近日，西北大学早期生命研究所研究人员与国际团队合作，发现了当前已知最古老的立方水母化石记录。相关成果日前发表于美国《科学公共图书馆·综合》（PLoS One）。
我国陕南寒武纪初期的宽川铺组以盛产小壳化石和微体动物胚胎化石而闻名于世。其中，五辐对称的动物胚胎化石比较常见，但其生物学属性长期以来一直是个谜。
上述研究团队发现，这些五辐对称的化石胚胎其实属于立方水母，是当前已知最古老的立方水母化石记录，距今大约5.35亿年，较先前从北美犹他州中寒武世地层中发现的四辐对称的立方水母化石至少早2000万年，比著名的澄江动物群刺胞动物还早1000万年以上。
据了解，这些微体胚胎化石是科研人员通过冰醋酸浸解技术获得的。此后，研究人员应用扫描电镜观察了其外貌，再借助微CT解析了其内部结构。研究结果显示，这些化石保存了非常精美、细致的原生内部结构，极少受到微生物降解的破坏。
论文作者之一韩健表示，这些立方水母胚胎化石虽然直径仅有0.6毫米左右，但不仅保存了细小触手、垂管、外伞等外部形态，还保存了呈五辐射对称排列的隔膜、生殖腺等微观内部结构，其中后三者为现代立方水母所特有。（生物谷Bioon.com）
生物谷推荐的英文摘要
PLoS One doi:10.1371/journal.pone.0070741
Early Cambrian Pentamerous Cubozoan Embryos from South China
Jian Han mail， Shin Kubota， Guoxiang Li， Xiaoyong Yao， Xiaoguang Yang， Degan Shu， Yong Li， Shunichi Kinoshita， Osamu Sasaki， Tsuyoshi Komiya， Gang Yan
Background
Extant cubozoans are voracious predators characterized by their square shape， four evenly spaced outstretched tentacles and well-developed eyes. A few cubozoan fossils are known from the Middle Cambrian Marjum Formation of Utah and the well-known Carboniferous Mazon Creek Formation of Illinois. Undisputed cubozoan fossils were previously unknown from the early Cambrian； by that time probably all representatives of the living marine phyla， especially those of basal animals， should have evolved.
Methods
Microscopic fossils were recovered from a phosphatic limestone in the Lower Cambrian Kuanchuanpu Formation of South China using traditional acetic-acid maceration. Seven of the pre-hatched pentamerous cubozoan embryos， each of which bears five pairs of subumbrellar tentacle buds， were analyzed in detail through computed microtomography (Micro-CT) and scanning electron microscopy (SEM) without coating.
Results
The figured microscopic fossils are unequivocal pre-hatching embryos based on their spherical fertilization envelope and the enclosed soft-tissue that has preserved key anatomical features arranged in perfect pentaradial symmetry， allowing detailed comparison with modern cnidarians， especially medusozoans. A combination of features， such as the claustrum， gonad-lamella， suspensorium and velarium suspended by the frenula， occur exclusively in the gastrovascular system of extant cubozoans， indicating a cubozoan affinity for these fossils. Additionally， the interior anatomy of these embryonic cubozoan fossils unprecedentedly exhibits the development of many new septum-derived lamellae and well-partitioned gastric pockets unknown in living cubozoans， implying that ancestral cubozoans had already evolved highly specialized structures displaying unexpected complexity at the dawn of the Cambrian. The well-developed endodermic lamellae and gastric pockets developed in the late embryonic stages of these cubozoan fossils are comparable with extant pelagic juvenile cubomedusae rather than sessile cubopolyps， whcih indicates a direct development in these fossil taxa， lacking characteristic stages of a typical cnidarian metagenesis such as planktonic planula and sessile polyps.
