Functional morphology of arborescent animals: strength and design of cheilostome bryozoan skeletons

Abstract

In cheilostome bryozoans with rigidly arborescent growth habits, resistance to breakage under forces generated by flowing water or collision with moving objects depends on the strength and design of supporting skeletal parts. To investigate the abilities of modern and Tertiary cheilostomes to resist breakage, we measured bending strength in fresh, preserved, and fossil material; measured drag on actual and model colonies; and calculated stress within colonies and breaking values under concentrated and uniform loads. Among nine modern, five Oligocene, and four Paleocene species, inferred live bending strength (24–85 MNm−2) and stiffness (42–65 GNm−2) appear to be species-specific properties overlapping those of similar mineral-organic composite skeletons of Mollusca. Unlike those of Mollusca, cheilostome skeletons appear isotropic in bending, with strength not clearly related to microstructure or composition. Bending strength and morphologic disposition of skeletal material combine to produce a wide range of abilities to resist breaking. At early growth stages, almost all species appear highly resistant; at later ones, some break at velocities <0.2 m sec−1or concentrated loads <2 g, whereas others remain unbroken at velocities >2 m sec−1or concentrated loads >100 g. A negative correlation between strength and resistance indicates that design is the more important factor in the abilities of these bryozoans to withstand forces of these kinds. An apparent trend toward more resistant designs from Paleocene to modern species is related chiefly to increasing rates at which branches thicken toward the colony base. The consequent change in design may even have permitted modern species to use weaker skeletal material while increasing their overall resistance.