Middle Ordovician – Late Ordovician; Middle Silurian
Homocrinidae is a family of stationary intermediate-level epifaunal suspension feeders
Description of the Family
- Monocyclic, with small, irregularly constructed calyx
- Branched arms
- Small, conical cup
- Cup is composed of five similar basals, two large simple radials and three composite radials
- Each composite radial consists of a large inferradial and a small superradial
- The two simple radials and three superradials each have a facet for arm attachment
- Homocrinidae is a family of monocyclic crinoids of the subclass Disparida (Fig. 2). All members of this taxon are characterized by simple morphology and usually small size. For this reason, at the end of 19th century, this group of crinoids was named Larviformia by Wachsmuth and Springer (1897). Small size and simple skeletal morphology point to a paedomorphic origin of this order and its numerous lineages. In contrast to its descendants, the Homocrinidae is a small group. Several homocrinid genera have similar plate arrangement of the cup and are distinguished by branching of the arms (Warn and Strimple, 1977). They recorded only from Laurentia and range only up to the Middle Silurian. The origin of the Pisocrinidae from the Homocrinidae in the Early Silurian is suggested by comparison of the skeletal morphology of these groups, ontogeny of their skeleton and stratigraphical distribution (Rozhnov, 1981).
- The main morphological differences between these families are observed in the plate arrangement of the cup. The homocrinid cup is composed of five similar basals, two large simple radials and three composite radials (Fig. 2a). Each composite radial consists of a large inferradial and a small superradial. The two simple radials and three superradials each have a facet for arm attachment. The five arms usually branch several times (2b). Pisocrinids have five basals differing in size and shape or three similar basals (in younger genera). They have three large radials contacting the basal circlet, two of which (A and D) have facets for arm attachment, while one supports two symmetrically positioned superradials (B and C). Another small radial with a facet for an arm attachment occurs in the E ray (Fig. 3a). Hence, all plates are arranged within the homocrinid plane of symmetry (Fig. 3b). Pisocrinids have unbranched arms (Fig. 3c,d).
Treatise on Invertebrate Paleontology, Part T, Vol. 2(2) (1978):
- Cup small, conical; B, C, and E radials compound; anal X incorporated in cup between C and D radials.
- Monocyclic, with small, irregularly constructed calyx and moderately long, simple or variously branched, arms; basals five, subequal; radials five, three compound, the right posterior and right and left anterior bisected transversely into super-and inferradials; proximal anal plate resting on the truncated shoulders of the posterior radials, followed by a series of quadrangular plates that forms the support of a tubular or balloon shaped ventral sac; rays five, rarely remaining undivided, but usually branching, the first time isotomously, the subsequent divisions variously unequal.