Three Cambrian fossils assembled into an extinct body plan of cnidarian affinity

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 stemgroup 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 totalgroup Cnidaria is greater than previously assumed.Xianguangia sinica | Chengjiangopenna wangii | Galeaplumosus abilus | ciliary suspension feeder | cnidarian stem group

Anoxia can increase the rate of decay for cnidarian tissue: UsingActinia equinato understand the early fossil record

“…Two‐dimensional anthozoan polyps can be found preserved as organic carbon and aluminosilicate biofilms, or as moulds (Hou, Stanley, Zhao, & Ma, ; Lei, Han, Ou, & Wan, ; Ou et al, ). Moulds typically preserve the gross outline of the organism and external morphology only.…”

“…The other three characters (Hypostome, Actinopharynx and Tentacles) varied their relative position depending on the conditions of the experiment. See Table 2 6.1 | 2D Preservation Two-dimensional anthozoan polyps can be found preserved as organic carbon and aluminosilicate biofilms, or as moulds (Hou, Stanley, Zhao, & Ma, 2005;Lei, Han, Ou, & Wan, 2014;Ou et al, 2017). Moulds typically preserve the gross outline of the organism and external morphology only.…”

Cambrian Sessile, Suspension Feeding Stem-Group Ctenophores and Evolution of the Comb Jelly Body Plan

“…Alternative hypotheses based on morphology have proposed that ctenophores are the sister group of Bilateria (together forming Acrosomata, named for an acrosome in the sperm [9]) or the deuterostome sister group, based on ciliary characteristics [10]. Morphological phylogenetic analyses either recovered Acrosomata [11], placed ctenophores in a polytomy with numerous bilaterians [12], or recovered Coelenterata [13].…”

Evolution: The Battle of the First Animals

“…The new taxon Daihua and the previously described Xianguangia [10] are both stalkless, but have a sclerotized organic skeleton supporting an expanded body region with long tentacles bearing pinnules with rows of cilia [8]. Xianguangia had previously been placed in the stem lineage of Cnidaria [10].…”

“…Another taxon that has yet to find a home in any animal phylum is Dinomischus, which is also tulip-like but has a sclerotized organic skeleton not found in the entirely soft-bodied Siphusauctum [9]. The new taxon Daihua and the previously described Xianguangia [10] are both stalkless, but have a sclerotized organic skeleton supporting an expanded body region with long tentacles bearing pinnules with rows of cilia [8]. Xianguangia had previously been placed in the stem lineage of Cnidaria [10].…”