Article: The interrelationship of early colony development, monticules and branches in Palaeozoic bryozoans

The generalized early colony development of Ordovician trepostome bryozoans includes two distinct stages: an earlier triangular colony, or protoecial cone, followed by a later circular colony, or ancestrular disc, in which vestiges of the protoecial cone are observable. Monticules, polymorphic zooid clusters characteristic of later astogeny, reproduce the structure of the zone of early development, including a replicate of the ancestrula, the monarchozooid. A second type of ancestrular replicate, the basilozooid, is found within the axial zones of colony branches. The evidence from colony structure and zooid morphology is here interpreted as illustrating the presence of a growth regulator. It is inferred that the ancestrula and its replicates, monarchozooids and basilozooids, exerted local dominance over their respective regions of the colony, and that this is reflected in radial zooid alignments and gradients in zooecial size. A colony in which several monticules were destroyed by borings shows disruption of normal zooecial alignments in the areas affected. The programme of early development is repeated many times, with modifications, to produce all the later stages of colony growth.