EDRIOASTEROID ENCRUSTATION PATTERNS ON BRACHIOPOD SHELLS AND BIVALVE INTERNAL MOLDS FROM THE VERULAM FORMATION (MIDDLE ORDOVICIAN) OF GAMEBRIDGE, ONTARIO, CANADA

Three-hundred-ninety edrioasteroids, an extinct clade of echinoderm, were collected in 1975 from the Verulam Formation at Gamebridge, Ontario, Canada. Of these specimens, 95.4% are Isorophusella incondita and 4.6% are Cryptogoleus chapmani. Both edrioasteroid species, which required a hard substrate for encrustation, exhibit preferential substrate attachment with 62% encrusting bivalve internal molds and 23.5% encrusting Rafinesquina alternata brachiopod shells. An additional 5.3% edrioasteroids are found encrusting other faunal skeletal elements, including bryozoans, trilobite fragments, and cephalopod internal molds. Further, 9.2% of the edrioasteroids are found attached directly to the limestone substrate, providing evidence that some areas of the generally muddy seafloor were lithified by carbonate cement prior to encrustation and subsequent burial.

Of the edrioasteroids colonizing bivalve molds and brachiopod shells, specimens of Isorophusella incondita encrust shells most frequently near the anterior margin (41%) where scouring of loose sediment via Bernoulli’s principle and sediment plucking would remove the matrix, thereby providing areas of refuge for the edrioasteroids. On the other hand, specimens of Cryptogoleus chapmani show a preference for the central region of the shell (42.9%), which provided a topographic high above the muddy seafloor.

Ninety-two percent of examined bivalve molds and brachiopod shells are encrusted by one edrioasteroid individual with few instances of multiple edrioasteroids encrusting the same shell noted. These results suggest space was not at a premium on this ancient seafloor. Although no instances of intraspecific interactions were noted, interspecific interactions between bryozoans and edrioasteroids occur with few examples of overgrowth and edrioasteroid marginal deformation. This provides evidence that edrioasteroids, once attached, lacked the ability to move.