Microcavities in lower Middle Ordovician bryozoan mounds from the Laval Formation (Chazy Group) near Montreal, Quebec contain evidence that endolithic (boring) sponges were present. Ramose borings with scalloped walls and swellings resembling endolithic sponge galleries, faceted carbonate grains similar to modern sponge chips, and siliceous spicules both in situ on the cavity wall or roof and in the sediment, all point to the activities of endolithic sponges in the hard substrate of the wall and roof.Coelobiontic (cavity-dwelling) endolithic sponges therefore infested cavities in skeletal mounds and reefs in the Middle Ordovician and appear to have exploited the cavity habitat very soon after the appearance of metazoan skeletal reefs in the Ordovician.

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