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

Abstract

Highlights d We describe a new early Cambrian fossil with a polypoid body plan from Chengjiang d It is related to the supposed cnidarian Xianguangia and the iconic Dinomischus d The fossils possess very large ciliary structures otherwise seen only in ctenophores d We show that these fossils form a grade of stem-group ctenophores

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

“…Xianguangia sinica is a tentaculate polyp known from the Chengjiang BST and generally interpreted as a stem group cnidarian (Ou et al, 2017). However, Xianguangia was recently reinterpreted as a benthic stem group Ctenophore (Zhao et al, 2019), though this interpretation may prove controversial because of the presence of mesenteries (Daley & Antcliffe, 2019). Analysis of the taphonomy of these Lagerstätte has shown that anoxic and reducing conditions are common and regularly cited as amongst the primary causative agents in the preservation of the soft-bodied animals (Allison, 1988a;1988b;Anderson, Tosca, Gaines, Mongiardino Koch, & Briggs, 2018;Butterfield, 1990;Gaines et al, 2008Gaines et al, , 2012Gaines et al, , 2005.…”

Evolution: The Battle of the First Animals

“…The most famous of these are the 'bizarre crab' Anomalocaris and the 'mind-bending' Hallucigenia. The fossils described by Zhao and colleagues [8] are just as enigmatic, marking this work as an important contribution to the field owing to the detailed descriptions and images of a new taxon, Daihua sanqiong, and of enlightening new specimens of previously described taxa, including Xianguangia, Dinomischus and Siphusauctum. This last taxon has colloquially been called the 'tulip animal', which is a fairly accurate visual description of its overall anatomy [9].…”

“…These provide another tangible record of evolution and are particularly abundant and well preserved during the first major burst of animal evolution, the Cambrian explosion. A new study in this issue of Current Biology by Yang Zhao, Jakob Vinther and colleagues [8] describes bizarre Cambrian fossils and places them in the ctenophore stem lineage, forming a close alliance between ctenophores and cnidarians ( Figure 1).…”

“…This last taxon has colloquially been called the 'tulip animal', which is a fairly accurate visual description of its overall anatomy [9]. The tulip-shaped body of Siphusauctum consists of a narrow stem topped by a rounded, bulbous main body chamber, which contains the mouth and gut, and is adorned with 'petals' represented by six radially-arranged tentacles covered in rows of hair-like protrusions, or cilia [8,9]. Siphusauctum was previously identified as a bilaterian but could not be placed in any specific phylum [9].…”

“…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].…”