Small-Shelly-Fauna
For the Small - Shelly Fauna (SSF ) - often erroneously referred to as " Tommotium Fauna " - is it to mineralized fossils, often only a few millimeters in size, by the final stages of the Ediacaran through to the end of the Early Cambrian periods can be detected almost completely. The term " Small Shelly Fauna " means small -shelled fauna, which is to be considered "small" in this case relative. The fossils are very different, and there is no official definition of " Small Shelly Fauna " or " Small Shelly Fossils ". They nearly all come from older rock layers than more well-known fossils such as trilobites. Since most SSF remained in that they were rapidly covered by phosphate and this type of preservation mainly confined to the late Ediacaran and early Cambrian periods, it is quite possible that the animals from which they originated, before this period showed up and then still existed.
Some of the fossils show complete skeletons of small organisms, including the mysterious Cloudina and some worm -like molluscs. The bulk of the fossils, however, consists fragments or shattered remains of larger organisms, including sponges, molluscs, snails similar Halkieriide, brachiopods, echinoderms and velvet worms - like that may have been closely related to the ancestors of the arthropods. An early explanation for the emergence of SSF - also for the development of mineralized skeletons - came from a sudden increase in calcium concentration in the oceans. On the other hand, there are many fossils of the SSF from other minerals such as silicon. But because the first representatives occurred at about the time when organisms first began to burrow to escape predators, it is more likely that they represent early steps of an evolutionary arms race between the predators and the prey always better protected. On the other hand, mineralized skeletons could have easily developed because they are stronger and cheaper to produce than completely organic skeletons as those of insects. Regardless, it is true in every case that animals use those minerals which were most accessible to them.
Although representatives of the SSF, due to their small size and often fragmentary condition difficult to identify and to classify, they nevertheless provide an important indication of how developed the major groups of marine invertebrates, but especially for the pace and sequencing of the Cambrian explosion. They include not only the earliest known representatives of some modern tribes, but also have the great advantage that they provide us with a complete record of early Cambrian organisms whose body already had hard parts.
Discovery history
References for the specified time: Ediacaran fauna Small Shelly Fauna, but it could have lasted longer Tommotium Cambrian Explosion
The term " Small Shelly Fossils " ( to German: small shell fossils ) was first introduced in 1975 by Samuel Matthews and VV Missarzhevsky used. Strictly speaking, it is a misnomer, since, according to Stefan Bengtson, " they are not always small, they often have no shells - and the description would also fit well with beach gastropods from the Pleistocene. " However, paleontologists found no better name and made their frustration with parodies such as "small silly fossils " ( to German: Small stupid fossils ) or "small smellies " ( to German: Little Stinker ) 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 designations made.
Single copies and sometimes even quite extensive collections of these fossils were discovered 1872-1967, but at first no one concluded that the early Cambrian would have additional features to the traditionally known trilobites, and other Archaeocyaths over a wider range of animals. In the late 1960s, Soviet paleontologists discovered even more extensive occurrence of SSF in layers below those containing Cambrian trilobites. Consequently, these animals had to be older than the trilobites. Unfortunately, the documents described these discoveries in Russian only were available, so that the documents of Matthews and Missarzhevsky until 1975 for the first time outside the Russian language area drew attention to the SSF.
There have already been a lively debate about the early evolution of animals. Preston Cloud claimed in 1948 and 1968, the process was " explosive ", and developed in the early 1970s, Niles Eldredge and Stephen Jay Gould their theory of punctuated equilibrium, which is interrupted by brief periods of rapid changes studied the evolution as long intervals of relative stasis. On the other hand, were of about the same time both Wyatt Durham and Martin Glaessner the view that the animal kingdom would have had a long history already in the Proterozoic, which had remained undiscovered only because of lack of fossils to date.
Dissemination
Accumulations with high biodiversity have been discovered in China, Mongolia, Kazakhstan, Australia and Antarctica, while other finds in India, Pakistan, Iran, Europe and North America have a mediocre diversity.
There are different views with regard to the period of the small- Shelly- fauna. The Russian discoveries of the late 1960s were associated with 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 erroneous term " Tommotium - fauna ". On the other hand, includes Bengtson in " SSF " ediacarische fossils such Cloudina and post- tommotische fossils such Microdictyon from the Maotianshan slate with a. Some fossils of the SSF were even found in layers together with fossils of trilobites. It was assumed that the mass extinction at the end of Botomium, an age of early Cambrian, most of the SSF would have destroyed. In 2004, however, was reported by halkieriiden fossils in rocks from the middle Cambrian, which was found in the Georgina Basin in Australia.
Type of preservation
Typical SSF are, if not always, preserved in phosphates, possibly also corresponds to their original mineral composition. They are separated by means of weak acids from limestone, usually acetic acid; the phosphatized fossils remain there after the rock has dissolved. Preservation by a pure phosphate layer seems to have been very common during the early Cambrian, but was less common with time due to the increasing mixing of the seabed by grave end living beings. However, had they not been preserved in this way, SSF would dissolve together with the rock in the acids that are used for the exposure. It would therefore be quite possible that the animals which left these fossils have lived before or after the period to which it assigns them today, but so far could not be successfully separated from the rock - the extinction of previously adopted the most SSF at the end of Cambrian has therefore did not have taken place. For decades it was assumed Halkieriide whose shield panels represent a typical type of SSF, had disappeared during the mass extinction at the end of Botomium; but was reported by halkieriiden shield plates in Australian rocks of the Middle Cambrian in 2004, a time more than 10 million years later.
Fossils SSF consist of a variety of minerals, and especially silicon, calcium phosphate and calcium carbonate. The range of minerals which used the individual organisms was initially influenced by the chemical composition of the oceans, in which developed these organisms. Later, these minerals were still used, regardless of their composition. So used, for example, during the period of the Ediacaran and in the transition period to the Cambrian animals, used the calcium carbonate in the aragonite. Animals other hand, used, first during the Tommotium appeared another mineral called calcite.
A recently discovered modern gastropod species that lives in the deep ocean near thermal springs, shows the influence of both former and current local chemical milieu: the shell of these animals is composed of aragonite, as you can also find it in the earliest fossil mollusks; but they also shield panels on the side of her foot, which are mineralized with the iron sulfides pyrite and greigite. These minerals could never previously detected in any living thing, but they are among the ingredients that are emitted in large quantities from the chimneys of thermal springs.
The methods of the shell structure are highly variable within the SSF, and in most cases the exact mechanisms are not known.
Evolutionary significance
At some localities have up to 20 % of Cloudina - fossils in a way holes. This is to holes, which are thought to have been produced by predators. Similar fossils of the species Sinotubulites, which were often found in the same localities, showed no such holes. In addition, the distribution of holes in Cloudina fossils on a selection as to size down, because the biggest holes are located in the largest shells. This indication of selective attacks by predators suggests that new species evolved in response to these same attacks, which is often presented as a possible cause for the rapid increase of biodiversity in the early Cambrian.
The SSF provide a relatively constant image throughout the early Cambrian, which is why they provide useful insights into the Cambrian explosion as deposits in other fossil sites. Although most of the SSF are difficult to identify, to give evidence in the form of fossils that have been filed already in modern taxa, or in the crown groups of their evolutionary " aunts " and " cousins ". With their help, scientists can estimate the form and speed took place the development of the animals. Such estimates show that the earliest representatives of the SSF are the simplest simultaneously. With the progress of time, they can be classified within a crown group in ever clearer tribes. The earliest ( ediakarischen ) Small shellies can be considered only partially as diploblastisch because they consist of at least two fabric layers. Subsequent shells are already clear as to call diploblastisch. Follow you with the Helcionelliden the first SSF, you can assign the mollusks.
A closer look at the recent SSF the advice for classification in certain strains are becoming clearer and with the beginning of the Atdabanium can become a crown group of the modern tribes, echinoderms, expected some SSF already. This one gets the impression that the first animals of the SSF already present from the late Ediakarium basic parts of the later tribes, the individual tribes rather gradually emerged in a fast, yet comprehensible and orderly manner as in a sudden confusion. This shows the actual speed of the Cambrian explosion.