| Chancelloriidae | |
|---|---|
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| Chancelloria | |
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| Life restoration ofAllonnia | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Order: | †Chancelloriida |
| Family: | †Chancelloriidae Walcott, 1920 |
| Genera | |
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TheChancelloriids 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 representsessile,radially symmetrical hollow bag-like organisms with a soft skin armored with star-shapedcalcareoussclerites 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-likehalkieriids, which are typically considered to be stem-groupmolluscs. While the sclerites of the two groups are very similar right down to the microscopic level, the large dissimilarity in the body plans of the two groups is difficult to reconcile with this hypothesis.[1] The proposed clade containing the two groups, "Coeloscleritophora", is generally not thought to bemonophyletic.[2][3] Recent research has suggested that chancellorids represent an independent group of basaleumetazoans.[4]
Chancelloriid fossils have been found in many parts of the world, including various parts ofAsia (e.g.Siberia, China,Mongolia), Australia's Georgina Basin, Canada'sBurgess Shale, and the United States. The earliest known fossils come from the small shelly fossil assemblage of theAnabarites trisulcatus Zone of the LowerNemakit-Daldynian Stage, Siberia and its analog in China is theAnabarites trisulcatus-Protohertzina anabarica Zone of the basalMeishucunian Stage. The fossil record suggests that chancelloriids declined rapidly during the Late Cambrian, and they were probably extinct by the end of the Cambrian.[5]
They were first described in 1920 byCharles Doolittle Walcott, who regarded them as one of the most primitive groups ofsponges.[6]
The chancelloriids had bag-like bodies with anorifice at the top, and show no evidence of internal organs. The differentspecies 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 calledsclerites, 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 evengenus.[5]
Individual sclerites had star-shaped bases that lay flat against the body and one spine projecting outwards at aright angle. The sclerites had internal cavities and in fact many are preserved ascastings of the cavities filled withphosphate.[5] It is thought that when the animals were alive these cavities were filled withtissues thatsecreted 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 differentcrystalline form. This suggests it was a slightly unstable material such asaragonite, a form ofcalcium carbonate. Some sclerites appear to be on top of the skin, other covered by it, and some appear partly covered.[5]
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;[5] they are often preserved in attachment to other organisms or shelly debris.[8] They were very likelyfilter-feeders.[5]
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 moderncacti.[9]
The classification of chancelloriids is difficult, contentious and important topaleontologists' view of the evolution ofmulti-celled animals. Walcott classified chancelloriids assponges,[6] a view that was first questioned by Bengston and colleagues, who considered the hollow, multi-part spicules to be quite unlike anything secreted by a sponge.[10] 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 ofspongin, acollagenprotein, in modern keratose (horny)demosponges such asDarwinella.[11]
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; thesilica-based spines of demosponges aresecreted by specialistsclerocyte 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 ofaragonite, which is not found in demosponges, and the only sponges that use aragonite are thesclerosponges, whose soft bodies cover hard, often massive skeletons made of either aragonite orcalcite, another form ofcalcium carbonate; sponges have loosely bound-together skins calledpinacoderms, which are only one cell thick, while the skins of chancellorids were much thicker and shows signs of connective structures called beltdesmosomes. In their opinion the presence of belt desmosomes made chancellorids members of theEpitheliazoa, the next highertaxon above thePorifera, to which sponges belong. They thought it was difficult to say whether chancellorids were members of theEumetazoa, "true animals" whose tissues are organized intoGerm 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 becomingsessilefilter-feeders.[5]
Porter (2008) argued that the sclerites of chancelloriids are extremely similar to those of thehalkieriids, mobilebilaterian animals that looked likeslugs inchain 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:[7]
