同步加速器X射线层析显微技术在化石胚胎Markuelia研究中的应用

运用同步加速器X射线层析显微技术,对化石胚胎Markuelia的内部构造进行了虚拟切片及三维重建,揭示出化石胚胎Markuelia同时存在左型和右型盘绕方式,身体分节数为64-70之间,初步证明了尾刺发育早于吻刺的观点,观察到的消化道比以前所描述的要长,并推断有可能保存有司翻吻运动的肌肉组织或脑部神经结。

寒武纪化石胚胎 Markuelia 的肌肉组织

利用同步加速器辐射X射线断层显微技术(SRXTM),研究湖南省王村化石库的化石胚胎Markuelia hunanensis。所研究的标本保存了头部和尾部表皮下的栅栏状结构,对这些结构进行三维重建后,发现栅栏结构为环形,呈两侧对称状排列。将头部和尾部的栅栏结构分别解释为可能的咽部和尾刺基部的肌肉组织。对可能的肌肉组织结构的进一步研究表明,Markuelia hunanensis最早的胚后阶段可能是小型底栖动物,生活在泥砂缝隙之间,并且可能是活跃的捕食者。

Internal Structure of Cambrian Fossil Embryo Markuelia Revealed in the Light of Synchrotron Radiation X-ray Tomographic Microscopy

In the light of Synchrotron Radiation X-ray Tomographic Microscopy （SRXTM）, the internal structure of Markuelia hunanensis is revealed. In one example, vitrification and peeling show the annuli hidden under the chorion. Sectioning and 3-D reconstruction display an intact digestive tract from the inverted introvert to the terminal anus. The inverted introvert forms a rugby cavum. The following digestive tract is rope-like coiling, parallel to the body axis, about 650 μm in length, and uniform in diameter （-80 μm）. An exquisitely preserved pipe-like structure is hidden in the middle of the rope-like structure, diameter 20--40 lam, with a length of -120 μm. We interpret this pipe-like structure as the possible epidermis of the gut and its surroundings as the possible residue of musculature, similar to that in Priapulans. The two symmetrical rod-shape structures connecting the body wall and digestive tract are interpreted as the possible retractor muscles. After comparing the well preserved Left-form and Right-form Body of Markuelia, we suggest that they may represent a dimorphism. Counted directly, one sample of Markuelia hunanensis possesses 62 annulations and the other 68.

寒武纪胚胎Markuelia的发育序列

基于最近在湘西中寒武-上寒武统发现的更多的精美保存的Markuelia hunanensis标本,并且借助于同步加速器辐射X射线层析显微技术,建立了寒武纪胚胎Markuelia从卵裂(cleavage)阶段经过器官形成(organogenesis)阶段至预孵化(pre-hatching)阶段的发育序列,特别是预孵化阶段的发育序列,即从吻刺和尾刺刚刚形成的最早时期直至即将孵化(just about hatching)的最后时期的发育序列.这个发育序列提供了一个胚胎预孵化阶段的发育模式,而这一发育模式在现生的Scalidophorans(曳鳃类、兜甲类和动吻类)中尚未建立.因而,它不仅加深了对Markuelia现生后裔胚胎发育的认识,而且为这类动物的演化和发育开拓了一个新的(研究)窗口.

Developmental sequence of Cambrian embryo Markuelia

Based on more exquisitely preserved specimens of Markuelia hunanensis recently recovered from Middle and Upper Cambrian in western Hunan and in the light of Synchrotron radiation X-ray tomo-graphic microscopy, the developmental sequence from cleavage through organogenesis to the pre-hatching of Cambrian embryo Markuelia, especially the developmental sequence during the pre-hatching stage, i.e. from the earliest period when the scalids and tail spines only took shape to the latest period (just about hatching), is established. This developmental sequence provides a pattern of embryonic development during the pre-hatching stage, which has not been established in the living scalidophorans (priapulids, loriciferans and kinorhynchs). Thus, it not only enriches our knowledge on the embryonic development of the extant descendants of Markuelia, but also opens a new window to the evolution and development of the animal.

寒武纪化石胚胎的解剖学、亲缘关系以及发育序列

Bengtson和Yue首次确定Olivooides和Markuelia为化石胚胎,建立了Olivooides的胚胎和胚后发育序列,并且提出Olivooides的亲缘关系是刺细胞类(Bengtson and Yue,1997;Yue and Bengtson,1999)。在过去的10年中,许多论述寒武纪化石胚胎的文章问世,极大地提高了人们的认识。其中,Steiner等识别出Pseudooides为化石胚胎,并且推想了Pseudooides的发育序列(Steiner et al.,2004a,b)。基于在中国湖南西部中、上寒武统发现的许多精美保存的标本,对Markuelia进行了解剖学、亲缘关系和支序分析研究,限定Markuelia的亲缘关系为Scalidophora(动吻动物门、兜甲动物门和曳鳃动物门的统称)的干群。尝试性地建立了Markuelia的发育序列,特别是预孵化阶段的发育序列,即从吻刺形成的最早时期直至即将孵化的最后时期的发育序列。因为最近发现了保存完美的Olivooides预孵化阶段、孵化阶段以及未成年个体阶段(Olivooides和Punctatus之间的过渡类型)的标本,重新建立了Olivooides的胚胎和胚后发育序列。尽管Olivooides的亲缘关系仍然有争议,但多数作者支持其亲缘关系为刺细胞类。然而,迄今所描述的大多数寒武纪化石胚胎的亲缘关系,特别是谜一般的化石胚胎——Pseudooides的亲缘关系仍然不得而知。由于Markuelia孵化后的幼虫、未成年个体或成年个体的标本还未发现,其胚后发育序列则无从谈起,令人信服的原肠胚阶段的化石胚胎也未见报道。今后,可靠的原肠胚阶段的化石的发现很可能要借助于同步加速器辐射X射线层析显微技术。总之,寒武纪化石胚胎的研究还有很多悬而未决的问题。

Cambrian Fossil Embryos from Western Hunan,South China

The exquisitely preserved fossil embryos of Markuelia recovered from the limestones of the Middle Cambrian Haoqiao Formation and Upper Cambrian Bitiao formation in western Hunan, South China are described and illustrated in detail for the first time. A new species Markuelia elegans sp. nov. is established based mainly on embryos from the Upper Cambrian. A few of animal＇s resting eggs, which are comparable with those of the Ediacaran Doushantuo Formation, have been also found in the Upper Cambrian of western Hunan. The membrane of one egg from the uppermost Cambrian has been replaced by pyrite and the overgrowth of the pyrite crystals exhibits a unique inorganic pattern termed herein ＇Pseudoembryo＇. The taphonomic setting in deeper water with possible strong reducing conditions promoted the excellent preservation for the Markuelia specimens. The study of Markuelia provides not only constraint on the anatomy, affinity, embryonic development and phylogenetic significance of this wormlike animal and but also opens a new window onto the evolution and development of the earliest animals.

应用光学显微镜揭示寒武纪化石胚胎的表面结构

寒武纪化石胚胎是演化发育生物学研究的热点。传统的扫描电子显微镜可以观察到胚胎表面的三维结构,同步辐射X射线断层扫描显微镜等技术可以揭示胚胎的内部结构,但是胚胎表面由于矿物颜色差异表现出的结构在扫描电子显微镜和同步辐射成像下均无法观察,而光学显微镜恰恰能清晰地揭示这种结构。本研究应用光学显微镜和扫描电子显微镜对比观察了寒武纪化石胚胎,证明了光学显微镜在"光球"类胚胎观察方面有无可比拟的优势。从而揭示了在化石胚胎Olivooides multisulcatus的胚胎发育过程中,五辐射对称的口部要先于星状结构出现,证明辐射对称是Olivooides multisulcatus胚胎发育早期就已经建立的基本体构。