Cloudinidae

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Cloudinidae
Temporal range: 555–529  Ma [1]
Cloudina.svg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: incertae sedis
Family:† Cloudinidae
Hahn and Pflug, 1985
Species
  • Cloudina Germs, 1972
    • Cl. hartmannaeGerms 1972
    • Cl. riemkeaeGerms 1972
    • Cl. lucianoiBeurlen & Sommer, 1957) Zaine & Fairchild, 1985
    • Cl. sinensisZhang, Li et Dung, 1992
    • Cl. carinataCortijo, Musa, Jensena et Palacios, 2009
    • Cl. ningqiangensisCai et al., 2017
    • Cl. xuanjiangpingensisCai et al., 2017
  • Conotubus Zhang and Lin 1986
    • Co. hemiannulatusZhang and Lin 1986
    • (others?)
  • AcuticocloudinaHahn and Pflug, 1985
Synonyms
  • Aulophycus lucianoiBeurlen & Sommer 1957 = C. waldeiHahn & Pflug, 1985 = C. lucianoiZaine & Fairchild, 1985

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]

Contents

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.

Morphology

Cutaway diagram of Cloudina showing "living space" within the shell. Cloudina internal 01.png
Cutaway diagram of Cloudina showing "living space" within the shell.

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]

Classification

Cloudina was originally classified in 1972 as a member of the Cribricyathea, a class known from the Early Cambrian. [4] Glaessner (1976) accepted this classification and also proposed that Cloudina was similar to the annelid worms, particularly serpulid polychaetes. [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. [12] [13] Some specimens of Cloudina hartmannae display budding, [4] which implies asexual reproduction. [14] On this basis Grant (1990) classified Cloudina as a coral-like cnidarian. [8] Since the tubes had an open base, creating a single living space rather than a series of separate chambers, Cloudina is more likely to be a stem group polychaete worm, [9] in other words an evolutionary "aunt" or "cousin" of more recent polychaetes. This interpretation is reinforced by the even distribution of bore-holes made by predators. [15] [16] 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. [8] [13] [17]

Ecology

Restoration of Cloudina hartmannae with speculative mouth parts Cloudina NT.jpg
Restoration of Cloudina hartmannae with speculative mouth parts

Cloudina is usually found in association with microbial stromatolites, which are limited to shallow water; their isotopic composition [18] 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. [19] 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. [9]

In many Cloudina specimens the ridges formed by the cones are of varying width, which suggests the organisms grew at a variable rate. 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. Kinks in the developing tube are easily explained by the mat falling slightly from the horizontal. [20] 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, [9] 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. [4] On the other hand, in some locations up to 20% of Cloudina fossils contain predatory borings ranging from 15 to 400 μm in diameter. [15] [16] 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, mainly buried in a microbial mat, and adds 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 similar in size to Cloudina. [10]

Fossil findings in the Nama Group, Namibia, suggest that Cloudina was one of the first reef-building animals, [21] [22] but machine-learning facilitated 3D tomography indicates that the 'reef-forming' fossils are in fact simply aggregations of solitary individuals. [23]

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. [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] Nevada, [31] central Spain, northwest Mexico and California, [8] in west and south Siberia. The Cloudina fossils found in association with late Precambrian-Early Cambrian anabaritids SSF and tubular agglutinated skeletal fossils Platysolenites and Spirosolenites in Siberia. [32] [33]

Paleontological importance

Although not the first small shelly fossil to be found, Cloudina is one of the earliest and most abundant. [34] 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. [34] [35] While predatory borings are common in Cloudina specimens, no such borings have been found in Sinotubulites , a similar shelly fossil sometimes found in the same beds. In addition, the diameters of borings in Cloudina are proportional to the sizes of specimens, which suggests that predators were selective about the size of their prey. These two indications that predators attacked selectively suggest the possibility of speciation in response to predation, which is often postulated as a potential cause of the rapid diversification of animals in the Early Cambrian. [16]

See also


Related Research Articles

<span class="mw-page-title-main">Exoskeleton</span> External skeleton of an organism

An exoskeleton is an external skeleton that both supports the body shape and protects the internal organs of an animal, in contrast to an internal endoskeleton which is enclosed under other soft tissues. Some large, hard protective exoskeletons are known as "shells".

<i>Dickinsonia</i> Extinct genus of early animals

Dickinsonia is a genus of extinct organism that lived during the late Ediacaran period in what is now Australia, China, Russia and Ukraine, most likely a basal animal. It is one of the best known members of the Ediacaran biota. The individual Dickinsonia typically resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; its mode of growth has been considered consistent with a stem-group bilaterian affinity, though various other affinities have been proposed. The discovery of cholesterol molecules in fossils of Dickinsonia lends support to the idea that Dickinsonia was an animal, though these results have been questioned.

<i>Ausia fenestrata</i> Genus of marine filter feeders

Ausia fenestrata is a curious Ediacaran period fossil represented by only one specimen 5 cm long from the Nama Group, a Vendian to Cambrian group of stratigraphic sequences deposited in the Nama foreland basin in central and southern Namibia. It has similarity to Burykhia from Ediacaran (Vendian) siliciclastic sediments exposed on the Syuzma River of Arkhangelsk Oblast, northwest Russia. This fossil is of the form of an elongate bag-like sandstone cast tapering to a cone on one end. The surface of the fossil is covered with oval depressions ("windows") regularly spaced over the surface in the manner of concentric/parallel rows. The taxonomic identity of Ausia is unresolved.

<span class="mw-page-title-main">Doushantuo Formation</span>

The Doushantuo Formation is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerstätten in Zigui in Hubei, Xiuning in Anhui, and Weng'an in Guizhou, as one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation, whose deposits date back to the Early and Middle Ediacaran, is of particular interest because it covers the poorly understood interval of time between the end of the Cryogenian geological period and the more familiar fauna of the Late Ediacaran Avalon explosion, as well as due to its microfossils' potential utility as biostratigraphical markers. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma at its base to about 551 Ma at its top, with the most fossiliferous layer predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon Peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion at the beginning of the Phanerozoic.

<span class="mw-page-title-main">Microfossil</span> Fossil that requires the use of a microscope to see it

A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil.

<i>Spriggina</i> Extinct genus of annelid worms

Spriggina is a genus of early bilaterian animals whose relationship to living animals is unclear. Fossils of Spriggina are known from the late Ediacaran period in what is now South Australia. Spriggina floundersi is the official fossil emblem of South Australia; it has been found nowhere else.

<span class="mw-page-title-main">Vendobionta</span> Group of extinct creatures that were part of the Ediacaran biota

Vendobionts or Vendozoans (Vendobionta) are a proposed very high-level, extinct clade of benthic organisms that made up of the majority of the organisms that were part of the Ediacaran biota. It is a hypothetical group and at the same time, it would be the oldest of the animals that populated the Earth about 580 million years ago, in the Ediacaran period. They became extinct shortly after the so-called Cambrian explosion, with the introduction of fauna formed by more recognizable groups and more related to modern animals. It is very likely that the whole Ediacaran biota is not a monophyletic clade and not every genus placed in its subtaxa is an animal.

Namacalathus is a problematic metazoan fossil occurring in the latest Ediacaran. The first, and only described species, N. hermanastes, was first described in 2000 from the Nama Group of central and southern Namibia.

<i>Swartpuntia</i> Extinct genus of Ediacaran fossil

Swartpuntia is a monospecific genus of erniettomorph from the terminal Ediacaran period, with at least three quilted, leaf-shaped petaloids — probably five or six. The petaloids comprise vertical sheets of tubes filled with sand. Swartpuntia specimens range in length from 12 to 19 cm, and in width from 11.5 to 140 cm. The margin is serrated, with a 1 mm wide groove. A 14 mm wide stem extends down the middle, tapering towards the top, and stopping 25 mm from the tip. The stem has a V shaped ornamentation on it. The original fossils were found at, and named after, the Swartpunt farm between Aus and Rosh Pinah in Namibia. The generic name comes from Swartpunt, meaning black point in reference to the colour of the rocks. The specific name germsi honours Gerard Germs, who studied the Nama formation of geological beds.

Namapoikia rietoogensis is among the earliest known animals to produce a calcareous skeleton. Known from the Ediacaran period, before the Cambrian explosion of calcifying animals, the long-lived organism grew up to a metre in diameter and resembles a colonial sponge. It was an encruster, filling vertical fissures in the reefs in which it originally grew.

<span class="mw-page-title-main">Ediacaran biota</span> All organisms of the Ediacaran Period (c. 635–538.8 million years ago)

The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.

Sinotubulites is a genus of small, tube-shaped shelly fossils from the Ediacaran period. It is often found in association with Cloudina.

<span class="mw-page-title-main">Cambrian substrate revolution</span> Diversification of animal burrowing

The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is a sudden diversification of animal burrowing during the early Cambrian period.

The end-Ediacaran extinction is a mass extinction believed to have occurred near the end of the Ediacaran period, the final period of the Proterozoic eon. Evidence suggesting that such a mass extinction occurred includes a massive reduction in diversity of acritarchs, the sudden disappearance of the Ediacara biota and calcifying organisms, and the time gap before Cambrian organisms "replaced" them. Some lines of evidence suggests that there may have been two distinct pulses of the extinction event, one occurring 550 million years ago and the other 539 million years ago.

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.

The Cambrian explosion, Cambrian radiation,Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately 538.8 million years ago in the Cambrian Period of early Paleozoic when there was a sudden radiation of complex life and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.

Ediacaran type preservation relates to the dominant preservational mode in the Ediacaran period, where Ediacaran organisms were preserved as casts on the surface of microbial mats.

<span class="mw-page-title-main">Nama Group</span>

The Nama Group is a 125,000 square kilometres (48,000 sq mi) megaregional Vendian to Cambrian group of stratigraphic sequences deposited in the Nama foreland basin in central and southern Namibia. The Nama Basin is a peripheral foreland basin, and the Nama Group was deposited in two early basins, the Zaris and Witputs, to the north, while the South African Vanrhynsdorp Group was deposited in the southern third. The Nama Group is made of fluvial and shallow-water marine sediments, both siliciclastic and carbonate. La Tinta Group in Argentina is considered equivalent to Nama Group.

<i>Arumberia</i> Trace fossil

Arumberia is an enigmatic fossil from the Ediacaran period originally described from the Arumbera Sandstone, Northern Territory, Australia but also found in the Urals, East Siberia, England and Wales, Northern France, the Avalon Peninsula and India. Several morphologically distinct species are recognized.

References

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  18. ↑ Ca/Mg ratios
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  28. ↑ by Yochelson and Herrera, 1974; they could have mistaken them for Salterella. See Grant 1990 for reference and discussion.
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