Neuroanatomy in a middle Cambrian mollisoniid and the ancestral nervous system organization of chelicerates

Abstract

Recent years have witnessed a steady increase in reports of fossilized nervous tissues among Cambrian total-group euarthropods, which allow reconstructing the early evolutionary history of these animals. Here, we describe the central nervous system of the stem-group chelicerate Mollisonia symmetrica from the mid-Cambrian Burgess Shale. The fossilized neurological anatomy of M. symmetrica includes optic nerves connected to a pair of lateral eyes, a putative condensed cephalic synganglion, and a metameric ventral nerve cord. Each trunk tergite is associated with a condensed ganglion bearing lateral segmental nerves, and linked by longitudinal connectives. The nervous system is preserved as reflective carbonaceous films underneath the phosphatized digestive tract. Our results suggest that M. symmetrica illustrates the ancestral organization of stem-group Chelicerata before the evolution of the derived neuroanatomical characters observed in Cambrian megacheirans and extant representatives. Our findings reveal a conflict between the phylogenetic signals provided by neuroanatomical and appendicular data, which we interpret as evidence of mosaic evolution in the chelicerate stem-lineage.

Cambrian origin of the arachnid brain reveals early divergence of Chelicerata

“…Our findings differ from prior descriptions that assume a linear mollisoniid brain (19,21) and propose seven nerve roots extending from a cerebral 'syncerebrum' of Mollisonia (19).…”

“…The lateral retinal neuropil extends a tract decorated by possibly three small neuropils (Fig. 2C), as previously suggested (19), to the second of the three cerebral domains (Fig. 2F), which we designate the ce2 domain.…”

Interpreting fossilized nervous tissues

“…Two Mollisonia specimens were later reexamined in that context, with description of nervous tissues. [ 7 ] The interpretations of Mollisonia MCZ 1811 and USNM 305093 [ 7 ] exhibit issues similar to leanchoiliid examples discussed before. USNM 305093 is a poorly preserved specimen with generic patterns of preservation of internal tissues.…”

“…When a more rigorous approach to tissue interpretation is applied and various extrapolations are subtracted, paleoneurological evidence provides a far less complete scenario than depicted by a number of recent studies, especially regarding the arachnomorph CNS. [ 3,6,7 ] We present here ideal situations where cephalic lacunae were not misinterpreted as nervous tissues, but this possibility should be considered as well and investigated further.…”

“…In the past decade, there has been a surge of studies presenting evidence of fossilized nervous tissues, mainly in fossil arthropods from Cambrian Konservat‐Lagerstätten, [ 1–10 ] but also, more rarely, from more recent deposits. [ 11 ] The question of whether such labile tissues—that is, prone to rapid decay—as ganglia and nerves can be preserved in settings of BST deposits has been discussed at length before.…”