Phylum Echinodermata (Precambrian -Recent)

Subphylum Crinozoa (Cambrian-Recent)

Class Crinoidea (Cambrian-Recent)

Subphylum Blastozoa (Cambrian-Permian)

Class Blastoidea (Silurian-Permian)

Class Rhombifera (Ordovician-Devonian)


Subphylum Crinozoa and Class Crinoidea

Most crinoids consist of a calyx with arms supported by a stem. The arrangement of arms and plates are the main criteria for subclass and lower division. The arms can be single or branched, and may be constructed of a single series of brachials (uniserial) or composed of an alternating series of brachials (biserial), and may also support pinnules.

The normal arrangement of plates consist of brachials, radials, and basals. They can be dicyclic (having infrabasals) as in the example or monocyclic (without infrabasals) as in the example.

Crinoids can be divided into 4 subclasses: the Inadunates, Camerates, Flexibles, and Articulates (see your text book), only the articulates survive to the Recent. For an example of how large crinoids can be, look at this specimen.

Subphylum Blastozoa

The two blasotzoan classes which are of importance include the Blastoidea and Rhombifera (= cystoids of some workers). Other blastozoan classes with a fossil record include the Diploporata, and Eocrinoidea. Together, all blastozoans have hydrospires, small folds under the lancet plates. The differences between each class is based primarily upon the pore and plate arrangement.

Class Blastoidea

Blastoids were attached by a stem to their substrate. The calyx of most species consists of 13 main plates: 5 deltoids, 5 radials, and 3 basals. Pores are restricted to the 5 ambulacral areas covered by lancet plates. The margins of the lancets were lined with brachioles. The mouth is centrally located on the top of the calyx (note Blastoids do not have a tegmen) and is surrounded by smaller holes for water vascular control called spiracles.

Figure 2 - Blastoid and Rhombiferoid Morphology

From McRoberts (1998)

Class Rhombiferia

The Rhombifereans on the other hand have an irregular calyx with irregular shaped plates. The pores are not restricted to the ambulacral areas, but are commonly distributed over the calyx in rhombahedral patterns ("Rhomb pores") which overlaps two adjacent plates.

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