The Nama assemblage was the last of the

Ediacaran The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

biotic assemblages. Following the

Avalon Avalon () is an island featured in the Arthurian legend. It first appeared in Geoffrey of Monmouth's 1136 ''Historia Regum Britanniae'' as a place of magic where King Arthur's sword Excalibur was made and later where Arthur was taken to recove ...

and

White Sea assemblage The White Sea assemblage was the second of the three Late Ediacaran biotic assemblages, following the Avalon assemblage and preceding the Nama assemblage. It spanned from 560 Ma to 550 Ma. Showing an increase in genus diversity from the Aval ...

s, it spanned from 550 Ma to 539 Ma, coinciding with the Terminal Ediacaran biozone. The assemblage was characterized by a faunal turnover, with the decline of the preexisting White Sea biota. The drop of diversity has been compared to the

mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms. It occ ...

s of the Phanerozoic. A second drop of diversity occurred at the Ediacaran–Cambrian boundary, concluding the Nama assemblages with the

end-Ediacaran extinction The end-Ediacaran extinction is a mass extinction believed to have occurred near the end of the Ediacaran period, the final period of the Proterozoic eon. Evidence suggesting that such a mass extinction occurred includes a massive reduction in dive ...

Etymology and definitions

The Nama assemblage is named after the

Nama Group The Nama Group is a megaregional Vendian to Cambrian group of stratigraphic sequences deposited in the Nama Basin, Nama foreland basin in central and southern Namibia. The Nama Basin is a peripheral foreland basin, and the Nama Group was deposited ...

from the Tsaus Mountains of Namibia, which preserves a Late Ediacaran record of soft-bodied fossils. The biota of the Namibian sites clusters with similar biotas found in the

Southwestern United States The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural list of regions of the United States, region of the United States that includes Arizona and New Mexico, along with adjacen ...

South China South China ( zh, s=, p=Huá'nán, j=jyut6 naam4) is a geographical and cultural region that covers the southernmost part of China. Its precise meaning varies with context. A notable feature of South China in comparison to the rest of China is ...

and

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

, leading to a Nama assemblage being first defined by Gehling in 2001. This definition of a biological assemblage usually includes the totality of the biota found in these

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

, which in the case of the Nama assemblage exhibits a strong temporal element. The full division of the Ediacaran biota into three separate assemblages was first postulated by Ben Waggoner in 2003 through

parsimony analysis In phylogenetics and computational phylogenetics, maximum parsimony is an optimality criterion under which the phylogenetic tree that minimizes the total number of character-state changes (or minimizes the cost of differentially weighted charact ...

endemicity In epidemiology, an infection is said to be endemic in a specific population or populated place when that infection is constantly present, or maintained at a baseline level, without extra infections being brought into the group as a result of tr ...

. This study relied on temporal, paleogeographical and paleoenvironmental data. This same clustering was later recovered through hierarchical clustering and non-metric

multidimensional scaling Multidimensional scaling (MDS) is a means of visualizing the level of similarity of individual cases of a data set. MDS is used to translate distances between each pair of n objects in a set into a configuration of n points mapped into an ...

. Similar methods, supplemented with a

network analysis Network analysis can refer to: * Network theory, the analysis of relations through mathematical graphs ** Social network analysis, network theory applied to social relations * Network analysis (electrical circuits) See also *Network planning and d ...

-based clustering of genera into paleocommunities, or recurrent associations of taxa, later hinted at an additional cluster (Miahoe) overlapping with the Avalon and White Sea assemblages. The Nama assemblage was also recovered and interpreted as a paleocommunity. A closely related Terminal Ediacaran

biozone In biostratigraphy, biostratigraphic units or biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxon, taxa, as opposed to a Lithostratigraphy, lithostratigraphic unit which is defined by the ...

, contrasting with the previous

Ediacaran biota The Ediacaran (; formerly Vendian) biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (). These were enigmatic tubular and frond-shaped, mostly sessile, organis ...

biozone, was recovered when including formations in the network analysis, comprising species-poor formations mostly consisting of Nama genera. A proposed definition of the Nama assemblage as an evolutionary fauna by Wood and coauthors restricts it to new morphogroups such as calcifying metazoans,

cloudinid The cloudinids, an early metazoan family containing the genera ''Acuticocloudina'', ''Cloudina'' and ''Conotubus'', lived in the late Ediacaran period about 550 million years ago and became extinct at the base of the Cambrian. They formed millim ...

s and complex trace fossils, excluding holdovers from previous faunas such as ''

Paracharnia ''Paracharnia'' is a reassessed genus of a fossil reported by Ding and Chen (1981). It is the first Ediacaran metazoan fossilized remains found in China, taken from the Shibantan Member, Dengying Formation, Sinian System in the Eastern Yangtze Go ...

''. Under this definition, the assemblage's lower boundary would be the oldest appearance of '' Cloudina'' in the fossil record, placing the boundary at 550 Ma. This definition distinguishes the Nama assemblage from the Terminal Ediacaran biozone, the latter of which includes both holdovers and newer taxa.

Paleohistory

The Nama assemblage is bounded from the earlier

and later

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

period by two major episodes of faunal turnover, considered to be pulses of the

. The genus diversity was lower than in the earlier Ediacaran and later

Fortunian The Fortunian age marks the beginning of the Phanerozoic Eon, the Paleozoic Era, and the Cambrian Period. It is the first of the two stages of the Terreneuvian series. Its base is defined as the first appearance of the trace fossil ''Treptichnus ...

biotas, a fact that has been shown to be independent of sampling bias. Nonetheless, the decline in

taxa was accompanied by a rise in sessile eumetazoans, with new developments such as the rise of

predation Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

and

biomineralization Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often resulting in hardened or stiffened '' mineralized tissues''. It is an extremely widespread phenomenon: all six taxonomic kingd ...

. Another decline in diversity has been also proposed around 545 Ma.

Early decline

The decline in

biodiversity Biodiversity is the variability of life, life on Earth. It can be measured on various levels. There is for example genetic variability, species diversity, ecosystem diversity and Phylogenetics, phylogenetic diversity. Diversity is not distribut ...

from the previous White Sea assemblage has been argued to have been caused by decreasing sea oxygen levels, favoring the survival of animals with a higher

surface-to-volume ratio The surface-area-to-volume ratio or surface-to-volume ratio (denoted as SA:V, SA/V, or sa/vol) is the ratio between surface area and volume of an object or collection of objects. SA:V is an important concept in science and engineering. It is use ...

. This was, however, contested by findings showing a decline in both hard-bodied and soft-bodied fauna starting before the fall in oxygen levels. Under this model, the widespread anoxia in deeper waters would have minimally affected the Ediacaran fauna, largely concentrated in shallow water areas in

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an islan ...

settings like the Nama Group. Conversely, ecological change may have been responsible for the decline in oxygen levels.

Biota

The soft-bodied

of the Nama assemblage, in decline compared to the White Sea assemblage, was dominated by erniettomorphs, although rangeomorphs, arboreomorphs and dipleurozoans were also present. While the Late Ediacaran assemblages are mostly temporally stratified, holdovers from the Avalon and White Sea assemblages were present later than 550 million years ago, and are usually assigned to the Nama assemblage on a chronological basis regardless of biological affinity. These include '' Hiemalora'', ''

Charnia ''Charnia'' is an extinct genus of frond-like lifeforms belonging to the Ediacaran biota with segmented, leaf-like ridges branching alternately to the right and left from a zig-zag medial suture (thus exhibiting glide reflection, or opposite is ...

'' and the arboreomorph ''

Arborea Arborea is a town and ''comune'' in the province of Oristano, Sardinia, Italy, whose economy is largely based on agriculture and cattle breeding with production of vegetables, rice, fruit and milk (notably the local milk product Arborea). Histo ...

''. However, some definitions exclude these organisms from the Nama assemblage, distinguishing it from the temporal Terminal Ediacaran biozone. The benthic, calcified ''

Namacalathus ''Namacalathus'' is a problematic metazoan fossil occurring in the latest Ediacaran. The first, and only described species, ''N. hermanastes'', was first described in 2000 from the Nama Group of central and southern Namibia. Discovery and nami ...

'' is only known from the Nama assemblage, although its affiliations remain disputed.

Tubular organisms

The first traces of the tubular cloudinids appear in the Nama assemblage, including both the mineralized ''Cloudina'' and the softer-bodied ''Conotubus''. Other tubular organisms are known, such as ''

'', ''

Sinotubulites ''Sinotubulites'' is a genus of small, tube-shaped shelly fossils from near the terminal boundary of the Ediacaran period in formations within China and North America. It is often found in association with '' Cloudina'', and much like ''Cloudina ...

'', '' Corumbella'' and ''

Gaojiashania ''Gaojiashania cyclus'' is a worm-like, soft bodied organism with an epibenthic mode of life. Composed of repeating ring-like units, ''G. cyclus'' is flexible, soft, and not easily preserved. Pyritization prior to decay of soft parts results in ...

''.

References

{{reflist, refs= {{Cite journal , last=Waggoner , first=Ben , date=1 February 2003 , title=The Ediacaran Biotas in Space and Time , url=https://academic.oup.com/icb/article-lookup/doi/10.1093/icb/43.1.104 , journal=Integrative and Comparative Biology , language=en , volume=43 , issue=1 , pages=104–113 , doi=10.1093/icb/43.1.104 , pmid=21680415 , issn=1540-7063 {{Cite journal , last1=Muscente , first1=A. D. , last2=Bykova , first2=Natalia , last3=Boag , first3=Thomas H. , last4=Buatois , first4=Luis A. , last5=Mángano , first5=M. Gabriela , last6=Eleish , first6=Ahmed , last7=Prabhu , first7=Anirudh , last8=Pan , first8=Feifei , last9=Meyer , first9=Michael B. , last10=Schiffbauer , first10=James D. , last11=Fox , first11=Peter , last12=Hazen , first12=Robert M. , last13=Knoll , first13=Andrew H. , date=2019-02-22 , title=Ediacaran biozones identified with network analysis provide evidence for pulsed extinctions of early complex life , journal=Nature Communications , language=en , volume=10 , issue=1 , pages=911 , doi=10.1038/s41467-019-08837-3 , pmid=30796215 , issn=2041-1723, pmc=6384941 , bibcode=2019NatCo..10..911M {{Cite journal , last1=Boag , first1=Thomas H. , last2=Darroch , first2=Simon A. F. , last3=Laflamme , first3=Marc , date=November 2016 , title=Ediacaran distributions in space and time: testing assemblage concepts of earliest macroscopic body fossils , url=https://www.cambridge.org/core/journals/paleobiology/article/abs/ediacaran-distributions-in-space-and-time-testing-assemblage-concepts-of-earliest-macroscopic-body-fossils/57AC111D4FA52AC4DECC56A1E489F621 , journal=Paleobiology , language=en , volume=42 , issue=4 , pages=574–594 , doi=10.1017/pab.2016.20 , bibcode=2016Pbio...42..574B , issn=0094-8373, url-access=subscription {{Cite journal , title=The Latest Ediacaran Wormworld Fauna: Setting the Ecological Stage for the Cambrian Explosion , url=https://rock.geosociety.org/gsatoday/archive/26/11/article/i1052-5173-26-11-4.htm , journal=Geological Society of America , volume=26 , issue=11 , date=November 2011 , first1=James D , last1=Schiffbauer , first2=John Warren , last2=Huntley , first3=Gretchen R., last3=O'Neil , first4=Simon A.F., last4=Darroch , first5=Marc , last5=Laflamme , first6=Yaoping , last6=Cai , pages=4–11 Ediacaran