Proclivity of nervous system preservation in Cambrian Burgess Shale-type deposits

Abstract

Recent investigations on neurological tissues preserved in Cambrian fossils have clarified the phylogenetic affinities and head segmentation in pivotal members of stem-group Euarthropoda. However, palaeoneuroanatomical features are often incomplete or described from single exceptional specimens, raising concerns about themorphological interpretation of fossilized neurological structures and their significance for early euarthropod evolution. Here, we describe the central nervous system (CNS) of the short great-appendage euarthropod Alalcomenaeus based on material from two Cambrian Burgess Shale-type deposits of the American Great Basin, the Pioche Formation (Stage 4) and the Marjum Formation (Drumian). The specimens reveal complementary ventral and lateral views of the CNS, preserved as a dark carbonaceous compression throughout the body. The head features a dorsal brain connected to four stalked ventral eyes, and four pairs of segmental nerves. The first to seventh trunk tergites overlie a ventral nerve cord with seven ganglia, each associated with paired sets of segmental nerve bundles. Posteriorly, the nerve cord features elongate thread-like connectives. The Great Basin fossils strengthen the original description-and broader evolutionary implications- of the CNS in Alalcomenaeus from the early Cambrian (Stage 3) Chengjiang deposit of South China. The spatio-temporal recurrence of fossilized neural tissues in Cambrian Konservat-Lagerstätten across North America (Pioche, Burgess Shale, Marjum) and South China (Chengjiang, Xiaoshiba) indicates that their preservation is consistent with the mechanism of Burgess Shaletype fossilization,without the need to invoke alternative taphonomic pathways or the presence of microbial biofilms.