FOSSILIZATION OF NARAOIIDS FROM THE KAILI BIOTA (GUIZHOU PROVINCE, CHAINA) BASED ON ARTHROPOD TAPHONOMY INDEX (ATI) – A NEW MODEL FOR EXCEPTIONALLY PRESERVED ARTHROPODS

Naraoia is one of the most widespread and long ranging (middle Early Cambrian to late Middle Cambrian) nonmineralizing arthropods from konservat fossil-lagerstätten. New materials from the Kaili Biota offer new insights into each step of fossilization and preservation of this Cambrian nonmineralizing arthropod Naraoia. Necrotic processes, entombment orientations, and diagenetic modifications of Kaili naraoiid specimens (n=69) are described. The decay rates of appendages, internal soft-parts, and dorsal sclerites are different and are treated as separate taphonomic elements. A new taphonomic model, the Arthropod Taphonomy Index (ATI), is used to distinguish most corpses from molts and to identify necrotic processes on nonmineralizing arthropods in the pre-burial stage. These exceptionally preserved fossils were deposited where scavengers were rare to absent and where bioturbation was not intense (e.g., Ichnofabric Index=0 to 1). Understanding fossil-diagenesis is necessary for accurate identification of anatomy in exceptionally preserved organisms, including naraoiids. Kaili naraoiids are buried in three orientations with respect to bedding: 1) parallel, 2) oblique, and 3) folded-over (specimens with noticeably shortened anterior shields). Isolated dorsal shields are rare (n=3), and all of them are preserved parallel to bedding. Although the burial mechanism for the entire Kaili Biota is still uncertain, all folded-over naraoiids (n=7) are preserved with visible midguts and appear to represent a portion of the indigenous community that experienced transport over only a short distance. Identified taphonomic modifications caused by post-burial processes include squeezed-out gut contents, three-dimensionally preserved internal organs, and sclerite fractures.