From Limbs to Jaws — the Nature of the Mouthparts of Anomalocaridids  

Anomalocaridids, known from North America, South China, Australia and Europe, have long been regarded as giant Cambrian predators.  The discovery of Schinderhannes bartelsi (showing an unusual combination of anomalocaridid and euarthropod characters) from the Devonian Hunsrück Slate greatly extends the stratigraphic range of this group of animals.  Characteristic features of the anomalocaridids include a pair of frontal (preoral) great appendages and a circular mouthpart.  Recently, intensive excavation in localities of the Chengjiang biota and the Burgess Shale led to the growing number of new specimens, which elucidate many aspects of anomalocaridid morphology, diversity, ecology and affinity.  Phylogenetic analyses popularly place anomalocaridids in the stem group of euarthropods.  However, the nature of the anomalocaridid mouthparts remains obscure. 

Among several genera of anomalocaridids, the mouthparts of Hurdia are most commonly preserved, which consist of a circlet of 32 plates, the inner margin of each plate armed with small projections (teeth), with four larger plates arranged perpendicularly and separated by seven smaller plates.  Examination of anomalocaridid specimens from the Chengjiang biota, the Kaili biota and the Burgess Shale suggest that the mouthparts have a limb nature.  Evidence includes the presence of articular membranes between adjacent plates, and successive overlap from the larger plates to smaller plates.  These are typical characters of segmented arthropod appendages, each plate representing a skeletal ring of podomere and two adjacent podomeres articulated by articular membranes.  Additionally, the smaller teeth fringing the inner margin of plates are probably homologous with endites of arthropod limbs and ventral spines in the frontal appendages of anomalocaridids.  The mouthparts of anomalocaridids are here interpreted as four pairs of fused, modified head appendages succeeding the frontal appendages.  Each of the four perpendicularly arranged larger plates forms by fusion of two limb bases: the anterior and posterior larger plates represent a fused structure between two bases of the second and fifth pair of head appendages, respectively; the right- and left-side larger plates are fusions of two bases of adjacent limbs (the third and forth pairs of head appendages) in the right and the left, respectively.  In Hurdia, also possibly in Parapeytoia, many rows of teeth are present within the central opening of the mouthparts.  These extra rows of teeth are interpreted as sclerotized structures lining the wall of the buccal cavity and the foregut, which are functioned as gastric mills for further processing the food.  The new interpretation of anomalocaridids bearing five pairs of head appendages further supports their relationships with arthropods.