Evolution of cephalic feeding structures and the phylogeny of Arthropoda

Abstract

Focusing on structural and functional changes during the evolution of Arthropoda, we based our approach on evidence provided by two major Cambrian lagerstätten yielding exceptional preservation: the Lower Cambrian Chengjiang lagerstätte, Yunnan Province, China, and the Lower to Upper Cambrian ‘Orsten’-type lagerstätten with their 3D-preserved fossils. We established a model of major steps in the evolution of the arthropod feeding system, with emphasis on the head region. Using fossils, we identify two major gaps in our knowledge about this evolutionary process to be filled in the future. One of them is how development progressed from the stem arthropod level toward that of Arthropoda s. str. The latter stage is known now from three Chengjiang taxa, which possess, besides other features, a head composed of only two segments, i.e. those bearing the compound eyes and the limb-shaped antennulae. The post-antennular trunk limbs are very simple and lack any feeding structures, spines or setae. With this, only the antennula could have been involved in food gathering. Another uncertainty concerns the transition from the Arthropoda s. str. level to that of the Euarthropoda. Euarthropoda embraces all those well-sclerotized arthropods with extant descendants, and its ground pattern includes a larger head tagma with four appendage-bearing segments and post-antennular limbs made of a rigid, but flat gnathobasic basipod carrying two rami. At this stage, feeding had become more elaborate than before, yet all post-antennular appendages remained serially designed. Crustacea changed their feeding system initially by modifying the anterior three cephalic appendages, and the mouth area and by developing a specific setation on various body parts. Subsequently, more appendages became involved within certain in-group taxa. Our model elucidates that changes of the functional system occurred at the macroscopic and the microscopic level. Although many allied features, such as the gut system or the appendage morphology, remained remarkably conservative over longer periods, feeding was most likely a significant driving force for evolutionary changes in the morphology of arthropods, particularly of the head region.