Chitinozoans were described for the first time in 1030  in Early Palaeozoic erratic boulders from East Prussia, which originated from more northern latitudes and were deposited by Quaternary glaciers. The term chitinozoa was introduced by Eisenack (1931)  because it was assumed their wall had a chitinous composition and that the vesicles were of a zoological origin. In these early years after their discovery, reports focused on length-width measurements and systematic notes. Publications in North-America , the United Kingdom  and France  extended the area where chitinozoans were described. Taxonomic notes suggested a wide array of possible theories concerning the origins of these enigmatic palynomorphs. Riding (1980)  summarizes the most prevalent theories which linked chitinozoans to many diverse life forms across different Kingdoms: Protozoa (Testacea, Foraminifera, Dinoflagellata), Metazoa (Annelida, Gastropoda, Graptolites) and Fungi. One of the most convincing arguments was that chitinozoans are eggs produced by a marine metazoan . Kozlowski’s study mainly focused on the genera Desmochitina and Cyathochitina which often occur in chains or clusters. Because of this clustering and because chitinozoans are in essence hermetically closed vesicles, it was proposed that they were protected eggs or cysts . Moreover, because of this particular grouping, which Kozlowski deemed to be too complex to be formed by protozoans, this author concluded that they must have a metazoan origin which is still considered as the most plausible theory by the majority of the more recent authors [e.g., 8-10] sometimes referred to as the ‘chitinozoan animal’ . The limiting factor for such an animal is the specific stratigraphic range of chitinozoans: early Ordovician (Tremadocian) to the latest Devonian (Fammenian) [12-13].
A chitinozoan vesicle can be described as a purse or flask-like structured formed by an organic wall, the test or tegument. Its total length varies from around 100 μm to a few hundred micrometers, although larger vesicles up to 2 mm have been described [e.g. 9]. The test consists of two separate layers of dark brown or black chitin-like matter with a combined thickness of about 5 μm. Despite its name, no chitin-related organic compounds have been described from the test of well-preserved chitinozoans .
A chitinozoan vesicle consists in general of a chamber connected by the shoulder and flexure with the neck and end aperture. Figure 1 shows the basic morphologic characteristics and chamber shapes that are encountered in most vesicles: around the aperture a collarette, a fragile membranous continuation of the vesicle wall can be found. Chitinozoans are hermetically closed by either a discoid operculum covering the chamber directly or a prosome, a segmented cylinder with horizontal septa situated in the neck. Both operculum and prosome are extended aborally by the rica, a variously developed membranous expansion lining the upper part of the chamber. Though sometimes found in clusters and chains (e.g. Figure 2 in ) most vesicles are found isolated though it is assumed they were part of a chain-like structure. It should be noted no internal communication between vesicles has been reported, meaning clusters are not considered to be a colony of organisms. The suprageneric and generic classification is mostly based on the wall ornamentation of individual vesicles .
Chitinozoans are one of the most useful index taxa for biozonation of Ordovician and Silurian, characterizing GSSP boundaries [e.g., 18-19], auxiliary GSSP boundaries [e.g., 20], and stage boundaries [e.g. 21-22]. The stratigraphic worth of chitinozoans is based on the fact that they occur in almost all types of marine deposits, even when the traditional biostratigraphic markers (like graptolites) appear to be absent. In this context, chitinozoans played a key role in the Late Ordovician extinction and biodiversification event [22-23]. More recently, the combination of geochemical techniques (stable isotope analysis) combined with chitinozoan palaeobiodiversity changes led to the understanding that chitinozoan distribution is controlled by latitudinal temperature gradients [23-24]. An excellent overview of the historic and recent uses of chitinozoans is provided by .