三峡地区寒武系纽芬兰统岩家河组小壳化石研究进展

埃迪卡拉纪—寒武纪转换时期小壳化石的突然出现是世人瞩目的重要演化事件之一,它见证了动物的爆发式快速演化过程,标志着以两侧对称动物为特征的寒武纪生态系统的开始。湖北三峡地区寒武系纽芬兰统岩家河组处于幸运阶—第二阶这一关键层段,是研究上述问题的理想地区之一。岩家河组小壳化石研究最近取得的进展,为寒武系纽芬兰统幸运阶/第二阶界线的标定,和寒武纪早期软体动物和刺胞动物的演化及个体发育提供了重要的化石依据,本文对其进行了总结。全球界线层型剖面和点位(GSSP)寒武系第二阶底界划分标志的Watsonella crosbyi和Aldanella attleborensis在岩家河组第5段底部的同时出现,意味着岩家河组第5段已归属寒武系第二阶。这两类软体动物化石均广泛分布于华南、蒙古、西伯利亚和北美等大陆碳酸盐岩相寒武系第二阶地层中,它们均是定义寒武系第二阶底界的有力竞争者。另外,W.crosbyi壳顶下方一对肌肉附着结构的发现提供了重要的软组织结构信息,证实了W.crosbyi为内腹型壳,归属为未扭曲的软体动物太阳女神螺类。新发现六方锥石类Septuconularia yanjiaheensis具侧向压扁、两辐射对称的、14个等大锥面的围鞘,是迄今所描述的六方锥石类中锥面数最多的类型。根据横肋疏密程度,围鞘由下向上可分为三部分,可能代表胚胎期、幼年期和成年期不同的生长阶段。Septuconularia显然由寒武系幸运阶Hexaconularia通过扇面扩增演变而来,证明Hexaconularia可能是Arthrochites和Septuconularia之间的中间过渡类型。新发现的橄榄蛋类Octapyrgites elongatus与幸运阶Olivooides和Quadrapyrgites十分相似,围鞘由方形的壳顶区和波纹状倒宝塔形的远顶区组成。然而Octapyrgites只有8个口折叶,有别于具12个口折叶的Quadrapyrgites。通过与幸运期水母类多种对称方式对比,发现在寒武系第二阶橄榄蛋类数量减少和五辐射对称骨状壳类缺失,表明在幸运期/第二期转换时期,底栖固着型的刺胞动物水母类多样性发生了明显衰落,甚至部分绝灭,而同期的两侧对称动物则迅速辐射演化。

Recent research progress on small shelly fossils from the Cambrian (Terreneuvian) Yanjiahe Formation in the Three Gorges area

The sudden appearance of small shelly fossils near the Ediacaran-Cambrian boundary represents one of the most conspicuous biomineralization events. It witnessed the explosive radiation of metazoans and signals the formation of the Cambrian ecosystem reigned by animals, especially bilaterians. The Yanjiahe Formation in the Three Gorges area represents one of the most important Cambrian stratigraphic sequences bridging the chronostratigraphic units between the Fortunian and Cambrian Stage 2. Here we summarize the recent progresses on small shelly fossils, particularly mollusks and cnidarians, from the Yanjiahe Formation to provide an important paleontological basis for defining the base of Cambrian Stage 2 and for better understanding the evolution and ontogeny of molluscs and medusozoans in early Cambrian oceans. The co-occurrence of Watsonella crosbyi and Aldanella attleborensis, both as the candidate GSSP markers for defining the base of Cambrian Stage 2, is documented for the first time at the base of Member 5 of the Yanjiahe Formation and assigned the age of Member 5 (Age 2). Watsonella crosbyi and A.attleborensis are widely distributed among Cambrian palaeocontinents (e.g., South China, Mongolia, Siberia, Avalonia) and range from low to high paleolatitudes in carbonate facies. The stratigraphic range is restricted to Cambrian Stage 2, which further supports the FAD of either W.crosbyi or A.attleborensis as a candidate GSSP marker for defining the base of this unit. Moreover, a single pair of cardinal processes below the apex of W.crosbyi most likely provide muscle attachment sites. This trait provides desiderative new data on the shell musculature of W.crosbyi, which suggests that this species is an untorted helcionelloid mollusc with an endogastrically coiled shell. The new hexangulaconulariid Septuconularia yanjiaheensis has a laterally compressed and biradially symmetrical deriderm, exhibiting fourteen gently tapered faces, thus representing a most specialized hexangulaconulariid taxon. Longitudinally, the periderm consists of three regions that probably correspond, respectively, to an embryonic stage, a transient juvenile stage, and a long adult stage. Septuconularia yanjiaheensis may have been derived from six-faced Hexaconularia(Fortunian Stage), which is mostly like a morphologically intermediate taxon between Arthrochites and Septuconularia. The new olivooid Octapyrgites elongatus is similar to the genera Olivooides and Quadrapyrgites from the Cambrian Fortunian Stage; its periderm consists of a quadrate apical region and a strongly corrugated reversed pagoda-like adapical region bearing folded apertural lobes. However, O.elongatus exhibits eight V-shaped apertural lobes while Quadrapyrgites has twelve. In contrast to the radiation-induced abundant medusozoans with diverse symmetry patterns during the Fortunian Age, the paucity of olivooids and absence of pentaradial cnidarians and carinachitids in Cambrian Stage 2 indicate a marked decline in the disparity of cnidarians during the Fortunian-Age 2 transition, when, by contrast, bilaterians underwent rapid diversification.