The cloudinids, an early metazoan family containing the genera Acuticocloudina, Cloudina and Conotubus, lived in the late Ediacaran period about 550 million years ago[2][3] and became extinct at the base of the Cambrian.

[5] Cloudinids had a wide geographic range, reflected in the present distribution of localities in which their fossils are found, and are an abundant component of some deposits.

The classification of the cloudinids has proved difficult: they were initially regarded as polychaete worms, and then as coral-like cnidarians on the basis of what look like buds on some specimens.

In 2020, a new study of pyritized specimens from the Wood Canyon Formation in Nevada showed the presence of Nephrozoan type guts, the oldest on record, supporting the bilaterian interpretation.

The most widely supported answer is that their shells are a defense against predators, as some Cloudina specimens from China bear the marks of multiple attacks, which suggests they survived at least a few of them.

These two points suggest that predators attacked in a selective manner, and the evolutionary arms race which this indicates is commonly cited as a cause of the Cambrian explosion of animal diversity and complexity.

[11] However, Hahn & Pflug (1985) and Conway Morris et al.. (1990) doubted both Germs' and Glaessner's suggested relationships, and were unwilling to classify it to anything more than its own family, Cloudinidae.

Adolf Seilacher suggests that they adhered to microbial mats, and that the growth phases represented the organism keeping pace with sedimentation—growing through new material deposited on it that would otherwise bury it.

An alternative is that the organism dwelt on seaweeds,[9] but until a specimen unquestionably in situ is discovered, its mode of life remains open to debate.

The even distribution is perhaps difficult to reconcile with an infaunal lifestyle, mainly buried in a microbial mat, and adds weight to Miller's suggestion that the animal lived on seaweeds or in a reef environment.

[24] First found in the Nama Group in Namibia,[4] Cloudina has also been reported in Oman,[13] China's Dengying Formation,[13][16] Canada,[25] Uruguay,[26][27] Argentina,[28] Antarctica,[29] Brazil,[30][31] Nevada,[32] central Spain, northwest Mexico and California,[8] in west and south Siberia.

[35] The evolution of external shells in the Late Ediacaran is thought to be a defence against predators, marking the start of an evolutionary arms race.