Abstract
Extant panarthropods (euarthropods, onychophorans and tardigrades) are hallmarked by stunning morphological and taxonomic diversity, but their central nervous systems (CNS) are relatively conserved. The timing of divergences of the ground pattern CNS organization of the major panarthropod clades has been poorly constrained because of a scarcity of data from their early fossil record. Although the CNS has been documented in three-dimensional detail in insects from Cenozoic ambers, it is widely assumed that these tissues are too prone to decay to withstand other styles of fossilization or geologically older preservation. However, Cambrian Burgess Shale-type compressions have emerged as sources of fossilized brains and nerve cords. CNS in these Cambrian fossils are preserved as carbon films or as iron oxides/hydroxides after pyrite in association with carbon. Experiments with carcasses compacted in fine-grained sediment depict preservation of neural tissue for a more prolonged temporal window than anticipated by decay experiments in other media. CNS and compound eye characters in exceptionally preserved Cambrian fossils predict divergences of the mandibulate and chelicerate ground patterns by Cambrian Stage 3 (ca 518 Ma), a dating that is compatible with molecular estimates for these splits.
Highlights
Panarthropods provide an intriguing test case for exploring the efficacy of neural characters for inferring phylogeny because the group presents the challenges of unrivalled species diversity and a vast spectrum of phenotypic variation
The nervous system has until recently been all but undocumented in the compression fossils that dominate discussion of early panarthropods, named Burgess Shale-type preservation [10,11,12] after the iconic example of the Burgess Shale in Canada. This dearth of neuroanatomical data from Cambrian fossils has meant that neuroanatomy and palaeontology have largely operated exclusively of each other in considerations of panarthropod evolution
Panarthropoda encompasses Tardigrada, Onychophora and Euarthropoda; crown-group Euarthropoda is composed of Chelicerata and Mandibulata; that clade and those stem-group euarthropod fossils that possess a structurally differentiated deutocerebral appendage are assigned to Deuteropoda
Summary
Panarthropods provide an intriguing test case for exploring the efficacy of neural characters for inferring phylogeny because the group presents the challenges of unrivalled species diversity and a vast spectrum of phenotypic variation. The nervous system has until recently been all but undocumented in the compression fossils that dominate discussion of early panarthropods, named Burgess Shale-type preservation [10,11,12] after the iconic example of the Burgess Shale in Canada. This dearth of neuroanatomical data from Cambrian fossils has meant that neuroanatomy and palaeontology have largely operated exclusively of each other in considerations of panarthropod evolution. Panarthropoda encompasses Tardigrada, Onychophora and Euarthropoda; crown-group Euarthropoda is composed of Chelicerata and Mandibulata; that clade and those stem-group euarthropod fossils that possess a structurally differentiated deutocerebral appendage are assigned to Deuteropoda
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