Jugatacaris And Isoxys (Arthropod) From The Chengjiang Fauna, Lower Cambrian

Phylum Arthropoda is the most extensive phylum in the animal kingdom, composed of more than three-fourths of all known species. In essence all of the major groups of arthropods that are known from the present day can be traced back to the Cambrian. They are mainly occurred in the exceptionally preserved Lagerstatten, which have provided a major contribution to our knowledge of early animals without mineralized skeletons. One of them is the Chengjiang biota, providing a comprehensive and very early window on the origin and evolution of arthropods.A new arthropod Jugatacaris agilis n. gen. and n. sp. is reported. Its unique carapace with a dorsal fin-like structure, covering two third of the trunk, readily distinguishes it from other Cambrian arthropods. The cephalon bears five pairs of appendages. The number of trunk segments varies from 55 to 65, each carrying a pair of biramous appendages except for the posteromost three to five segments. Trunk is ended with a conical telson and a pair of broad furcal rami. The morphological features indicate that J. agilis has a filter-feeding mode of life. The animal is assigned to the Branchiopoda, due to possess the filter-feeding apparatus resembling that of living branchiopods. The specialization of limbs suggests that J. agilis was most probably a crown group crustacean.Isoxys has been well known for its widespread geographical distribution and 17 species has been established. However, morphological comparison demonstrates that species of Isoxys have a quite broad morphospace. It has cast doubt on the current taxonomy that assigned all species to a single genus. In addition, Isoxys curvirostratus has recently been placed into the great appendage arthropods, since carries a pair of grasping appendages. Such placement might be inadequate because the homology of the great appendages can not be established. Many arthropods share the raptorial "great appendage" functionally, but their homology assumption is disputable. The venom glands are first reported here. New finding indicates that venomous predation has much early origin. Associated with evidence previously reported, it is very likely that almost all the feeding styles known in present-day ecosystems probably existed in the Early Cambrian and organs adapted for this purpose had already reached a high level of diversity and anatomical sophistication.The presence of the reticulate ornament on the carapace was a fundamental diagnosis for Isoxys auritus. Any cases without the reticulation have been attributed to the poor preservation. However, close examination indicates that the overall shapes of the two morphs are different. We believe that these two morphs represent dimorphism. The trunk segments and appendages increase from 7 to 11 during the growth. Allometry is evident in the growth of I. auritus:the length/height ration of the carapace increases with size, the ratio of the size of eye and antenna to the carapace reduce. If we take Kunmingella as potential ancestor, the development of I.auritus progresses beyond that of the ancestor in both size and shape. This process can be described as hypermorphosis.