Francevillian biota

The Francevillian biota (Also referred to as Gabon macrofossils, Gabonionta or Francevillian group fossils) are a collection of 2.1-billion-year-old Palaeoproterozoic macroscopic structures, controversially suggested to be fossils, known from the Francevillian B Formation in Gabon, a black shale province notable for its lack of any noticeable metamorphism.[1] The structures have been postulated by some authors to be evidence of the earliest form of multicellular life, and of eukaryotes.[1][2] They were discovered by an international team led by Moroccan-French geologist Abderrazak El Albani, of the University of Poitiers, France. While they have yet to be assigned to a formal taxonomic position, they have been informally and collectively referred to as the "Gabonionta", including by the Natural History Museum Vienna in 2014.[3] The status of the structures as fossils has been questioned.[4][5]

Part of a series on
The Francevillian basin
Geology and localities
Fossils
(Also see Akouemma)
Evolutionary concepts
Example of macroscopic structure referred to the Francevillian biota

Morphology
Members of the Francevillian biota
Francevillian biota macroscopic structure at display at the Natural History Museum, Vienna.
Two adjacent macrosopic Francevillian Biota structures

The structures are up to 17 centimetres (6.7 in) in size.[6][7] They form flattened disks with a characteristic morphology, including circular and elongated specimens. A spherical to ellipsoidal central body is bounded by radial structures. The structures show three-dimensionality and purportedly coordinated growth.[6]

A newer 2014 study by El Albani et al. describes multiple types of structures with different morphologies. There are convoluted tubes, and "string of pearls"-like structures that terminate in a "flower". This is similar to dictyostelid slime molds, amoebal organisms that form multicellular assemblies to migrate. However, dictyostelids are terrestrial, not marine organisms so the structures cannot simply be dictyostelids. Among known fossils, the Ediacaran Nemiana and Beltanelloides are most similar when compared to the "string of pearls".[1]

In 2023, more structures were studied by El Albani and colleagues, which were characterized by the authors as eukaryotes. They appear to be flattened lenticular disks reaching up to 4.5 cm in diameter, with a chambered interior surrounded by a notched ridge about 1/6th of the diameter in width. The authors hypothesized this to play a role in their movement through the water column, as they suggested the organisms were likely planktonic. The structures were found to have an unusual concentration of zinc compared to the surrounding sediments, an element performing key function in eukaryote biochemistry.[2]

Locality
Geology of the Francevillian basin

The findings come from shales of the Franceville basin with a high density of up to 40 structures per square meter. The authors proposed that the organisms survived at the bottom in shallow sea water in colonies. The geochemistry of the site indicates that the structures formed in sediment under an oxygenated water column of a prograding delta, and if they were biological might have engaged in aerobic respiration.[6]

Interpretations

In describing the structures, El Albani and colleagues described them as colonial organisms with possible affinities to eukaryotes, akin to microbial mats, albeit unlike any known structures in the fossil record, yet noting the complexity of the structures and presence of sterane as suggestive of possible eukaryote identity. In a concurrent news report in Nature, paleontologist Philip Donoghue of Bristol University advocates a more conservative approach pending further evidence before calling them eukaryotes.

Another view, held by Yale's Adolf Seilacher, interprets the fossils as not organisms at all, but rather pseudofossils of inorganic pyrites.[8] El Albani et al. (2014) explicitly disputed Seilacher's interpretation.[1] A 2016 study of similar structures In Michigan, around 1.1 billion years old found them to be concretions, which the authors suggested cast doubt in the biogenicity of the Francevillian structures.[9] In a 2017 review paper Emmanuelle J. Javaux & Kevin Lepot stated that the biogenic nature of the macroscopic structures was "questionable".[4] Miao et al. 2019 stated that due to the "simple morphology and lack of diagnostic features, their eukaryotic affiliation still remains uncertain".[10] A 2023 review suggested that the structures were potentially artifacts of diagenesis, and that reliabliy distinguishing between biogenic and abiogenic structures in Paleoproterozoic rocks could be "extremely difficult", and therefore the Francevillian Biota and other supposed multicellular fossils of a similar age "currently fail to pass the stringent criteria for these structures to be viewed as bona fide fossils".[5] A 2023 isotopic analysis of the structures found that they were enriched in zinc, cobalt and nickel isotopes, with the zinc being preferentially enriched in light isotopes, which the authors suggested could represent eukaryotic metabolism. However they noted that the Francevillian Biota is still 400 million years older than is currently widely accepted for the earliest known eukaryotes.[11]

See also
Wikimedia Commons has media related to Gabonionta.

References
  1. El Albani, Abderrazak; Bengtson, Stefan; Canfield, Donald E.; Riboulleau, Armelle; Rollion Bard, Claire; Macchiarelli, Roberto; et al. (2014). "The 2.1 Ga Old Francevillian Biota: Biogenicity, Taphonomy and Biodiversity". PLOS ONE. 9 (6): e99438. Bibcode:2014PLoSO...999438E. doi:10.1371/journal.pone.0099438. PMC 4070892. PMID 24963687.
  2. El Albani, Abderrazak (2023). "A search for life in Palaeoproterozoic marine sediments using Zn isotopes and geochemistry". Earth and Planetary Science Letters. 623: 118169. Bibcode:2023E&PSL.61218169E. doi:10.1016/j.epsl.2023.118169. S2CID 258360867.
  3. Experiment Life – the Gabonionta. (Press Release). 4 March 2014. Naturhistorisches Museum Wien
  4. Javaux, Emmanuelle J.; Lepot, Kevin (January 2018). "The Paleoproterozoic fossil record: Implications for the evolution of the biosphere during Earth's middle-age". Earth-Science Reviews. 176: 68–86. Bibcode:2018ESRv..176...68J. doi:10.1016/j.earscirev.2017.10.001. S2CID 37069547. The identity of the [Francevillian biota] macrostructures remains unknown and their biogenicity is questionable
  5. Fakhraee, Mojtaba; Tarhan, Lidya G.; Reinhard, Christopher T.; Crowe, Sean A.; Lyons, Timothy W.; Planavsky, Noah J. (May 2023). "Earth's surface oxygenation and the rise of eukaryotic life: Relationships to the Lomagundi positive carbon isotope excursion revisited". Earth-Science Reviews. 240: 104398. Bibcode:2023ESRv..24004398F. doi:10.1016/j.earscirev.2023.104398. S2CID 257761993.
  6. El Albani, Abderrazak; Bengtson, Stefan; Canfield, Donald E.; Bekker, Andrey; Macchiarelli, Roberto; Mazurier, Arnaud; Hammarlund, Emma U.; et al. (2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago" (PDF). Nature. 466 (7302): 100–104. Bibcode:2010Natur.466..100A. doi:10.1038/nature09166. PMID 20596019. S2CID 4331375.
  7. "Une vie complexe il y a 2 milliards d'années : l'hypothèse se confirme !". futura-sciences.com (in French). 26 June 2014. Retrieved 18 November 2017.
  8. Maxmen, Amy (30 June 2010). "Ancient macrofossils unearthed in West Africa". Nature. doi:10.1038/news.2010.323.
  9. Anderson, Ross P.; Tarhan, Lidya G.; Cummings, Katherine E.; Planavsky, Noah J.; BjøRnerud, Marcia (July 2016). "MACROSCOPIC STRUCTURES IN THE 1.1 Ga CONTINENTAL COPPER HARBOR FORMATION: CONCRETIONS OR FOSSILS?". PALAIOS. 31 (7): 327–338. doi:10.2110/palo.2016.013. ISSN 0883-1351.
  10. Miao, Lanyun; Moczydłowska, Małgorzata; Zhu, Shixing; Zhu, Maoyan (2019-02-01). "New record of organic-walled, morphologically distinct microfossils from the late Paleoproterozoic Changcheng Group in the Yanshan Range, North China". Precambrian Research. 321: 172–198. Bibcode:2019PreR..321..172M. doi:10.1016/j.precamres.2018.11.019. ISSN 0301-9268. S2CID 134362289.
  11. Ossa Ossa, Frantz; Pons, Marie-Laure; Bekker, Andrey; Hofmann, Axel; Poulton, Simon W.; Andersen, Morten B.; Agangi, Andrea; Gregory, Daniel; Reinke, Christian; Steinhilber, Bernd; Marin-Carbonne, Johanna; Schoenberg, Ronny (June 2023). "Zinc enrichment and isotopic fractionation in a marine habitat of the c. 2.1 Ga Francevillian Group: A signature of zinc utilization by eukaryotes?". Earth and Planetary Science Letters. 611: 118147. doi:10.1016/j.epsl.2023.118147.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.