Aspidella

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Aspidella
Cyclomedusa fossils. Large aspidella look virtually identical, but have either concentric rings or radial rays.

Cyclomedusa fossils . Large aspidella look virtually identical, but have either concentric rings or radial rays.

Temporal occurrence
Ediacarium
610 to 555 million years
Locations
Systematics
incertae sedis
Genre : Aspidella
Scientific name
Aspidella
Billings , 1872
Art
  • Aspidella terranovica

Aspidella is a disc-shaped fossil of unclear systematic position from the geological age of the Ediacarian .

etymology

The generic name Aspidella is derived from the Latin diminutive of the ancient Greek ἀσπίς - Aspis - meaning "round shield". The species name terranovica is Latin ("coming from the new land") and denotes the type locality Newfoundland .

First description and research history

Aspidella terranovica was discovered in 1868 by the Scottish geologist Alexander Murray . In 1872 Elkanah Billings first described fossils of Aspidella terranovica from a Precambrian black schist outcrop on Duckworth Street in St. John's , Newfoundland. Billings was then chief paleontologist with the Geological Survey of Canada . However, his find was doubted by Charles Doolittle Walcott , who took the view that these were inorganically formed concretions . Others explained the fossil finds as gas bubbles escaping from the sediment or even of supernatural origin.

Aspidella terranovica is the first Ediacara biota fossil to be scientifically described.

For decades, Aspidella and related fossil finds were not recognized as Precambrian. It was not until 1946, with Reginald Claude Sprigg's research in the Ediacara Hills in South Australia , that the pre-Cambrian age of the fossils could no longer be in doubt. Since then, comparable fossils have been found at several sites around the world and are considered reliable evidence of multicellular life in the late Precambrian.

description

The genus Aspidella is a disc-shaped fossil with concentric rings and / or radial rays. The diameter of the disc can be 1 to 180 millimeters. Usually diameters of 4 to 10 millimeters are achieved. The disks can also assume an elliptical shape with a long semiaxis of 30 to 80 millimeters and a short semiaxis of 10 to 40 millimeters. There is usually a protrusion in the center. The disc edges are highlighted by distinct concentric rings and radial rib patterns.

distribution

Aspidella is found next to the type locality St. John's ( Fermeuse Formation of the St. John's Group ) on the Bonavista Peninsula and at Mistaken Point ( Mistaken Point Formation of the Conception Group ) in Newfoundland . Other finds come from the Twitya Formation in British Columbia and from the Albemarle Group of North Carolina . The taxon is also known from the Rawnsley Quartzite in South Australia .

Age

Aspidella fossils are found in layers that are dated to 610 to 555 million years BP . For example, the fossils from the Mistaken Point Formation are estimated to be 565 ± 3 million years BP and the Fermeuse Formation is estimated to be 560 million years BP. Somewhat questionable finds from Kazakhstan are even ascribed an age of 770 million years to BP.

ecology

The rarity of large individuals suggests that Aspidella was an r-strategist who left numerous offspring, but which died at a young age. Most often, Aspidella occurs in sediments of the deep water area. Aspidella is found in almost all Taphozoenoses of the Ediacarian, including in communities that were located above the storm wave base . Occasionally, up to 3000 individuals can be found on one square meter.

Taphonomy

The lower and upper sides of the fossils are characterized by a specific chemical composition that is similar to petrified biofilm coatings. The sediment enclosed in the fossil is enriched in certain elements compared to the enclosing rock matrix. Such a distribution can hardly be explained by means of diagenetic processes after the organism has died. It can therefore be assumed that the element enrichment (and thus also the sediment enrichment) took place while still alive.

Family position

Like Ediacaria (see below), Aspidella was initially regarded as an umbrella jellyfish (Scyphozoa). However, this assignment has now been rejected. Rather, it is assumed that the disks were anchors of organisms that rose up in sea water and broke off before they were embedded (in some specimens, stalk remains can be seen instead of the central protrusion). The common interpretation as anchoring discs of fern frond organisms (Frondomorpha) comes from Gehling and colleagues (2000). However, some researchers advocate an interpretation as microbial colonies. Peterson and colleagues (2003) believe that Aspidella and the similar taxa Charnia and Charniodiscus are similar to sponges . Menon and colleagues (2013) recently refer to horizontal and vertical traces of movement in the sediment caused by Aspidella and therefore conclude that a representative of the cnidarians (Cnidaria) was responsible.

Taxonomy

Forms which differed morphologically from Aspidella were called Ediacaria or Spriggia . The differences between Aspidella and the flat, ringed Spriggia wadea and the large, radial, convex protrusion form Ediacaria can obviously be traced back to different taphonomic conditions. In particular Spriggia and Ediacaria belong to a single taxon - Spriggia was fossilized in compact, fine-grained clay, whereas Ediacaria was settled on sandy sediments.

In addition, Aspidella has numerous other synonyms :

Individual evidence

  1. ↑ Hibbard, JP et al .: Significance of New Ediacaran Fossils and U ‐ Pb Zircon Ages from the Albemarle Group, Carolina Terrane of North Carolina . In: The Journal of Geology . tape 117 (5) , 2009, pp. 487. , Doi : 10.1086 / 600863 .
  2. ↑ Benus, AP: Sedimentological context of a deep-water Ediacaran fauna (Mistaken Point Formation, Avalon Zone, eastern Newfoundland) . In: Landing, E. et al., Trace fossils, small shelly fossils and the Precambrian-Cambrian boundary (Ed.): Bulletin of the New York State Museum . tape 463 , 1988, pp. 1-81 .
  3. ↑ Meert, JG et al: Glaciation and ~ 770 Ma Ediacara (?) Fossils from the Lesser Karatau Microcontinent, Kazakhstan . In: Gondwana Research . tape 19 (4) , 2010, pp. 867-880 , doi : 10.1016 / j.gr.2010.11.008 .
  4. ↑ Laflamme, M., Schiffbauer, JD, Narbonne, GM and Briggs, DEG: Microbial biofilms and the preservation of the Ediacara biota . In: Lethaia . 2010, doi : 10.1111 / j.1502-3931.2010.00235.x .
  5. ^ Narbonne, GM: The Ediacara Biota: Neoproterozoic Origin of Animals and Their Ecosystems . In: Annual Review of Earth and Planetary Sciences . tape 33 , 2005, pp. 421-442 , doi : 10.1146 / annurev.earth.33.092203.122519 .
  6. ^ Gehling, JG et al.: The first named Ediacaran body fossil, Aspidella terranovica . In: Palaeontology . tape 43 , 2000, pp. 427-456 .
  7. ↑ Grazhdankin, D. and Gerdes, G .: Ediacaran microbial colonies . In: Lethaia . tape 40 (3) , 2007, pp. 201-210 , doi : 10.1111 / j.1502-3931.2007.00025.x .
  8. ↑ Menon, LR et al .: Evidence for Cnidaria-like behavior in approx. 560 Ma Ediacaran Aspidella . In: Geology . 2013, doi : 10.1130 / G34424.1 .

Web links

Commons : Aspidella (syn. Cyclomedusa )  - collection of images, videos and audio files