Chancelloriidae

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Chancelloriidae
Temporal range: Early Cambrian–Late Cambrian
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Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: incertae sedis
Class: Coeloscleritophora
Order: Chancelloriida
Family: Chancelloriidae
Walcott, 1920
Genera
Chancelloria eros fossil, Wheeler Shale, House Range, Utah Chancelloria eros fossil, Middle Cambrian, Millard County UT.jpg
Chancelloria eros fossil, Wheeler Shale, House Range, Utah

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 sclerites from which radiate sharp spines.

Contents

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. [1]

Occurrence

Chancelloriid fossils have been found in many parts of the world, including various parts of Asia (e.g. Siberia, China, Mongolia), Australia's Georgina Basin, Canada's Burgess Shale, and the United States. 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. [2]

They were first described in 1920 by Charles Doolittle Walcott, who regarded them as one of the most primitive groups of sponges. [3]

Description

The chancelloriids had bag-like bodies with an orifice 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 centimetres (1.6 to 2.4 in) long and 1.5 to 2 centimetres (0.59 to 0.79 in) in diameter at its widest point; Allonnia junyani formed a disk or cylinder usually 6 to 7 centimetres (2.4 to 2.8 in) in diameter, and the tallest were about 20 centimetres (7.9 in) long. [2]

Most of the fossils consist of collections of mineralized hard parts called sclerites, 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. [2]

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 castings of the cavities filled with phosphate. [2] It is thought that when the animals were alive these cavities were filled with tissues that secreted the hard outer coverings. [4] It is not clear what the hard substance of the walls was since it has been replaced or converted to a different crystalline 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. [2]

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; [2] they are often preserved in attachment to other organisms or shelly debris. [5] They were very likely filter-feeders. [2]

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 have 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. [6]

Classification

The classification of chancelloriids is difficult, contentious and important to paleontologists' view of the evolution of multi-celled animals. Walcott classified chancelloriids as sponges, [3] 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. [7] 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) demosponges such as Darwinella . [8]

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 sclerosponges, 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 pinacoderms, which are only one cell thick, while the skins of chancellorids were much thicker and shows signs of connective structures called belt desmosomes. 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 layers; 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. [2]

Halkieriid sclerite structure 300.png
= organic skin
= aragonite fibers
= organic flesh
Halkieriid sclerite structure 300.png
Coelosclerite structure. This diagram is from a microscopic examination of a halkieriid sclerite, but the internal structures of chancelloriid sclerites are almost identical despite the difference in external shapes. [4]

Porter (2008) argued that the sclerites of chancelloriids are extremely similar to those of the halkieriids, mobile bilaterian animals that looked like slugs 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: [4]

Related Research Articles

Burgess Shale Rock formation in the Canadian Rockies with exceptional preservation of fossils soft parts

The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At 508 million years old, it is one of the earliest fossil beds containing soft-part imprints.

Hyolitha

Hyoliths are animals with small conical shells, known as fossils from the Palaeozoic Era. They are lophophorates, a group which includes the brachiopods.

Sponge Animals of the phylum Porifera

Sponges, the members of the phylum Porifera, are a basal Metazoa (animal) clade as a sister of the Diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. The branch of zoology that studies sponges is known as spongiology.

Sclerite hardened body part

A sclerite is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly to the hardened parts of arthropod exoskeletons and the internal spicules of invertebrates such as certain sponges and soft corals. In paleontology, a scleritome is the complete set of sclerites of an organism, often all that is known from fossil invertebrates.

<i>Wiwaxia</i> Genus of Cambrian animals

Wiwaxia is a genus of soft-bodied animals that were covered in carbonaceous scales and spines that protected it from predators. Wiwaxia fossils – mainly isolated scales, but sometimes complete, articulated fossils – are known from early Cambrian and middle Cambrian fossil deposits across the globe. The living animal would have measured up to 5 cm (2 inch) when fully grown, although a range of juvenile specimens are known, the smallest being 2 millimetres (0.079 in) long.

<i>Ottoia</i>

Ottoia is a stem-group archaeopriapulid worm known from Cambrian fossils. Although priapulid-like worms from various Cambrian deposits are often referred to Ottoia on spurious grounds, the only clear Ottoia macrofossils come from the Burgess Shale of British Columbia, which was deposited 508 million years ago. Microfossils extend the record of Ottoia throughout the Western Canada Sedimentary Basin, from the mid- to late- Cambrian.

Halkieriid Family of molluscs

The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.

The Coelosclerithophorans are a polyphyletic group of organisms bearing hollow sclerites made of aragonite, and with an allegedly distinctive microstructure.

<i>Odontogriphus</i> Genus of soft-bodied animals from middle Cambrian

Odontogriphus is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as 12.5 centimetres (4.9 in) in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot, and a "shell" on its back that was moderately rigid but of a material unsuited to fossilization.

<i>Orthrozanclus</i>

Orthrozanclus is a genus of sea creatures from the Middle Cambrian Burgess shale. Animals in this genus were one to two centimeters long, with spikes protruding from their armored bodies. The placement of this genus into a specific family is not universally accepted.

<i>Choia</i> Genus of sponges

Choia is a genus of extinct demosponge ranging from the Cambrian until the Lower Ordovician periods. Fossils of Choia have been found in the Burgess Shale in British Columbia; the Maotianshan shales of China; the Wheeler Shale in Utah; and the Lower Ordovician Fezouata formation. It was first described in 1920 by Charles Doolittle Walcott.

Halwaxiida

Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.

<i>Chancelloria</i>

Chancelloria is a genus of early animals known from the Middle Cambrian Burgess Shale, the Comley limestone, the Wheeler Shale, the Bright Angel Shale and elsewhere. It is named after Chancellor Peak. It was first described in 1920 by Charles Doolittle Walcott, who regarded them as one of the most primitive groups of sponges. This appears unlikely, and it is currently placed in the enigmatic group Coeloscleritophora. 178 specimens of Chancelloria are known from the Greater Phyllopod bed, where they comprise 0.34% of the community.

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the Late Ediacaran and Early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.

Since 1990 there has been intense debate among paleontologists about the evolution in the Early Cambrian period of the "super-phylum" Lophotrochozoa, which is thought to include the modern molluscs, annelid worms and brachiopods, as well as their evolutionary "aunts" and "cousins".

The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification of other organisms.

Australohalkieria is an extinct genus of halkieriid from Australia and Antarctica.

The fossils of the Burgess Shale, like the Burgess Shale itself, formed around 505 million years ago in the Mid Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 60,000 specimens in a series of field trips up from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to discover new species, and statistical analysis suggests discoveries will continue for the foreseeable future. Stephen Jay Gould's book Wonderful Life describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution has been contested.

<i>Hazelia</i>

Hazelia is a genus of spicular Cambrian demosponge known from the Burgess Shale, the Marjum formation of Utah, and possibly Chengjiang. It was described by Charles Walcott in 1920.

Eiffelia is an extinct genus of sponges known from the Middle Cambrian Burgess Shale as well as several Early Cambrian small shelly fossil deposits. It is named after Eiffel Peak, which was itself named after the Eiffel Tower. It was first described in 1920 by Charles Doolittle Walcott. It belongs in the Hexactinellid stem group. 60 specimens of Eiffelia are known from the Greater Phyllopod bed, where they comprise 0.11% of the community.

References

  1. Bengtson, Stefan; Collins, Desmond (August 2009). "Burgess Shale Chancelloriids – A Prickly Problem" (PDF). In Smith, Martin R.; O'Brien, Lorna J.; Caron, Jean-Bernard (eds.). Abstract Volume. International Conference on the Cambrian Explosion (Walcott 2009). Toronto, Ontario, Canada: The Burgess Shale Consortium (published 31 July 2009). ISBN   978-0-9812885-1-2.
  2. 1 2 3 4 5 6 7 8 Janussen, D.; Steiner, M. & Zhu, M-Y. (2002). "New Well-preserved Scleritomes of Chancelloridae from the Early Cambrian Yuanshan Formation (Chengjiang, China) and the Middle Cambrian Wheeler Shale (Utah, USA) and paleobiological implications". Journal of Paleontology. 76 (4): 596–606. doi:10.1666/0022-3360(2002)076<0596:NWPSOC>2.0.CO;2 . Retrieved 2008-08-04. Free full text without images at Janussen, Dorte (2002). "(as above)". Journal of Paleontology. Retrieved 2008-08-04.
  3. 1 2 Walcott, C. D. (1920). "Cambrian geology and paleontology IV:6—Middle Cambrian Spongiae". Smithsonian Miscellaneous Collections . 67: 261–364.
  4. 1 2 3 4 5 6 7 8 Porter, S.M (2008). "Skeletal microstructure indicates Chancelloriids and Halkieriids are closely related". Palaeontology. 51 (4): 865–879. doi:10.1111/j.1475-4983.2008.00792.x.
  5. Bengtson, Stefan; Collins, Desmond (August 2009). "Burgess Shale Chancelloriids – A Prickly Problem" (PDF). In Smith, Martin R.; O'Brien, Lorna J.; Caron, Jean-Bernard (eds.). Abstract Volume. International Conference on the Cambrian Explosion (Walcott 2009). Toronto, Ontario, Canada: The Burgess Shale Consortium (published 31 July 2009). ISBN   978-0-9812885-1-2.
  6. Bengtson, S. (2002). "Origins and early evolution of predation". In Kowalewski, M.; Kelley, P.H. (eds.). The fossil record of predation. The Paleontological Society Papers 8. The Paleontological Society. pp. 289–317.
  7. Bengtson, S; Missarzhevsky, VV (1981). "Coeloscleritophora—a major group of enigmatic Cambrian metazoans". US Geological Survey Open-file Report: 81–743.
  8. Butterfield, N. J.; Nicholas, C. J. (1996). "Burgess Shale-Type Preservation of Both Non-Mineralizing and 'Shelly' Cambrian Organisms from the Mackenzie Mountains, Northwestern Canada". Journal of Paleontology. 70 (6): 893–899. doi:10.1017/s0022336000038579. JSTOR   1306492.
  9. Fedonkin, M.A.; Waggoner, B.M. (1997). "The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism". Nature. 388 (6645): 868. Bibcode:1997Natur.388..868F. doi:10.1038/42242.
  10. Bengtson, S. "Mineralized skeletons and early animal evolution". In Briggs, D.E.G. (ed.). Evolving form and function: fossils and development. New Haven, CT: Peabody Museum of Natural History, Yale University. p. 288.

Further reading