Possible ctenophoran affinities of the precambrian “sea‐pen” Rangea

Abstract

The Namibian Kuibis Quartzite fossils of Rangea are preserved three-dimensionally owing to incomplete collapse of the soft tissues under the load of instantaneously deposited sand. The process of fossilization did not reproduce the original external morphology of the organism but rather the inner surface of collapsed organs, presumably a system of sacs connected by a medial canal. The body of Rangea had tetraradial symmetry, a body plan shared also by the White Sea Russian fossil Bomakellia and possibly some other Precambrian frond-like fossils. They all had a complex internal anatomy, smooth surface of the body, and radial membranes, making their alleged colonial nature unlikely. Despite a different style of preservation, the Middle Cambrian Burgess Shale frond-like Thaumaptilon shows several anatomical similarities to Rangea. The body plan of the Burgess Shale ctenophore Fasciculus, with its numerous, pinnately arranged comb organs, is in many respects transitional between Thaumaptilon and the Early Cambrian ctenophore Maotianoascus from the Chengjiang fauna of South China. It is proposed that the irregularly distributed dark spots on the fusiform units of the petaloid of Thaumaptilon represent a kind of macrocilia and that the units are homologous with the ctenophoran comb organs. These superficial structures were underlain by the complex serial organs, well represented in the fossils of Rangea. The Precambrian "sea-pens" were thus probably sedentary ancestors of the ctenophores.

Convergent evolution of neural systems in ctenophores

“…Fossil records in the later Precambrian and lower Cambrian (Chen et al, 2007;Dzik, 2002;Shu et al, 2006;Tang et al, 2011) further support early ctenophore ancestry. For example, the ctenophore-type Eoandromeda is dated at 580-551 Mya (Tang et al, 2011), before the appearance of distinct sponge-type fossils around 548 Mya (Penny et al, 2014).…”

“…According to 18S rRNA analysis by Podar and colleagues (Podar et al, 2001), this could have occurred at the K-T boundary 66 million years ago -yet the paleontological data suggest the presence of pre-Cambrian, Cambrian and Devonian ctenophore fossils with extensive comb organization (see Chen et al, 1991;Chen et al, 2007;Conway Morris and Collins, 1996;Dzik, 2002;Erwin and Valentine, 2013;Shu et al, 2006;Stanley and Stürmer, 1983;Tang et al, 2011). There is a possibility that the last common ancestor of extant ctenophores shared neuronal toolkits with other eumetazoans (Cnidaria and Bilateria) but this scenario, regardless of phylogenetic reconstructions (Moroz, 2014), still implies a situation…”

Elements of a ‘nervous system’ in sponges

“…The fossil record does not give any insight into early ctenophore body plans -except for the idea that frond-like animals of the Ediacaran may have had ctenophoran affinities (Dzik, 2002) -but if ctenophores were predatory as extant species are, then what would they have eaten? The environment in which the first multicellular animals evolved was presumably oxygenated at the surface, as a result of photosynthesis and turbulence, but the only food would have been picoplankton -flagellates, bacteria and viruses (Lenton et al, 2014).…”

A uniquely preserved Ediacaran fossil with direct evidence for a quilted bodyplan

“…2B), horizontal trace fossils ( Fig. 2 B and C), and a rare Ediacara fossil described as Paracharnia dengyingensis (22,24,25). The biomineralized tubular fossil Sinotubulites baimatuoensis occurs in the uppermost Shibantan or lowermost Baimatuo members (24,26), and Cloudina hartmannae occurs in correlative strata elsewhere in South China (27,28).…”