Small shelly fauna

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Part of a series on
The Cambrian explosion
Fossil localities
Key organisms
Ediacaran biota
Burgess-type
Small shelly fauna
Evolutionary concepts
Trends
Themes

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF)

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.

Some of the fossils represent the entire

molluscs. However, the bulk of the fossils are fragments or disarticulated remains of larger organisms, including sponges, molluscs, slug-like halkieriids, brachiopods, echinoderms, and onychophoran-like organisms that may have been close to the ancestors of arthropods
.

One of the early explanations for the appearance of the SSFs – and therefore the evolution of mineralized skeletons – suggested a sudden increase in the ocean's concentration of

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 skeletons may have evolved simply because they are stronger and take less energy 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 invertebrates 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.

History of discovery

Axis scale: millions of years ago.


References for dates:
Ediacara biota[2]
Small shelly fauna[3], but may have been longer[4][5]
Tommotian age[6]
Cambrian explosion[7]

Maotianshan shales[8]

The term "small shelly fossils" was coined by Samuel Matthews and V. V. Missarzhevsky in 1975.

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".[3]

The great majority of all the morphological features of later shelled organisms appear among the SSFs.[3][5] No-one has attempted a formal definition of "small shelly fauna", "small shelly fossils" or other similar phrases.[10]

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 trilobites, archaeocyathans, 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.[3]

There was already a vigorous debate about the early evolution of animals. Preston Cloud argued in 1948 and 1968 that the process was "explosive",[11] 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.[12] 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.[3][13]

Occurrence

Rich collections have been found in China,Russia,

age was thought to have wiped out most of the SSF, with the exception of the halkieriids, wiwaxiids and Pojetaia.[15]

Mode of preservation

Small shelly fossils are typically, although not always, preserved in

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.[18]

Minerals used in shells

Small shelly fossils are composed of a variety of minerals, the most important being

Tommotian age used another form, calcite.[19]

A recently discovered modern

metazoan but whose ingredients are emitted in large quantities by the vents.[3]

Methods of constructing shells vary widely among the SSF, and in most cases the exact mechanisms are not known.[3]

Evolution of skeletons and biomineralization

  Biomineralized
No Yes
Skeleton No Dickinsonia[20]
Halkieria sclerites[21]
Yes Kimberella[22] Helcionellids[23]
Part of a series related to
Biomineralization
General
Exoskeletons (shells)
Endoskeletons (bones)
Teeth, scales, tusks etc
Calcification
Silicification
Other forms
Related

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 skeletons. 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.[3]

It has often been suggested that biomineralization evolved as a response to an increase in the concentration of

silica appeared virtually simultaneously in a range of environments.[17]

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.[17] 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.[25] 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.[17]

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 take less energy 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

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.[26]

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.[24]

Evolutionary significance

In some locations, up to 20% of

Cloudina fossils show borings, holes that are thought to have been made by predators.[27][28] 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.[28]

Stem groups[29]
  • — = Lines of descent
  •   = Basal node
  •   = Crown node
  •   = Total group
  •   = Crown group
  •   = Stem group

The small shellies provide a relatively continuous record throughout the early Cambrian, and thus provide a more useful insight into the

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
Atdabanian, some SSFs can be assigned to the crown group of a modern phylum, echinoderms.[30]
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",[30]: 163  and thus reveals the true pace of the Cambrian explosion.[30]

Types of small shelly fossil

Further information:
List of small shelly fossils

Ediacaran forms

Cloudina
Computer model of Namacalathus

The few collections of SSF from the

Cloudina's "tube", which was 8 to 150 millimetres (0.31 to 5.91 in) long, consisted of nested cones that were mineralized with calcium carbonate but left unmineralized gaps between the cones.[31][32] Sinotubulites built long thin tubes that were more flexible but probably had mineralized ridges.[33]

Namapoikia was probably either a sponge or a coral-like organism, and built dwellings up to 1 metre (39 in) across out of calcium carbonate.[34]

silica, and are thought to be the remains of sponges.[3][35][36]

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.[37]

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

brachiopods, echinoderms, and onychophoran-like organisms that may have been close to the ancestors of arthropods.[3] A multitude of problematic tubular fossils, such as anabaritids, Hyolithellus or Torellella
characterize the earliest Cambrian Small Shelly Fossil skeletal assemblages.

Halkieria, showing numerous sclerites
on the sides and back, and the cap-like shells at both ends.

Most of the Cambrian SSF consists of

Maikhanella.[40]

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.[3] 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.[3][38] 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,[3] although they may have been more closely related to halkieriids.[41]

mollusc
. This restoration shows water flowing in under the shell, over the gills and out through the "exhaust pipe".

Some sclerites are mineralized with

lobopods, animals that looked like worms with legs and are thought to be close to the ancestors of arthropods.[3]

hyoliths, small animals that had conical shells and may have been molluscs or worm-like Sipuncula.[44]

Small arthropods with bivalve-like shells have been found in early Cambrian beds in China,[46] and other fossils (Mongolitubulus henrikseni) represent spines that snapped off bivalved arthropod carapaces.[47]

Post-Cambrian forms

SSFs after the Cambrian start to pick up more recognizable and modern groups. By the mid-Ordovician, the majority of SSFs simply represent larval molluscs, mostly gastropods.[48]

See also

Notes

  1. S2CID 133538050
    .
  2. PMID 15326344
    . Retrieved 2008-07-18.
  3. ^ a b c d e f g h i j k l m n o p q r s t u v w Bengtson, S. (2004). Lipps, J.H.; Waggoner, B.M. (eds.). "Early skeletal fossils" (PDF). Neoproterozoic- Cambrian Biological Revolutions. Paleontological Society Papers. 10: 67–78. Retrieved 2008-07-18.
  4. ^
    doi:10.1669/0883-1351(2004)019<0178:CTPWTF>2.0.CO;2
    . Retrieved 2009-04-22.
  5. ^ a b c Dzik, J. (1994). "Evolution of 'small shelly fossils' assemblages of the early Paleozoic". Acta Palaeontologica Polonica. 39 (3): 27–313. Retrieved 2008-08-01.
  6. ^ "The Tommotian Age". Retrieved 2008-07-30.
  7. ISBN 0-632-04444-6
    .
  8. ^ Hou, X-G; Aldridge, R.J.; Bengstrom, J.; Siveter, D.J. & Feng, X-H (2004). The Cambrian Fossils of Chengjiang, China. Blackwell Science. p. 233.
  9. S2CID 140660306
    . Retrieved 2008-07-18.
  10. doi:10.1016/j.palaeo.2007.03.046
    .
  11. PMID 18122310
    . and Cloud, P. E. (1968). "Pre-metazoan evolution and the origins of the Metazoa.". In Drake, E. T. (ed.). Evolution and Environment. New Haven, Conn.: Yale University Press. pp. 1–72.
  12. ^ Eldredge, N. & Gould, S. J. "Punctuated equilibria: An alternative to phyletic gradualism.". In Schopf, T. J. M. (ed.). Models in Paleobiology. San Francisco, CA.: Freeman, Cooper & Co. pp. 82–115.
  13. ^ Durham, J. W. (1971). "The fossil record and the origin of the Deuterostomata". Proceedings of the North American Paleontological Convention, Part H: 1104–1132. and Glaessner, M.F. (1972). "Precambrian palaeozoology". In Jones, J. B.; McGowran, B. (eds.). Stratigraphic Problems of the Later Precambrian and Early Cambrian. Vol. 1. University of Adelaide. pp. 43–52.
  14. doi:10.1139/E05-119
    .
  15. S2CID 131557288
    .
  16. ^
    doi:10.1130/B30346.1
    .
  17. ^
    OCLC 156823511
    , retrieved 2008-08-01
  18. S2CID 131557288
    .
  19. S2CID 27418253
    .
  20. S2CID 17181699
    . Retrieved 2007-11-24.
  21. S2CID 4324153
    .
  22. OCLC 156823511
  23. ^
    S2CID 46429653
    .
  24. ^
    S2CID 55178329. Archived from the original
    (PDF) on 2009-02-26. Retrieved 2008-08-24.
  25. S2CID 133260258
    .
  26. doi:10.1111/j.1095-8312.2005.00507.x
    . Retrieved 2008-08-01.
  27. S2CID 131590949
    .
  28. ^
    S2CID 6710335
    .
  29. ^ Craske, A.J. & Jefferies, R.P.S. (1989). "A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion". Palaeontology. 32: 69–99.
  30. ^
    PMID 21680420
    .
  31. PMID 11538690. Archived from the original
    on 2011-05-22. Retrieved 2008-07-19.
  32. doi:10.2475/ajs.272.8.752
    .
  33. doi:10.1111/j.1502-3931.2007.00040.x
    .
  34. S2CID 9515357
    .
  35. doi:10.1130/0091-7613(1997)025<0303:ESSCFS>2.3.CO;2
    . Retrieved 2008-07-19.
  36. S2CID 130802211
    .
  37. S2CID 52231115
    . Retrieved 2008-07-19.
  38. ^
    doi:10.1098/rstb.1995.0029
    .
  39. doi:10.1006/zjls.1996.0065
    .
  40. doi:10.1111/j.1502-3931.1992.tb01644.x
    .
  41. doi:10.1111/j.1475-4983.2008.00792.x
    . Retrieved 2008-08-07.
  42. PMID 18577500. There is a reconstruction of Micrina at "Catalogue of Organisms: Back to the Scleritome – Tommotiids Revealed!"
    . Retrieved 2008-07-23.
  43. doi:10.1130/G24385A.1
    . Retrieved 2008-07-23.
  44. ^
    ISSN 0022-3360
    . Retrieved 2008-07-30.
  45. ISBN 978-0-521-40242-2
    .
  46. ISSN 0091-7613
    .
  47. S2CID 129309039
    .
  48. ^ Dzik, J. 1994. Evolution of "small shelly fossils" assemblages of the early Paleozoic. Acta Palaeontologica Polonica, 39, 247–313.

External links

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