct C isotope excursions, which occur both in slope carbonates in South China and in shallow - water carbonates in North China, have also been recognized in Cambrian sections on other continents, and they coincide with global mass extinctions of trilobites. The two excursions evidently reflect global changes of Cambrian sea level, and they have utility for Cambrian subdivisions and for both regional and global stratigraphic correlation. In addition, a negative carbon excursion below the base of the Ptychagnostus atavus Zone in the Wangcun section supports previous suggestions that the FAD of P. atavus can be considered as a global correlatable horizon within the middle Cambrian.     

寒武纪早期宽川铺生物群中圆管螺化石显微结构及显微谱学分析

陕南寒武系底部宽川铺组不仅保存了大量的小壳化石,还以磷酸盐化的方式立体保存了多种软躯体动物的胚胎和成体化石,为研究早期动物矿化模式、个体发育方式以及动物躯体构型的早期辐射过程提供了非常关键的信息。然而宽川铺生物群的研究大多聚焦于化石系统分类及亲缘关系探索,对生物群的沉积背景、埋藏过程和保存模式的分析甚少。本文以宽川铺生物群的核心产出层位——陕西省西乡县大河镇宽川铺组下部磷质碎屑灰岩为研究对象,对其中的一类管状化石圆管螺及其围岩进行了显微结构学和显微谱学综合分析,获得了化石显微结构和关键结构对应的元素、矿物成分等信息,讨论了管状化石圆管螺的埋藏过程和保存模式,并在此基础上对其生物学特征进行了初步探讨。该研究对理解宽川铺生物群的埋藏机制提供了新的线索。     

First evidence for Cambrian glaciation provided by sections in Avalonian New Brunswick and Ireland: Additional data for Avalon–Gondwana separation by the earliest Palaeozoic

The first evidence for Cambrian glaciation is provided by two successions on the Avalon microcontinent. The middle lowest Cambrian (middle Terreneuvian Series and Fortunian Stage–Stage 2 boundary interval) has an incised sequence boundary overlain by a fluvial lowstand facies and higher, olive green, marine mudstone on Hanford Brook, southern New Brunswick. This succession in the lower Mystery Lake Member of the Chapel Island Formation may be related to melting of an ice sheet in Avalon. The evidence for this interpretation is a muddy diamictite with outsized (up to 10 cm in diameter), Proterozoic marble and basalt clasts that penetrated overlying laminae in the marine mudstone. That eustatic rise was associated with the mudstone deposition is suggested by an approximately coeval rise that deposited sediments with Watsonella crosbyi Zone fossils 650 km away in Avalonian eastern Newfoundland. A sea-level rise within the Watsonella crosbyi Chron, at ca. 535 Ma, may correspond to a unnamed negative ¹³C excursion younger than the basal Cambrian excursion (BACE) and the ZHUCE excursion in Stage 2 of the upper Terreneuvian Series. Cambrian dropstones are now also recognized on the northern (Gander) margin of Avalon in continental slope–rise sedimentary rocks in southeast Ireland. Although their age (Early–Middle Cambrian) is poorly constrained, dropstones in the Booley Bay Formation provide additional evidence for Cambrian glaciation on the Avalon microcontinent. Besides providing the first evidence of Cambrian glaciation, these dropstone deposits emphasize that Avalon was not part of or even latitudinally close to the terminal Ediacaran–Cambrian, tropical carbonate platform successions of West Gondwana.     

Shell microstructures of the Early Cambrian Anabarella and Watsonella as new evidence on the origin of the Rostroconchia

Internal moulds of the laterally compressed monoplacophoran Anabarella plana Vostokova, 1962, and likely earliest rostroconch Watsonella Grabau, 1900, from the Early Cambrian of the Siberian Platform, show similar microstructures. The moulds are covered with a thin phosphatic crust replicating the inner morphology and microstructure of the shells. The shells were completely removed during etching of the samples in 10% acetic acid, except for some moulds of Watsonella sp., which retained an incompletely preserved and recrystallized wall. Three types of microtexture were found in moulds of Anabarella and Watsonella : polygonal, spiny and step-wise. The polygonal texture is well exposed in the apical area and dorsal margin and is interpreted to represent a prismatic outer layer. The polygons can be partially overlapped by spiny and stepwise textures that may represent a crossed-lamellar inner layer. Prisms and lamellae were first-order structural units, probably consisting of fibers. The similar shell microstructures of Anabarella and Watsonella , especially at the dorsal margins, support the hypothesis that a laterally compressed monoplacophoran such as Anabarella plana was the first evolutionary step from monoplacophorans towards Early Cambrian bivalves via the earliest rostroconch-like Watsonella.     

陕西宁强地区早寒武世磷酸盐化Punctatus新材料

描记在陕西南部宽川铺地区的灯影组宽川铺段的震旦-寒武系界线附近发现的具五辐对称特征磷酸盐化Punctatus新种———Punctatus triangulicostalissp.nov.(三角隆脊瘤点壳)。新种的锥体由5条放射状排列的锥脊组成,柱体由5纵列等间隔排列的三角隆脊组成,构成了完美的五辐对称生物体。同时描记的有PunctatusemeiensisHe,1980,新种与它的显著区别是具有与锥顶5条放射脊相对应的5纵列三角隆脊。新种呈三维立体保存,暗示生物体内部已具有一定的硬化程度,而锥体表层及柱状体表面的三角隆脊为磷酸盐化软组织。形态功能分析表明,Punctatus在分类上可能更接近于腔肠动物的水螅型。     

Proetid trilobites from the middle Telychian (Llandovery,Silurian) Ningqiang Formation,Shaanxi, China

Four proetid trilobite species, Chuanqianoproetus guizhouensis Yin in Yin and Li, 1978, C. shuangheensis Wu, 1977, C. sp., and Latiproetus guizhouensis Yin in Yin and Li, 1978, are described from the Ningqiang Formation in Ningqiang County, Shaanxi, China. They are middle Telychian (Llandovery, Silurian) in age. Chuanqianoproetus and Latiproetus are both regarded as members of the subfamily Warburgellinae; the former has been considered to be a junior subjective synonym of Astroproetus, but on the basis of its morphology we argue here that they represent two independent genera. The poorly sorted bioclastic grainstones containing trilobite sclerites may indicate their deposition in turbulent conditions on an open marine platform.