Box Echinoderm classification

In broad terms, the Echinodermata may be divided into two main sister groups - the stalked pel-matozoans and the mobile eleutherozoans. But there are a number of more bizarre Lower Paleozoic forms, known from only a few specimens at single localities, which are difficult to classify at present. The classification is based on a number of key features: the main body of the animal, enclosed by plates (the theca or test), areas bearing tube feet (ambulacra) with perforations or holes (brachioles) and, in the case of the pelmatozoans, the possession of a cup (calyx) and arms (brachia).

Subphylum PELMATOZOA

Class EOCRINOIDEA

• Globular or flat theca with 2-5 ambulacra bearing brachioles

• Cambrian (Lower) to Silurian

Class PARACRINOIDEA

• Irregularly arranged plates comprising globular to lenticular theca; 2-5 ambulacra commonly with pinnules. Hydropore adjacent to mouth. Stem attached to three basal plates

• Ordovician (Darriwilian to Hirnantian)

Class BLASTOIDEA

• Flask-shaped theca commonly with three basal plates; ambulacra with elongate lancet plate and rows of side plates

• Ordovician (Katian) to Permian (Tatarian)

Class DIPLOPORITA

• Globular theca with many plates in an irregular to regular pattern; 3-5 food grooves with brachioles. Diplopores perforate thecal plates

• Cambrian (Middle) to Devonian (Eifelian)

Class RHOMBIFERA

• Globular theca with 2-5 ambulacra extending from the mouth to, commonly, the edge of the upper surface. Pore structures cross adjacent thecal plates arranged in a rhomboid pattern, and comprise the respiratory system

• Cambrian (Upper) to Devonian (Frasnian)

Class CRINOIDEA

• Calyx with lower cup and upper tegmen. Sea lilies and feather stars

• Ordovician (Tremadocian) to Recent.

Subphylum ELEUTHEROZOA

Class EDRIOASTEROIDEA

• Disk-shaped thecae with straight or curved ambulacra with the mouth situated centrally and the anus sited on the interambulacra

• ?Precambrian (Ediacaran), Cambrian (Lower) to Carboniferous (Gzelian)

Class ASTEROIDEA

• Between 5 and 25 arms with large tube feet extend from a central disk. Starfishes or sea stars

• Ordovician (Floian) to Recent

Class OPHIUROIDEA

• Five long, thin, flexible arms, consisting of vertebrae and with small tube feet, extend from large, circular central disk. Brittle stars or basket stars

• Ordovician (Floian) to Recent

Class ECHINOIDEA

• Test is usually globular with plates differentiated into ambulacral and interambulacral areas. Mouth on underside, anus on upperside or sited posteriorly. Sea urchins, heart urchins and sand dollars

• Ordovician (Katian) to Recent

Class HOLOTHUROIDEA

• Body is cucumber-shaped with leathery skin with muscular mesoderm and spicules. A ring of modified tube feet surround the mouth. Sea cucumbers

• Ordovician (Floian) to Recent

Box 15.2 Origin of the echinoderms and the status of the helicoplacoids

During the major Early Cambrian radiation of echinoderms, many rather bizarre forms appeared suddenly with very different morphologies. At least nine genera were present, of which about half had pentameral symmetry, but the others were not pentameral at all. One such non-pentameral group, the helicoplacoids (Fig. 15.2), is unique in having only three ambulacral areas with tube feet wrapped around their spindle-shaped bodies. Moreover, the group lacked appendages and individuals probably lived with their shorter ends anchored to the sediment. However, helicoplacoids have many plates with the distinctive stereom structure, ambulacra and a mouth sited laterally together with an apical anus. The helicoplacoids have thus been interpreted as primitive echinoderms, surviving by suspension feeding in the sessile benthos. Helicoplacus may be very close to the stem group of all subsequent Echinodermata, and something like this animal might have given rise to the pel-matozoan and eleutherozoan body plans. Other groups of echinoderms were already diverse and widespread during the Early Cambrian, but the helicoplacoids were apparently restricted to western North America, where they were very abundant during only the Early Cambrian. Their extinction may have been a very important ecological signal. Such groups of unattached "sediment stickers" were well adapted to the algal mat substrates of the Neoproterozoic. Perhaps they could not cope with the increased bioerosion and bioturbation of soft substrates that were part of the move away from seafloors covered by microbial mats that prompted the Cambrian substrate revolution (Bottjer et al. 2000).

Classification Echinoderme

Figure 15.2 Helicoplacus from the Lower Cambrian (x10). (Based on Treatise on Invertebrate Paleontology, Part S. Geol. Soc. Am. and Univ. Kansas Press.)

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