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 the morphological 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 Shale-type fossilization, without the need to invoke alternative taphonomic pathways or the presence of microbial biofilms.
Highlights
The Cambrian fossil record has produced fundamental insights into the morphology and initial diversification of animal phyla, with euarthropod evolution standing as a prime example of the impact of palaeontological data towards reconstructing the origin of major extant groups [1,2,3]
In addition to limbs [11,12], eyes [13,14], guts [15,16], muscles [17,18] and circulatory systems [19,20], recent studies have reported the preservation of neurological tissues including the condensed dorsal brain, optic neuropils and the ventral nerve cord (VNC) with segmental nerves [21,22,23,24,25,26,27,28,29,30]
The preservation potential of nervous tissues in Cambrian fossils has come under intense scrutiny, as actualistic taphonomic experiments demonstrate that the ecdysozoan nervous system is prone to rapid decay, relative to other tissues, under controlled laboratory (a) ms royalsocietypublishing.org/journal/rspb Proc
Summary
The Cambrian fossil record has produced fundamental insights into the morphology and initial diversification of animal phyla, with euarthropod evolution standing as a prime example of the impact of palaeontological data towards reconstructing the origin of major extant groups [1,2,3]. In addition to limbs [11,12], eyes [13,14], guts [15,16], muscles [17,18] and circulatory systems [19,20], recent studies have reported the preservation of neurological tissues including the condensed dorsal brain, optic neuropils and the ventral nerve cord (VNC) with segmental nerves [21,22,23,24,25,26,27,28,29,30] These discoveries directly impact hypotheses concerning the ancestral organization of the brain in extant euarthropods, and more broadly, the evolution of the head and nervous systems in Panarthropoda [31,32,33]. We describe the nervous system in the leanchoiliid Alalcomenaeus from two Cambrian sites of the western USA, namely, the Stage 4 Pioche Formation and the Drumian Marjum Formation, and explore the broader implications for the preservation of neurological tissues in Burgess Shale-type deposits
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