Chitinozoan - Ecology

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.

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: perhaps more plausible is that they acted to attach to other organisms. 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.

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. 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.

Corals in Gotland with daily growth markings have been found in association with abundant chitinozoans, which allow the detection of seasonal variation in chitinozoan abundance. A peak in abundance during the late autumn months is observed, with the maxima for different species occurring on different dates. Such a pattern is also observed in modern-day tropical zooplankton. 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. Chitinozoans also become rarer in shallower water - although the reverse is not necessarily true. They cannot survive freshwater input.