Four competing hypotheses of the phylogenetic relationship of extant echinoderms.

<p>Most phylogenetic studies place Echinoidea and Holothuroidea as sister groups. The difficulty lies in the placement of Ophiuroidea; different methods favor different positions for brittle stars. A) The predominant hypotheses of how extant echinoderms are related are the Asterozoan and Cryptosyringid hypotheses. B) Two other alternate hypotheses based on mitochondrial gene alignments.