Cambrian Artiopoda Reveals a Constraint in Euarthropod Brain Evolution

Abstract

Cambrian stem mandibulates and chelicerates have provided robust evidence of brains that conform to a cerebral ground pattern also defining extant euarthopods. But information about the brains of trilobites or their immediate relatives remains a mystery. Here we describe the detailed organization of the specialized brain of Xandarella spectaculum, a member of the subphylum Trilobitomorpha which went extinct at the end Permian. While retaining the pre-Cambrian cephalic ground pattern as three seriate domains, empirical evidence indicates clade-specific accentuation of some and loss of other neural arrangements typifying extant mandibulates. The trilobitomorph deutocerebral organization is dominated by exaggerated glomerular centers whereas its prominent lateral eyes supply minute visual centers. Paired ocelli are associated with the prominent prosocerebral bridge of the forebrain, an essential center for path integration. Together these modifications indicate neural organization typifying some extant benthic crustaceans and even hexapods that engage in foraging behaviors. With reference to existing neurological attributes of living pancrustaceans major divergences of the trilobitomorph brain from the ancestral cerebral ground pattern may have weakened its resilience to geo-ecological disruptions and hence the final extinction of the clade to which trilobitomorphs belong.