Coleolid

Coleolids are Cambrian Small shelly fossils with an aragonite skeleton.^[1] Their affinity is unknown but they were probably produced by an organism of the annelid grade of complexity.^[2]:37

References

1. ^ Porter, S. M. (Jun 2007). "Seawater chemistry and early carbonate biomineralization". Science 316 (5829): 1302–1301. Bibcode 2007Sci...316.1302P. doi:10.1126/science.1137284. ISSN 0036-8075. PMID 17540895.  edit
2. ^ Palaeobiology II By Derek E. G. Briggs, Peter R. Crowther.