EXCEPTIONALLY PRESERVED EXCEPTIONAL FOSSILS

Cambrian Burgess Shale-type Konservat-Lagerstätten with exceptionally well-preserved soft tissues have provided significant insights into the origin and early evolution of metazoans. These fossils are suggested to share a similar preservational pathway but some non-biomineralized parts of organisms (e.g. cuticles, appendages and the alimentary canal) are more commonly well-preserved than others, and the preservation of individual specimens can vary hugely between localities, or even within the same locality and taxon. In exceptional cases, some rare anatomical features (e.g. detailed anatomy of the visual and nervous systems) are also preserved, providing vital information for understanding these ancient animals and their phylogenetic context. Applying micro-computed tomography on Burgess Shale-type specimens, we reveal the most complete neuroanatomical profile known from a Cambrian animal from the head and trunk of a ‘great appendage’ arthropod, Alalcomenaeus sp. (Chengjiang Lagerstätte). MicroCT and elemental analyses reveal one optic neuropil separate from a protocerebrum contiguous with four head ganglia, succeeded by eight contiguous ganglia in an eleven-segment trunk. This arrangement corresponds most closely to the nervous system of Chelicerata, and direct evidence is provided for deutocerebral innervation of the great appendage. The fossil neuroanatomy and neural cladistics that includes characters visible in the fossil material together support the assignment of ‘great appendage’ arthropods to the chelicerate total group and corroborate segmental and structural homology between the great appendage and chelicera. Taphonomic experiments on annelids contradict the widespread assumption that neural tissue is especially prone to decay and unable to be retained during fossilization.