Chitinozoan
Chitinozoan | |
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Burgsvik beds , showing its flask-like shape
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Scientific classification | |
Domain: | Eukaryota |
Phylum: | |
Class: | Chitinozoa Eisenack 1931
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Subgroups | |
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Chitinozoa (singular: chitinozoan, plural: chitinozoans) are a group of
Their bizarre form has made
Chitinozoan ecology is also open to speculation; some may have floated in the water column, where others may have attached themselves to other organisms. Most species were particular about their living conditions, and tend to be most common in specific paleoenvironments. Their abundance also varied with the seasons.
Anatomy
Chitinozoa range in length from around 50 to 2000
The base of the chitinozoan lies at the opposite end from the aperture. The base may involve various ornamentation derived from the internal layer. The edge of the base (basal margin) may extend into a sharp radial plate, the carina. Alternatively, it could send out large spines or branches, known as processes. In chitinozoans which attach to substrates or each other in large chains, the center of the base is augmented with apical structures which project down to assist attachment.[3]
External ornamentation is often preserved on the surface of the fossils, in the form of hairs, loops or protrusions, which are sometimes as large as the chamber itself. The range and complexity of ornament increased with time, against a backdrop of decreasing organism size. The earliest Ordovician species were large and smooth-walled;[7] by the mid-Ordovician a large and expanding variety of ornament, and of hollow appendages, was evident. While shorter appendages are generally solid, larger protrusions tend to be hollow, with some of the largest displaying a spongy internal structure.[8] However, even hollow appendages leave no mark on the inner wall of the organisms: this may suggest that they were secreted or attached from the outside.[8] There is some debate about the number of layers present in the organisms' walls: up to three layers have been reported, with the internal wall often ornamented; some specimens only appear to display one such wall layer. The multitude of walls may indeed reflect the construction of the organism, but could be a result of the preservational process.[8]
"Immature" or juvenile examples of chitinozoans have not been found; this may suggest that either they did not "grow", that they were
Many chitinozoans are found as isolated fossils, but chains of multiple tests, joined from aperture to base, have been reported for all genera.[7] Very long chains twist into helical (spring-shaped) forms. Occasionally, clusters or condensed chains are found, packed in an organic "cocoon".
Classification
Alfred Eisenack's original description of the chitinozoans placed them in three families, spanning seven genera,[10] based on morphological grounds. Further genera were identified, at first on an annual basis, as time progressed.[11] Since its publication in 1931, Eisenack's original classification has been much honed by these additional discoveries, as well as advances in microscopy. The advent of the scanning electron microscope in the 1970s allowed the improved detection of surface ornamentation which is hugely important in identification—as can be appreciated by a comparison of the images on this page. Even the light microscope image here is of far greater quality than could have been achieved earlier in the century, using poorly preserved specimens and less advanced microscopes.[11]
The original three families proposed by Eisenack represented the best classification possible with available data, based largely on the presence or absence of chains of organisms and the chamber's shape. The orders were subsequently revised to conform better to Linnean taxonomy, placing related organisms more closely together. This was made possible as scientific advances permitted the identification of distinctive traits in organisms across Eisenack's groups. Features of the base and neck, the presence of spines, and perforations or connections are now considered the most useful diagnostic features.[7][11]
Chitinozoans are placed into two orders. The order Operculatifera includes those with an operculum over the aperture and no distinct neck. The order Prosomatifera includes those with a clearly discernable neck and an internal prosome.[3]
Affinities
Young graptolites
The graptolites are colonial organic walled fossils which also occurred from the Ordovician to the Devonian; only part of their life cycle is known and it is not clear how they reproduced. It has been suggested that the Chitinozoa may represent the pre-sicula stages of graptolites—the period between the colony's sexual reproduction, and the formation of a new colony.[7] This hypothesis appears to be supported by the co-occurrence of graptolite and chitinozoan fossils, whose abundances appear to mirror one another. The similar chemical composition of the fossils has been seized by both sides of the argument. Proponents suggest that the use of the same chemical framework is an indicator that the two may be related. However, this factor means that situations favouring the preservation of one will also tend to preserve the other—and the preparation techniques used to extract the fossils will also favour or disfavour the two groups equally. Therefore, the apparent co-occurrence of the two fossils may merely be an artifact of their similar composition.[7][8] The hypothesis struggles to explain the continuing abundance of chitinozoans after the middle Devonian, when graptolites became increasingly rare.[8]
Eggs
The test of the Chitinozoa was fixed—there was no scope for any parts of it to move or rotate. This makes it seem likely that the tests were containers, to protect whatever was inside—whether that was a "hibernating" or encysted organism, or a clutch of hatching eggs.[8] There are several arguments behind an association of the chitinozoans with
Recent excavations of the
Protists
Arguments put forwards by Obut (1973) proposed that the organisms were one-celled "plants" similar to the dinoflagellates, which would now be grouped into the
The cyst forms of a particular group of
In 2020, exceptionally preserved remains of chitinozoans were described, showing the remains of smaller tests within larger ones, suggesting asexual reproduction.[6]
Ecology
It is not immediately clear what mode of life was occupied by these improbably shaped fossils, and an answer only becomes apparent after following several lines of reasoning.
The fossils' restriction to marine sediments can be taken as sound evidence that the organisms dwelt in the Palæozoic seas—which presents three main modes of life:
- Infaunal – living within the sediment—the "burrowers"
- Benthic – dwelling upon the sea floor, perhaps anchored in place—the "sitters"
- Pelagic – free-floating in the water column—the "drifters"
An infaunal mode of life can be quickly ruled out, as the fossils are sometimes found in alignment with the depositing current; as nothing attached them to the bottom, they must have fallen from the water column.[8]
The ornament of the chitinozoans may cast light on the question. Whilst in some cases a defensive role—by making the vessel larger, and thus less digestible by would-be predators—seems probable, it is not impossible that the protrusions may have anchored the organisms to the sea floor. However, their low-density construction makes this unlikely:[8] perhaps more plausible is that they acted to attach to other organisms.[8] Longer spines also make the organisms more buoyant, by decreasing their Rayleigh number (i.e. increasing the relative importance of water's viscosity)—it is therefore possible that at least the long-spined chitinozoans were planktonic "floaters". On the other hand, the walls of some chitinozoans were probably too thick and dense to allow them to float.[8]
Whilst little is known about their interactions with other organisms, small holes in the tests of some chitinozoans are evidence that they were hosts to some parasites.
Stratigraphic application
Since Alfred Eisenack first recognised and named the group
The oldest know chitinozoans appear to be
References
- .
- ^ ISBN 0-444-00267-7.
- ^ S2CID 241650725, retrieved 2022-07-12
- S2CID 128673881. Retrieved 8 November 2022.
- ^ S2CID 129236534.
- ^ ISSN 0091-7613.
- ^ JSTOR 3687298.
- ^ ISSN 0300-9491.
- PMID 31362642.
- ^ one of which, Mirachitina, is no longer recognised as a chitinozoan
- ^ JSTOR 1301963.
- ^ a b Kozlowski, R. (1963). "Sur la nature des chitinozoaires". Acta Palaeontologica Polonica. 8: 425–45.
- ^ Reid, P. C. and A. W. G. John: A possible relationship between chitinozoa and tintinnids. Rev. Paleobot. Palynol. 34, 251-262 (1981).
- S2CID 23050887.
- ^ Eisenack, A. (1968). "Uber Chitinozoen des baltischen Gebietes". Palaeontographica, Abteilung A (in German). 131: 137–98.
- ^ Martin, F. (197). "Palynofacies et microfacies du Silurien inférieur a Deerlijk". Institut Royal des Sciences Naturelles de Belgique, Sciences de la Terre, Bulletin (in French). 47 (10): 11–12 (of 26).
- ISBN 0-08-021551-3.
- PMID 11164220.
- ^ Sutherland, S.J.E.; Palaeontographical Society Monographs (1994). Ludlow Chitinozoans from the Type Area and Adjacent Regions. Palaeontographical Society. pp. 1–124.
- ^ After "their chitinoid appearance"
- ISSN 0091-7613.
External links
- Commission Internationale de Microflore du Paléozoique (CIMP), international commission for Palaeozoic palynology.