Maotianshan-Shale nemathelminths — Morphology, biology, and the phylogeny of Nemathelminthes
Maotianshan-Shale nemathelminths — Morphology, biology, and the phylogeny of Nemathelminthes
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DOI：
10.1016/j.palaeo.2007.03.019
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发表时间：
2007-10
期刊：
影响因子：
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通讯作者：
A. Maas;Diying Huang;Jun‐yuan Chen;D. Waloszek;A. Braun
中科院分区：
文献类型：
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作者：
A. Maas;Diying Huang;Jun‐yuan Chen;D. Waloszek;A. Braun
本文对昆明下寒武统猫天山-页岩生物群（澄江、海口和山口动物群）中个体丰富的线虫类进行了形态、生活习性及其在早期海洋浅水生态系统中的可能作用的研究。它们的身体形状可以分为四组：长的蠕虫状形式，囊状形式和管状形式，具有圆锥形渐缩的坚硬躯干，以及由葡萄状前部和类似形状的躯干组成的形式，躯干具有沿着身体长轴的角质板。后一种形状与现存的幼虫priapulids和幼虫和成虫loriciferans的有皮躯干相匹配，两者都是scalidophoran Nemathelminthes的成员。所有寒武纪的分类群都与Scalidophora有几个共同的特征，包括Loricifera，Priapulida和Kinorhyncha，例如咽衬有内部弯曲的刺和身体前部区域，内向，同样有钩状刺。活着的烧鳞兽使用它们的内向来移动，用咽来吞咽猎物，化石形式也被认为是这种情况。它们向内弯曲的咽刺防止猎物逃脱，而内向的刺，即所谓的鳞片，在将身体向前拉时可以将身体锚在水面上。生物体可以将其咽部缩回并向内深深地插入体内，这只能在下寒武世的咽部中得到证明。详细的安排和数量的咽和内向刺被认为是有用的功能，系统定位下寒武统类群。结构本身可能已经演变，但是，早于scalidophoran地面模式，并可能已经表征干种cycloneuralian线虫，因为存在的两个类群的线虫，nematomorphs（在幼虫）和线虫（口周感器）的相似功能。内向者在Nematoida中有六个鳞屑环，而在Scalidophora中有四或五倍的环，这表明化石分类群的可能位置靠近Scalidophora。我们认为，寒武纪的龟甲类类群（第四组）的衍生物的树干谱系的Loricifera。这意味着，微小的尺寸，轻微的双边化，一个特定的穿刺装置发展沿着这些动物的进化路径，朝着生活在间隙，冠群。重建化石分类群的生活习性，在我们看来，我们调查的形式最有可能是表生或底生的，生活在泥泞的沉积环境中。在沉积物中垂直挖洞似乎不太可能，而寒武纪线虫类的掠食性生活也是合理的。我们还讨论了这些化石的意义方面的系统地位的线虫。
We investigated individual-rich nemathelminth species from the Lower Cambrian Maotianshan-Shale (Chengjiang) biota comprising Chengjiang, Haikou and Shankou faunas near Kunming, southern China, with regard to morphology, life habits and their possible role in the early marine shallow-water ecosystems. Their body shapes can be grouped into four sets: long, worm-shaped forms, sac-like forms and tubiform forms with a conically tapering, stiff trunk, and forms with a body consisting of a grape-shaped anterior part and a similarly shaped trunk with cuticular plates along the long axis of the body. This latter shape matches the loricate trunk of extant larval priapulids and larval and adult loriciferans, both members of the scalidophoran Nemathelminthes. All Cambrian taxa share several features with the Scalidophora, comprising Loricifera, Priapulida and Kinorhyncha, such as the pharynx lined with internally curved spines and the frontal body region, introvert, which is similarly armed with hook-like spines. Living scalidophorans use their introvert for locomotion and the pharynx to swallow prey, which is assumed to be the case for the fossil forms too. Their inwardly curved pharyngeal spines prevent prey from escaping, while the introvert spines, the so-called scalids, may serve to anchor the body to the surface when pulling it forward. The living forms can withdraw their pharynx and introvert deeply into the body, which can be demonstrated only for the pharynx in the Lower Cambrian forms. Details of the arrangement and number of pharynx and introvert spines are considered useful features for a systematic positioning for the Lower Cambrian taxa. The structures themselves might have evolved, however, earlier than in the scalidophoran ground pattern and probably characterised already the stem species of cycloneuralian nemathelminths because of the presence of similar features in both taxa of the Nematoida, the nematomorphs (in the larvae) and the nematodes (circum-oral sensilla). Introverts with rings of six scalids in Nematoida opposite to rings of multiples of four or five in Scalidophora indicate a possible position of the fossil taxa close to the Scalidophora. We argue that the loricate Cambrian taxa (the fourth set) are derivatives of the stem lineage of the Loricifera. This implies that minute size, slight bilateralisation, a specific piercing apparatus developed along the evolutionary path of these animals toward life in the interstitial, characterise the crown group. Reconstructing the life habits of the fossil taxa, it seems to us that the forms investigated by us were most likely epi- or inbenthic, living in muddy sedimentary environments. A vertical burrowing into the sediment does not seem very likely, while a predatory life is plausible also for the Cambrian nemathelminths. We furthermore discuss the significance of these fossils with regard to the systematic status of the Nemathelminthes.