Клаудиналар: ежелгі теңіз жануарларының туысы

Genus of aquatic animals

The cloudinids, an early metazoan family containing the genera Acuticocloudina, Cloudina and Conotubus, lived in the late Ediacaran period about 550 million years ago. They formed millimetre scale conical fossils consisting of calcareous cones nested within one another; the appearance of the organism itself remains unknown. The name Cloudina honors the 20th century geologist and paleontologist Preston Cloud. 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. They never appear in the same layers as soft bodied Ediacaran biota, but the fact that some sequences contain cloudinids and Ediacaran biota in alternating layers suggests that these groups had different environmental preferences. It has been suggested that cloudinids lived embedded in microbial mats, growing new cones to avoid being buried by silt. However no specimens have been found embedded in mats, and their mode of life is still an unresolved question. 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. Current scientific opinion is divided between classifying them as polychaetes and regarding it as unsafe to classify them as members of any broader grouping. 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. Cloudinids are important in the history of animal evolution for two reasons. They are among the earliest and most abundant of the small shelly fossils with mineralized skeletons, and therefore feature in the debate about why such skeletons first appeared in the Late Ediacaran. 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. The holes made by predators are approximately proportional to the size of the Cloudina specimens, and Sinotubulites fossils, which are often found in the same beds, have so far shown no such holes. 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.

Cloudina varies in size from a diameter of 0.3 to 6.5 mm, and 8 to 150 mm in length. Fossils consist of a series of stacked vase like calcite tubes, whose original mineral composition is unknown, but inferred to be high magnesium calcite. Each cone traps a significant pore space beneath it, and stacks eccentrically in the one below. This results in a ridged external appearance. The overall tube is curved or sinuous, and occasionally branches. The tube walls are 8 to 50 micrometers thick, usually lying in the range 10 to 25 μm. 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. On this basis Grant (1990) classified Cloudina as a coral like cnidarian.

Cloudina is usually found in association with microbial stromatolites, which are limited to shallow water; their isotopic composition suggests that water temperatures were relatively cool. They have also been found in normal sea floor sediments, suggesting that they were not only restricted to dwelling on microbial mounds. On the other hand, Cloudina has never been found in the same layers as the soft bodied Ediacara biota, but Cloudina and Ediacara biota have been found in alternating layers. This suggests that the two groups of organisms had different environmental preferences. Because of its small size, Cloudina would be expected to be found in situ in the microbial mat, especially if, as Seilacher suggests, sedimentation built up around it during its lifetime. But all the many specimens discovered to date have only been found having been washed out of their places of growth. A further argument against Seilacher's hypothesis is that the predatory borings found in many specimens are not concentrated at what would be the top end, as one would expect if the animal was mainly buried. An alternative is that the organism dwelt on seaweeds, but until a specimen unquestionably in situ is discovered, its mode of life remains open to debate. The tubes often appear to form colonies, although they are sometimes found in more isolated situations. The frequent appearance of large and sometimes single species colonies has been attributed to the lack of significant predation. Fossil findings in the Nama Group, Namibia, suggest that Cloudina was one of the first reef building animals, but machine learning facilitated 3D tomography indicates that the 'reef forming' fossils are in fact simply aggregations of solitary individuals.