Nama assemblage
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The Nama assemblage was the last of the Ediacaran biotic assemblages. Following the Avalon and White Sea assemblages, it spanned from c. 550 Ma to c. 539 Ma, coinciding with the Terminal Ediacaran biozone.[3] 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 extinctions of the Phanerozoic. A second drop of diversity occurred at the Ediacaran–Cambrian boundary, concluding the Nama assemblages with the end-Ediacaran extinction.[4][5][6]
Etymology and definition[edit]
The Nama assemblage is named after the Nama Group in Namibia, preserving a Late Ediacaran record of soft-bodied fossils. While the earliest fossils in the Nama Group, known from the Tsaus Mountains environment, date back to before 550.5 Ma, they represent holdover taxa from the earlier Avalon and White Sea assemblages. A proposed definition of the Nama assemblage's lower boundary by Wood et al. refers to the oldest appearance of Cloudina in the fossil record, placing the boundary at 550 Ma.[3]
Paleohistory[edit]
The Nama assemblage is bounded from the earlier White Sea assemblage and later Cambrian period by two major episodes of faunal turnover, considered to be pulses of the end-Ediacaran extinction. Another decline in diversity has been also proposed around c. 545 Ma.[3]
The decline in biodiversity 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.[5] 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 settings like the Nama Group.[7][3] Conversely, ecological change may have been responsible for the decline in oxygen levels.[8]
Biota[edit]
The Nama biota was dominated by soft-bodied 'vendozoan' taxa such as erniettomorphs, although rangeomorphs, arboreomorphs and dipleurozoans were also present.[3]
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 and the arboreomorph Arborea. However, some definitions exclude these organisms from the Nama assemblage, distinguishing it from the temporal Terminal Ediacaran biozone.[3]
The benthic, calcified Namacalathus is only known from the Nama assemblage, although its affiliations remain disputed.[3]
Tubular organisms[edit]
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 Namacalathus, Sinotubulites, Corumbella and Gaojiashania.[3]
References[edit]
- ^ Shen, Bing; Dong, Lin; Xiao, Shuhai; Kowalewski, Michal (4 January 2008). "The Avalon explosion: evolution of Ediacara morphospace". Science. 319 (5859): 81–84. Bibcode:2008Sci...319...81S. doi:10.1126/science.1150279. ISSN 1095-9203. PMID 18174439.
- ^ Shi, Wei; Li, Chao; Luo, Genming; Huang, Junhua; Algeo, Thomas J.; Jin, Chengsheng; Zhang, Zihu; Cheng, Meng (24 January 2018). "Sulfur isotope evidence for transient marine-shelf oxidation during the Ediacaran Shuram Excursion". Geology. 46 (3): 267–270. doi:10.1130/G39663.1.
- ^ a b c d e f g h Wood, Rachel; Bowyer, Fred T.; Alexander, Ruaridh; Yilales, Mariana; Uahengo, Collen-Issia; Kaputuaza, Kavevaza; Ndeunyema, Junias; Curtis, Andrew (September 2023). "New Ediacaran biota from the oldest Nama Group, Namibia (Tsaus Mountains), and re-definition of the Nama Assemblage". Geological Magazine. 160 (9): 1673–1686. Bibcode:2023GeoM..160.1673W. doi:10.1017/S0016756823000638. hdl:20.500.11820/bc8c23b0-d59c-4230-a45b-db854a8ad0f3. ISSN 0016-7568.
- ^ Bowyer, Fred T.; Uahengo, Collen-Issia; Kaputuaza, Kavevaza; Ndeunyema, Junias; Yilales, Mariana; Alexander, Ruaridh D.; Curtis, Andrew; Wood, Rachel A. (15 October 2023). "Constraining the onset and environmental setting of metazoan biomineralization: The Ediacaran Nama Group of the Tsaus Mountains, Namibia". Earth and Planetary Science Letters. 620: 118336. Bibcode:2023E&PSL.62018336B. doi:10.1016/j.epsl.2023.118336. ISSN 0012-821X.
- ^ a b c Evans, Scott D.; Tu, Chenyi; Rizzo, Adriana; Surprenant, Rachel L.; Boan, Phillip C.; McCandless, Heather; Marshall, Nathan; Xiao, Shuhai; Droser, Mary L. (15 November 2022). "Environmental drivers of the first major animal extinction across the Ediacaran White Sea-Nama transition". Proceedings of the National Academy of Sciences. 119 (46): e2207475119. Bibcode:2022PNAS..11907475E. doi:10.1073/pnas.2207475119. ISSN 0027-8424. PMC 9674242. PMID 36343248.
- ^ Bottjer, David J.; Clapham, Matthew E. (2006). Xiao, Shuhai; Kaufman, Alan J. (eds.). Evolutionary Paleoecology of Ediacaran Benthic Marine Animals. Dordrecht: Springer Netherlands. pp. 91–114. doi:10.1007/1-4020-5202-2_4. ISBN 978-1-4020-5202-6.
- ^ Tostevin, Rosalie; Clarkson, Matthew O.; Gangl, Sophie; Shields, Graham A.; Wood, Rachel A.; Bowyer, Fred; Penny, Amelia M.; Stirling, Claudine H. (2019-01-15). "Uranium isotope evidence for an expansion of anoxia in terminal Ediacaran oceans". Earth and Planetary Science Letters. 506: 104–112. doi:10.1016/j.epsl.2018.10.045. ISSN 0012-821X.
- ^ Lenton, Timothy M.; Daines, Stuart J. (2018). "The effects of marine eukaryote evolution on phosphorus, carbon and oxygen cycling across the Proterozoic–Phanerozoic transition". Emerging Topics in Life Science. 2 (2): 267–278. doi:10.1042/ETLS20170156. PMC 7289021. PMID 32412617.