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 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.

Of Circuits and Brains: The Origin and Diversification of Neural Architectures

“…The most accepted time for the origin of the centralized nervous system is during the Ediacaran Period, when signs of burrowing substrates in the ocean appear, implying the directional movement and, thus, the control of body's maneuvrability. In fact, some clear signs of nervous system fossilization (not without associated polemics) have been revealed in exceptionally preserved biotas in Cambrian deposits (Strausfeld et al, 2016;Ortega-Hernández et al, 2019). In 2017, Budd and Jensen described the temporal and ecological context in which the early bilaterians arose, probably slightly later than 560 million years ago, at the Ediacaran-Cambrian boundary (Budd and Jensen, 2017).…”

The origin and evolution of the euarthropod labrum

“…The neuroanatomy of the megacheiran Alalcomenaeus sp. is presented by Tanaka et al (2013) and Ortega-Hernández et al (2019). Although there must be some uncertainty in its interpretation, the reconstruction of Tanaka et al (2013) shows a protocerebral mass innervating the eyes, from which paired connectives lead to the posterior; then a large ganglion that corresponds in position to the insertion of the great appendage; and then smaller ganglia posterior to this that they interpret as being ventral neuromeres that innervate the posterior cephalic appendages.…”

“…The potential innervation of the labrum/LA is not considered in this paper, nor is there any preserved evidence of the STNS. In Ortega-Hernández et al (2019), another view of the CNS of Alalcomenaeus is presented, from specimens from the Pioche and Marjum formations. In the Marjum material, a general overview of the CNS is given.…”

Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan

“…Neural tissues in the Chengjiang Biota are well known among arthropods [ 30 – 34 ], and have also been reported in priapulans [ 35 ]. The veracity of these interpretations has recently been supported by similar reports of temporally contemporaneous neural preservation in North American deposits [ 36 ].…”