Order: Disparida (Moore & Laudon, 1943)
Cincinnatian Families: Heterocrinidae , Homocrinidae , Iocrinidae
Early Ordovician – Late Permian
Disparida is an extinct order of stationary intermediate-level epifaunal suspension feeders
Characteristics of the Order
- Most disparids are small and characterized by simplicity in structure of the crown and cup
- Many with diameter and height of the theca lesss than 2 mm
- Weak to very prominent bilateral symmetry developed in different planes
Treatise on Invertebrate Paleontology, Part T, Vol. 2(2) (1978):
- Monocyclic inadunates with weak to very prominent bilateral symmetry developed in different planes (E-BC in Homocrinacea, Calceocrinacea, Pisocrinacea, Allagecrinacea; D-AB in Heterocrinacea; C-EA in Myelodactylacea, Anomalocrinacea; and A-CD in Belemnocrinacea, Hybocrinacea).
- Most disparids are small and characterized by simplicity in structure of the crown and cup; many are microcrinoids, with diameter and height of the theca less than 2 mm. Chief plates are radials, which are main components of the cup and orals of the tegmen. The arms are slender and uniserial, branched at several heights above the cup or unbranched; pinnules are lacking.
- A well-marked tendency toward development of bilateral symmetry is observed, oriented in different planes. These are respectively designated according to the ray and opposite interray in which they are placed, namely 1)homocrinoid (through the E ray), 2) heterocrinoid (through the D ray), 3) iocrinoid (through the C ray), and belemnocrinoid (through the A ray). The last is also caleld crinoid plane, since it is widely prevalent in cladid Inadunata, Flexibilia, Camerata, and Articulata. Genera having perfect pentamerous symmetry are bilaterally symmetrical with respect to planes passing through any ray and its opposite interray. Such symmetry is found in some crinoid groups, but not in Disparida.
- A feature having generally importance in the evolution, and hence classification of crinoids, is fusion of skeletal elements and another is change in structural relationships of morphological elements. Both of these result in simplicites derived from complexities. Among the disparid crinoids illustrations are offered by 1) reduction in number of basals from five to three to one, 2)lateral fusion of axillary brachials with each other and subjacent radials to produce the large multifaceted radials of Catillocrinidae, Anamesocrinidae, and Calceocrinidae (Fig.320), 3) fusion of infer-and superradials to yiel simple undeivided radials as in Haplocrinitidae and Pisocrinidae derived from Herocrinacea and Iocrinidae of unknown ancestry, and 4) disappearance of proximal brachials fixed in the cup of Homocrinidae and Heterocrinidae, evidently by change of their status from fixed to free brachials. Cladid inadunates evolved similarly in the direction of reducing the nmber and complexity of thecal and arm components. For example, the Aethocrinidae, near-oldest (Lower Ordovician) of all known crinoids are followed soon (Middle Ordovician – Upper Ordovician) by much more simply organized genera
Moore & Laudon (1943):
- Under the ordinal name Diaparata are grouped monocyclic inadunate crinoids having a dorsal cup composed only of basals (BB), radials (RR), and generally an X plate and radianal (RA) or, in place of the latter an infraradianal (iRA) and superradianal (sRA). The arms are free above the radials. Known representatives of the order range from Ordovician to Permian.
- This order, equivalent to Bather’s Monocyclica Inadunata, is characterized generally by dissimilarity of structures in different rays. Certain RR differs in size and shape from others; some may be compound, and a fBr or more than one may occur in a ray. The arrangement of arms in one ray may be unlike that in adjoining ones. In some Ordovician representatives of the order, PBrBr may be firmly joined to the RR and sRA.