BRYOZOA or "moss animals" are very small creatures that live together in colonies that may reach a considerable size. In the warm Ordovician seas, they added greatly to the limestone-building process of that period. As each little animal secretes a hard wall around its body, compound skeletons are built up. Durable fossils result from these hard walls of lime which enclosed a "U-shaped" digestive tract. Retractor muscles at the base were able to pull the exposed anal end and the tentacled mouth into the safety of the tubular compartment.
The evolution of the bryozoa was rapid, and they are useful for determining the age of strata of sedimentary rock. They were of rare occurrence in the Cambrian Period, but there was a profusion of the "long-celled" type during the Ordovician Period. In Silurian time, whole colonies a spreading leaf-like shape instead of the more massive long-celled colonies.
The SMALL BRANCHES in picture one were collected by chipping them from the sedimentary rocks in the river bed below Baby Point. They are early Ordovician. The LARGER BRANCHES in the second and third pictures were found loose in strata higher up the bank (some from a steeper bank north from the Bloor Street West bridge). All of the GLOBULAR CLUMPS and APPENDAGES were collected loose from higher strata in the latter location, and they represent a late Ordovician form. Surfaces on the flatter LEAF-LIKE COLONY have the same texture as the other Bryozoan colonies; this was collected from a sedimentary rock in a stratum lower than the rounder clumps and might have been a prototype of those living during the Silurian Period!
The leaf-like structures of Bryozoa colonies, described as being from the Silurian Period, were found at the low level of the Humber River bed. The clump-like structures, sometimes with appendages, were collected at much higher levels of the present river bank (i.e. at much higher levels of the ancient shores of the late Ordovician Oceans). Since today's ocean shores have also changed, it would not be unreasonable to assume that bryozoa colonies adapted to the changes which occurred between the Ordovician and Silurian Periods in Southern Ontario.