Merocrinidae

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Treatise on Invertebrate Paleontology, Part T, Vol. 2(2) (1978):

• Radial articular facets wide. Infer- and axillary superradial present in C-ray, supporting anal X and longitudinal series of anal plates on left side and branched arm on right.
• The Merocrinidae, composed so far as now known of the single genus Merocrinus, differ from most other dendrocrinines in having an inferradianal and superradianal. This character indicates a primitive evolutionary stage. The cup is perfectly symmetrical, inasmuch as the inferradianal coresponds exactly in size and shape to the four adjacent radial plates. The superradianal and X plates are not incorporated in the cup, but there is no indication that they have been displaced upward from an original position within the cup. As in Iocrinus, the radials and inferradianal of Merocrinus may be interpreted to represent the most archaic of all structural plans among inadunate crinoids, in which there is no differentiation of compound radials (consisting of unfused primitive radials, otherwise known as inferradials, and primitive first brachials, otherwise called superradials) and “simple” radials (consisting of fused lower elements of the rays). If this diagnosis is correct, it is pertinent to call attention to the cyclic course of normal evolutionary changes in the cup of inadunate crinoids, that is, from initial or near-initial perfect pentamerous symmetry to bilateral symmetry , and finally back to perfect pentamerous symmetry.
• The radial articular facets of the Merocrinidae are wide (plenary), which suggests origin of the group in an unknown stock that surely stands far apart from the early Cyathocrinina. The characters of the cup, arms, and strikingly armlike anal tube most closely resemble those of some families of Disparida, but this similarity is not known to have significance as to genesis of Merocrinus. The repeated isotomous branching of the arms indicates an early evolutionary stage, but not the most primitive one, in development of this part of the organism. No descendants of the Merocrinidae are known.

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