Chitinozoa (singular: chitinozoan, plural: chitinozoans) are a group of flask-shaped, organic walled marine microfossils produced by an as yet unknown organism. 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.
Their bizarre form has made classification and ecological reconstruction difficult. Since their discovery in 1931, suggestions of protist, plant, and fungal affinities have all been entertained. 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.
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 micrometres. They appear dark to almost opaque when viewed under an optical microscope. 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. This plug may be called an operculum (if it lies at the tip of the aperture) or a prosome (if it lies deep within the narrowed region or neck). The rim of the aperture, known as the collarette, often has a distinctive form or texture.
Many chitinozoans are found as isolated fossils, but chains of multiple tests, joined from aperture to base, have been reported for all genera. based on morphological grounds. Further genera were identified, at first on an annual basis, as time progressed.
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. and it is not impossible that the chitinozoans are a convergent phenomenon laid by both groups. In fact, the spirally coiled nature of chitinozoan chains has been used to suggest that they were laid by a spirally coiled organism, such as the gastropods; were this inference true, uncoiled chains could be attributed to the (straight) annelid worms or other organisms. but polychaete worms, Promissum conodonts and orthocone cephalopods remain as likely candidates. However, further evidence connecting chitinozoans to any of these groups is circumstantial at best.
In 2020, exceptionally preserved remains of chitinozoans were described, showing the remains of smaller tests within larger ones, suggesting asexual reproduction. Although some forms have been reinterpreted as "pock-marks" caused by the disintegration of the diagenetic mineral pyrite, the clustering of cylindrical holes around the chamber—where the flesh of the organism was likely to be concentrated—is evidence for a biological cause. The diversity of living habits is also reflected by the depth of water and distance from the shore. Different species are found in highest abundance at different depths. While deeper waters around 40 km from the shoreline are generally the optimal environment, some species appear to prefer very shallow water. 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. They cannot survive freshwater input.
Stratigraphic application
Since Alfred Eisenack first recognised and named the group in 1930, the Chitinozoa have proven incredibly useful as a stratigraphic markers in biostratigraphy during the Ordovician, Silurian and Devonian periods. Their utility is due to the rapidity of their morphological evolution, their abundance—the most productive samples bearing almost a thousand tests per gram
The oldest known chitinozoans appear to be phosphatized remains tentatively referred to the genus Eisenackitina. They were recovered from the Middle Cambrian (Stage 5)–age Gaotai Formation, more than 20 million years before the group is found elsewhere in the Ordovician. Chitinozoans appear to have become extinct at the end of the Devonian; rare Carboniferous and Permian remains may represent reworked fossils or fungal spores.