Cloudinidae

Cloudina Temporal range: Ediacaran
Scientific classification
Kingdom:	Animalia
Phylum:	Archaeocyatha? Vologdin 1937
Class:	Cribriocyathea? Vologdin 1961
Order:	Pterocyathida? Jankauskas 1967
Family:	Cloudinidae Hahn and Pflug, 1985
Genus:	Cloudina Germs 1972
Species
C. hartmannae Germs 1972 C. riemkeae Germs 1972

Cloudinids are an extinct metazoan family containing the genus Cloudina. They formed small conical fossils consisting of calcareous tube nested within one another. What the organism itself looked like remains unknown. Cloudinids were widely distributed. They are quite abundant in some deposits. They are amongst the earliest animals to produce a calcareous shell.

The name Cloudina honors the 20th century geologist and paleontologist Preston Cloud.

Morphology

Cloudina varies in size from a diameter of 0.3 to 6.5 mm, and 8 to 150 mm in length.. It consists of a series of stacked vase-like tubes, made from microscopic calcite crystals, probably embedded in an organic matrix. 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. The two species of Cloudina are only distinguished on the basis of diameter, and it is possible that they in fact represent male and female forms of the same species.
Whilst it used to be thought that the tubes had test-tube like bases, detailed three-dimentional reconstruction has shown that the tubes had an open base. This evidence suggests that Cloudina is not a coral-like cnidarian, but is worm-like.

Cloudina occurred in calcium carbonate rich areas of stromatolite reefs. It is found with Namacalathus, which like Cloudina was "weakly skeletal" and solitary, and Namapoikia, which was "robustly skeletal" and and formed sheets on open surfaces.

Geological significance

Cloudina are characteristic of the so-called "Small Shelly Fauna" in the final Neoproterozoic period, called the Ediacaran, and disappeared in the extinction event that marks the Precambrian - Cambrian boundary and the rise of readily-fossilised life, currently dated at 542 million years old. The extinctions set the stage for the Cambrian explosion of life-forms.

Exactly what kind of animal the Cloudinids were is unknown; it resembles serpulid polychaete annelid worms and pogonophoran worms, and has similarities to the trace fossils Salterella and Cornulites. Its lack of bilateral or radial symmetry rules out Coelenteratan affinities. As with so many Ediacaran life forms, there is great debate as to its classification, and it may be unwise to classify it between the kingdom and family level.

Ecology

The tubes often appear to form colonies, although they are sometimes found in more isolated situations. The assimilation of large, sometimes monospecific, colonies can be attributed to the lack of significant predation.

Cloudina is often found in association with microbial stromatolites which are limited to shallow water; their isotopic composition suggests that water temperatures were relatively cool.

Their growth shows a periodic structure, with new layers added periodically; the ridges formed are often of varying width, suggesting a non-constant growth rate. Adolf Seilacher suggests that the organisms 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. Kinks in the developing tube are easily explained by the mat falling slightly from the horizontal. The small size of Cloudina means that it is usually found having been washed out of its place of growth, and until a specimen unquestionably in life position is discovered, its mode of life is open to debate; it has been suggested that the organism may in fact have dwelt on seaweeds.

Some specimens of Cloudina hartmannae display budding, which implies asexual reproduction.

In some locations, up to 20% of Cloudina fossils contain predatory borings ranging from 15 to 400 µm in diameter. The boreholes are rather evenly distributed along the tube length, and some tubes had been bored multiple times - hence the organism could survive attacks (since predators do not attack empty shells). This may indicate that the animal could vary its position in the tube in response to predation, or that it occupied the full length - but not the full width - of the tube. The even distribution is perhaps difficult to reconcile with an infaunal lifestyle, adding weight to Miller's suggeston that the animal lived on seaweeds or in a reef environment. Interestingly, the co-occuring and very similar shelly fossil Sinotubulites was not affected by borings. In addition, the distribution of borings suggests selection for size. This evidence of predator selectivity shows the possibility of speciation in response to predation, often postulated as a potential cause of the rapid diversification event during the Cambrian.

Fossil locations

First found in the Nama Formation in Namibia, Cloudina has also been reported in Oman, China's Dengying Formation, Canada, Argentina, Antarctica, Nevada, central Spain, northwest Mexico, California and Brazil. It is never found in association with fossils of Cambrian age, and whilst it is never in the same bed as "Vendozoan-type" (soft-bodied) Ediacaran fossils, it is sometimes interbedded with them, suggesting that the two classes of fossil may have occupied different habitats in close proximity. This co-existance stands against Mark McMenamin's hypothesis that the advent of skeletonisation caused the extinction of soft-bodied Ediacaran forms.