Taphonomy of Naraoiids (Arthropoda) from the Middle Cambrian Kaili Biota, Guizhou Province, South China

Naraoiids in the Kaili biota (Taijiangian Stage, Wulingian Series, lower Middle Cambrian) are preserved in a variety of taphonomic states. Decomposition, entombment, and fossil diagenesis of Kaili naraoiid specimens (n = 69) are described. The decay rates of appendages, internal organs, and dorsal sclerites are different, and should be treated as separate taphonomic elements. A classification scheme to delineate the preservational states is proposed here to distinguish carcasses from molts in deposits with exceptional preservation. For example, at least 35% of Kaili naraoiids represent carcasses. Among them, ∼21% of individuals are interpreted as having been buried alive (Type A preservational state) and 79% showed some evidence of decay prior to burial (Type B preservational state). 65% of naraoiids can be either molts or decayed carcasses, and molts typically are articulated (Type C preservational state). Disarticulated (Type D preservational state) specimens are rare. Kaili naraoiids were buried in three orientations with respect to bedding: (1) parallel, (2) oblique, and (3) folded-over (specimens with noticeably shortened anterior shields). Due to the nature of the naraoiid bauplan, most specimens (∼77%) tend to be buried parallel to bedding. All naraoiid specimens with folded-over anterior shields (10%) represent carcasses. Identified taphonomic modifications caused by post-burial processes include extruded gut contents, three-dimensionally preserved internal organs, and sclerite fractures. Preservation of three-dimensional soft-parts accompanied by flattened hard-parts is interpreted here as the result of syndiagenetic anaerobic microbial decay. The relief and size of preserved midguts depend on: (1) availability of volatile tissues, (2) abundance and types of microbes, and (3) duration of anaerobic microbial decay prior to compactional dewatering.