Middle Ordovician – Late Ordovician
Merocrinus is an extinct genus of stationary upper-level epifaunal suspension feeders
Identification in Hand Sample:
- Small, short cup extending laterally only a short distance beyond the uppermost columnal
- Infrabasals wide, low, and are visible in side view; basals are short
- Arms long, bifurcating isotomously several times, 6 or more primibrachs to a ray
- Stem transversely round, stout, composed of quite thin columnals
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- The generic diagnosis of Moore et al. (1978, p. T627) is applicable except in two respects: 1. The nonpinnulate arms of Merocrinus branch isotomously on the primibrachs, but the higher branches can be isotomous or bilaterally heterotomous as discussed later. 2. Following Ausich et al. (2002, p. 987), the plate in the radial position in the C-ray is termed inferradianal; this plate is overlain by the C-ray radial, which bears the anal X on its left shoulder and an arm on its right shoulder.
Meyer et al. (2002):
- Restricted to the lower most few meters of the Kope Formation; probably comparable to the disparids in having a low filtration fan density.
- Merocrinus is frequently visualized as a primitive cladid which shows a CD interray that is very similar to some disparids like Iocrinus (e.g., Moore, 1962; Moore et al., 1978). This idea is not consistent with the cladogram which puts Merocrinus within the moderately advanced Ordovician cladids. Consequently, the similarity of anal structures between Merocrinus and disparids is caused by convergence and not common ancestry.
Treatise on Invertebrate Paleontology, Part T, Vol. 2(2) (1978):
- Characters of the family. Cup small, short, extending laterally only short distance beyond uppermost columnal; infrabasals wide, low, visible in side view; basals short. Arms long, bifurcating isotomously several times, 6 or more primibrachs to a ray. Cup plate in C-ray, which is identical to normal radials of other rays, is homologous to inferradianal and next higher plate with form and position of axillary; first primibrach bears a normal arm on its right shoulder and strong longitudinal series of anal tube plates on its left shoulder, is equivalent to a superradianal. This latter plate has been termed anibrachial. The presence of two radianals in the C-ray has been long recognized as an archaic structure correlated with a primitive evolutionary character. Stem transversely round, stout, composed of quite thin columnals.
- Except for the presence of an infrabasal circlet and a larger stem, Merocrinus is structurally identical to the monocyclic genus Iocrinus. Both genera exhibit an axillary primibrachial plate on the right posterior radial, which bears an arm on the right and an unbranched series of brachials (brachianals) on the left supporting an anal sac. Although Merocrinus is dicyclic, elements of the posterior are considered more significant in classification, and the genus is believed to be closely related to monocyclic inadunates.