Information about Cloudina

Cloudina Fossil 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 early metazoan family containing the genus Cloudina, which became extinct at the base of the Cambrian period. They formed millimetre-scale conical fossils consisting of calcareous tube nested within one another; the appearance of the organism itself remains unknown. Cloudinids had a wide geographic range, reflected in the present distribution of fossiliferous localites; they are an abundant component of some deposits. They are among 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.^[1] Fossils comprise a series of stacked vase-like calcite tubes, whose original mineralogy is unknown.^[2] 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.^[2] 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.

Although it used to be thought that the tubes had test-tube like bases, detailed three-dimensional reconstruction has shown that the tubes had an open base. This evidence suggests that Cloudina is not a coral-like cnidarian, as its budding morphology might suggest,^[0] but that it is more likely to be a stem group polychaete worm,^[0] an interpretation that is reinforced by the even distribution of predator bore-holes.^[0][0] However, as with so many Ediacaran life forms, there is great debate surrounding its position in the tree of life, and classification between the kingdom and family level may be unwise.^[0][3]

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.^[0]

Cloudina is often found in association with microbial stromatolites which are limited to shallow water; their isotopic composition^[4] 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.^[5] 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. That, of the numerous specimens discovered to date, it has only been found having been washed out of its place of growth argues against Seilacher's sedimentation-driven growth hypothesis; it has instead been suggested that the organism dwelt on seaweeds.^[6] Until a specimen unquestionably in situ is discovered, its mode of life remains open to debate.

Some specimens of Cloudina hartmannae display budding,^[0] which implies asexual reproduction.^[7]

In some locations, up to 20% of Cloudina fossils contain predatory borings ranging from 15 to 400 µm in diameter.^[8][2] 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 suggestion that the animal lived on seaweeds or in a reef environment. If modern-day molluscs are a suitable analogy, the size distribution of the borings suggests that the predator was a similar size to Cloudina.^[9] Interestingly, the co-occurring and very similar shelly fossil Sinotubulites was not affected by borings. In addition, the distribution of borings suggests selection for size.^[2] 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

Cloudina occurred in calcium carbonate rich areas of stromatolite reefs. It is found in association with Namacalathus, which like Cloudina was "weakly skeletal" and solitary, and Namapoikia, which was "robustly skeletal" and formed sheets on open surfaces.^[10]

First found in the Nama Formation in Namibia,^[0] Cloudina has also been reported in Oman,^[11] China's Dengying Formation,^[2][12] Canada,^[13] Uruguay,^[14][15] Argentina,^[16] Antarctica,^[17] Brazil,^[18] Nevada,^[19] central Spain, northwest Mexico and California.^[2] It is never found in association with fossils of Cambrian age,^[20] and although 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-existence stands against Mark McMenamin's hypothesis that the advent of skeletonisation caused the extinction of soft-bodied Ediacaran forms; however the disappearance of Cloudina at the Ediacaran - Cambrian boundary suggests that an extinction event may have occurred, which may have been related to the subsequent Cambrian explosion of life-forms. Although no Cloudinid body fossils have been found after the Ediacaran period, it has been suggested that the enigmatic trace fossils Salterella and Cornulites were formed by Cloudinids.

References and footnotes