Comb jellies are some of the most evocative marine animals, with transparent tissues enclosing blue combs in constant motion. Their fossil record goes back to the Cambrian, where they appear with essentially modern forms. A new research paper documents "an exquisitely preserved late-stage embryo of a ctenophore ("comb jelly")" in sediments that are right at the base of the Cambrian. "This specimen represents the earliest firm evidence of the Ctenophora now known".
"The ctenophoran affinities of this embryo are firmly supported by the presence of eight sets of comb rows and comb plates, and of meridional and aboral canals, all having the same relative positions as those in modern ctenophores. The presence of an adult body plan and of adult-like structures, even in its prehatching embryonic condition, suggests that this ancient ctenophore embryo would have likely directly matured into a planktonic juvenile like that of most modern ctenophores.[. . .] Its lack of tentacles and the presence of branched meridional canals, however, are characters shared by the embryos of extant beroidian ctenophores, with which this fossil therefore seems more closely allied."
The authors consider the common ancestor of these organisms. Their discovery rules out several previously proposed candidates. Links with Precambrian faunas are also rejected: "the suggestion by Shu et al. that the Ctenophora occupies an intermediate evolutionary position between sponges and cnidarians is inconsistent with numerous lines of evidence, both anatomical and molecular."
So we are left with the thought that the earliest ctenophore was rather like "extant beroidian ctenophores" and some of us might question whether there ever was a branch linking ctenophores to the elusive Darwinian Tree of Life.
Raman spectra of a Lower Cambrian ctenophore embryo from southwestern Shaanxi, China
Jun-Yuan Chen, J. William Schopf, David J. Bottjer, Chen-Yu Zhang, Anatoliy B. Kudryavtsev, Abhishek B. Tripathi, Xiu-Qiang Wang, Yong-Hua Yang, Xiang Gao, and Ying Yang.
Proc. Natl. Acad. Sci. USA, (10 April, 2007), 104(15), 6289-6292. DOI: 10.1073/pnas.0701246104 (OPEN ACCESS ARTICLE)
The Early Cambrian (~540 million years old) Meishucun fossil assemblage of Ningqiang County (Shaanxi Province), China, contains the oldest complex skeletonized organisms known in the geological record. We here report the finding in this assemblage of an exquisitely preserved late-stage embryo of a ctenophore ("comb jelly"), its fine structure documented by confocal laser scanning microscopy and shown by Raman spectroscopy to be composed of carbonaceous kerogen permineralized in apatite. In its spheroidal morphology, the presence of eight comb rows and the absence of tentacles, this embryo resembles an adult ctenophore (Maotianoascus octonarius) known from the immediately younger Chengjiang fauna of Yunnan, China. The oldest ctenophore and the only embryonic comb jelly known from the fossil record, this exceptionally well preserved specimen provides important clues about the early evolution of the phylum Ctenophora and of metazoans in general.
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