[1] Common from the Ordovician to Devonian periods (i.e. the mid-Paleozoic), the millimetre-scale organisms are abundant in almost all types of marine sediment across the globe.

The organisms have been better understood as improvements in microscopy facilitated the study of their fine structure, and it has been suggested that they represent either the eggs or juvenile stage of a marine animal.

Their anatomy is based around the broad chamber, a radially symmetrical region involving a central cavity encased by two layers of a chitin-like substance.

The chamber narrows towards the main opening (the aperture), though a circular plug prevents direct contact between the central cavity and its surroundings.

While shorter appendages are generally solid, larger protrusions tend to be hollow, with some of the largest displaying a spongy internal structure.

[8] "Immature" or juvenile examples of chitinozoans have not been found; this may suggest that either they did not "grow", that they were moults (unlikely), or that the fossilisable parts of the organism only formed after the developmental process was complete.

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.

[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.

[3] 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.

[8] Recent excavations of the Soom Shale, an Ordovician konservat-lagerstätten in South Africa, have yielded chitinozoans alongside a wide range of other organisms.

Most organisms present in the shale can be ruled out for a variety of reasons,[5] but polychaete worms, Promissum conodonts and orthocone cephalopods remain as likely candidates.

[5] Alfred Eisenack's original guess was that the Chitinozoa were amoebae, specifically the rhizopod order Testacea, since similar chitin-based tests were produced by the extant members of this group.

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.

[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 whole, chitinozoans are less abundant in turbulent waters or reef environments, implying an aversion to such regimes when alive, if it is not an effect of sedimentary focusing.

[19] Since Alfred Eisenack first recognised and named the group[20] in 1930, the Chitinozoa have proven incredibly useful as a stratigraphic markers in biostratigraphy during the Ordovician, Silurian and Devonian periods.

They are also widely distributed and appear in a variety of marine depositional settings, making correlation easier; better still, they can often be recognised in even quite strongly metamorphosed rocks.

Aside from the acritarchs, chitinozoans were the only reliable means of correlating palæozoic units until the late 1960s, when the detailed study of conodonts and graptolites fully unleashed their stratigraphic potential.

[21] Chitinozoans appear to have become extinct at the end of the Devonian; rare Carboniferous and Permian remains may represent reworked fossils or fungal spores.