Abstract
Molecular clock analyses estimate that crown-group animals began diversifying hundreds of millions of years before the start of the Cambrian period. However, the fossil record has not yielded unequivocal evidence for animals during this interval. Some of the most promising candidates for Precambrian animals occur in the Weng'an biota of South China, including a suite of tubular fossils assigned to Sinocyclocyclicus, Ramitubus, Crassitubus and Quadratitubus, that have been interpreted as soft-bodied eumetazoans comparable to tabulate corals. Here, we present new insights into the anatomy, original composition and phylogenetic affinities of these taxa based on data from synchrotron radiation X-ray tomographic microscopy, ptychographic nanotomography, scanning electron microscopy and electron probe microanalysis. The patterns of deformation observed suggest that the cross walls of Sinocyclocyclicus and Quadratitubus were more rigid than those of Ramitubus and Crassitubus. Ramitubus and Crassitubus specimens preserve enigmatic cellular clusters at terminal positions in the tubes. Specimens of Sinocyclocyclicus and Ramitubus have biological features that might be cellular tissue or subcellular structures filling the spaces between the cross walls. These observations are incompatible with a cnidarian interpretation, in which the spaces between cross walls are abandoned parts of the former living positions of the polyp. The affinity of the Weng'an tubular fossils may lie within the algae.
Highlights
Molecular clock analyses estimate that metazoans diversified in the Ediacaran or Cryogenian tens or hundreds of millions of years before the beginning of the Cambrian [1,2]
We selected examples from each of the four tubular genera described in detail by Liu et al [25], which are primarily distinguished by differences in overall morphology, in the nature of their cross walls, and in the presence or absence of a sheath
We find that the patterns of deformation observed in the Doushantuo tubular fossils suggest that the cross walls of Sinocyclocyclicus and Quadratitubus were more rigid than those of Ramitubus and Crassitubus
Summary
Molecular clock analyses estimate that metazoans diversified in the Ediacaran or Cryogenian tens or hundreds of millions of years before the beginning of the Cambrian [1,2]. The Weng’an biota [6] from the Doushantuo Formation of South China has yielded a suite of Early Ediacaran microfossils described as embryonic [7] and adult [8] animals, including putative stem-group metazoans [9], sponges [10,11], cnidarians [12] and bilaterians [8,13,14] These are found alongside fossils assigned to acritarchs [15,16] and algae [17]. [25,26,27] from the Doushantuo, assigned to the genera Sinocyclocyclicus, Ramitubus, Crassitubus and Quadratitubus, are rare examples of widely accepted Precambrian metazoans [28,29,30] These fossil taxa have been interpreted as a closely interrelated group of soft-bodied animals comparable to tabulate corals, and they have been marshalled as evidence for cnidarian-like eumetazoans at this time [19,25]. We assess the implications of these new data for understanding their phylogenetic affinity
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