Meeting Abstract
The panarthropod head represents a complex body region that has evolved through the integration and functional specialization of the anterior appendage-bearing segments. Advances in the developmental biology of diverse extant organisms have led to a substantial clarity regarding the relationships of segmental homology between Onychophora, Tardigrada, and Euarthropoda. The improved understanding of the segmental organization in panarthropods offers a novel perspective for interpreting the Cambrian fossil record of these successful animals. A combined palaeobiological and developmental approach to the study of the panarthropod head through deep time leads to propose a consensus hypothesis for the intricate evolutionary history of this tagma. The contribution of exceptionally preserved brains in Cambrian fossils – together with the recognition of segmentally informative morphological landmarks – illuminate the character polarity for major anatomical features. The euarthropod stem-lineage provides a detailed view of the step-wise acquisition of critical characters leading to the formation of a multiappendicular head formed by the fusion of several segments, such as the transformation of the ancestral protocerebral limb pair into the labrum, following the postero-ventral migration of the mouth opening. Stem-group onychophorans demonstrate an independent ventral migration of the mouth, as well as the differentiation of the deutocerebral limbs as expressed in extant representatives. The anterior organization of crown-group Tardigrada retains several ancestral features, such as an anterior-facing mouth and one-segmented head. A proposed model aims to clarify contentiuous issues on the evolution of the panarthropod head, and lays the foundation from which to further address this complex subject in the future.
