Ediacaran type preservation relates to the dominant preservational mode in the Ediacaran period, where Ediacaran organisms were preserved as casts on the surface of microbial mats.
All but the smallest fraction of the fossil record consists of the robust skeletal matter of decayed corpses. Hence, since Ediacaran biota had soft bodies and no skeletons, their abundant preservation is surprising. The absence of burrowing creatures living in the sediments undoubtedly helped; [1] since after the evolution of these organisms in the Cambrian, soft-bodied impressions were usually disturbed before they could fossilize.
Microbial mats are areas of sediment stabilised by the presence of colonies of microbes, which secrete sticky fluids or otherwise bind the sediment particles. They appear to migrate upwards when covered by a thin layer of sediment, but this is an illusion caused by the colony's growth; individuals do not, themselves, move. If too thick a layer of sediment is deposited before they can grow or reproduce through it, parts of the colony will die, leaving behind fossils with a characteristically wrinkled "elephant skin" texture. [2] Most Ediacaran strata with the "elephant skin" texture characteristic of microbial mats contain fossils, and Ediacaran fossils are almost never found in beds that do not contain these microbial mats. Although microbial mats were once widespread, the evolution of grazing organisms in the Cambrian vastly reduced their numbers, [3] and these communities are now limited to inhospitable refugia where predators cannot survive long enough to eat them.
The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms, they would normally not fossilise. Unlike later soft-bodied fossil biota (such as the Burgess Shale, or Solnhofen Limestone) the Ediacara biota is not found in a restricted environment subject to unusual local conditions: they were a global phenomenon. The processes that were operating must have been systemic and worldwide. There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind. It is thought that the fossils were preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived. [4] However, it is more common to find Ediacaran fossils under sandy beds deposited by storms or high-energy, bottom-scraping ocean currents known as turbidites. [4] Soft-bodied organisms today almost never fossilise during such events.
Three different preservational modes are known: [5] [6] [7]
The presence of widespread microbial mats probably aided preservation by stabilising their impressions in the sediment below, [9] in combination with the formation of iron sulfides and pyrite to form a "death mask" mantling the organisms. [10]
Many models suggest that overlying sediment mineralized before the underlying organism decayed, causing the un-mineralized underlying sediment to fill the void after decay. [11]
One mode of early sediment mineralization, which accounts for the occurrence of this preservational mode into the Cambrian and its increasing scarcity thereafter, is silicification: this links the preservation of the fossils to the higher silica content of oceans before sponges, diatoms and other silica sinks became widespread. [12]
This hypothesis struggles to account for a number of observations, particularly in the Flinders and White Sea deposits; it is therefore difficult to argue that it formed a necessary component of Ediacara type preservation. [13]
In Flinders-style preservation, the overlying sediment always has a larger grain size than the sediment layer beneath. Because sediments with smaller grain sizes are more fluid, these can be squeezed up into a void that forms as organic material begins to decay. This model, proposed by Mary Wade in 1969, has found experimental support from cardboard-containing Death Star ice cubes. [14]
The rate of cementation of the overlying substrate, relative to the rate of decomposition of the organism, determines whether the top or bottom surface of an organism is preserved. Most disc-shaped fossils decomposed before the overlying sediment was cemented, and the ash or sand slumped in to fill the void, leaving a cast of the underside of the organism.
Conversely, quilted fossils tend to decompose after the cementation of the overlying sediment; hence their upper surfaces are preserved. Their more resistant nature is reflected in the fact that in rare occasions, quilted fossils are found within storm beds, the high-energy sedimentation not having destroyed them as it would have the less-resistant discs. Further, in some cases, the bacterial precipitation of minerals formed a "death mask", creating a mould of the organism. [15]
Dickinsonia is an extinct genus of basal animal that lived during the late Ediacaran period in what is now Australia, China, India, Russia and Ukraine. The individual Dickinsonia typically resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; its mode of growth is consistent with a stem-group bilaterian affinity, though some have suggested that it belongs to the fungi, or even an "extinct kingdom". The discovery of cholesterol molecules in fossils of Dickinsonia lends support to the idea that Dickinsonia was an animal.
Cyclomedusa is a circular fossil of the Ediacaran biota; it has a circular bump in the middle and as many as five circular growth ridges around it. Many specimens are small, but specimens in excess of 20 cm are known. The concentric disks are not necessarily circular, especially when adjacent individuals interfere with each other's growth. Many radial segment lines — somewhat pineapple-like — extend across the outer disks. A few specimens show what might be a stem extending from the center in some direction or other.
Kimberella is an extinct 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 gastropods, although its affinity with this group is contentious.
Tribrachidium heraldicum is a tri-radially symmetric fossil animal that lived in the late Ediacaran (Vendian) seas. In life, it was hemispherical in form. T. heraldicum is the best known member of the extinct group Trilobozoa.
Vendobionts or Vendozoans (Vendobionta) are a group of benthic beings made up of the majority of extinct creatures that were part of the Ediacaran biota. It is a hypothetical group and at the same time, it would be the oldest of the animals that populated the Earth about 580 million years ago, in the Ediacaran or Vendic period. They became extinct when the so-called Cambrian explosion appeared, with the introduction of fauna formed by more recognizable groups and more related to modern animals. It is very likely that it isn't a monophyletic clade and not every genus placed in its subtaxa is an animal.
Yorgia waggoneri is a discoid Ediacaran organism. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of 25 cm (9.8 in). It is classified within the extinct animal phylum Proarticulata.
Ediacaria is a fossil genus dating to the Ediacaran Period of the Neoproterozoic Era. Unlike most Ediacaran biota, which disappeared almost entirely from the fossil record at the end of the Period, Ediacaria fossils have been found dating from the Baikalian age of the Upper Riphean to 501 million years ago, well into the Cambrian Period. Ediacaria consists of concentric rough circles, radial lines between the circles and a central dome, with a diameter from 1 to 70 cm.
Cephalonega stepanovi is a fossil organism from Ediacaran deposits of the Arkhangelsk Region, Russia. It was described by Mikhail A. Fedonkin in 1976
Rangea is a frond-like Ediacaran fossil with six-fold radial symmetry. It is the type genus of the rangeomorphs.
Aspidella is an Ediacaran disk-shaped fossil of uncertain affinity. It is known from the single species A. terranovica.
The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were composed of enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.
Ernettia is an extinct genus of Ediacaran organisms with an infaunal lifestyle. Fossil preservations and modeling indicate this organism was sessile and “sack”-shaped. It survived partly buried in substrate, with an upturned bell-shaped frill exposed above the sediment-water interface. Ernietta have been recovered from present-day Namibia, and are a part of the Ediacaran biota, a late Proterozoic radiation of multicellular organisms. They are among the earliest complex multicellular organisms and are known from the late Ediacaran. Ernietta plateauensis remains the sole species of the genus.
The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is the diversification of animal burrowing during the early Cambrian period.
Evidence suggesting that a mass extinction occurred at the end of the Ediacaran period, 542 million years ago, includes:
A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, and also just bacterial. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts. A few are found as endosymbionts of animals.
Eoandromeda is an Ediacaran organism consisting of eight radial spiral arms, and known from two taphonomic modes: the standard Ediacara type preservation in Australia, and as carbonaceous compressions from the Doushantuo formation of China, where it is abundant.
Arumberia is an enigmatic fossil from the Ediacaran period originally described from the Arumbera Sandstone, Northern Territory, Australia but also found in the Urals, East Siberia, England and Wales, Northern France, the Avalon Peninsula and India. Several morphologically distinct species are recognized.
Beltanelliformis is a genus of discoid fossil from the Ediacaran period, sometimes ascribed to the Ediacaran Biota. The chemical signature obtained from organically-preserved specimens points to a cyanobacterial affinity. Depending on its preservation, it is sometimes referred to as Nemiana or Beltanelloides.
The Ediacaran fossil Hallidaya, a close relative of Skinnera lived in Belomorian of the Late Ediacaran period prior to the Cambrian explosion and thrived in the marine strata on the ocean floor of what is now considered Australia. These fossils were disk-shaped organisms that were slightly dome shaped with tri-radial symmetry. These Ediacaran organisms thrived by living in low-energy inner shelf, in the wave- and current-agitated shoreface, and in the high-energy distributary systems.
Modes of preservation in the Cambrian | ||
|---|---|---|
| Exceptional |  | |
| Conventional | ||