扬子板块西北缘下泥盆统平驿铺组双壳类遗迹化石

四川北部广元地区下泥盆统平驿铺组产出丰富的双壳类遗迹化石，但尚未开展过系统的遗迹学工作，也未进行过行为学与古生态学分析。文中针对四川广元马家剖面平驿铺组中部地层开展沉积学和系统遗迹学研究，在陆棚至近滨带下部沉积中共识别出3个属种的双壳类遗迹化石，包括Lockeia siliquaria,Protovirgularia rugosa,Ptychoplasma vagans,以及其他无脊椎动物门类遗迹化石Cruziana problematica,Dimorphichnus isp.,Lophoctenium isp.等。对双壳类遗迹化石的出现方式、形态以及保存特征的分析表明，平驿铺组中部曾出现过2种类型的双壳类，分属具分叉足(原鳃目)和楔状足的类群。原鳃目分布于过渡带和陆棚区，营沉积物摄食生活；受高能事件或沉积物掩埋影响，它们由停息状态变为“逃逸”状态的行为产生了Pro.rugosa。具楔状足的双壳类依生态类型可分为2类：在过渡带与陆棚区，漫游的、营沉积物摄食的类型产生了Pty.vagans;而在近滨下部，滤食性双壳类居群则在不同期次高能事件的间隙对沉积物进行大规模殖居，产生了L.sil...

Bivalvian trace fossils from the Lower Devonian Pingyipu Formation in northwestern Yangtze Platform

Abundant bivalve trace fossils are found in the Lower Devonian Pingyipu Formation in Guangyuan area,northern Sichuan Province,but systematic ichnological work and behavioral and paleoecological analysis have not been carried out. Close observations on the occurrence,preservation style and morphological features of bivalvian traces provide invaluable information to guide the interpretation of their trace maker,ethological and paleoecological features. Here we carried out sedimentary and systematic ichnology studies on the middle part of the Lower Devonian Pingyipu Formation at the Majia section,Guangyuan area. The results suggest occurrence of bivalvian trace fossils,including Lockeia siliquaria,Protovirgularia rugosa, Ptychoplasma vagans as well as trace fossils of other invertebrate groups such as Cruziana problematica, Dimorphichnus isp.,and Lophoctenium isp. in the lower shoreface to off shore settings. The overall morphological and taphonomic features of the trace fossils suggest that,two major groups,including the cleft-footed(protobranchia)and wedge-footed bivalves occurred in the middle part of the Pingyipu Formation. The protobranchia were deposit-feeding bivalves distributed in the transitional zone and off shore settings. The transition from resting to “escape”state of these bivalves,probably due to erosive events or a higher sedimentation rates,produced the trace fossil Pro. rugosa. The wedge-footed bivalves could be further divided into two groups. The vagile,deposit-feeding type occupied the transitional zone and off shore settings. Their crawling behavior,probably linked to an effort to search for edible material from the sediment generated the trace fossil Pty. vagans. While in the lower shoreface setting,colonies of suspension-feeding bivalves colonized the sediment during relatively tranquil intervals between erosional events. The vertical transition of trace fossil composition in the middle Pingyipu Formation suggests a substitution of Cruziana ichnofacies developed on a relative stiff substrate by the latter Skolithos ichnofacies in an upward shallowing succession.