New data on the palaeobiology of the enigmatic yunnanozoans from theChengjiangBiota,LowerCambrian,China

Abstract

Interpretation of the enigmatic soft‐bodied yunnanozoans from the Lower Cambrian (Stage 3, Series 2) Chengjiang Biota has remained controversial for decades because of their strange body plan and their variable taphonomic alteration. Proposed affinities have ranged from stem bilaterian to stem vertebrate. A study of over seven hundred slabs, many newly collected, from five sections at two localities demonstrates that yunnanozoans have sclerotized dorsal and axial segments, a body cavity, a unique feeding region and a coiled alimentary canal. Although two genera have been previously discriminated, all specimens have the same number of filamentous arches (seven pairs) and of ventral circular structures (four pairs); this indicates that Haikouella Chen, Huang and Li, 1999 is a junior synonym of Yunnanozoon Hou, Ramsköld and Bergström, 1991. Our analysis reveals new details of the putative pharyngeal pores of Yunnanozoon lividum, and although there are similarities with the gill slits of deuterostomes, the question of their homology remains moot. The filamentous arches and their supporting rods were probably sclerotized and represent a specialized feeding structure. Yunnanozoans exhibit characters that are open to a number of alternative interpretations of putative homology, encompassing comparisons with deuterostomes and protostomes (including members of the Platyzoa). Given their unique features, and the current lack of secure homologies, the affinities of yunnanozoans should continue to be considered in a wider bilaterian context.

A critical appraisal of appendage disparity and homology in fishes

“…On the one hand, this taxon might be a stem deuterostome (Shu, 2003(Shu, , 2008Shu et al, 2010;. Based on a more recent investigation of over 700 specimens, Cong, Hou, Aldridge, Purnell, and Li (2015) argued that yunnanozoan characters can be open to alternative interpretations and that their affinities to other groups should conservatively be considered within a wider bilaterian context. Some phylogenetic analyses have resolved Haikouella at the base of craniates (Holland & Chen, 2001;, suggesting that yunnanozoans (i.e., Yunnanozoon + Haikouella) may be the sister group of all other craniates.…”

“…Alternatively, it might be a stem craniate if the interpretation of structures identified as a brain, eyes, a post-anal tail and median fins are correct . Notwithstanding the uncertain phylogenetic position of yunnanozoans, it is generally agreed that Yunnanozoon and Haikouella are either close relatives or even possibly synonyms (Cong, Hou, Aldridge, Purnell, & Li, 2015;Turner et al, 2010). Based on a more recent investigation of over 700 specimens, Cong, Hou, Aldridge, Purnell, and Li (2015) argued that yunnanozoan characters can be open to alternative interpretations and that their affinities to other groups should conservatively be considered within a wider bilaterian context.…”

“…Alternatively, it might be a stem craniate if the interpretation of structures identified as a brain, eyes, a post-anal tail and median fins are correct . Some phylogenetic analyses have resolved Haikouella at the base of craniates (Holland & Chen, 2001;, suggesting that yunnanozo- or even possibly synonyms (Cong, Hou, Aldridge, Purnell, & Li, 2015;Turner et al, 2010).…”

Cambrian Chordates and Vetulicolians

“…Taken together, these chordate and vetulicolian genera document an important part of the explosive radiation of deuterostomes at the base of the Cambrian. Although debate is likely to continue, the preponderance of the evidence does suggest that vetulicolians and yunnanozoans are indeed deuterostomes [47,50,52,53,[67][68][69][70][71]. Although deuterostomes, the extremely odd character set in vetulicolians also suggests that they may very well represent a higher taxon of phylum rank.…”

Naked chancelloriids from the lower Cambrian of China show evidence for sponge-type growth

“…The application of cladistic techniques in phylogenetic reconstruction has led to the incorporation of many of these 'problematic' fossils into the stem groups of higher taxa [1]. Others, however, are resistant to phylogenetic placement, either because of a simple lack of preserved, identifiable homologies [2] or because of their genuinely perplexing character combinations [3]. In the latter category are chancelloriids, an extinct group of Cambrian animals known from exceptionally preserved whole-body fossils in Burgess Shale-type Lagerstätten, and as disarticulated skeletal elements (sclerites) in assemblages of small shelly fossils (SSFs).…”