Ernietta

Fossil specimens show individuals to have lived partly buried in the substrate, as well as filled to some degree by substrate material. An exposed frill extended out of the substrate and was thought to have conducted feeding in the water column.[3][5] Modeling based on fossil specimens show that the frill possessed an “upturned bell” shape.[3][6] Water and nutrients circulated within this bell cavity, and the organism is thought to have engaged in suspension feeding.[3][6] It is possible that appendages which carried out feeding were not preserved in fossils.[6][7] Previously, Ernietta were thought to have obtained nutrients by passive absorption, however, this is currently unsupported given the high volume to surface area ratio observed in Ernietta[3][6]. Alternatively Ernietta may have lived from associated symbiotic algae.[8]

Hydrodynamic modeling carried out by Gibson et al. in 2019 assumed that Ernietta inhabited shallow marine environments in aggregations.[3] Nutrient delivery was found to be optimized when individuals were situated in “clumped” formations, with multiple individuals aggregated in groups located upstream or downstream from one another. This formation enhanced both vertical mixing and the direction of nutrient-rich currents to the bodies of downstream individuals. This may thus be one of the earliest examples of commensalism, in which organisms act to mutual benefit.[6]

Ernietta plateauensis from the Kliphoek Member of the Dabis Formation. Farm Aar, Namibia.

Cross-sectional view of Ernietta plateauensis from the Ediacaran Kliphoek Member of the Dabis Formation. Farm Aar, near Aus, Namibia.

The body of Ernietta is composed of a layer of tubes (with some preservations indicating a double-layer of tubes).[2][3] Perpendicular to these tubes is an equatorial seam.[3][2] The body is asymmetrical along either side of this seam. The presence of this seam and offset symmetry unites the Ernettiomorpha, which includes taxa more similar to Ernietta (for example, Pteridinium, Swartpuntia, and Mietta) than to the rangeomorphs.[2]

Ernietta has been considered a benthic shallow marine fossil comparable with an anemone, however there is evidence for freshwater environments from its low boron content compared with other Ediacaran fossils.[9] Specimens have also been found covered with thin layers of wind-drift sand of alluvial levees.[10]

All occurrences of Ernietta are known from the Nama Group (specifically, the Kuibis and Schwarzrand subgroups) of present-day Namibia.[4] The Nama Group consists of fluvial and shallow marine sediments that span the Ediacaran to the Cambrian (approximately spanning from 570-543 Ma). Formations of the Nama Group outcrop across southern Namibia. Most specimens of Ernietta are preserved in sandstones, with a single occurrence in a siltstone.[4] Fossils of the Aar Member of the Nama Group in particular are preserved within beds of cross-stratified sandstones.[5] A significant discovery of this taxon was reported in 2016, at the Farm Aar field site in southern Namibia, which recovered a number of specimens preserved in life position in the water column.[2] This depositional environment of this site has been interpreted as subtidal, with periodic influxes of storm-induced sands.[2]

Other reports of Ernietta exist outside Namibia, including Nevada, USA [11] An occurrence in the Windermere group of British Columbia, Canada has not yet been described.[2]

Ediacaran fossils are difficult to assign to taxonomic classifications based on modern organisms, as they have no living representatives. As such, these fossils are often allied by morphological groupings, known as clades.[2] As such, Ernietta belongs to the Ernettiomorpha.[12] As a whole, the Ediacaran biota have been variously defined as early marine animals,[13] cnidarians,[14] lichens,[8] bacterial colonies,[15] or an intermediate between plants and animals.[16]

The Ernietta genus contains a sole species: Ernietta plateauensis. This species is the type species of the Ernettiomorpha and was first described in 1966 by Pflug.[17] In 1972, Pflug split this species into 13 genera and 29 species, all of which were later synonymized and united in a single species, Ernietta plateauensis.[2][12] These synonymized taxa include:

Congregation of multiple individuals of Ernietta plateauensis from the Ediacaran Kilhoek Member of the Dabis Formation. Farm Aar, near Aus, Namibia.

• List of Ediacaran genera
• Ediacaran biota
• Ediacaran
• Geology of Namibia

• Life on a Young Planet: The First Three Billion Years of Evolution on Earth (Princeton Science Library) by Andrew H. Knoll
• Exceptional Fossil Preservation by David J. Bottjer, Walter Etter, James W. Hagadorn, and Carol M. Tang
• The Ecology of the Cambrian Radiation by Andrey Zhuravlev and Robert Riding
• J. Dzik (1999). "Organic membranous skeleton of the Precambrian metazoans from Namibia" (PDF). Geology. 27 (6): 519–522. Bibcode:1999Geo....27..519D. doi:10.1130/0091-7613(1999)027<0519:OMSOTP>2.3.CO;2.