Aspidella
Encyclopedia
Aspidella is an Ediacaran
disk-shaped fossil
.
s, 3-8 cm long and 1-4 cm wide. Most have a central pimple. The rim of all specimens is made up by ridge-edged rays and/or concentric rings.
s, producing numerous offspring of which most died young. It is most common in deep-water sediments, but is a constituent of most Ediacaran fossil assemblages, including those deposited above storm wave-base. The organisms can reach densities of 3000 m−2.
(see also below), Aspidella has initially been considered a scyphozoa
n jellyfish. This initial designation has been refuted; some specimens have been shown to be the holdfast
of some organism, the main body of which extended into the open water but broke off before fossilization (a few specimens bearing stubs of stalks opposed to the central pimple support this); whereas others represent microbial colonies.
Peterson, et al. have proposed that Aspidella (and the similar Charnia
and Charniodiscus
) were analogous to fungi
.
described Aspidella terranovica fossils from Duckworth Street, St. John's
, Newfoundland
. They were in a Precambrian
outcrop of black shale
. Billings was the head paleontologist with the Geological Survey of Canada at the time. Even so, his findings were disputed by Charles Doolittle Walcott
, who claimed that the shapes in the rocks were concretion
s formed inorganically. Other explanations offered at the time were that the circles were gas escape bubbles, or fakes planted by God to lure those with little faith into error. They were the first Ediacaran (Vendian) fossils described by a scientist.
For decades Aspidella and its partner fossils were not considered to be Precambrian life forms. This lasted until the work of Reg Sprigg
who discovered the Ediacara Hills
fossils. Fossils were found in many other parts of the world in rocks of about the same age and became accepted as genuine remains of life forms.
diminutive
of Ancient Greek
aspis
(Ασπις, a round shield). terranovica is Latin for "from the new land" (ie Newfoundland).
Different morphological
forms have been called Ediacaria
or Spriggina
wadea. However, the differences between the small elliptical "typical" Aspidella, the flat, ringed "Spriggina" wadea and the large, pimpled and rayed Ediacaria seem to be due to different taphonomic
conditions. For example, "S." wadea and Ediacaria appear to be remains of the same animals, only that the former was fossilized in more compact, fine-grained clay
whereas the latter is known from rocks that originally were predominantly sand
y sediment.
Numerous other taxa may also be junior synonyms
of Aspidella:
Due to its nondescript nature, Aspidella might be considered a form taxon
, an artificial assemblage of similar-looking and similar-living organisms without indicating of a phylogenetic
relationship. In this case, some presumed synonyms (such as Ediacaria or Cyclomedusa) would remain valid. The type specimens were of the ellipsoid type (they are lost, but a cast remains). Thus, if Aspidella in the loose sense turns out to be an assemblage of more or less related taxa, the genus name would apply only to the smallish ellipsoid specimens.
and Mistaken Point in Newfoundland
, and the Twitya formation in British Columbia
; see also below.
Aspidella fossils are found from , with putative representatives dating to .
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
disk-shaped fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
.
Morphology
Aspidella consists of disk-shaped fossils, with concentric rings and/or centripetal rays. The diameter of circular Aspidella varies from 1 to 180 mm. Most individuals are between 4 and 10 mm, however; smaller animals would presumably have decayed before they could fossilize. Other Aspidella take the form of ellipseEllipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
s, 3-8 cm long and 1-4 cm wide. Most have a central pimple. The rim of all specimens is made up by ridge-edged rays and/or concentric rings.
Ecology
The rarity of large individuals probably indicates that Aspidella were r-strategistR/K selection theory
In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity or quality of offspring...
s, producing numerous offspring of which most died young. It is most common in deep-water sediments, but is a constituent of most Ediacaran fossil assemblages, including those deposited above storm wave-base. The organisms can reach densities of 3000 m−2.
Affinity
Just like EdiacariaEdiacaria
Ediacaria is a fossil genus dating to the Ediacaran Period of the Neoproterozoic Era. Unlike most Ediacaran biota which disappeared almost entirely from the fossil record at the end of the Period, Ediacaria fossils have been found dating from the Baikalian age of the Upper Riphean to 501 million...
(see also below), Aspidella has initially been considered a scyphozoa
Scyphozoa
Scyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
n jellyfish. This initial designation has been refuted; some specimens have been shown to be the holdfast
Holdfast
A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate. ...
of some organism, the main body of which extended into the open water but broke off before fossilization (a few specimens bearing stubs of stalks opposed to the central pimple support this); whereas others represent microbial colonies.
Peterson, et al. have proposed that Aspidella (and the similar Charnia
Charnia
Charnia is the genus name given to a frond-like Ediacaran lifeform with segmented ridges branching alternately to the right and left from a zig-zag medial suture. The genus Charnia was named after Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found.- Diversity...
and Charniodiscus
Charniodiscus
Charniodiscus is an Ediacaran fossil that was probably a stationary filter feeder that lived anchored to a sandy sea bed. The organism had a holdfast, stalk and frond. The holdfast was bulbous shaped, and the stalk was flexible. The frond was segmented and had a pointed tip...
) were analogous to fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
.
Taphonomy
The upper and lower surfaces of the fossils have a distinct elemental composition that resembles that of fossilized biofilms. The sediment within the fossils also has a distinct composition, being enriched in certain elements with respect to the rock matrix. Since it is difficult to account for such a distribution of elements by post-mortem diagenetic processes, it would appear that the elements (and thus the sediment) were incorporated into the organism whilst it was alive.History of research
In 1872, Elkanah BillingsElkanah Billings
Elkanah Billings is often referred to as Canada's first paleontologist. Billings was born on a farm by the Rideau River outside Bytown, now known as Billings Estate. His parents were named Lamira and Braddish Billings. His family included an older sister named Sabra and an older brother Braddish II...
described Aspidella terranovica fossils from Duckworth Street, St. John's
St. John's, Newfoundland and Labrador
St. John's is the capital and largest city in Newfoundland and Labrador, and is the oldest English-founded city in North America. It is located on the eastern tip of the Avalon Peninsula on the island of Newfoundland. With a population of 192,326 as of July 1, 2010, the St...
, Newfoundland
Newfoundland and Labrador
Newfoundland and Labrador is the easternmost province of Canada. Situated in the country's Atlantic region, it incorporates the island of Newfoundland and mainland Labrador with a combined area of . As of April 2011, the province's estimated population is 508,400...
. They were in a Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
outcrop of black shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
. Billings was the head paleontologist with the Geological Survey of Canada at the time. Even so, his findings were disputed by Charles Doolittle Walcott
Charles Doolittle Walcott
Charles Doolittle Walcott was an American invertebrate paleontologist. He became known for his discovery in 1909 of well-preserved fossils in the Burgess Shale of British Columbia, Canada.-Early life:...
, who claimed that the shapes in the rocks were concretion
Concretion
A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...
s formed inorganically. Other explanations offered at the time were that the circles were gas escape bubbles, or fakes planted by God to lure those with little faith into error. They were the first Ediacaran (Vendian) fossils described by a scientist.
For decades Aspidella and its partner fossils were not considered to be Precambrian life forms. This lasted until the work of Reg Sprigg
Reg Sprigg
Reginald Claude Sprigg, AO, HonDSc ANU, HonDSc Flinders, MSc Adelaide, FTSE was an Australian geologist and conservationist. At age 17 he became the youngest Fellow of the Royal Society of South Australia. In 1946, in the Ediacara Hills, South Australia he discovered the Ediacara biota, an...
who discovered the Ediacara Hills
Ediacara Hills
Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around 650 km north of Adelaide. The area has many old copper and silver mines from mining activity in the late 19th century...
fossils. Fossils were found in many other parts of the world in rocks of about the same age and became accepted as genuine remains of life forms.
Systematics and taxonomy
Aspidella is derived from the LatinLatin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
diminutive
Diminutive
In language structure, a diminutive, or diminutive form , is a formation of a word used to convey a slight degree of the root meaning, smallness of the object or quality named, encapsulation, intimacy, or endearment...
of Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
aspis
Aspis
"Aspis" is the generic term for the word shield. The aspis, which is carried by Greek infantry of various periods, is often referred to as a hoplon .According to Diodorus Siculus:-Construction:...
(Ασπις, a round shield). terranovica is Latin for "from the new land" (ie Newfoundland).
Different morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
forms have been called Ediacaria
Ediacaria
Ediacaria is a fossil genus dating to the Ediacaran Period of the Neoproterozoic Era. Unlike most Ediacaran biota which disappeared almost entirely from the fossil record at the end of the Period, Ediacaria fossils have been found dating from the Baikalian age of the Upper Riphean to 501 million...
or Spriggina
Spriggina
Fossils of Spriggina are known from the Ediacaran period, around . The segmented organism reached about 3 cm in length and may have been predatory...
wadea. However, the differences between the small elliptical "typical" Aspidella, the flat, ringed "Spriggina" wadea and the large, pimpled and rayed Ediacaria seem to be due to different taphonomic
Taphonomy
Taphonomy is the study of decaying organisms over time and how they become fossilized . The term taphonomy was introduced to paleontology in 1940 by Russian scientist Ivan Efremov to describe the study of the transition of remains, parts, or products of organisms, from the biosphere, to the...
conditions. For example, "S." wadea and Ediacaria appear to be remains of the same animals, only that the former was fossilized in more compact, fine-grained clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
whereas the latter is known from rocks that originally were predominantly sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
y sediment.
Numerous other taxa may also be junior synonyms
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
of Aspidella:
- Beltanella
- CyclomedusaCyclomedusaCyclomedusa is a circular fossil of the Ediacaran biota; it has a circular bump in the middle and as many as five circular growth ridges around it. Many specimens are small, but specimens in excess of 20cm are known. The concentric disks are not necessarily circular, especially when adjacent...
- Glaessneria
- Irridinitus
- Jampolium
- Madigania
- Medusinites
- Paliella
- Paramedusium
- Planomedusites
- Protodipleurosoma
- Tateana
- Tirasiana
- Vendella
Due to its nondescript nature, Aspidella might be considered a form taxon
Form taxon
Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships...
, an artificial assemblage of similar-looking and similar-living organisms without indicating of a phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
relationship. In this case, some presumed synonyms (such as Ediacaria or Cyclomedusa) would remain valid. The type specimens were of the ellipsoid type (they are lost, but a cast remains). Thus, if Aspidella in the loose sense turns out to be an assemblage of more or less related taxa, the genus name would apply only to the smallish ellipsoid specimens.
Occurrence
Other places where Aspidella are found is the Bonavista PeninsulaBonavista Peninsula
The Bonavista Peninsula is a large peninsula on the east coast of the island of Newfoundland in the Canadian province of Newfoundland and Labrador....
and Mistaken Point in Newfoundland
Newfoundland and Labrador
Newfoundland and Labrador is the easternmost province of Canada. Situated in the country's Atlantic region, it incorporates the island of Newfoundland and mainland Labrador with a combined area of . As of April 2011, the province's estimated population is 508,400...
, and the Twitya formation in British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
; see also below.
Aspidella fossils are found from , with putative representatives dating to .