Patterns of early colony development (astogeny) in four genera of trepostome bryozoans from the Upper Ordovician

Abstract

The early astogeny of specimens representing the bryozoan genera Dekayia, Parvohallopora, Heterotrypa, and Homotrypella follows the same pattern documented in other Ordovician trepostomes. After larval settlement and metamorphosis, typically three primary, periancestrular, autozooids bud distally from the founder zooid, the ancestrula, with secondary zooids budding from the primaries, tertiary zooids from the secondaries, and so on. Progressive lateral displacement of buds, through backbudding, eventually produces a subcircular ancestrular disk. Significant intergeneric differences in this basic pattern do not appear to exist. Intrageneric variability in budding pattern occurs during colony development. That variability might have resulted from any or all of the following sources: within-genus plasticity, the development of multiple ancestrulae, or environmental variation. The basic pattern of three primary buds developed from individual ancestrulae has been recognized in other trepostomes, some cyclostomes, and two suborders of cryptostomes. Cladistically, the trait must be considered polyphyletic until more is known about the number of primary buds in the remaining bryozoan suborders. Ancestrular dimensions are statistically similar across genera except between the smaller Dekayia and larger Parvohallopora, but ancestrular sizes in all four Dillsboro genera are consistent with the inference that they possessed a polyembryonic lecithotrophic larval type.