As for most soft-bodied animals, holothuroids have a poor fossilrecord. Published accounts exist of body fossils for about 19 species,though at least that many body-fossil species lay undescribed on museumshelves. Most ancient holothuroids are known from fossils of isolatedossicles. This complicates the taxonomy somewhat since ossicles can differeven within an individual depending on age, habitat and geography. Howthen does one identify a single species? As a result, most fossilholothuroids have been described as paraspecies based on unique ossicletypes. Entire or isolated elements of the calcareous ring are also known,though less work has been done on these potentially informative structures. The rarity of holothuroid fossils in part may be due to a lack ofcollecting effort, since the microscopic ossicles require specialcollecting methods, and there are few specialists working on the group. Inaddition, isolated ring elements may sometimes be confused with the robustplates of other echinoderms.
Figure 4. Isolated pieces of the calcareous rings of fossil holothurians.
Left: Interradial pieces; Center: Radial pieces; both from apodid holothurians from the Upper Liassic of Germany, approx. 180 Ma;
Right: Pieces from fossil molpadiid holothurians from the Hauterivian of Germany, approx. 130 Ma.
Photographs copyright © 2000 Mike Reich.
Holothuroids probably evolved by at least the Lower Silurian, most likelyfrom a little-known group of extinct Palaeozoic echinoderms calledophiocistioids. However, the oldest reported body fossil of a holothuroidis from the Lower Devonian, while the oldest undoubted ossicle is from theUpper Silurian. Plate ossicles are known from the Ordovician, but theiridentity as holothuroid is uncertain because they resemble the plates ofother echinoderms. Still, plate ossicles ascribable to holothuroids arewell known and, when combined with the phylogenetic evidence, suggest thatseveral groups of ancient plated forms existed that are only distantlyrelated to living plated dendrochirotes and dactylochirotes.Alternatively, these living forms have retained their ancient armour andHolothuroidea has had a long and repeated history of losing a platedmorphology.
A comprehensive account of holothurian palaeontology is found in Gilliland(1993), while an up-to-date bibliography and other palaeontologicalinformation is available from Mike Reich's Fossil Holothuroidea Page.
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