End-Ediacaran extinction
Encyclopedia
Evidence suggesting that a mass extinction occurred at the end of the Ediacaran
period, , includes:
" of soft-bodied organisms, preserved by microbial mats; and calcifying organisms such as Cloudina and Namacalathus
, which had a carbonate skeleton. Because both these groups disappear abruptly at the end of the Ediacaran period, , their disappearance cannot simply represent the closure of a preservational window, as had previously been suspected.
. However, the Cambrian explosion
of animals that gave rise to body fossils did not happen instantaneously. This implies that the "explosion" did not represent animals "replacing" the incumbent organisms, and pushing them gradually to extinction; rather, the data are more consistent with a radiation of animals to fill in vacant niches, left empty as an extinction cleared out the pre-existing fauna.
The concept that all Ediacarans became extinct at the start of the Cambrian is dealt a death knell if any post-Ediacaran survivors are found. Organisms from the lower Cambrian, such as Thaumaptilon
, were once thought to be Ediacarans, but this hypothesis no longer has many adherents. One possible Ediacaran survivor whose status is still open to scrutiny is Ediacaria
booleyi, a purported holdfast structure known from the upper Cambrian. If this does turn out to be a true Ediacaran, the biota cannot have disappeared completely. Disbelievers have claimed that the fossils don't actually have a biological origin, which doesn't seem to be the case—evidence is mounting to suggest that it is an organism (or at least of biological origin, perhaps a microbial colony), just not one that is related to the Ediacara biota.
.
Other apparent "survivors", such as Thaumaptilon
, have been reinterpreted as unrelated to the Ediacaran biota.
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, , includes:
- A mass extinction of acritarchAcritarchAcritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
s - The sudden disappearance of the Ediacara biotaEdiacara biotaThe 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...
and calcifying organisms; - The time gap before Cambrian organisms "replaced" them.
Pre-Ediacaran organisms
During the Ediacaran period, two main groups of organisms are found in the fossil record: the "Ediacara biotaEdiacara 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...
" of soft-bodied organisms, preserved by microbial mats; and calcifying organisms such as Cloudina and 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 had a carbonate skeleton. Because both these groups disappear abruptly at the end of the Ediacaran period, , their disappearance cannot simply represent the closure of a preservational window, as had previously been suspected.
Post-Ediacaran organisms
The fossil record of the earliest Cambrian, just after the Ediacaran period, shows a sudden increase in burrowing activity and diversityCambrian substrate revolution
The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is the diversification of animal burrowing during the early Cambrian period....
. However, 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...
of animals that gave rise to body fossils did not happen instantaneously. This implies that the "explosion" did not represent animals "replacing" the incumbent organisms, and pushing them gradually to extinction; rather, the data are more consistent with a radiation of animals to fill in vacant niches, left empty as an extinction cleared out the pre-existing fauna.
The concept that all Ediacarans became extinct at the start of the Cambrian is dealt a death knell if any post-Ediacaran survivors are found. Organisms from the lower Cambrian, such as Thaumaptilon
Thaumaptilon
Thaumaptilon is a fossil from the middle Cambrian Burgess Shale which some authors have compared to members of the Ediacaran biota, generally believed to have disappeared at the start of the Cambrian,...
, were once thought to be Ediacarans, but this hypothesis no longer has many adherents. One possible Ediacaran survivor whose status is still open to scrutiny is Ediacaria
Ediacaria
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...
booleyi, a purported holdfast structure known from the upper Cambrian. If this does turn out to be a true Ediacaran, the biota cannot have disappeared completely. Disbelievers have claimed that the fossils don't actually have a biological origin, which doesn't seem to be the case—evidence is mounting to suggest that it is an organism (or at least of biological origin, perhaps a microbial colony), just not one that is related to the Ediacara biota.
Survivors
Some organisms clearly survived the extinction—life continued. However, very few organisms are known from both sides of the Ediacaran-Cambrian boundary. One such organism is the agglutinated foramanifera PlatysolenitesPlatysolenites
Platysolenites is an agglutinated foramanifera known from Ediacaran and lower Cambrian assemblages.-External links:*...
.
Other apparent "survivors", such as Thaumaptilon
Thaumaptilon
Thaumaptilon is a fossil from the middle Cambrian Burgess Shale which some authors have compared to members of the Ediacaran biota, generally believed to have disappeared at the start of the Cambrian,...
, have been reinterpreted as unrelated to the Ediacaran biota.