40 



OHIO FOSSILS 



Phylum Echinoderma 



GENERAL. The starfish, sea-urchins, and their relatives are familiar examples of this 

 phylum which includes also many less familiar animals. The echinoderms all have five-fold 

 symmetry; they are all built on a pattern of five rays, though this is considerably modified in 

 some cases. The whole body may have the form of a five-pointed star, or there may be only a 

 pattern of five grooves on a round body, or the mantle may be surrounded by five arms, or arms 

 in multiples of five. They may be attached to the bottom directly or by a stem (subphylum 

 Pelmatozoa, including among others, the classes Edrioasteroidea, Cystoidea, and Crinoidea) or 

 may be free -moving and stemless (subphylum Eleutherozoa, including among others, the classes 

 Stelleroidea, Echinoidea, and Holothuroidea). Only the more important classes are described in 

 detail here. The echinoderms are exclusively marine; their geologic range is Cambrian to 

 present. 



CLASS EDRIOASTEROIDEA. These are the seat -stars which look like starfish attached 

 to a cushion-like disk covered with plates. They are only distantly related to the starfish, much 

 more closely related to the cystids and crinoids. They are an extinct group which lived from 

 early Cambrian to Mississippian times. 



CLASS CYSTOIDEA. This group of extinct (Ordovician to Permian) echinoderms with or 

 without stem, may be distinguished from the crinoids (see below) by their poorly developed arms. 

 The subclass Blastoidea, formerly considered a class, have regularly arranged plates and per- 

 fect five-fold symmetry in the arrangement of the food grooves. 



The irregular cystids are seldom abundant but many kinds have been found in the Ordovi- 

 cian and Silurian rocks of Ohio. The regular forms (blastoids) are fairly common in some Devon- 

 ian beds of Ohio. None has been found in the Pennsylvanian or Permian rocks of the state. 



CLASS 



CRINOIDEA . Most crinoids or sea-lilies have a stem, which anchors the animal 

 there are a few free -swimming or, more exactly, floating forms. Crinoids (fig. 



to the bottom; 



45) have a globular body or cup to which are attached five arms which may branch several times 

 and which may be much longer than the cup. After death, the plates of the body are often scat- 

 tered and the stem is preserved in sections or separated into pieces (called columnals or ossi- 

 cles); crinoidal limestones are made up of dissociated plates and columnals. The geologic range 



of the crinoids is Ordovician to present; they were much 

 more abundant and varied in the Paleozoic than in the Meso- 

 zoic and Cenozoic. 



Complete crinoids are a rarity everywhere; a few good 

 specimens turn up now and then in the Ordovician, Silurian, 

 and Devonian of Ohio. Crinoidal limestones are found in all 

 systems, with the exception of the Permian and Pleistocene. 



CLASS STELLEROIDEA. These are the starfish and 

 their relatives. They are characterized by the star-shaped 

 body with a central disk and five rays or arms covered with 

 plates. They are rare as fossils and range from the Ordo- 

 vician to the present. 



Complete starfish are even rarer than crinoids except 

 in a few favored localities. A few specimens are found from 

 time to time in the Ordovician, Silurian, and Devonian of 

 Ohio. 



CLASS ECHINOIDEA . The sea-urchins are echino- 

 derms with a globular or disk-shaped body which is covered 

 in life with movable spines. Fossil sea-urchins (Ordovician 

 to present) are rare in Paleozoic, more abundant in younger 

 rocks. 



Fig. 45 



