Lower Cambrian Vendobionts from China and Early Diploblast Evolution

Abstract

Ediacaran assemblages immediately predate the Cambrian explosion of metazoans and should have played a crucial role in this radiation. Their wider relationships, however, have remained refractory and difficult to integrate with early metazoan phylogeny. Here, we describe a frondlike fossil, Stromatoveris (S. psygmoglena sp. nov.), from the Lower Cambrian Chengjiang Lagerstätte (Yunnan, China) that is strikingly similar to Ediacaran vendobionts. The exquisite preservation reveals closely spaced branches, probably ciliated, that appear to represent precursors of the diagnostic comb rows of ctenophores. Therefore, this finding has important implications for the early evolution of this phylum and related diploblasts, some of which independently evolved a frondose habit.

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…”

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

“…These putative species have been assigned to three different animal groups: hexacorals (16,(19)(20)(21)(22)(23)(24), octocorals (17,25), and hemichordates (18), respectively. However, alternative views suggest that X. sinica was a ctenophore (23, 26), a lophophorate (27), an Ediacaran survivor (28), or a metazoan of unknown affinity (29,30).…”

Raman spectra of a Lower Cambrian ctenophore embryo from southwestern Shaanxi, China

“…In contrast with this pelagic interpretation, Shu et al (19) recently hypothesized an evolutionary link between the benthic, frondose Chengjiang fossil Stromaveris psygmoglena and modern ctenophores as well as some Ediacaran vendobionts. To link these groups, they proposed that the early evolution of ctenophores was marked by a shift from a benthic, sessile existence to a pelagic habit coupled with a change in the function of their cilia from feeding to locomotion.…”

“…Such a shift would involve major changes in basic morphology and ecology and would require many (undocumented) intermediate stages. Furthermore, their interpretation of S. psygmoglena as a stem-group ctenophore is based heavily on the presence of closely spaced branches that because they are ''probably ciliated'' were inferred to represent precursors of the diagnostic comb rows of ctenophores (19). Given that cilia are of widespread occurrence, not only in metazoans but in protists as well, and that they have diverse functions, not only for locomotion or feeding, use of the presence of probable cilia as a prime character by which to infer a ctenophore affinity for S. psygmoglena is problematic.…”