中三叠世安尼期罗平生物群海生爬行类研究新进展

摘要

2007年,中国地质调查局成都地质调查中心云南1∶5万区域地质调查项目组在距罗平县城东南15 km的罗雄镇大洼子村附近中三叠统关岭组二段地层中发现了罗平生物群。罗平生物群目前已经发现有海生爬行类、鱼类、节肢动物、棘皮动物、菊石、双壳、腹足类、腕足类、植物等十多个大类的化石,完好地体现了当时海洋生物的多样性,是目前已知化石分异度最高的三叠纪海生化石库之一。根据含化石地层中发现的微体化石牙形石Nicoraella Kockeli带,确认罗平生物群时代为中三叠世安尼期Pelsonian亚期。罗平生物群目前发现的海生爬行类种类丰富,基本涵盖了三叠纪时期主要的海生爬行类别,包括鱼龙类、鳍龙类、原龙类以及初龙类。中生代是对海生爬行类至关重要的时期。由于一直没有发现它们留下的遗迹化石,它们的运动模式还一直存在争议。云南罗平生物群海生爬行类研究中发现数百枚保存完好的幻龙类足迹——罗平双桨迹Dikoposichnus luopingensis,为研究其运动模式提供了直接的证据。另外罗平生物群中巨型捕食者张氏幻龙Nothosaurus zhangi的发现也支持了罗平双桨迹造迹者的存在。张氏幻龙拥有三叠纪世界上最大的头骨,这些处于食物链顶层的海生爬行类的繁盛说明当时生态系统已经很完善,代表了二叠纪末生物大灭绝之后海洋生态系统的全面复苏。%The Luoping Biota was discovered in 2007 by the Chengdu Center of China Geological Survey during 1:50000 regional mapping works at Dawazi Village, Luoxiong Town, 15 km southeast of Luoping City, Yunnan Province. The interval containing the Luoping biota is the middle to upper part of Member II of the Guanling Formation. The fossil assemblage of the Luoping biota is a mixture of marine animals, terrestrial plants and a few terrestrial animals. To date, more than ten fossil groups have been recovered, which include marine reptiles, fishes, arthropods, echinoderms, ammonites, bivalves, gastropods, lingulid brachiopods, foraminifers, and plants. The Luoping Biota is one of the most diverse Triassic marine fossil Lagerstätten records in the world. The age of the Luoping biota is assigned to the Pelsonian Substage of the Middle Triassic Anisian Stage based on the index conodontNicoraella Kockeli. Well-preserved, diverse marine reptiles are one of the highlights of the Luoping biota, including ichthyosaurs, sauropterygians, protorosaurs and archosauromorphs. Mesozoic is remarkable for marine reptiles, whose swimming modes, however, remain a topic of much controversy because of the absence of their tracks. Hundreds of seabed tracks found in Luoping Biota, which were assigned toDikoposichnus luopingensis, provide new materials for the study of their locomotive mode. Besides, the macropredator Nothosaurus zhangireported from Luoping Biota also supports the existence of the makers ofDikoposichnus luopingensis. The skull ofNothosaurus zhangi is the largest sauropterygian ever reported in Triassic. The occurrence of a different assemblage of marine reptiles as top predators in the food web indicates a well developed marine ecosystem, showing full rebuilding of the marine ecosystem after the mass extinction that happened about 250 million years ago.