Fig. 1

Hypotheses about the evolution of skeletogenesis across echinoderm taxa with a possible ancestral function of non-genomic thyroid hormone signaling. Recent evidence suggests a single origin of larval skeletogenesis followed by loss in Asteroids (sea stars; [32]), although independent origins of larval skeletogenesis in Ophiuroids (brittle stars) and Echinoids/Holothuroids (sea urchins/sea cucumbers) have also been proposed [12, 28, 70]. We test the hypothesis that THs, predominantly T4, regulate skeletogenesis in Ophiuroids, Echinoids, and Asteroids, as a result of evolutionarily ancient regulation of skeletogenesis by THs. We suggest that TH regulation was co-opted to regulate larval skeletogenesis when the gene regulatory apparatus allowing for skeletogenesis in adults became expressed and co-opted by the larval stage of some echinoderms