Cambrian lobopodians have attracted much attention in that they are considered to have close affinities with the origin of arthropods, and recent studies agree that they sample the stem-groups of both Arthropoda and Onychophora. A new... more
Cambrian lobopodians have attracted much attention in that they are considered to have close affinities with the origin of arthropods, and recent studies agree that they sample the stem-groups of both Arthropoda and Onychophora. A new lobopodian species, Diania cactiformis Liu et al., 2011, was recently reported from the Chengjiang Lagerstätte (south-west China; Cambrian Series 2). This animal was interpreted as bearing arthropod-like articulated appendages and was resolved as the sister taxon to Arthropoda in a cladistic analysis. A reanalysis of the published dataset did not reproduce the phylogenetic placement of D. cactiformis, and the interpretation of its appendage morphology is controversial. New observations on D. cactiformis are made using newly collected material, rejecting the evidence for sclerotized, segmented and articulated appendages; instead the appendages compare more closely to lobopods. The criteria for defining arthropodized appendages are further discussed through comparative studies among fossil lobopodians, anomalocaridids and arthropods. Our study challenges the previous description of anteroposterior orientation, terminal claws and trunk tagmosis as expressed by differentiated anterior and posterior appendages. A synthesis of four recent datasets for lobopodians and stem-group arthropods is used to revisit the phylogeny of stem arthropods. Irrespective of how characters are weighted, D. cactiformis is resolved at a much more basal position to arthropods, indicating that the morphology of this animal's appendages does not fundamentally inform on the evolution of arthropodization.
Liu et al.1 described an ‘armoured’ lobopodian, Diania cactiformis, from the Chengjiang Lagerstätte (China; Cambrian, stage 3); this fossil bears potentially arthropod-like articulated and possibly sclerotized appendages, but lacks a... more
Liu et al.1 described an ‘armoured’ lobopodian, Diania cactiformis, from the Chengjiang Lagerstätte (China; Cambrian, stage 3); this fossil bears potentially arthropod-like articulated and possibly sclerotized appendages, but lacks a sclerotized body. A cladistic analysis resolved Diania as sister-taxon to arthropods. From this phylogenetic position the authors tentatively inferred that arthropodization (sclerotization of limbs) may have preceded arthrodization (sclerotization of body elements) in arthropod evolution. Although we concur with the reasoning behind this inference, it rests on a phylogenetic placement that our analysis of the published data set does not reproduce.
