Chancelloriidae
| Chancelloriidae Temporal range:
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|---|---|
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| Chancelloria | |
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| Life restoration of Allonnia | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Order: | †Chancelloriida |
| Family: | †Chancelloriidae Walcott, 1920 |
| Genera | |
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The Chancelloriids are an extinct family of superficially sponge-like animals 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
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
Occurrence
Chancelloriid fossils have been found in many parts of the world, including various parts of
They were first described in 1920 by
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.[5]
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.[5]
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.[5] It is thought that when the animals were alive these cavities were filled with tissues that secreted the hard outer coverings.[7] 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.[5]
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;
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
Classification
The classification of chancelloriids is difficult, contentious and important to
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
Porter (2008) argued that the sclerites of chancelloriids are extremely similar to those of the
- 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 kinorhynchs.[7]
- 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 542 million years ago, while Kimberella from 555 million years ago, which shows no evidence of sclerites, was almost certainly a bilaterian.[7][12]
- One solution to this small shelly fossils by coatings of phosphate 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 542 million years ago and 521 million years ago, but only one between 555 million years ago and 542 million years ago.[7]
- 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.[7][13]
- It is also possible that the morphology of the spines is plesiomorphic for Coeloscleritophora paraphyletic rather than polyphyletic.[2]
References
- ISBN 978-0-9812885-1-2.
- ^ ISSN 1094-8074.
- S2CID 218872873.
- S2CID 234069516.
- ^ S2CID 129127213. Retrieved 2008-08-04. Free full text without images at Janussen, Dorte (2002). "(as above)". Journal of Paleontology. Archived from the originalon 2008-12-10. Retrieved 2008-08-04.
- ^ Smithsonian Miscellaneous Collections. 67: 261–364.
- ^ .
- ISBN 978-0-9812885-1-2.
- ^ 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.
- ^ Bengtson, S; Missarzhevsky, VV (1981). "Coeloscleritophora—a major group of enigmatic Cambrian metazoans". US Geological Survey Open-file Report: 81–743.
- S2CID 133427906.
- S2CID 4395089.
- ^ 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
- Randell, R.D.; Lieberman, B.S.; Hasiotis, S.T.; and Pope, M.C. (Sep 2005). "New Chancelloriids from the Early Cambrian Sekwi Formation with a comment on Chancelloriid affinities". Journal of Paleontology. 79 (5): 987–996. S2CID 130292492.