早寒武世马哈螺类的骨骼化特征、分类和演化意义

马哈螺类（maikhanellids)是已知最古老的单板类，以往缺乏系统研究，并在分类上争议较大，文中以滇东早寒武世梅树村阶微小骨骼化石的丰富材料为依据，详细阐述该类群的形态特征和超微结构，探讨其亲缘关系和演化趋势，研究表明，马哈螺类由6个属组成，包括Maikhanella,Ramenta,Ramentoides gen.nov.,Mediata gen.nov.,Yunnanopleure和Purella。各属之间壳形变化较大，但裸露的壳顶和普遍发育的鳞片状或条状突起表明它们在明显的亲缘关系，并显示两个演化方向，即Maikhanella Ramentoides gen.nov.Ramenta和Maikhanella Mediata gen.nov.Yunanopleura-Purella。马哈螺类分布于梅树村期早，中期，Maikhanella是最原始的马哈螺，最先出现在梅树村阶小歪头山段，也是中谊村段下磷矿层唯一的马哈螺，而在该段上磷矿层马哈螺类发生了明显的属种分异，在大少段所有的马哈螺类突然消失，马哈螺类的壳体曾被描述为：骨针状的外骨骼”，“骨片状外骼”和“包裹在中间基质中的骨针”，但当前研究表明，以Maikhanella pristinis和Ramenta cambrina为代表的马哈螺类的内壳层具有早期单板类中较常见的水平交错纤状结构，指示了马哈螺类具有致密成层的壳壁，文中新建2亚科，2属和5种，即Maikhanellinae subfamily nov.,Purellinae subfamily nov.,Maikhanella Perelegans sp.nov.,Ramenta superata sp.nov.,Ramentoides latispinus gen.et sp.nov.,Mediata kunyangensis gen.et sp.nov.,Yunnanopleura longidens sp.nov。

SKELETALIZATION CHARACTERS, CLASSIFICATION AND EVOLUTIONARY SIGNIFICANCE OF EARLY CAMBRIAN MONOPLACOPHORAN MAIKHANELLIDS

Maikhanellids represent a group of the oldest monoplacophorans. Their classification and evolutionary significance have long been in dispute because of the lack of systematic research. The present study elaborates the morphological features and ultrastructures of maikhanellid shells and, on this basis, deals with their classification, phylogeny and evolutionary trend, with a rich and well-preserved collection from the Early Cambrian Meishucunian Stage in eastern Yunnan of China.This paper shows that the family Maikhanellidae consists of two subfamilies and all together six genera, including Maikhanella, Ramenta, Ramentoides gen. nov. and Mediata gen. nov. under the Maikhanellinae subfamily nov. and Yunnanopleura and Purella under the Purellinae subfamily nov. Although these genera are obviously different in shape and some other aspects of morphology, they are all characterized by possessing a distinct bare apex, with the rest of outer surface ornamented by closely distributed projective granular structures. Their phylogenic relationships and evolutionary trends are recognized as developing in two directions, one from Maikhanella, Ramentoides gen. nov. to Ramenta, and the other from Maikhanella, Mediata gen. nov., Yunnanopleura to Purella.Maikhanellids occurred mainly in the early and middle Meishucunian Stage. The most primitive form is Maikhanella, which first appeared in the Xiaowaitoushan Member of the early Meishucunian Stage and then developed as the only kind of maikhanellids in the lower part of Zhongyicun Member. Abundant and diverse maikhanellids appeared in the upper part of the Zhongyicun Member of the middle Meishucunian Stage, but all six genera disappeared suddenly in the succeeding Dahai Member without drastic changes in sedimentation.Maikhanellid shells were previously described as consisting of “spicule exoskeleton”， “sclerite exoskeleton” or “spicules embedded in an intermediate matrix” by different authors. The present research, however, indicates that the wall of maikhanellid shells, represented by Maikhanella pristinis and Ramenta cambrina, is layered and that the inner layer of shell is composed of horizontally crossed fibrous structures, a feature that is common in the Monoplacophora. Described in this paper are two new subfamilies, two new genera and five new species, viz., Maikhanellinae subfamily nov., Purellinae subfamily nov., Maikhanella perelegans sp. nov., Ramenta superata sp. nov., Ramentoides latispinus gen. et sp. nov. and Mediata kunyangensis gen. et sp. nov., Yunnanopleura longidens sp.nov.