The growth of the food-gathering systems of Ordovician crinoids illustrates the solutions evolved to avoid or minimize problems of suspension feeding and maintenance. The column has low metabolic requirements and may even be self-sufficient; little or no food need be diverted from the crown to the column. The tissue that the food-gathering system supplies with food is all or mostly in the crown. The volume of tissue in the crown is isometric with its volume. Distal arm branches, ramules, or pinnules form throughout growth of most taxa. The number of arm branches is constant in hybocrinids and porocrinids. The length and number of plates in the food-gathering system show marked positive allometry relative to crown volume. Food grooves become wider in larger crinoids but growth rates vary greatly. Branch density declines in all species studied. The tubefoot spacing can be constant, increase, or decrease in larger crinoids. Food-gathering capacity is the number of food-catching tubefeet multiplied by the average food-groove width, and it is positively allometric in terms of crown volume. The food-gathering ratio (food-gathering capacity: crown volume) declines somewhat, remains the same, or increases slightly during growth. The food-gathering parameters are correlated with arm structure. Crinoids with pinnulate arms exhibit the highest food-gathering ratios, high values of tubefoot spacing and branch density, and narrow food grooves, and they catch small food particles with small and closely spaced tubefeet. The lowest food-gathering ratios are seen in crinoids with unbranched and nonpinnulate arms which possess low tubefoot spacings and branch densities and wide food grooves. On average these organisms ate larger food items collected by larger and more widely spaced tubefeet. Larger food-gathering ratios are associated with greater resistance to fluid flow through the arms and tubefeet. The allometry of the food-gathering systems of Ordovician crinoids suggests that their metabolic rates could be proportional to body mass0.75 as in many living animals and plants.
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