Chancelloriidae
Encyclopedia
The Chancelloriids are an extinct family of animal common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism. Other specimens appear to be more complete and to represent sessile
, bag-like organisms with a soft skin armored with star-shaped calcareous
sclerite
s from which radiate sharp spines.
Classifying the chancelloriids is difficult. Some paleontologists classify them as sponges, an idea which chancelloriids' sessile lifestyle and simple structure make plausible. Other proposals suggest that they were more advanced, or at least originated from more advanced ancestors; for example chancelloriids' skins appear to be much more complex than those of any sponge. It has been suggested that chancelloriids were related to the "chain mail" armored slug-like halkieriids, which are important in analyses of the Cambrian explosion
. While the sclerites of the two groups are very similar right down to the microscopic level, and therefore unlikely to have evolved independently
, the large dissimilarity in the body plans of the two groups creates a puzzle which is hard to resolve.
(e.g. Siberia
, China
, Mongolia
), Australia
's Georgina Basin, Canada
's Burgess Shale
, and the USA. The earliest known fossils come from the small shelly fossil assemblage of the Anabarites trisulcatus Zone of the Lower Nemakit-Daldynian Stage, Siberia and its analog in China is the Anabarites trisulcatus-Protohertzina anabarica Zone of the basal Meishucunian Stage. The fossil record suggests that chancelloriids declined rapidly during the Late Cambrian, and they were probably extinct by the end of the Cambrian.
They were first described in 1920 by Charles Doolittle Walcott
, who regarded them as one of the most primitive groups of sponges.
at the top, and show no evidence of internal organs. The different species
show a variety of shapes and sizes, for example: Chancelloria eros was a slim cone with the narrow end at the bottom, typically 4 to 6 cm (1.6 to 2.4 in) long and 1.5 to 2 cm (0.590551181102362 to 0.78740157480315 in) in diameter at its widest point; Allonnia junyani formed a disk or cylinder usually 6 to 7 cm (2.4 to 2.8 in) in diameter, and the tallest were about 20 centimetres (7.9 in) long.
Most of the fossils consist of collections of mineralized hard parts called sclerite
s, and an assembly that is thought to have belonged to one individual is called a scleritome. Many specimens consist only of scattered sclerites, whose form is used to classify them, and some specimens have not yet been assigned to a species or even genus
.
Individual sclerites had star-shaped bases that lay flat against the body and one spine projecting outwards at a right angle
. The sclerites had internal cavities and in fact many are preserved as casting
s of the cavities filled with phosphate
. It is thought that when the animals were alive these cavities were filled with tissue
s that secreted
the hard outer coverings. It is not clear what the hard substance of the walls was since it has been replaced or converted to a different crystal
ine form. This suggests it was a slightly unstable material such as aragonite
, a form of calcium carbonate
. Some sclerites appear to be on top of the skin, other covered by it, and some appear partly covered.
Since the sclerites were external and non-interlocking, they could not have functioned as supporting "struts". Since the body was sessile and attached to the sea-bed, the sclerites would not aided locomotion by increasing traction. So the only conceivable function for the sclerites appears to be defence against predators, rather similar to the spines on modern cacti
.
view of the evolution of multi-celled animals. Walcott classified chancelloriids as sponges, a view that was first queried by Bengston and colleagues, who considered the hollow, multi-part spicules to be quite unlike anything secreted by a sponge. Butterfield and Nicholas (1996) argued that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites is similar to that of fibers of spongin
, a collagen
protein
, in modern keratose (horny) demosponge
s such as Darwinella.
However Janussen, Steiner and Zhu (2002) opposed this view, arguing that: spongin does not appear in all Porifera, but may be a defining feature of the demosponges; the silica-based spines of demosponges are secreted
by specialist sclerocyte
cells that surround them, while mineralized chancellorid sclerites were hollow and filled with soft tissues connected to the rest of the animal by restricted openings in the bases of the sclerites; chancellorid sclerites were probably made of aragonite
, which is not found in demosponges, and the only sponges that use aragonite are the sclerosponge
s, whose soft bodies cover hard, often massive skeletons made of either aragonite or calcite
, another form of calcium carbonate
; sponges have loosely bound-together skins called pinacoderm
s, which are only one cell thick, while the skins of chancellorids were much thicker and shows signs of connective structures called belt desmosome
s. In their opinion the presence of belt desmosomes made chancellorids members of the Epitheliazoa, the next higher taxon
above the Porifera, to which sponges belong. They thought it was difficult to say whether chancellorids were members of the Eumetazoa
, "true animals" whose tissues are organized into Germ layer
s; chancellorids' lack of organs such as sense organs, muscles and a gut would seem to exclude them from the Eumetazoa; but possibly chancellorids descended from Eumetazoans that lost these features after becoming sessile
filter-feeders
.
Porter (2008) argued that the sclerites of chancelloriids are extremely similar to those of the halkieriids, mobile bilaterian animals that looked like slug
s in chain mail
and whose fossils are found in rocks from the very Early Cambrian to the Mid Cambrian. The hollow "coelosclerites" of halkieriids and chancelloriids resemble each other at all levels: both have a thin external organic layer, and an internal "pulp cavity" that is connected to the rest of the body by a narrow channel; the walls of both are made of the same material, aragonite
; the arrangement of the aragonite fibers in each is the same, running mainly from base to tip but with each being closer to the surface at the end nearest the tip. Porter thought it extremely improbable that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related. This dilemma, she suggested, may be resolved in various ways:
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
, bag-like organisms with a soft skin armored with star-shaped calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
sclerite
Sclerite
A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....
s from which radiate sharp spines.
Classifying the chancelloriids is difficult. Some paleontologists classify them as sponges, an idea which chancelloriids' sessile lifestyle and simple structure make plausible. Other proposals suggest that they were more advanced, or at least originated from more advanced ancestors; for example chancelloriids' skins appear to be much more complex than those of any sponge. It has been suggested that chancelloriids were related to the "chain mail" armored slug-like halkieriids, which are important in analyses of the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
. While the sclerites of the two groups are very similar right down to the microscopic level, and therefore unlikely to have evolved independently
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
, the large dissimilarity in the body plans of the two groups creates a puzzle which is hard to resolve.
Occurrence
Chancelloriid fossils have been found in many parts of the world, including various parts of AsiaAsia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
(e.g. Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, Mongolia
Mongolia
Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest...
), Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
's Georgina Basin, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
's Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
, and the USA. The earliest known fossils come from the small shelly fossil assemblage of the Anabarites trisulcatus Zone of the Lower Nemakit-Daldynian Stage, Siberia and its analog in China is the Anabarites trisulcatus-Protohertzina anabarica Zone of the basal Meishucunian Stage. The fossil record suggests that chancelloriids declined rapidly during the Late Cambrian, and they were probably extinct by the end of the Cambrian.
They were first described in 1920 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 regarded them as one of the most primitive groups of sponges.
Description
The chancelloriids had bag-like bodies with an orificeBody orifice
-External orifices:In a typical mammalian body such as the human body, the external body orifices are:* The nostrils, for breathing and the associated sense of smell.* The eyes, for the sense of sight and crying....
at the top, and show no evidence of internal organs. The different species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
show a variety of shapes and sizes, for example: Chancelloria eros was a slim cone with the narrow end at the bottom, typically 4 to 6 cm (1.6 to 2.4 in) long and 1.5 to 2 cm (0.590551181102362 to 0.78740157480315 in) in diameter at its widest point; Allonnia junyani formed a disk or cylinder usually 6 to 7 cm (2.4 to 2.8 in) in diameter, and the tallest were about 20 centimetres (7.9 in) long.
Sclerite
A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....
s, and an assembly that is thought to have belonged to one individual is called a scleritome. Many specimens consist only of scattered sclerites, whose form is used to classify them, and some specimens have not yet been assigned to a species or even genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
.
Individual sclerites had star-shaped bases that lay flat against the body and one spine projecting outwards at a right angle
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...
. The sclerites had internal cavities and in fact many are preserved as casting
Casting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
s of the cavities filled with phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
. It is thought that when the animals were alive these cavities were filled with tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
s that secreted
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
the hard outer coverings. It is not clear what the hard substance of the walls was since it has been replaced or converted to a different crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
ine form. This suggests it was a slightly unstable material such as aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
, a form of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
. Some sclerites appear to be on top of the skin, other covered by it, and some appear partly covered.
Lifestyle
Chancelloriids probably lived on muddy sea-floors, as their sclerites increase in size from the bottom to the top, and all had thickenings at the bases, which are regarded as anchors; they are often preserved in attachment to other organisms or shelly debris. They were very likely filter-feeders.Since the sclerites were external and non-interlocking, they could not have functioned as supporting "struts". Since the body was sessile and attached to the sea-bed, the sclerites would not aided locomotion by increasing traction. So the only conceivable function for the sclerites appears to be defence against predators, rather similar to the spines on modern cacti
Cacti
-See also:* RRDtool The underlying software upon which Cacti is built* MRTG The original Multi Router Traffic Grapher from which RRDtool was "extracted".* Munin -External links:******...
.
Classification
The classification of chancelloriids is difficult, contentious and important to paleontologists'Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
view of the evolution of multi-celled animals. Walcott classified chancelloriids as sponges, a view that was first queried by Bengston and colleagues, who considered the hollow, multi-part spicules to be quite unlike anything secreted by a sponge. Butterfield and Nicholas (1996) argued that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites is similar to that of fibers of spongin
Spongin
Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. They are secreted by sponge cells known as spongocytes...
, a collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
, in modern keratose (horny) demosponge
Demosponge
The Demospongiae are the largest class in the phylum Porifera. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges...
s such as Darwinella.
However Janussen, Steiner and Zhu (2002) opposed this view, arguing that: spongin does not appear in all Porifera, but may be a defining feature of the demosponges; the silica-based spines of demosponges are secreted
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
by specialist sclerocyte
Sclerocyte
Sclerocytes are spicule secreting cells, found in sponges. They secrete calcareous or siliceous spicules which are found in the mesohyl layer of sponges. The sclerocytes produce spicules via formation of a cellular triad. The triad of cells then undergo mitosis, creating six sclerocytes...
cells that surround them, while mineralized chancellorid sclerites were hollow and filled with soft tissues connected to the rest of the animal by restricted openings in the bases of the sclerites; chancellorid sclerites were probably made of aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
, which is not found in demosponges, and the only sponges that use aragonite are the sclerosponge
Sclerosponge
|frame|Ceratoporella nicholsoni, cut specimen from [[Pedro Bank]], [[Caribbean Sea]], about 15 cm wide. Brown sponge tissue covers white massive [[aragonite]] [[skeleton]]....
s, whose soft bodies cover hard, often massive skeletons made of either aragonite or calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, another form of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
; sponges have loosely bound-together skins called pinacoderm
Pinacoderm
The pinacoderm is the outer most layer of cells in the phylum Porifera , equivalent to the epidermis in other organisms....
s, which are only one cell thick, while the skins of chancellorids were much thicker and shows signs of connective structures called belt desmosome
Desmosome
A desmosome , also known as macula adherens , is a cell structure specialized for cell-to-cell adhesion...
s. In their opinion the presence of belt desmosomes made chancellorids members of the Epitheliazoa, the next higher taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
above the Porifera, to which sponges belong. They thought it was difficult to say whether chancellorids were members of the Eumetazoa
Eumetazoa
Eumetazoa is a clade comprising all major animal groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues organized into germ layers, and an embryo that goes through a gastrula stage...
, "true animals" whose tissues are organized into Germ layer
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...
s; chancellorids' lack of organs such as sense organs, muscles and a gut would seem to exclude them from the Eumetazoa; but possibly chancellorids descended from Eumetazoans that lost these features after becoming sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
filter-feeders
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
.
Porter (2008) argued that the sclerites of chancelloriids are extremely similar to those of the halkieriids, mobile bilaterian animals that looked like slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
s in chain mail
Chain Mail
"Chain Mail" is a single by Mancunian band James, released in March 1986 by Sire Records, the first after the band defected from Factory Records. The record was released in two different versions, as 7" single and 12" EP, with different artworks by John Carroll and, confusingly, under different...
and whose fossils are found in rocks from the very Early Cambrian to the Mid Cambrian. The hollow "coelosclerites" of halkieriids and chancelloriids resemble each other at all levels: both have a thin external organic layer, and an internal "pulp cavity" that is connected to the rest of the body by a narrow channel; the walls of both are made of the same material, aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
; the arrangement of the aragonite fibers in each is the same, running mainly from base to tip but with each being closer to the surface at the end nearest the tip. Porter thought it extremely improbable that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related. This dilemma, she suggested, may be resolved in various ways:
- One possibility is that chancelloriids evolved from bilaterian ancestors but then adopted a sessile lifestyle and rapidly lost all unnecessary features. However the gut and other internal organs have not been lost in other bilaterians that lost their external bilateral symmetry, such as echinodermEchinodermEchinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, priapulids, and kinorhynchs. - On the other hand perhaps chancelloriids are similar to the organisms from which bilaterians evolved. That would imply that the earliest bilaterians had similar coelosclerites. However there are no fossils of such sclerites before , while KimberellaKimberellaKimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
from , which shows no evidence of sclerites, was almost certainly a bilaterian. - One solution to this dilemmaDilemmaA dilemma |proposition]]") is a problem offering two possibilities, neither of which is practically acceptable. One in this position has been traditionally described as "being on the horns of a dilemma", neither horn being comfortable...
may be that preservation of small shelly fossils by coatings of phosphatePhosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
was common only for a relatively short time, during the Early Cambrian, and that coelosclerite-bearing organisms were alive several million years before and after the time of phosphatic preservation. In fact there are over 25 cases of phosphatic preservation between and , but only one between and . - Alternatively, perhaps the ancestors of both chancelloriids and halkieriids had very similar but unmineralized coelosclerites, and at some intermediate time independently incorporated aragonite into these very similar structures.