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- The calcichordate hypothesis holds that each separate lineage of chordate (Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate, and thus the echinoderms and the chordates are sister groups, with the hemichordates as an out-group. It was formulated by British Museum paleontologist Richard Jefferies. The hypothesis has been disproven as of 2019; exceptional preservation of soft tissues in the single appendage of the stylophorans Thoralicystis and Hanusia revealed clear traces consistent with a water vascular system—an ambulacral canal with tube feet—covered by movable plates, where the calcichordate hypothesis would require the anatomy be a tail containing a notochord protected by fixed plates. The enlarged area at the base of the appendage, which in the calcichordate hypothesis would contain muscles to move the tail, contains an extension of the body cavity. The findings also disprove the hypothesis that the group were stalked echinoderms. Based on the internal anatomy, "calcichordates" (stylophorans) are revealed as echinoderms with a single, somewhat starfish-like arm that gathered food with tube feet and transferred it to a mouth at the base of the arm. The food-gathering arm may also have been used for locomotion, as in starfish. The fossils from the Bou Izargane Lagerstätte from the Lower Ordivician of Morocco were unknown to Jeffries; they had not been discovered when he formulated the hypothesis in the early 1960s based on the examination of a group of mitrate fossils, and were still unknown when he wrote subsequent papers in 1981 and 1997 defending the hypothesis. Stylophorans are classed as echinoderms based on their possession of at least two shared and unique features (apomorphies) of the phylum; stereom plates and a water vascular system. Because they show no sign of radial symmetry, the position of stylophorans within Echinodermata remains unresolved as of 2019. (en)
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- The calcichordate hypothesis holds that each separate lineage of chordate (Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate, and thus the echinoderms and the chordates are sister groups, with the hemichordates as an out-group. (en)
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- Calcichordate hypothesis (en)
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