Most modern echinoderms assume a mouth-down feeding and locomotion posture.Members of the class Crinoidea are an exception.They are attached,mouth-up,and feed using tube feet to trap food from the water column.Recent molecular analyses show the Crinoidea to be basal to other extant classes.Which of the following help to explain these observations?
A)Feeding and locomotion posture is not a phylogenetically significant character as all fossil echinoderms assumed a mouth-down posture.
B)The mouth-up posture must be an ancestral character.The mouth-down posture evolved secondarily.
C)The molecular analyses must be in error.The mouth-up posture is phylogenetically significant and must have been derived secondarily.
D)There is no reasonable explanation for these observations at this time.

Question

Quizplus · Accepted Answer

The observation that members of the Crinoidea class have a mouth-up posture and feed using tube feet to trap food from the water column can be explained by the possibility that this is an ancestral character, and the mouth-down posture evolved secondarily in other echinoderm classes. This is supported by recent molecular analyses that show the Crinoidea to be basal to other extant classes.

Most Modern Echinoderms Assume a Mouth-Down Feeding and Locomotion Posture