Cloudinidae
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.[1] 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.[4]
Cloudinids comprise two genera: Cloudina itself is mineralized, whereas Conotubus is at best weakly mineralized, whilst sharing the same "funnel-in-funnel" construction.[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. 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.[3]
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.[4] Fossils consist of a series of stacked vase-like calcite tubes, whose original mineral composition is unknown,[6] but inferred to be high-magnesium calcite.[7] 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.[8] Although it used to be thought that the tubes had test-tube like bases,[4] detailed three-dimensional reconstruction has shown that the tubes had an open base.[9] There is evidence that the tube was flexible.[10]
