Small shelly fauna
Encyclopedia
The small shelly fauna or small shelly fossils, abbreviated to SSF, are mineralized fossil
s, 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 trilobite
s. 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.
Some of the fossils represent the entire skeleton
s of small organisms, including the mysterious Cloudina and some snail-like molluscs. However, the bulk of the fossils are fragments or disarticulated remains of larger organisms, including sponges, molluscs, slug-like halkieriids, brachiopods, echinoderm
s, and onychophoran-like organisms that may have been close to the ancestors of arthropod
s.
One of the early explanations for the appearance of the SSFs – and therefore the evolution of mineralized skeleton
s – suggested a sudden increase in the ocean's concentration of calcium
. However, many SSFs are constructed of other minerals, such as silica. Because the first SSFs appear around the same time as organisms first started burrowing to avoid predation, it is more likely that they represent early steps in an evolutionary arms race
between predators and increasingly well-defended prey. On the other hand mineralized skeleton
s may have evolved simply because they are stronger and cheaper to produce than all-organic skeletons like those of insects. Nevertheless it is still true that the animals used minerals that were most easily accessible.
Although the small size and often fragmentary nature of SSFs makes it difficult to identify and classify them, they provide very important evidence for how the main groups of marine invertebrate
s evolved, and particularly for the pace and pattern of evolution in the Cambrian explosion
. Besides including the earliest known representatives of some modern phyla
, they have the great advantage of presenting a nearly continuous record of Early Cambrian organisms whose bodies include hard parts.
since, as Stefan Bengtson says, "they are not always small, they are commonly not shelly – and the term might equally well apply to Pleistocene
periwinkles." Paleontologists have been unable to invent a better term and have vented their frustration in parodies such as "small silly fossils" and "small smellies". The term is often abbreviated to "small shellies" or "SSF".
The great majority of all the morphological features of later shelled organisms appear among the SSFs. No-one has attempted a formal definition of "small shelly fauna", "small shelly fossils" or other similar phrases.
Specimens and sometimes quite rich collections of these fossils were discovered between 1872 and 1967, but no-one drew the conclusion that the Early Cambrian contained a diverse range of animals in addition to the traditionally recognized trilobite
s, archaeocyatha
ns, etc. In the late 1960s Soviet
paleontologists discovered even richer collections of SSFs in beds below and therefore earlier than those containing Cambrian trilobites. Unfortunately the papers that described these discoveries were in Russian
, and the 1975 paper by Matthews and Missarzhevsky first brought the SSFs to the serious attention of the non-Russian-reading world.
There was already a vigorous debate about the early evolution of animal
s. Preston Cloud
argued in 1948 and 1968 that the process was "explosive", and in the early 1970s Niles Eldredge
and Stephen Jay Gould
developed their theory of punctuated equilibrium
, which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change. On the other hand around the same time Wyatt Durham and Martin Glaessner
both argued that the animal kingdom had a long Proterozoic
history that was hidden by the lack of fossils.
, Mongolia
, Kazakhstan
, Australia
, and Antarctica; and moderately diverse
ones in India
, Pakistan
, Iran
, Europe
and North America
. There are different views about the time range of the SSFs. The Russian discoveries of the late 1960s were assigned to the Tommotian age of the Cambrian
period, and for some time the term "small shelly fauna" was applied only to that age. On the other hand Bengston includes in "SSF" Ediacaran
fossils like Cloudina and post-Tommotian fossils like Microdictyon
from the Maotianshan shales
lagerstätte
. SSFs have been found in layers that also contain fossil trilobite
s. The mass extinction at the end of the Cambrian period's Botomian age was thought to have wiped out most of the SSF, with the exception of the halkieriids, wiwaxiids and Pojetaia.
. Whilst some shellies were originally phosphatic, in most cases the phosphate represents a replacement of the original calcite. They are usually extracted from limestone by placing the limestone in a weak acid, typically acetic acid
; the phosphatized fossils remain after the rock is dissolved away. Preservation of microfossils by phosphate seems to have become less common after the early Cambrian, perhaps as a result of increased disturbance of sea-floors by burrowing animals
. Without this fossil-forming mode, may small shelly fossils may not have been preserved – or been impossible to extract from the rock; hence the animals that produced these fossils may have lived beyond the Early Cambrian – the apparent extinction of most SSFs by the end of the Cambrian
may be an illusion. For decades it was thought that halkieriids, whose "armor plates" are a common type of SSF, perished in the end-Botomian mass extinction; but in 2004 halkieriid armor plates were reported from Mid Cambrian rocks in Australia, a good 10 million years more recent than that.
and calcium carbonate
. The minerals used by each organism are influenced by the chemistry of the oceans the organism first evolved in, but then continue to be used even if the ocean chemistry changes. For example in the Ediacaran
period and the Nemakit-Daldynian age of the Cambrian
, those animals that used calcium carbonate
used the form called aragonite
. On the other hand animals that first appeared in the following Tommotian age used another form, calcite
.
A recently-discovered modern gastropod that lives near deep-sea hydrothermal vent
s illustrates the influence of both earlier and contemporary local chemical environments: its shell is made of aragonite
, which is found in the earliest fossil molluscs; but it also has armor plates on the sides of its foot, and these are mineralized with the iron sulfides pyrite
and greigite
, which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents.
Methods of constructing shells vary widely among the SSF, and in most cases the exact mechanisms are not known.
Biomineralization is the production of mineralized parts by organisms. Hypotheses to explain the evolution of biomineralization include physiological adaptation to changing chemistry of the oceans, defense against predators and the opportunity to grow larger. The functions of biomineralization in SSFs vary: some SSFs are not yet understood; some are components of armor; and some are skeleton
s. A skeleton is any fairly rigid structure of an animal, irrespective of whether it has joints and irrespective of whether it is biomineralized. Although some SSFs may not be skeletons, SSFs are biomineralized by definition, being shelly. Skeletons provide a wide range of possible advantages, including : protection, support, attachment to a surface, a platform or set of levers for muscles to act on, traction when moving on a surface, food handling, provision of filtration
chambers and storage of essential substances.
It has often been suggested that biomineralization evolved as a response to an increase in the concentration of calcium
in the seas, which happened around the Ediacaran
-Cambrian
boundary, and that biomineralization's main benefit was to store harmlessly minerals that might have disrupted organisms' internal processes. For example Mikhail A. Fedonkin
suggested that an increase in the length of food chain
s may have contributed, as animals higher up the food chain accumulate greater amounts of waste products and toxins relative to their size, and biomineralization may have been a way of isolating excess carbonates or silicates consumed with prey. However, biomineralizing a skeleton is a fairly expensive way to dispose safely of excess minerals, as the main construction cost is the organic matrix, mostly protein
s and polysaccharide
s, with which minerals are combined to form composite material
s. The idea that biomineralization was a response to changes in ocean chemistry is also undermined by the fact that small shelly fossils made of calcite
, aragonite
, calcium phosphate
and silica appeared virtually simultaneously in a range of environments.
Organisms started burrowing to avoid predation
at around the same time. Jerzy Dzik suggested that biomineralization of skeletons was a defense against predators, marking the start of an evolutionary arms race
. He cited as another example of hardened defenses from this time the fact that the earliest protective "skeletons" included glued-together collections of inorganic objects — for example the Early Cambrian worm Onuphionella built a tube covered with mica
flakes. Such a strategy required both anatomical adaptations that allowed organisms to collect and glue objects and also moderately sophisticated nervous systems to co-ordinate this behavior.
On the other hand Bernard Cohen argued that biomineralized skeletons arose for "engineering" reasons rather than as defenses. There are many other defensive strategies available to prey animals including mobility and acute senses, chemical defenses, and concealment. Mineral-organic composites are both stronger and cheaper to build than all-organic skeletons, and these two advantages would have made it possible for animals to grow larger and, in some cases, more muscular — in animals beyond a certain size, the larger muscles and their greater lever
age produce forces all-organic skeletons are not rigid enough to withstand. The development of modern brachiopod
s includes a progression from all-organic to mineral-organic composite shells, which may be a clue to their evolutionary development. The evolution of rigid biomineralized exoskeletons may then have started an arms race in which predators developed drills or chemical weapons capable of penetrating shells, some prey animals developed heavier, tougher shells, etc.
Fedonkin suggested another explanation for the appearance of biomineralization around the start of the Cambrian: the Ediacara biota
evolved and flourished in cold waters, which slowed their metabolisms and left them with insufficient spare energy for biomineralization; but there are signs of global warming around the start of the Cambrian, which would have made biomineralization easier. A similar pattern is visible in living marine animals, since biomineralized skeletons are rarer and more fragile in polar waters than in the tropics.
s, holes that are thought to have been made by predators. The very similar shelly fossil Sinotubulites
, which is often found in the same locations, was not affected by borings. In addition, the distribution of borings in Cloudina suggests selection for size – the largest holes appear in the largest shells. This evidence of selective attacks by predators suggests that new species may have arisen in response to predation, which is often presented as a potential cause of the rapid diversification of animals in the Early Cambrian.
The small shellies provide a relatively continuous record throughout the early Cambrian, and thus provide a more useful insight into the Cambrian explosion
than instances of exceptional preservation
. Although most of the SSFs are difficult to identify, those assigned positions in modern taxa
, or in their stem groups of evolutionary "aunts" or "cousins", enable scientists to assess the pattern and speed of animal evolution on the strength of the small shelly evidence.
Such an assessment shows that the earliest small shellies are the most basal. As time goes on, they can be placed in the stem group to an ever smaller clade. In other words, the earliest (Ediacaran) small shellies can be tentatively considered diploblastic, in other words made of two main tissue layers. Later shellies are more convincingly diploblastic. Subsequently the Helcionellid
s are the first shelly fossils that can be placed in the stem group to a phylum (mollusca
). As one looks at more recent SSFs, the arguments for stem group placements become stronger, and by the Atdabanian, some SSFs can be assigned to the crown group of a modern phylum, echinoderm
s.
This gives the impression that the first SSF animals, from the late Ediacaran, were basal members of later clades, with the phyla subsequently appearing in a "rapid, but nevertheless resolvable and orderly" fashion, rather than as a "sudden jumble", and thus reveals the true pace of the Cambrian explosion
.
The few collections of SSF from the Ediacaran
period have a limited range of forms Fully and partially mineralized tubes are common and form a really mixed collection: the structures and compositions of their walls vary widely; specimens have been classified as members of a wide range of clades
including foraminifera
ns, cnidarians, polychaete
and pogonophoran
annelid
s, sipunculids and others. Cloudina’s "tube", which was 8 to 150 mm (0.31496062992126 to 5.9 in) long, consisted of nested cones that were mineralized with calcium carbonate
but left unmineralized gaps between the cones. Sinotubulites
built long thin tubes that were more flexible but probably had mineralized ridges.
Namapoikia
was probably either a sponge or a coral
-like organism, and built dwellings up to 1 metres (39.4 in) across out of calcium carbonate.
Spicule
s are spines or star-like combinations of spines, made of silica, and are thought to be the remains of sponges.
Namacalathus
, which may have been a cnidarian, closely related to jellyfish
and coral
s, built goblet-like dwellings with stalks up to 30 millimetres (1.2 in) long. This type of shape is known as a "stalked test", since "test" in biology means a roughly spherical shell.
s, and onychophoran-like organisms that may have been close to the ancestors of arthropods.
Most of the Cambrian SSF consists of sclerites, fragments that once made up the external armor of early animals, such as Halkieria
or "scale worms". Fairly complete and assembled sets, which are rare, are called "scleritomes". In many cases the body shapes of sclerites' creators and the distribution of sclerites on their bodies are not known. The "coat of mail
" generally disintegrated once the animal died, and its fragments became dispersed and sometimes fossilized. Reconstructing these elements usually relies upon a fully articulated fossil being found in an exceptionally preserved lagerstatte
. Such discoveries may in turn enable paleontologists to make sense of other similar fragments, such as those labelled Maikhanella
.
Many sclerites are of the type called "coelosclerites", which have a mineralized shell around a space originally filled with organic tissue and which show no evidence of accretionary growth. It is not clear whether coelosclerites evolved independently
in different groups of animals or were inherited from a common ancestor.
Halkieriids produced scale- or spine-shaped coelosclerites, and complete specimens show that the animals were slug
-shaped, and had cap-shaped shell plates at both ends in addition to the sclerites. Chancelloriids
produced star-shaped composite coelosclerites. They are known to have been animals that looked like cacti
and have been described as internally like sponges, although they may have been more closely related to halkieriids.
Some sclerites are mineralized with calcium phosphate
rather than calcium carbonate
. Tommotiids have a wide range of sclerite shapes and internal structures, and may in fact represent a polyphyletic set of lineages, in other words they may have independently developed phosphatic scleritomes rather than inheriting them from a common ancestor. On the other hand they may be closely related to the ancestors of modern brachiopod
s, animals that at first sight look like bivalve molluscs, but brachiopods stand on fleshy stalks and their internal anatomy is different. Some sclerites and small pieces of "debris" are regarded as the remains of echinoderm
s. Other phosphatic sclerites include tooth-shaped, hook-shaped and plate-like objects, mostly of unknown origin. However it is known that some, including Microdictyon
, were produced by lobopods, animals that looked like worms with legs and are thought to be close to the ancestors of arthropod
s.
Univalved and bivalved shells are fairly common. Some cap-shaped shells are thought to be the only sclerite covering their creators, while others are known to be parts of a more complex armor system like Halkieria
’s. The Helcionellid
s are thought to be early molluscs with somewhat snail-like shells. Some have horizontal "exhaust pipes" on the concave edges of their shells, and there is debate about whether these pointed forwards or backwards. Hyoliths left small conical shells. These animals may have been molluscs or worm-like Sipuncula
. Other molluscan univalved shells have been found in Canada
. Some bivalve shells have been found with both parts still joined, and include both brachiopod
s and bivalve molluscs. Fossils have been found that resemble the opercula
("lids") used by snails to close the openings in their armor, and are attributed to hyoliths, small animals that had conical shells and may have been molluscs or worm-like Sipuncula
.
Small arthropod
s with bivalve-like shells have been found in Early Cambrian beds in China, and other fossils (Mongolitubulus
henrikseni) represent spines that snapped off bivalved arthropod carapaces.
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
to the end of the Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
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 trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s. Since most SSFs were preserved by being covered quickly with phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
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.
Some of the fossils represent the entire skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s of small organisms, including the mysterious Cloudina and some snail-like molluscs. However, the bulk of the fossils are fragments or disarticulated remains of larger organisms, including sponges, molluscs, slug-like halkieriids, brachiopods, echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, and onychophoran-like organisms that may have been close to the ancestors of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s.
One of the early explanations for the appearance of the SSFs – and therefore the evolution of mineralized skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s – suggested a sudden increase in the ocean's concentration of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
. However, many SSFs are constructed of other minerals, such as silica. Because the first SSFs appear around the same time as organisms first started burrowing to avoid predation, it is more likely that they represent early steps in an evolutionary arms race
Evolutionary arms race
In evolutionary biology, an evolutionary arms race is an evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other, resembling an arms race, which are also examples of positive feedback...
between predators and increasingly well-defended prey. On the other hand mineralized skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s may have evolved simply because they are stronger and cheaper to produce than all-organic skeletons like those of insects. Nevertheless it is still true that the animals used minerals that were most easily accessible.
Although the small size and often fragmentary nature of SSFs makes it difficult to identify and classify them, they provide very important evidence for how the main groups of marine invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s evolved, and particularly for the pace and pattern of evolution in the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
. Besides including the earliest known representatives of some modern phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
, they have the great advantage of presenting a nearly continuous record of Early Cambrian organisms whose bodies include hard parts.
History of discovery
The term "small shelly fossils" was coined by Samuel Matthews and V.V. Missarzhevsky in 1975. It is quite a misnomerMisnomer
A misnomer is a term which suggests an interpretation that is known to be untrue. Such incorrect terms sometimes derive their names because of the form, action, or origin of the subject becoming named popularly or widely referenced—long before their true natures were known.- Sources of misnomers...
since, as Stefan Bengtson says, "they are not always small, they are commonly not shelly – and the term might equally well apply to Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
periwinkles." Paleontologists have been unable to invent a better term and have vented their frustration in parodies such as "small silly fossils" and "small smellies". The term is often abbreviated to "small shellies" or "SSF".
The great majority of all the morphological features of later shelled organisms appear among the SSFs. No-one has attempted a formal definition of "small shelly fauna", "small shelly fossils" or other similar phrases.
Specimens and sometimes quite rich collections of these fossils were discovered between 1872 and 1967, but no-one drew the conclusion that the Early Cambrian contained a diverse range of animals in addition to the traditionally recognized trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s, archaeocyatha
Archaeocyatha
The Archaeocyatha or archaeocyathids were sessile, reef-building marine organisms of warm tropical and subtropical waters that lived during the early Cambrian period. It is believed that the centre of the Archaeocyatha origin is in East Siberia, where they are first known from the beginning of...
ns, etc. In the late 1960s Soviet
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
paleontologists discovered even richer collections of SSFs in beds below and therefore earlier than those containing Cambrian trilobites. Unfortunately the papers that described these discoveries were in Russian
Russian language
Russian is a Slavic language used primarily in Russia, Belarus, Uzbekistan, Kazakhstan, Tajikistan and Kyrgyzstan. It is an unofficial but widely spoken language in Ukraine, Moldova, Latvia, Turkmenistan and Estonia and, to a lesser extent, the other countries that were once constituent republics...
, and the 1975 paper by Matthews and Missarzhevsky first brought the SSFs to the serious attention of the non-Russian-reading world.
There was already a vigorous debate about the early evolution of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s. Preston Cloud
Preston Cloud
Preston Ercelle Cloud, Jr. was an American paleontologist, geographer, and professor. He was best-known for his work on the geologic time scale and the origin of life on Earth.-Early life:...
argued in 1948 and 1968 that the process was "explosive", and in the early 1970s Niles Eldredge
Niles Eldredge
Niles Eldredge is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972.-Education:...
and Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
developed their theory of punctuated equilibrium
Punctuated equilibrium
Punctuated equilibrium is a theory in evolutionary biology which proposes that most species will exhibit little net evolutionary change for most of their geological history, remaining in an extended state called stasis...
, which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change. On the other hand around the same time Wyatt Durham and Martin Glaessner
Martin Glaessner
Martin Fritz Glaessner AM was a geologist and palaeontologist. Born and educated in Austro-Hungarian Empire, he spent the majority of his life in working for oil companies in Russia, and studying the geology of the South Pacific in Australia...
both argued that the animal kingdom had a long Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
history that was hidden by the lack of fossils.
Occurrence
Rich collections have been found in ChinaChina
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, Mongolia
Mongolia
Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest...
, Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, and Antarctica; and moderately diverse
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
ones in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
, Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. There are different views about the time range of the SSFs. The Russian discoveries of the late 1960s were assigned to the Tommotian age of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period, and for some time the term "small shelly fauna" was applied only to that age. On the other hand Bengston includes in "SSF" Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
fossils like Cloudina and post-Tommotian fossils like Microdictyon
Microdictyon
Microdictyon is an extinct "armored worm" coated with dot-likescleritic scales, known from the Early CambrianMaotianshan shale of Yunnan China. Microdictyon is sometimes included...
from the Maotianshan shales
Maotianshan shales
The Maotianshan Shales are a series of lower Cambrian deposits in the Chiungchussu formation, famous for their Konservat Lagerstätten, or high number of fossils preserved in place...
lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
. SSFs have been found in layers that also contain fossil trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s. The mass extinction at the end of the Cambrian period's Botomian age was thought to have wiped out most of the SSF, with the exception of the halkieriids, wiwaxiids and Pojetaia.
Mode of preservation
Small shelly fossils are typically, although not always, preserved in phosphatePhosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
. Whilst some shellies were originally phosphatic, in most cases the phosphate represents a replacement of the original calcite. They are usually extracted from limestone by placing the limestone in a weak acid, typically acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
; the phosphatized fossils remain after the rock is dissolved away. Preservation of microfossils by phosphate seems to have become less common after the early Cambrian, perhaps as a result of increased disturbance of sea-floors by burrowing animals
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
. Without this fossil-forming mode, may small shelly fossils may not have been preserved – or been impossible to extract from the rock; hence the animals that produced these fossils may have lived beyond the Early Cambrian – the apparent extinction of most SSFs by the end of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
may be an illusion. For decades it was thought that halkieriids, whose "armor plates" are a common type of SSF, perished in the end-Botomian mass extinction; but in 2004 halkieriid armor plates were reported from Mid Cambrian rocks in Australia, a good 10 million years more recent than that.
Minerals used in shells
Small shelly fossils are composed of a variety of minerals, the most important being silica, calcium phosphateCalcium phosphate
Calcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
and calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
. The minerals used by each organism are influenced by the chemistry of the oceans the organism first evolved in, but then continue to be used even if the ocean chemistry changes. For example in the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period and the Nemakit-Daldynian age of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
, those animals that used calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
used the form called aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
. On the other hand animals that first appeared in the following Tommotian age used another form, calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
.
A recently-discovered modern gastropod that lives near deep-sea hydrothermal vent
Hydrothermal vent
A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both...
s illustrates the influence of both earlier and contemporary local chemical environments: its shell is made of aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
, which is found in the earliest fossil molluscs; but it also has armor plates on the sides of its foot, and these are mineralized with the iron sulfides pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
and greigite
Greigite
Greigite is an iron sulfide mineral with formula Fe3S4. It is the sulfur equivalent of the iron oxide magnetite . It was first described in 1964 for an occurrence in San Bernardino County, California, and named after the mineralogist and physical chemist Joseph W...
, which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents.
Methods of constructing shells vary widely among the SSF, and in most cases the exact mechanisms are not known.
Evolution of skeletons and biomineralization
| Biomineralized | |||
| No | Yes | ||
| Skeleton | No | Dickinsonia Dickinsonia Dickinsonia is an iconic fossil of the Ediacaran biota. It resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; most interpretations consider it to be an animal, although others suggest it may be fungal, or a member of an "extinct kingdom".-Species variety:A... |
Halkieria Halkieria Halkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages... sclerites |
| Yes | Kimberella Kimberella Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in... |
Helcionellid Helcionellid The Helcionellids are fossil shells of very small marine mollusks that are preserved as shells. These animals are first found about in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician... s |
|
Biomineralization is the production of mineralized parts by organisms. Hypotheses to explain the evolution of biomineralization include physiological adaptation to changing chemistry of the oceans, defense against predators and the opportunity to grow larger. The functions of biomineralization in SSFs vary: some SSFs are not yet understood; some are components of armor; and some are skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s. A skeleton is any fairly rigid structure of an animal, irrespective of whether it has joints and irrespective of whether it is biomineralized. Although some SSFs may not be skeletons, SSFs are biomineralized by definition, being shelly. Skeletons provide a wide range of possible advantages, including : protection, support, attachment to a surface, a platform or set of levers for muscles to act on, traction when moving on a surface, food handling, provision of filtration
Filtration
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
chambers and storage of essential substances.
It has often been suggested that biomineralization evolved as a response to an increase in the concentration of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
in the seas, which happened around the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
-Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
boundary, and that biomineralization's main benefit was to store harmlessly minerals that might have disrupted organisms' internal processes. For example Mikhail A. Fedonkin
Mikhail A. Fedonkin
Dr. Mikhail Aleksandrovich Fedonkin is an awarding winning paleontologist specializing in documentation of the earliest animals' body fossils, tracks, and trails. He was the first to describe several fossils including Hiemalora, Onega stepanovi, and Nimbia occlusa.Fedonkin is fluent in English and...
suggested that an increase in the length of food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
s may have contributed, as animals higher up the food chain accumulate greater amounts of waste products and toxins relative to their size, and biomineralization may have been a way of isolating excess carbonates or silicates consumed with prey. However, biomineralizing a skeleton is a fairly expensive way to dispose safely of excess minerals, as the main construction cost is the organic matrix, mostly protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s and polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
s, with which minerals are combined to form composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
s. The idea that biomineralization was a response to changes in ocean chemistry is also undermined by the fact that small shelly fossils made of calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
, calcium phosphate
Calcium phosphate
Calcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
and silica appeared virtually simultaneously in a range of environments.
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
at around the same time. Jerzy Dzik suggested that biomineralization of skeletons was a defense against predators, marking the start of an evolutionary arms race
Evolutionary arms race
In evolutionary biology, an evolutionary arms race is an evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other, resembling an arms race, which are also examples of positive feedback...
. He cited as another example of hardened defenses from this time the fact that the earliest protective "skeletons" included glued-together collections of inorganic objects — for example the Early Cambrian worm Onuphionella built a tube covered with mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
flakes. Such a strategy required both anatomical adaptations that allowed organisms to collect and glue objects and also moderately sophisticated nervous systems to co-ordinate this behavior.
On the other hand Bernard Cohen argued that biomineralized skeletons arose for "engineering" reasons rather than as defenses. There are many other defensive strategies available to prey animals including mobility and acute senses, chemical defenses, and concealment. Mineral-organic composites are both stronger and cheaper to build than all-organic skeletons, and these two advantages would have made it possible for animals to grow larger and, in some cases, more muscular — in animals beyond a certain size, the larger muscles and their greater lever
Lever
In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...
age produce forces all-organic skeletons are not rigid enough to withstand. The development of modern brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s includes a progression from all-organic to mineral-organic composite shells, which may be a clue to their evolutionary development. The evolution of rigid biomineralized exoskeletons may then have started an arms race in which predators developed drills or chemical weapons capable of penetrating shells, some prey animals developed heavier, tougher shells, etc.
Fedonkin suggested another explanation for the appearance of biomineralization around the start of the Cambrian: the Ediacara biota
Ediacara biota
The Ediacara biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms which lived during the Ediacaran Period . Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.Simple multicellular organisms such as...
evolved and flourished in cold waters, which slowed their metabolisms and left them with insufficient spare energy for biomineralization; but there are signs of global warming around the start of the Cambrian, which would have made biomineralization easier. A similar pattern is visible in living marine animals, since biomineralized skeletons are rarer and more fragile in polar waters than in the tropics.
Evolutionary significance
In some locations, up to 20% of Cloudina fossils show boringBoring
Boring often refers to anything that causes boredom.Boring may also refer to:Making holes*Boring , the drilling of holes or tunnels in the earth**Tunnel boring machine machine used in boring tunnels or shafts...
s, holes that are thought to have been made by predators. The very similar shelly fossil Sinotubulites
Sinotubulites
Sinotubulites is a genus of small, tube-shaped shelly fossils from the Ediacaran period. It is often found in association with Cloudina.Its tube has a "tube-in-tube" structure composed of several thin layers...
, which is often found in the same locations, was not affected by borings. In addition, the distribution of borings in Cloudina suggests selection for size – the largest holes appear in the largest shells. This evidence of selective attacks by predators suggests that new species may have arisen in response to predation, which is often presented as a potential cause of the rapid diversification of animals in the Early Cambrian.
The small shellies provide a relatively continuous record throughout the early Cambrian, and thus provide a more useful insight into the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
than instances of exceptional preservation
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
. Although most of the SSFs are difficult to identify, those assigned positions in modern taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, or in their stem groups of evolutionary "aunts" or "cousins", enable scientists to assess the pattern and speed of animal evolution on the strength of the small shelly evidence.
Such an assessment shows that the earliest small shellies are the most basal. As time goes on, they can be placed in the stem group to an ever smaller clade. In other words, the earliest (Ediacaran) small shellies can be tentatively considered diploblastic, in other words made of two main tissue layers. Later shellies are more convincingly diploblastic. Subsequently the Helcionellid
Helcionellid
The Helcionellids are fossil shells of very small marine mollusks that are preserved as shells. These animals are first found about in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician...
s are the first shelly fossils that can be placed in the stem group to a phylum (mollusca
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...
). As one looks at more recent SSFs, the arguments for stem group placements become stronger, and by the Atdabanian, some SSFs can be assigned to the crown group of a modern phylum, echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s.
This gives the impression that the first SSF animals, from the late Ediacaran, were basal members of later clades, with the phyla subsequently appearing in a "rapid, but nevertheless resolvable and orderly" fashion, rather than as a "sudden jumble", and thus reveals the true pace of the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
.
Ediacaran forms
 |
 |
The few collections of SSF from the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period have a limited range of forms Fully and partially mineralized tubes are common and form a really mixed collection: the structures and compositions of their walls vary widely; specimens have been classified as members of a wide range of clades
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
including foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
ns, cnidarians, polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
and pogonophoran
Siboglinidae
Siboglinidae, also known as the beard worms, is a family of polychaete annelid worms whose members made up the former phyla Pogonophora and Vestimentifera. They are composed of about 100 species of vermiform creatures and live in thin tubes buried in sediments at ocean depths from 100 to 10,000 m...
annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s, sipunculids and others. Cloudina’s "tube", which was 8 to 150 mm (0.31496062992126 to 5.9 in) long, consisted of nested cones that were mineralized with calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
but left unmineralized gaps between the cones. Sinotubulites
Sinotubulites
Sinotubulites is a genus of small, tube-shaped shelly fossils from the Ediacaran period. It is often found in association with Cloudina.Its tube has a "tube-in-tube" structure composed of several thin layers...
built long thin tubes that were more flexible but probably had mineralized ridges.
Namapoikia
Namapoikia
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 organism grew up to a metre in diameter and resembles a colonial sponge or cnidarian...
was probably either a sponge or a coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
-like organism, and built dwellings up to 1 metres (39.4 in) across out of calcium carbonate.
Spicule
Spicule
Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....
s are spines or star-like combinations of spines, made of silica, and are thought to be the remains of sponges.
Namacalathus
Namacalathus
NamacalathusNama, from the geological group where it was described, + Greek κάλαθος, kalathos, meaning "basket in the shape of a lily", or "wine goblet" is a problematic metazoan fossil occurring in the latest Ediacaran. The single species N...
, which may have been a cnidarian, closely related to jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
and coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s, built goblet-like dwellings with stalks up to 30 millimetres (1.2 in) long. This type of shape is known as a "stalked test", since "test" in biology means a roughly spherical shell.
Cambrian forms
In finds from the early Cambrian, tubes and spicules become more abundant and diverse, and new types of SSF appear. Many have been attributed to well-known groups such as molluscs, slug-like halkieriids, brachiopods, echinodermEchinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, and onychophoran-like organisms that may have been close to the ancestors of arthropods.
Halkieria
Halkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
or "scale worms". Fairly complete and assembled sets, which are rare, are called "scleritomes". In many cases the body shapes of sclerites' creators and the distribution of sclerites on their bodies are not known. The "coat of mail
Mail (armour)
Mail is a type of armour consisting of small metal rings linked together in a pattern to form a mesh.-History:Mail was a highly successful type of armour and was used by nearly every metalworking culture....
" generally disintegrated once the animal died, and its fragments became dispersed and sometimes fossilized. Reconstructing these elements usually relies upon a fully articulated fossil being found in an exceptionally preserved lagerstatte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
. Such discoveries may in turn enable paleontologists to make sense of other similar fragments, such as those labelled Maikhanella
Maikhanella
Maikhanella is a genus of small shelly fossil; it is a 'scaly' shell whose scales are equivalent to sclerites of the genus Siphogonuchites. Not just an agglutination of sclerites, the shells have a calcified matrix. Juvenile shells appear not to incorporate sclerites....
.
Many sclerites are of the type called "coelosclerites", which have a mineralized shell around a space originally filled with organic tissue and which show no evidence of accretionary growth. It is not clear whether coelosclerites evolved independently
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
in different groups of animals or were inherited from a common ancestor.
Halkieriids produced scale- or spine-shaped coelosclerites, and complete specimens show that the animals were slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
-shaped, and had cap-shaped shell plates at both ends in addition to the sclerites. Chancelloriids
Chancelloriidae
The Chancelloriids are an extinct family of animal common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism...
produced star-shaped composite coelosclerites. They are known to have been animals that looked like cacti
Cactus
A cactus is a member of the plant family Cactaceae. Their distinctive appearance is a result of adaptations to conserve water in dry and/or hot environments. In most species, the stem has evolved to become photosynthetic and succulent, while the leaves have evolved into spines...
and have been described as internally like sponges, although they may have been more closely related to halkieriids.
Calcium phosphate
Calcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
rather than calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
. Tommotiids have a wide range of sclerite shapes and internal structures, and may in fact represent a polyphyletic set of lineages, in other words they may have independently developed phosphatic scleritomes rather than inheriting them from a common ancestor. On the other hand they may be closely related to the ancestors of modern brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s, animals that at first sight look like bivalve molluscs, but brachiopods stand on fleshy stalks and their internal anatomy is different. Some sclerites and small pieces of "debris" are regarded as the remains of echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s. Other phosphatic sclerites include tooth-shaped, hook-shaped and plate-like objects, mostly of unknown origin. However it is known that some, including Microdictyon
Microdictyon
Microdictyon is an extinct "armored worm" coated with dot-likescleritic scales, known from the Early CambrianMaotianshan shale of Yunnan China. Microdictyon is sometimes included...
, were produced by lobopods, animals that looked like worms with legs and are thought to be close to the ancestors of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s.
Univalved and bivalved shells are fairly common. Some cap-shaped shells are thought to be the only sclerite covering their creators, while others are known to be parts of a more complex armor system like Halkieria
Halkieria
Halkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
’s. The Helcionellid
Helcionellid
The Helcionellids are fossil shells of very small marine mollusks that are preserved as shells. These animals are first found about in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician...
s are thought to be early molluscs with somewhat snail-like shells. Some have horizontal "exhaust pipes" on the concave edges of their shells, and there is debate about whether these pointed forwards or backwards. Hyoliths left small conical shells. These animals may have been molluscs or worm-like Sipuncula
Sipuncula
The Sipuncula or Sipunculida is a group containing 144-320 species of bilaterally symmetrical, unsegmented marine worms...
. Other molluscan univalved shells have been found in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. Some bivalve shells have been found with both parts still joined, and include both brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s and bivalve molluscs. Fossils have been found that resemble the opercula
Operculum (gastropod)
The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...
("lids") used by snails to close the openings in their armor, and are attributed to hyoliths, small animals that had conical shells and may have been molluscs or worm-like Sipuncula
Sipuncula
The Sipuncula or Sipunculida is a group containing 144-320 species of bilaterally symmetrical, unsegmented marine worms...
.
Small arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s with bivalve-like shells have been found in Early Cambrian beds in China, and other fossils (Mongolitubulus
Mongolitubulus
Mongolitubulus is a genus of small shelly fossil. It is potentially a synonym with Rushtonites, and owing to the similarity of the two genera they are both dealt with herein.- Morphology :...
henrikseni) represent spines that snapped off bivalved arthropod carapaces.