Fuxianhuiid ventral nerve cord and early nervous system evolution in Panarthropoda

Abstract

Panarthropods are typified by disparate grades of neurological organization reflecting a complex evolutionary history. The fossil record offers a unique opportunity to reconstruct early character evolution of the nervous system via exceptional preservation in extinct representatives. Here we describe the neurological architecture of the ventral nerve cord (VNC) in the upper-stem group euarthropod Chengjiangocaris kunmingensis from the early Cambrian Xiaoshiba Lagerstätte (South China). The VNC of C. kunmingensis comprises a homonymous series of condensed ganglia that extend throughout the body, each associated with a pair of biramous limbs. Submillimetric preservation reveals numerous segmental and intersegmental nerve roots emerging from both sides of the VNC, which correspond topologically to the peripheral nerves of extant Priapulida and Onychophora. The fuxianhuiid VNC indicates that ancestral neurological features of Ecdysozoa persisted into derived members of stem-group Euarthropoda but were later lost in crown-group representatives. These findings illuminate the VNC ground pattern in Panarthropoda and suggest the independent secondary loss of cycloneuralian-like neurological characters in Tardigrada and Euarthropoda.stem-group Euarthropoda | Onychophora | phylogeny | Cambrian Explosion | Xiaoshiba Lagerstätte T he nervous system represents a critical source of phylogenetic information and has been used extensively for exploring the evolutionary relationships of extant Panarthropoda (i.e., Onychophora, Tardigrada, Euarthropoda) (1-7). Identification of fossilized nervous tissues has provided a unique perspective on early euarthropod brain neuroanatomy and suggests that broad patterns of extant neurological diversity were already in place by the Cambrian (8-11). The ventral nerve cord (VNC) reflects fundamental aspects of panarthropod body organization that complement the organization of the brain and together illuminate the evolution of the CNS (1-3, 5, 7, 12-16). The early evolutionary history of the panarthropod postcephalic CNS, however, remains obscure due to the exclusive preservation of brains in most available fossils (8,10,11). Moreover, the unresolved phylogenetic relationships within Panarthropoda complicate accurate reconstruction of the CNS ground pattern (16)(17)(18)(19)(20)(21)(22). In this study, we demonstrate the exceptional preservation of postcephalic neurological features in the early Cambrian fuxianhuiid Chengjiangocaris kunmingensis, an upper stem-group euarthropod (17) from the Xiaoshiba Lagerstätte, South China (23). These fossils clarify the neurological organization of the VNC in early euarthropod ancestors, thereby polarizing the evolution of the panarthropod CNS.

Preservation and phylogeny of Cambrian ecdysozoans tested by experimental decay of Priapulus

“…This includes the recent high-profile interpretations of neural and vascular tissues in Burgess Shale-type preserved ecdysozoans20212223242526. An inconsistency is identified between those anatomical interpretations and current understanding of preservation.…”

Soft‐Bodied Fossils Are Not Simply Rotten Carcasses – Toward a Holistic Understanding of Exceptional Fossil Preservation

“…Yang et al [102] identified well organized segmental ganglia in a total group euarthropod from the Chengjiang biota. Sansom [18] argued that this interpretation was implausible based on the rapid loss of nervous system morphology in his decay experiments on priapulids.…”

“…Sansom [18] argued that this interpretation was implausible based on the rapid loss of nervous system morphology in his decay experiments on priapulids. However, it is difficult to conceive how shrinkage of other anatomical features could generate the wellorganized features [103][104][105] and serially repeated structures [102] interpreted as fossil nervous systems. Shrinking a cuticle would not be expected to generate a rope-ladder morphology that was the primary basis for identification as a nerve cord.…”

Exceptionally Preserved Fossil Assemblages Through Geologic Time and Space

“…1) -represent a critical resource for studying the ancient biosphere. Fossils of this type include some of the earliest organisms on Earth (Knoll, 1985) as well as remains of integuments (Manning et al, 2009;Navalón et al, 2015), exoskeletons (McNamara et al, 2012a), feathers (Colleary et al, 2015), eyes (Lee et al, 2011), muscles (Martill, 1990), "jellies" (Chen et al, 2007;Stanley and Sturmer, 1987), internal organs (Zhang et al, 2015), nerves (Yang et al, 2016), cells (Xiao et al, 1998), and sub-cellular structures , which offer many opportunities for work on anatomy, physiology, and systematics. Furthermore, geologic deposits containing exceptionally-preserved fossils-such as certain Konservat-Lagerstätten (Seilacher, 1970)-offer relatively complete snapshots for exploring the ecology and dynamics of ancient ecosystems, as they generally contain remains of both biomineralizing and non-mineralizing taxa (Conway Morris, 1986).…”