Heads, Hox and the phylogenetic position of trilobites

The Arachnomorpha or Arachnata concepts have resolved Trilobita as most closely related to Chelicerata amongst extant Arthropoda. An alternative position of trilobites in the stem lineage of Mandibulata is suggested by their pattern of head tagmosis. The antennae of trilobites and Mandibulata are considered non-homologous with the antennae of Onychophora and stem lineage Euarthropoda: they represent ‘secondary’ and ‘primary antennae’, respectively. In extant taxa, ‘secondary antennae’ are deutocerebral, post-ocular, and are connected to deutocerebral olfactory neuropils, whereas ‘primary antennae’ are pre-ocular and connected to protocerebral olfactory neuropils. In fossils, an insertion at the anterolateral margin of the hypostome rather than more anteriorly on the head allows ‘secondary antennae’ to be identified. A deutocerebral mouthpart, of which the onychophoran jaw and the chelicera are examples, is regarded as plesiomorphic for Arthropoda. A loss of ‘primary antennae’ and modification of the deutocerebral mouthpart into a sensory antenna defines the Mandibulata. Trilobites share a ‘secondary antenna’ and a clearly-delimited head tagma with mandibulates. Given the extensive homoplasy forced by the Arachnata concept (reversals in pycnogonids and arachnids), a trilobite/mandibulate alliance may be better supported. Dedicated to Fred Schram on the occasion of his retirement. Our article challenges canonical views about arthropods, and we were forced to question ideas that we have long considered the best explanation of facts. In doing so, we venture into a territory from which Fred Schram has never shied away. Fred’s synthesis of data from living and fossil arthropods, his efforts to integrate classical morphological and evo-devo perspectives, and his willingness to explore dangerous ideas have inspired our reappraisal of the trilobite problem.