However, there is yet little unequivocal paleontological evidence of Precambrian sponges.
Here, we present a newly discovered 600-Myr-old fossil preserved at cellular resolution, displaying multiple poriferan features.
Because sponges do not display tissue-grade anatomical characters or organs beyond a general gross, sponge-like structure, this requirement would essentially demand preservation of recognizable cell types in the fossil.The character set evinced by this specimen, ranging from general anatomy to cell type, uniquely indicates that this specimen is a fossil of probable poriferan affinity. gode gratis dating sider Vesthimmerlands So far, we have only this single specimen, and although its organized and complex cellular structure precludes any reasonable interpretation that its origin is abiogenic, confirmation that it is indeed a fossilized sponge will clearly require discovery of additional specimens.Extensive phylogenomic analyses have converged on a distant Precambrian vintage for the origin of Metazoa, an evolutionary event that must have long predated the last common ancestor of Porifera (sponges) and Eumetazoa (cnidarians bilaterians), given the organismal and genomic complexity of these major groups.In modern phylogenomic analyses, elegant computational algorithms are applied to sequence divergence data from very large sets of genomic protein and other sequences so as to establish evolutionary branching order, and thus patterns of relatedness among animal clades.
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In addition, the recovery from Cryogenian-aged rocks of an organic sterol biomarker that is synthesized by marine demosponges suggested the early presence of this poriferan clade (12), although the conclusion that the provenance of this fossil sterol indicates the existence of Cryogenian sponges is now also questioned (7).Despite phylogenomic extrapolations that indicate divergence of sponge lineages from eumetazoan lineages deep in Precambrian time, lack of substantial paleontological evidence directly supporting this prediction has thus remained frustrating.Phylogenomic extrapolations indicate the last common ancestor of sponges and eumetazoans existed deep in the Cryogenian, perhaps 200 million years (Myr) before the Cambrian (541 Ma). anzeigen sie sucht ihn Jena This inference implies a long Precambrian history of animals phylogenetically allied with sponges.Key branch points can then be referred to real time by use of extrapolations from calibrations afforded by the dated fossil record.
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Various analyses of this type have been published, as well as an authoritative recent review by Erwin and Valentine (1).
Tantalizing paleontological claims of the Precambrian fossils that display one or another sponge-like characteristic have appeared in recent years, as critically reviewed by Antcliffe et al. We leave aside isolated objects that are claimed to be fossilized siliceous spicules, because these objects have little morphological information content, and also the numerous claims of fossilized sponge-grade organisms from several parts of the world dating to about 555 Ma, close to the Precambrian/Cambrian boundary.
There remain, however, several proposed sponge-like fossils of sufficiently deep temporal provenance to be relevant in principle to the phylogenomic predictions, although it must be noted that none are accepted as fossil sponges in the study by Antcliffe et al. Among these putative fossilized sponges are thin-walled, hollow microfossils, perforated by various openings, found in rocks as old as 760 million years (Myr) old (9); asymmetrical ellipsoid forms that appear to be perforated with interconnecting canals of pre-Marinoan Cryogenian age when tomographically reconstructed from images of serial surfaces (10); and a macroscopic triangular impression fossil interpreted as the remains of a conical sponge-like form of Ediacaran age, about 575 Myr old (11).
All calibrated phylogenomic analyses agree on a remotely Precambrian evolutionary origin of sponges.
Although earlier studies portrayed the four extant sponge classes as polyphyletic, technically sophisticated recent analyses indicate Porifera to be monophyletic, that is, to have had a single common evolutionary origin (2, 3).