Abstract
The early Cambrian problematica Xianguangia sinica, Chengjiangopenna wangii, and Galeaplumosus abilus from the Chengjiang biota (Yunnan, China) have caused much controversy in the past and their phylogenetic placements remain unresolved. Here we show, based on exceptionally preserved material (85 new specimens plus type material), that specimens previously assigned to these three species are in fact parts of the same organism and propose that C. wangii and G. abilus are junior synonyms of X. sinica Our reconstruction of the complete animal reveals an extinct body plan that combines the characteristics of the three described species and is distinct from all known fossil and living taxa. This animal resembled a cnidarian polyp in overall morphology and having a gastric cavity partitioned by septum-like structures. However, it possessed an additional body cavity within its holdfast, an anchoring pit on the basal disk, and feather-like tentacles with densely ciliated pinnules arranged in an alternating pattern, indicating that it was a suspension feeder rather than a predatory actiniarian. Phylogenetic analyses using Bayesian inference and maximum parsimony suggest that X. sinica is a stem-group cnidarian. This relationship implies that the last common ancestor of X. sinica and crown cnidarians was probably a benthic, polypoid animal with a partitioned gastric cavity and a single mouth/anus opening. This extinct body plan suggests that feeding strategies of stem cnidarians may have been drastically different from that of their crown relatives, which are almost exclusively predators, and reveals that the morphological disparity of total-group Cnidaria is greater than previously assumed.
Highlights
Lophophorate [27], an Ediacaran survivor [28], or a metazoan of unknown affinity [29, 30]
Our results revealed that specimens, which were previously assigned to three different taxa including X. sinica, C. wangii, and G. abilus, are parts of the same species
Reexamination of specimen ELI-Seapen05–001 (SI Appendix, Fig. S4 C and D), originally named C. wangii and assigned to crown-group octocorals [17, 25], suggests that it is an isolated tentacle of X. sinica
Summary
Lophophorate [27], an Ediacaran survivor [28], or a metazoan of unknown affinity [29, 30]. We demonstrate that these three problematica are conspecifics The assemblage of this Cambrian puzzle revealed the existence of a “feathered polyp,” most likely representing an offshoot of the cnidarian stem lineage that diverged early during the Cambrian radiation of animal body plans. Fossil problematica are extinct taxa that have defied unambiguous phylogenetic interpretations. They are enigmatic weirdos that have caused taxonomic headaches or have been unsatisfactorily shoehorned into one or another extant group. Deciphering fossil problematica from this crucial interval of evolution might provide pivotal insights into the origin and early radiation of metazoan body plans [1]. Clarifying the anatomy and phylogenetic placement of some of these problematica has greatly improved our understanding of the evolution of major animal groups such as cnidarians Clarifying the anatomy and phylogenetic placement of some of these problematica has greatly improved our understanding of the evolution of major animal groups such as cnidarians (e.g., refs. 2 and 3), ctenophores (e.g., ref. 4), molluscs [5, 6], annelids [7], panarthropods (e.g., refs. 8 and 9), lophophorates (e.g., ref. 10), hemichordates (e.g., ref. 11), and chordates [12,13,14,15]
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