Evolution of the colonial growth habit in the ordovician bryozoans of the class stenolaemata: Feeding adaptations (Leningrad Region, Russia)

Abstract

Based on the study of the growth habits and the relief of the colony surface in bryozoans of the class Stenolaemata from the Lower (Latorp horizon) and Middle (Volchov and Kunda horizons) Ordovician of the Leningrad Region, these bryozoans are shown to develop from the simple, unilaminate colonies (B _I ^β ) to the massive colonies with a nodular surface and smooth columnar colonies (B _II ^α ), which subsequently evolved into the columnar-spiral (B _II ^β ) and more complex erect branching and fenestrate constructions (B _II ^γ ), and subsequently into the branching, articulate colonies (B _III ^α ). The apertures of autozooecia and the character of their arrangement on the colony surface changed correlatively from the circular (B _I ^β ) to polygonal and roundedpolygonal, randomly arranged apertures, and subsequently to the oval apertures (B _III ^α ) arranged in strictly regular longitudinal or longitudinal-diagonal rows or in a quincuncial pattern. Thus, the development of growth habits in the bryozoans under consideration has a progressive character. It is expressed in the progressive increase in the complexity of growth habits of colonies and in the more regular arrangement of apertures and other structures on the colony surface. The directionality of morphological changes in the growth habits of colonies of Ordovician bryozoans was apparently closely associated with the development of more complex environmental interactions of these bryozoans, especially with water currents supplying food particles. It is suggested that the high competitive ability of bryozoans of the class Stenolaemata at early stages of its development in the basin of Baltoscandia was apparently due to the better use of food resources.