Small shelly fauna
The Small Shelly Fauna (SSF) - often incorrectly referred to as the "Tommotium Fauna" - are mineralized fossils , often only a few millimeters in size, from the last stages of the Ediacarian to the end of the early Cambrian with almost no gaps can be proven. The term “small-shelly fauna” means “ small-shell fauna ”, whereby “small” is to be viewed relatively in this case. The fossils are extremely diverse and there is no official definition of "small shelly fauna" or "small shelly fossils". Almost all of them come from older rock strata than better known fossils such as B. the trilobites . Since most of the SSF were preserved because they were quickly covered by phosphate and this type of preservation is mainly limited to the late Ediacarian and early Cambrian periods, it is quite possible that the animals from which they emerged already existed emerged during this period and continued to exist afterwards.
Some of the fossils show full skeletons of small organisms, including the mysterious cloudina and some snail-like molluscs . However, the mass of fossils consists of fragments or shattered remains of larger organisms, including sponges , mollusks, snails similar Halkieriide , brachiopods , echinoderms and velvet worms -like that may closely related to the ancestors of the arthropods were. An early explanation for the emergence of the SSF - including the development of mineralized skeletons - was based on a sudden increase in the calcium concentration in the oceans. On the other hand, many SSF fossils consist of other minerals such as B. silicon . But because the first representatives appeared around the time when organisms first began to burrow to evade predators, it is more likely that they represent early steps in an evolutionary arms race between the predators and the increasingly protected prey. On the other hand, mineralized skeletons may have evolved simply because they are stronger and cheaper to produce than completely organic skeletons like those of insects. Regardless, it is always true that animals used the minerals that were most easily accessible to them.
Although representatives of the SSF are difficult to identify and classify due to their small size and often fragmentary condition, they nonetheless provide important evidence of how the main groups of marine invertebrates developed, but above all of the pace and course of the Cambrian explosion . Not only do they include the earliest known representatives of some modern tribes , but they also have the great advantage that they give us a complete record of early Cambrian organisms whose bodies already had hard parts.
Discovery story
References for the times:
Ediacaran fauna
Small shelly fauna, but could have lasted longer
Tommotium
Cambrian Explosion
The term "Small Shelly Fossils" (in German: kleinschalige fossils) was first used in 1975 by Samuel Matthews and VV Missarzhevsky. Strictly speaking, it is a misnomer, because, says Stefan Bengtson, "they are not always small, they frequently have no shells - and the description would be as well to beach snails from the Pleistocene . Fit" paleontologists, however, found no better name and made their Frustration about it with parodies like "small silly fossils" (in German: small stupid fossils) or "small smellies" (in German: small stinkers) air. The term is often abbreviated to “small shellies” or simply “SSF”.
The vast majority of all morphological features of later shellfish can already be seen in the SSF. So far, no one has attempted a formal definition of the terms “Small Shelly Fauna”, “Small Shelly Fossils” or other similar terms.
Individual specimens and sometimes even quite extensive collections of these fossils were discovered between 1872 and 1967, but initially no one concluded that the early Cambrian had a wider range of animals in addition to the traditionally known trilobites, archaeocyathids and others. In the late 1960s, Soviet paleontologists discovered even greater occurrences of SSF in layers below that containing Cambrian trilobites. As a result, these animals had to be older than the trilobites. Unfortunately, the documents describing these discoveries were only available in Russian , so it was not until 1975 that Matthews and Missarzhevsky's documents first drew attention to the SSF outside the Russian-speaking world.
There has been a lively debate about the early evolution of animals . Preston Cloud claimed 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 viewed evolution as long intervals of relative stasis interrupted by short periods of rapid change. On the other hand, at around the same time, both Wyatt Durham and Martin Glaessner took the view that the animal kingdom had a long history as early as the Proterozoic , which had only gone undiscovered due to the lack of fossils.
distribution
Collections of high biodiversity have been found in China , Mongolia , Kazakhstan , Australia and the Antarctic , while other finds in India , Pakistan , Iran , Europe and North America show a rather mediocre diversity.
There are different views on the time span of the small shelly fauna. The Russian finds of the late 1960s were assigned to the Tommotium , an age from the Cambrian period, and for some time the name "Small Shelly Fauna" was used exclusively for this age, which explains the misnomer "Tommotium fauna" . On the other hand, Bengtson in "SSF" includes Ediacaric fossils such as Cloudina and post-tommotic fossils such as Microdictyon from the Maotianshan schist . Some SSF fossils have even been found in layers along with fossils of trilobites. It was believed that the mass extinction at the end of the Botomian , an age of the Early Cambrian, would have wiped out most of the SSF. However, in 2004 there were reports of Halkieriid fossils in rocks from the Middle Cambrian, which were found in the Georgina Basin in Australia .
Type of conservation
Typical SSF are, although not always, preserved in phosphates , which may also correspond to their original mineral composition. They are separated from the limestone using weak acids, usually acetic acid ; the phosphated fossils are left over after the rock has dissolved. Conservation by a pure phosphate layer seems to have been very common during the early Cambrian, but became rarer over time due to the increasing mixing of the seabed with burrowing creatures . However, if they had not been preserved in this way, SSF would dissolve along with the rock in the acids used for the exposure. It is therefore entirely possible that the animals that left these fossils lived before or after the period to which they are assigned today, but have not yet been successfully separated from the rock - the previously assumed extinction of most SSF at the end of the Cambrian may therefore not have happened at all. For decades it was assumed that Halkieriids , whose shield plates represent a typical species of SSF, had disappeared during the mass extinction at the end of the botomium ; but in 2004 Halberid shield plates were reported in Australian rocks of the Middle Cambrian, a time well over 10 million years later.
Mineral components of the shells
SSF fossils consist of a variety of minerals, but mainly silicon , calcium phosphate and calcium carbonate . The choice of minerals that each organism used was initially influenced by the chemical makeup of the oceans in which these organisms evolved. Later, however, these minerals continued to be used regardless of their composition. For example, during the Ediacarian and the transition to the Cambrian, animals that used calcium carbonate used aragonite . On the other hand, animals that first appeared during the Tommotium used another mineral called calcite .
A recently discovered modern gastropod species that lives near thermal springs in the deep sea shows the influence of both earlier and current local chemical milieus: The shell of these animals consists of aragonite, as can be found in the earliest fossil molluscs; but they also have shield plates on the side of their foot, which are mineralized with the iron sulfides pyrite and greigite . These minerals have never been detected in any living being before, but they are among the ingredients that are expelled in large quantities from the chimneys of the thermal springs.
The methods of shell construction are extremely variable within the SSF and in most cases the exact mechanisms are not known.
Evolutionary meaning
At some sites, up to 20% of the Cloudina fossils have some kind of drilling. These are holes that are believed to have been created by predators. Similar fossils of the species Sinotubulites , which were often discovered at the same sites, did not show any such holes. In addition, the distribution of the holes in Cloudina fossils indicates a selection in terms of size, because the largest holes are in the largest shells. This evidence of selective predator attacks suggests that new species evolved in response to these attacks, often portrayed as a possible cause of the rapid increase in biodiversity in the early Cambrian.
The SSF provide a relatively constant picture throughout the Early Cambrian, making them more useful insights into the Cambrian explosion than occurrences in other fossil sites . Although most of the SSF are difficult to identify, clues emerge in the form of fossils that have already been classified in modern taxa or in the crown groups of their evolutionary "aunts" or "cousins". With their help, scientists can estimate in what form and speed the development of the animals took place. Such estimates show that the earliest representatives of the SSF are also the simplest. As time progresses, they can be classified into more and more distinct tribes within a crown group. The earliest (ediakaric) small shellies can only be regarded as diploblastic to a limited extent , as they consist of at least two layers of tissue. Later shells can be more clearly described as diploblastic. They are followed by the Helcionellids, the first SSF that can be assigned to the mollusks .
On closer inspection of the younger SSF the indications for a classification into certain strains become clearer and with the beginning of the Atdabanium some SSF can already be counted to a crown group of the modern strains, the Echinoderms . From this one gets the impression that the first animals of the SSF from the late Ediakarium already represent fundamental parts of the later tribes, with the individual tribes appearing gradually in a quick, but nevertheless understandable and orderly manner rather than in a sudden mess. This shows the real speed of the Cambrian explosion.
literature
- Stephen Jay Gould: Chance Man. The miracle of life as a game of nature. ISBN 3-423-30389-1
- Bengtson, S. (2004): Early skeletal fossils. In: Lipps, JH, & Wagoner, BM: Neoproterozoic-Cambrian Biological Revolutions , Paleontological Society Papers 10: 67-78, PDF
Individual evidence
- ↑ M. Brasier, J. Antcliffe: Decoding the Enigma Ediacaran . In: Science . tape 305 , no. 5687 , August 20, 2004, p. 1115–1117 , doi : 10.1126 / science.1102673 ( online [accessed July 18, 2008]).
- ↑ a b c d e f g h S. Bengtson, Editor JH Lipps and BM Wagoner: Neoproterozoic-Cambrian Biological Revolutions . In: Paleontological Society Papers . tape 10 , 2004, p. 67–78 ( online [PDF; accessed July 18, 2008]). Online ( Memento of the original dated February 11, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ The Tommotian Age. Retrieved July 30, 2008 .
- ^ R. Cowen: History of Life . Ed .: Blackwell Science. 3. Edition. 2000, ISBN 0-632-04444-6 , pp. 63 .
- ↑ XG Hou, RJ Aldridge, J. Bengstrom, DJ Siveter, XH Feng: The Cambrian Fossils of Chengjiang, China . Ed .: Blackwell Science. 2004, p. 233 .
- ^ SC Matthews, VV Missarzhevsky: Small Shelly Fossils of Late Precambrian and Early Cambrian Age: a Review of Recent Work . In: Journal of the Geological Society . tape 131 , no. 3 , 1975, p. 289-304 , doi : 10.1144 / gsjgs.131.3.0289 ( online [accessed July 18, 2008]).
- ↑ M. Steiner, G. Li, Y. Qian, M. Zhu and B.-D. Erdtmann: Neoproterozoic to Early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China) . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 254 , no. 1-2 , October 2007, pp. 67-99 , doi : 10.1016 / j.palaeo.2007.03.046 .
- ^ Cloud, PE (1948): Some problems and patterns of evolution exemplified by fossil invertebrates. Evolution 2 (4): 322-350. & Cloud, PE (1968), Pre-metazoan evolution and the origins of the Metazoa. In: Drake, ET: Evolution and Environment , New Haven, Conn .: Yale University Press, pp. 1-72
- ^ Eldredge, N., & Gould, SJ: Punctuated equilibria: An alternative to phyletic gradualism. In: Schopf, TJM: Models in Paleobiology. San Francisco, CA .: Freeman, Cooper & Co., pages 82-115
- ^ Durham, JW (1971): The fossil record and the origin of the Deuterostomata. Proceedings of the North American Paleontological Convention, Part H: 1104-1132. and Glaessner, MF (1972): Precambrian palaeozoology. In: Jones, JB, & McGowran, B .: Stratigraphic Problems of the Later Precambrian and Early Cambrian , 1, University of Adelaide, pp. 43-52
- ↑ CB Skovsted: Small shelly fossils from the basal Emigrant Formation (Cambrian, uppermost Dyeran Stage) of Split Mountain, Nevada . In: Canadian Journal of Earth Sciences . tape 43 , no. 4 , April 2006, p. 487-496 , doi : 10.1139 / E05-119 ( online [accessed July 23, 2008]).
- ↑ SM Porter: Halkieriids in Middle Cambrian Phosphatic Limestones from Australia . In: = Journal of Paleontology . tape 78 , no. 3 , May 2004, pp. 574-590 ( online [accessed August 1, 2008]).
- ^ A b The Rise and Fall of the Ediacaran Biota. Geological Society, London, Special Publications, 286, 405-414. Dzik J, The Verdun Syndrome: simultaneous origin of protective armor and infaunal shelters at the Precambrian – Cambrian transition (PDF; 520 kB), accessed on August 1, 2008
- ↑ SM Porter: Closing the Phosphatization Window: Testing for the Influence of Taphonomic Megabias on the Pattern of Small Shelly Fossil Decline . In: Palaios . tape 19 , no. April 2 , 2004 ( online [accessed July 30, 2008]).
- ↑ J. Dzik: Evolution of 'small shelly fossils' assemblages of the early Paleozoic . In: Acta Palaeontologica Polonica . tape 39 , no. 3 , 1994, p. 27-313 ( online [accessed August 1, 2008]).
- ↑ SM Porter: Halkieriids in Middle Cambrian Phosphatic Limestones from Australia . In: Journal of Paleontology . tape 78 , no. 3 , May 2004, pp. 574-590 ( online [accessed August 1, 2008]).
- ^ S. Porter: Seawater Chemistry and Early Carbonate Biomineralization . In: Science . tape 316 , no. 5829 , 2007, pp. 1302 , doi : 10.1126 / science.1137284 .
- ^ H. Hua, co-authors BR Pratt, LUYI Zhang: Borings in Cloudina Shells: Complex Predator-Prey Dynamics in the Terminal Neoproterozoic . In: Palaios . tape 18 , 2003, p. 454 , doi : 10.1669 / 0883-1351 (2003) 018 <0454: BICSCP> 2.0.CO; 2 .
- ^ A b S. Bengtson, co-authors Y. Zhao: Predatorial Borings in Late Precambrian Mineralized Exoskeletons . In: Science . tape 257 , no. 5068 , July 17, 1992, p. 367 , doi : 10.1126 / science.257.5068.367 , PMID 17832833 .
- ↑ a b c d Graham E. Budd: The Cambrian Fossil Record and the Origin of the Phyla . In: Integrative and Comparative Biology . tape 43 , no. 1 , 2003, p. 157–165 , doi : 10.1093 / icb / 43.1.157 ( full text online ). Full text online ( Memento of the original from December 9, 2012 in the web archive archive.today ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
