“Cystoids” are Middle Ordovician (about 460 million years old) to Late Devonian (about 560 million years old) echinoderms that lived attached to the ocean floor via a column (stem). Modern echinoderms are starfish, brittle stars, sand dollars, etc. As indicated by the use of quote marks around the name “cystoids,” their classification in a state of flux and has been for many years. They are currently regarded as consisting of two classes: the rhomboiferan “cystoids,” and the much less common diploporites “cystoids” (Prothero, 2004).
The rhomboiferan “cystoids” were never very abundant, but they are common in some Middle Ordovician rocks.
Both classes of “cystoids” have an ovoid shape exoskeleton. Their food-catching ambulacral arms (also called brachioles) extend into the water column rather than being fused to the sides of their calyxes (bodies) as found on blastoid echinoderms (e.g., Pentrimites--see my previous post). The calyx plates of cystoids are characterized by being perforated by tiny tubes or canals (thecal pores). The arrangements of these thecal pores (they can be concealed or raised) is very important in the classification of cystoids.
The image above shows a partial “cystoid” exoskeleton with the basal part of the exoskeleton and its accompanying main-body section (slightly crushed, diameter 23 mm), including its minute thecal pores. But, only the lower parts of some of its segmented ambulacral arms are present.
An even more incomplete “cystoid” shows only the lower part of the basal area (diameter 12 mm); (note: the minute thecal pores present) and a small part of the column.
The diploporites “cystoids” are characterized by having a very flexible prehensile column that could wrap around a foreign stationary object.
Reference:
Prothero, D.R. 2004. Bringing fossils to life: an introduction to paleobiology. McGraw Hill. 503 pp.
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