the Early Cambrian Chengjiang fauna, from . Fossils found in the Canadian Northwest Territories dating to may be sponges; if this finding is confirmed, it suggests the first animals appeared before the Neoproterozoic oxygenation event. Freshwater sponges appear to be much younger, as the earliest known fossils date from the Mid-Eocene period about . Although about 90% of modern sponges are demosponges, fossilized remains of this type are less common than those of other types because their skeletons are composed of relatively soft spongin that does not fossilize well. Earliest sponge symbionts are known from the early Silurian. A chemical tracer is 24-isopropylcholestane, which is a stable derivative of 24-isopropylcholesterol, which is said to be produced by demosponges but not by eumetazoans ("true animals", i.e. cnidarians and bilaterians). Since choanoflagellates are thought to be animals' closest single-celled relatives, a team of scientists examined the biochemistry and genes of one choanoflagellate species. They concluded that this species could not produce 24-isopropylcholesterol but that investigation of a wider range of choanoflagellates would be necessary in order to prove that the fossil 24-isopropylcholestane could only have been produced by demosponges. Although a previous publication reported traces of the chemical 24-isopropylcholestane in ancient rocks dating to , recent research using a much more accurately dated rock series has revealed that these biomarkers only appear before the end of the Marinoan glaciation approximately , and that "Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, ~ in Oman)". While it has been argued that this 'sponge biomarker' could have originated from marine algae, recent research suggests that the algae's ability to produce this biomarker evolved only in the Carboniferous; as such, the biomarker remains strongly supportive of the presence of demosponges in the Cryogenian. Archaeocyathids, which some classify as a type of coralline sponge, are very common fossils in rocks from the Early Cambrian about , but apparently died out by the end of the Cambrian . It has been suggested that they were produced by: sponges; cnidarians; algae; foraminiferans; a completely separate phylum of animals, Archaeocyatha; or even a completely separate kingdom of life, labeled Archaeata or Inferibionta. Since the 1990s archaeocyathids have been regarded as a distinctive group of sponges. It is difficult to fit chancelloriids into classifications of sponges or more complex animals. An analysis in 1996 concluded that