常山美丽饰边虫Euloma changshanense Lu(三叶虫)的蜕壳及其古环境意义

Techniques of exuviations in Euloma changshanense Lu (Trilobita) and its palaeoenvironmental significance

Euloma changshanense Lu采自浙江江山下奥陶统,是一个蜕壳老壳的部分及自由颊腹边缘的印痕。从这后期破损的残壳分析其蜕壳过程,认为两自由颊在头前中间连接,蜕壳时在虫体拱曲时脱落于头前,但还在新壳之上,后被新的虫体顶走,只残存自由颊腹边缘在头前的印痕。这反映了三叶虫在蜕壳过程中有一停顿休息时间,而新的虫体常常顶走或拖走老的头壳上的自由颊和唇瓣,使老壳留下缺失的背壳部件。蜕壳过程是瞬间发生的,即只有原地埋葬才能保存这一状态,说明当时三叶虫处于比较宁静的水中,又处于沉积物供给充足的环境,在蜕壳过程中或不久即被沉积物掩埋。浙江江山印渚埠组的厚度较大,上面直接覆盖着大湾阶底部的Azygograptus suecicus带;而相邻不远的江西玉山印渚埠组顶部,上覆为玉山阶的底部Tetragraptus approxi- matus带,证明印渚埠组是穿时的。而两个地方的印渚埠组顶部都常产有Euloma changshanense Lu,说明此三叶虫虽为下奥陶统常见化石,但延续时间较长,在特马豆克晚期到整个玉山阶都存在,也说明印渚埠组沉积厚度较大,但整个沉积环境变化不大,即使三叶虫Euloma changshanense等种属也很少变化。这是研究三叶虫蜕壳及其生态替换、演变的好层位。

The Euloma changshanense Lu, which is a trilobita collected from the Lower Ordoviclan in Jiangshan, Zhejiang, is a part of old shield of exuviations. It is an imprint of librigenal doublure.This paper analyzes its process of exuviations through observing the deuterogenous remnants shield and considers that two librigenal cheeks link up in the middle of foreside of cephala. During the exuviations, the body of the trilobite climbed up, separated itself from the foreside of cephala, but the librigenal cheeks were still on the new shield. Then, they were propped up by the new trilobite, the imprint of librigenal doublure in the foreside of cephala was preserved only. It reflects that there was a period of break when the trilobites were exuviating. During the exuviations, there was a timeout. Furthermore the new trilobites usually carried or hauled the librigenal cheeks and hypostoma on the old cephala, which made the old shield leave hehind the carapace. The status of exuviations hap pened instantly,and it belonged to autochthonous remains, indicating that trilobite was in the status of sedentary water with abundant sediment supplying. The sediments would bury the trilobite during the exuviations or exuviations shortly. The Yinzhufu Formation at jiangshan is fairly thick. It is overlain by the Azygoraptus suecicus Zone which is at the lower part of the Dawanian Stage. In contrast, at Yushan （in Jiangxi Province）, not far away form jiangshan, the Yinzhufu Formation is overlain by the Tetragraptus approximates Zone which is at the lower part of the Yushanian Stage. This indicates that the Yinzhufu Formation is diachronous. However the Euloma changshanensis Lu is always found at the uppermost part of the Yinzhufu Formation on both places, indicating that these trilobites belong to the Lower Ordovician, but they thrived over a long period of time from the Late Tremadoc to the end of Yushanian Stage. It also indicates that though the Yinzhutu Formation is rehmtively thick, the sedimentary environment did nor change much and, even if the trilobites, such as Euloma changshunense, changed little. Such a formation is a good strtigraphical interval to study the exuviations of trilobite and the evolutionary progress of the ecological replacement.