【作者】 蔡耀平；

【导师】 华洪；

【作者基本信息】 西北大学 ， 地球生物学， 2011， 博士

【摘要】 产于我国上扬子区陕西宁强—勉县境内埃迪卡拉纪晚期的高家山生物群(最大时限范围：551-541 Ma),主要产出多种类型的宏体管状化石。其中,Conotubus和Gaojiashania这两个属目前在世界范围内只见于高家山生物群中,而且只在陕西宁强境内的几个剖面中被发现。虽然Conotubus hemiannulatus与Gaojiashania cyclus的古生物学意义引起了前人的关注,然而它们的化石埋藏学与古生态学却没有得到系统深入的研究。本文主要对Conotubus hemiannulatus与Gaojiashania cyclus的化石埋藏学和古生态学进行研究。化石埋藏学部分的研究主要包括：(1)对赋存这两种化石的埃迪卡拉系灯影组高家山段地层在横向上的沉积学与岩石学特征变化进行对比分析；(2)对高家山剖面的高家山段在垂向上的沉积序列进行了详细的沉积微相研究；(3)对赋存这两种化石的地层序列开展了详细的厘米级保存特征(分散、破碎)分析并划分出两个埋藏相；(4)对生物遗体沉积学过程进行复原；(5)对三种不同风化程度的粉砂岩—泥岩层中的化石成岩矿物的主要特征和成岩期次关系进行对比分析并讨论了它们可能的风化过程；(6)对Conotubus hemiannulatus与Gaojiashania cyclus的三种化石成岩方式(黄铁矿化、有机碳质压膜、铝硅酸盐矿物交代)进行了对比研究。古生态学部分的研究主要包括：(1)对Conotubus hemiannulatus的系统古生物学进行厘定并对其形态学以及可能的发育序列进行复原；(2)对Conotubus hemiannulatus的各种生物—沉积物相互作用关系及其生活方式进行了分析讨论；(3)对Gaojiashania cyclus的系统古生物学进行厘定并对其形态学进行复原；(4)对Gaojiashania cyclus的生活方式进行了分析讨论。通过本文的研究我们认为：(1) Conotubus hemiannulatus与Gaojiashania cyclus赋存的地层序列中广泛发育细粒碎屑事件沉积。事件沉积不仅为生物的快速埋藏提供了先决条件,同时也为其快速矿化起到了关键作用。这些细粒碎屑事件沉积物主要为毫米级厚的、由发育正粒序的粉砂岩层及均一块状的泥岩层构成的层偶,它们分别属于事件沉积早期和晚期阶段的产物。化石主要保存在发育正粒序的粉砂岩层内,极少出现在均一块状泥岩层内。(2)黄铁矿—有机碳质压膜—铝硅酸盐矿物的混合保存方式与寒武纪澄江型化石库中的化石成岩方式非常类似。黄铁矿保存了最多的化石形态学信息和最好的化石立体形态,而有机碳质压膜—铝硅酸盐矿物只保存了有限的形态学信息和较差的三维形态。高家山生物群中的实证材料表明,世界范围内广泛分布的寒武纪细粒碎屑岩中的特异保存方式可以追溯到前寒武纪。高家山生物群以及澄江生物群中的化石保存特征都体现了黄铁矿化作用在化石化过程中的首要作用。(3)Conotubus hemiannulatus是一种厘米级大小的锥形管体,较细的一段封闭,较粗的一端开口。管体由一系列筒状的管壁套合而成。第一管节由一个尖端封闭的光滑锥形管壁构成,其余的管节都由两端开口的具横纹的筒状管壁构成。这种管体结构与世界范围内广布的锥管状化石Cloudina中发育的非常类似,二者可能具有较近的亲缘关系。Conotubus hemiannulatus可能采用直立固着(年青个体)+匍匐平躺(老年个体)的生态方式固着到发育微生物席的泥质基底上,但靠近开口端的管体部分始终保持直立伸入水体中,表明其可能是一种滤食生物。(4)Gaojiashania cyclus是一种厘米级大小的直筒状化石,两端开口,管体中空。管体上发育密集的环纹以及相对较厚而坚硬的O形圆环。Gaojiashania cyclus可能采用匍匐平躺的生活方式,依靠相对坚硬的圆环固着到发育微生物席的泥质基底上。除了圆环,其余的管体部分都富有柔韧性,因此管体可以发生自由的弯曲、伸长、以及收缩。这表明Gaojiashania cyclus可能具有较强的运动能力,能够在水体—沉积物界面上进行水平方向的自由运动。(5)前人对Conotubus和Gaojiashania这两个属的化石分别做了一次初始描述和一次补充性的描述。然而,本文的研究认为前人划分属种的部分依据属于埋藏原因(压实、压扁、黄铁矿的再生长或重结晶、不完整保存)造成的假象。除了Conotubus varius被移除Conotubus属外,前人建立的所有种在本文中被归并为模式种Conotubushemiannulatus(半环纹锥管虫)。除了Gaojiashania haihaoliangensis被移除Gaojiashania属外,前人建立的所有种在本文中被归并为模式种Gaojiashania cyclus(圆环高家山虫)。更多还原

【Abstract】 The upper Ediacaran Gaojiashan Lagerstatte (551-541 Ma) in southern Shaanxi Province of South China hosts a variety of soft-bodied or lightly biomineralized tubular fossils (Shaanxilithes, Sinotubulites, and Cloudina) and calcareous microfossils (Protolagena), among which Conotubus and Gaojiashania are only known in this Lagerstatte worldwide. The paleobiological significance of the Gaojiashan biota has been discussed extensively, mostly in the Chinese literature. However, the taphonomy and paleoecology of this biota has not been studied in detail, limiting the interpretative power of these exceptionally preserved fossils.This study focuses on the taphonomy and paleoecology of Conotubus hemiannulatus and Gaojiashania cyclus. Integrated paleoecological, sedimentological, and taphonomic analysis shows that event deposition played an important role in the biostratinomy of the Gaojiashan Lagerstatte. Gaojiashan fossils, particularly pyritized fossils, preferentially occur in mm-thick normally-graded calcisiltite/siltstone layers, which are interpreted as. distal event deposits, but are rarely present in calcilutite and mudstone beds. Two taphofacies are recognized in the middle Gaojiashan calcisiltites/siltstones, on the basis of biostratinomic data (e.g., fossil fragmentation and disarticulation). Both taphofacies contain fossils (Conotubus, Gaojiashania, and Protolagena) that were buried in-situ by distal event deposits below the average storm wave base (SWB); evidence for in-situ burial includes oblique orientation, rejuvenation, growth interruption, and reorientation. This research represents one of the few biostratinomic studies of Precambrian Lagerstatten. The results show that the Gaojiashan Lagerstatte, like many Phanerozoic Konservat-Lagerstatten, was strongly influenced by event deposition which provides an obrution mechanism for quick burial.Early Cambrian Chengjiang-type Konservat-Lagerstatten owe their exceptional preservation to two primary modes of preservation:pyritization and carbonaceous compression with associated aluminosilicate clay coats. This study indicates that the late Ediacaran Gaojiashan Lagerstatte of southern Shaanxi Province, South China, shares a similar taphonomic array. Environmental scanning electron microscopic (ESEM) and energy dispersive X-ray spectroscopic (EDS) analyses of preserved soft tissues of representative Gaojiashan fossils, Conotubus hemiannulatus and Gaojiashania cyclus, indicate that authigenic pyritization is the primary taphonomic pathway within this deposit, with ca.80% of the collected fossils preserved through pyritization. Burgess Shale-type (BST) carbonaceous compression preservation and corresponding, yet unsettled, relationships with aluminosilicate clay coats are also present in this Lagerstatte, but may have played an auxiliary role in their exceptional preservation. Our taphonomic study documents that, like those in the Chengjiang-type deposits, authigenic pyritization tends to preserve more labile tissues than those preserved as carbonaceous compressions. We predict further discoveries of siliciclastic Lagerstatten sharing similar taphonomic modes to the early Cambrian Chengjiang and the late Ediacaran Gaojiashan deposits in which authigenic pyritization is dominantly responsible for exceptional preservation.Conotubus hemiannulatus is a cm-sized conical tube that tapers adapically from an aperture to a rounded apex. The tube consists of a series of nested cylindrical-to-funnel-shaped tube walls. The nested cylindrical tubes are broadly similar to the construction of the late Ediacaran fossil Cloudina. Integrated morphological, taphonomic, and paleoecological data suggest that Conotubus hemiannulatus occupied an epibenthic life-mode, with the apex anchoring to microbially-bound muddy substrate and the aperture extending upwards into the water column. It was probably a suspension-feeding organism. This soft bottom dweller employed specialized paleoecological strategies to rejuvenate and self-right the tubes after being subjected to sediment obrution, indicating strong burial-resistant capabilities. The tube likely provided space for the living organism, with additional larger cylinders added episodically to accommodate growth. Currently available evidence suggests that Conotubus hemiannulatus had non-biomineralized or weakly biomineralized tubes, and its phylogenetic affinity remains unresolved.Gaojiashania cyclus is a phylogenetically problematic tubular fossil that is only known from the late Ediacaran Gaojiashan Lagerstatte in southern Shaanxi Province, South China. It is a cm-sized tube that consists of a series of rigid and flexible couplets of tube walls that are edge-to-edge contacted with each other. The rigid wall is thicker (1-3 mm in thickness) and relatively shorter (1-2 mm in length), forming a circular ring; whereas the flexible wall is thinner (0.5-1 mm in thickness), elongated (0.5-5 mm in flexible and thus variable length) and ornamented with transverse annuli. The flexibility of the tubes allows the creature to curve, extend, and constrict at free angles. The tube construction of G. cyclus is unique in both fossil and modern tubular organisms hitherto known, and strikingly differs from that of other Ediacaran and Cambrian tubular fossils. Integrated morphological, taphonomic, and paleoecological data suggest that G. cyclus may have been a procumbent epibenthos, possibly with the rings anchoring to microbially-bound muddy substrate. The preserved tube is interpreted as the exterior part of the animal rather than a dwelling tube. Currently available evidence suggests that G. cyclus had non-mineralized or weakly mineralized tubes, and its phylogenetic affinity remains unresolved.更多还原