Branching, biomechanics and bryozoan evolution

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Abstract
Late Cretaceous and early Tertiary radiations in cheilostome bryozoans gained major impetus from the evolutionary development of rigidly erect, branching growth habits from ancestral encrusting forms. Consideration of the skeletal strength, branching morphology, and behaviour in water flow of living and fossil arborescent cheilostomes suggests that: (1) the earliest species of the Cretaceous depended on high-strength skeletal material to resist failure, but were limited in growth by weak designs resulting from inability to thicken branches and thus augment skeletal cross sections toward the colony base; (2) with increasing ability to enhance design through branch thickening, a growing number of Cainozoic species have become capable of avoiding failure in likely régimes, even though they commonly have skeletons of weaker material; and (3) concurrently with the waning importance of mechanical limitations, spatial constraints have become more significant, so that most species now have branches diverging at angles that minimize crowding as a colony reaches maximum size. Morphological data from a total of 70 species ranging from the Cenomanian to the present provide statistically significant support for these trends. Preliminary data on detailed changes through time suggest that the trends arose through selection among species of generally advantageous morphological differences that are species-specific and fluctuate randomly within species.