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

Abstract

Aspects of functional morphology are examined and lifestyles proposed for seven exceptionally well-preserved crinoids from the Middle Ordovician Lebanon Limestone, Middle Tennessee. 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 crassusandTryssocrinus endotomitus), a hybocrinid (Hybocrinus bilateralis), reteocrinids (Reteocrinus polkiand R.variabilicaulis), and rhodocrinitid camerates (Archaeocrinus snyderiandGustabilicrinus plektanikaulos). Using attachment structures as the basis, these crinoids can be divided into two groups: encrusters and commensals. Encrusters are the most diverse and include all taxa except the rhodocrinitids. They have a small discoidal holdfast and xenomorphic column. Proxistele-mesisteles were holomeric or pentameric. Flexure potential of this region of the column in disparids was increased by wider ligament gaps at nodal-internodal articulations; in other taxa flexure was enhanced by increasing the frequency of columnals. Short dististeles are polymeric or a plate mosaic. This region functioned as a resilient buffer between the mesistele and holdfast. Columns as a whole had moderate flexure potential. Feeding-structure design was varied, although all butHybocrinus bilateralishad pinnulate or nonpinnulate parabolic filtration fans.Hybocrinus bilateralishad atomous arms arrayed in a bilaterally symmetrical pattern; the symmetry plane was oriented parallel to prevailing currents. Commensals in the Lebanon are the rhodocrinitids. Their attachment structures are characterized by a long distally tapering column terminating in a coil. Densely pinnulate arms formed a well-developed parabolic filtration fan.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 characterized by high diversity, particularly among crinoids and bryozoans. They are strikingly tiered according to elevation above the seafloor (and probably additionally by feeding limitations). This assemblage was the result of longterm coevolution in association with hardgrounds.Ontogenetic development of the column of encrusting crinoids indicates that the evolution of holomeric columnals occurred through a simpler sequence than previously proposed and need not have included gradual alignment of offset pentameres as intermediate steps. Columnals evolved from a poorly organized plate mosaic through a polymeric stage.