Figure 1. Backscattered SEM image of Lenica spicules with core replaced by aluminosilicates and outer layer by pyrite (bright), with: A, Reconstruction of spicule structure; B, Interpretation of spicule evolution in sponges. Scale bar 200 μm.
Sponges are among the most ancient of all animals, appearing in the fossil record at least 540 million years ago. Their early evolution is difficult to interpret, however, because their delicate skeletons do not fossilise easily. Ideas of early sponge evolution are currently based as much on molecular evidence from living species as they are from the fossil record, but research into the structure of the skeletons in early fossils is providing surprising insights.
Most modern sponges fall into two easily separated groups: Silicea, with a skeleton of minute spicules made of opal (silica), and Calcarea, with spicules made of calcite. The secretion of these spicules is complex and varied, but in general, the two groups use different chemical and cellular processes to secrete their spicules, and it has been widely assumed that the two types of spicule evolved independently.
This concept has been rejected by new work by a NIGPAS-based team: researcher Dr. Joseph P. Botting, with Dr. Lucy A. Muir, Professor Shuhai Xiao, Li Xiangfeng and Professor Jih-Pai Lin. The group has been working on early Cambrian (~520 million-year-old) sponges from the Hetang Biota of Anhui. In this paper, published in the journal Lethaia, they have studied the enigmatic sponge Lenica using Backscattered Electron Microscopy and elemental mapping. Lenica is classified as a protomonaxonid, a group of sponges with uncertain relationships to the modern classes of sponge, and which may have evolved earlier than the living groups.
Lenica is unusual in having extremely large spicules, up to 10 cm long, and in the Hetang Biota they are preserved in a variety of ways. Although none of the spicules are preserved in pristine condition, it has been possible to obtain near-comprehensive information from the different preservational types.
The results show that Lenica spicules consisted of a silica spicule with an axial filament (as in modern Silicea), surrounded by a second mineral layer that was probably calcium carbonate. There was also a thin, probably organic layer on the outside of the spicule, as seen in modern Calcarea. The spicules of Lenica therefore appear to consist of calcarean spicules around a core consisting of a silicean spicule. This shows that spicules are almost certainly shared between Calcarea and Silicea, supporting previous work by the first author but in contrast to standard views in modern biology. This has profound implications for the interpretation of the fossil record of sponges, and means that many early sponges should be assigned to primitive stem groups rather than living classes.
Related information of this paper: Botting, J.P., Muir, L.A., Xiao, S., Li, X.-F. & Lin, J.-P. 2012. Evidence for spicule homology in calcareous and siliceous sponges: biminerallic spicules in Lenica sp. (Porifera; ?Protomonaxonida) of early Cambrian age (535-520 Ma) from South China. Lethaia 45, 463-475.
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