Paleoecology of hardground encrusting and commensal crinoids, Middle Ordovician, Tennessee

Aspects of functional morphology are examined and lifestyles proposed for seven exceptionally well-preserved crinoids from the Lebanon Limestone. Potential ranges of motion are based on plate-shapes, articular surfaces of attachment, and feeding structures. This diverse group of crinoids includes disparid inadunates (Columbicrinus crassus and Tryssocrinus endotomitus), a hybocrinid (Hybocrinus bilateralis), reteocrinids (Reteocrinus polki and R. variabilicaulis), and rhodocrinitid camerates (Archaeocrinus snyderi and Gustabilicrinus plektanikaulos). Using attachment structures as the basis, these crinoids can be divided into two groups: encrusters and commensals. Encrusters were limited to hardgrounds or skeletal substrata. Commensals were more opportunistic, utilizing a variety of substrates, and they were widespread in Lebanon subenvironments including those with both soft and hard seafloors. Lebanon hardground assemblages are charcterized by high diversity, particularly among crinoids and bryozoans. They are tiered according to elevation above the seafloor (and probably additionally by feeding limitations).