Brain structure resolves the segmental affinity of anomalocaridid appendages.

Area of Science:

• Paleontology
• Evolutionary Biology
• Neuroscience

Background:

• Anomalocaridids, prominent Cambrian fossils, have uncertain evolutionary relationships within moulting animals (Ecdysozoa).
• Previous studies debated their placement as stem-group euarthropods, crown-group euarthropods, or a separate lineage due to unclear frontal appendage homologies.

Purpose of the Study:

• To investigate the affinities of anomalocaridids (Radiodonta) by examining the neuroanatomy of a new species, Lyrarapax unguispinus.
• To clarify the segmental organization of the anomalocaridid head and the origin of their frontal appendages.

Main Methods:

• Description of Lyrarapax unguispinus from the early Cambrian Chengjiang biota, preserving soft tissues.
• Analysis of carbon-rich areas in fossilized brain tissue to infer neural organization and appendage connections.

Main Results:

• Lyrarapax unguispinus exhibits a dorsal, segmented brain with paired pre-protocerebral ganglia linked to frontal appendages.
• Neuroanatomical evidence supports an alliance between radiodonts and arthropods, distinct from cycloneuralians.
• Brain organization parallels that of Onychophora, suggesting pre-ocular frontal appendages are ancestral traits for euarthropods and onychophorans.

Conclusions:

• Radiodonta are positioned on the euarthropod stem-lineage.
• The paired frontal appendages of anomalocaridids are homologous to the labrum of crown-group euarthropods.
• This study provides direct evidence for the segmental composition of the anomalocaridid head and its implications for arthropod evolution.