Echmatocrinus from the Burgess Shale: A Crinoid or an Octocoral?  

Echmatocrinus brachiatus from the Middle Cambrian Burgess Shale was originally described as the earliest known crinoid-like fossil, and was generally accepted as an echinoderm and crinoid precursor or sister group.  An alternative interpretation, as an early skeletized octocoral (Cnidaria), has also been proposed. 

Morphologic features of Echmatocrinus indicating likely echinoderm affinities include: (1) the plated stalk, conical cup, and erect, uniserial, articulated “arms” that resemble those in other early crinoids, (2) the apparent sutured, polygonal plates covering all parts of the body, (3) the reticulate ornament that resembles other known Burgess Shale echinoderms, (4) the resemblance of this ornament to stereom, although with a larger pore size, (5) the presence of aligned crystals in the thin pyrite film, possibly indicating single-crystal original plating, (6) the soft appendages (tube feet?) branching alternately off the arms in the holotype, (7) the curled tips of the arms like those seen in some living crinoids, (8) the apparent internal ligaments holding the arm segments together, (9) tiny plates lying alongside many arms that appear to be open cover plates, and (10) a disklike attachment structure at the distal tip of the stalk, similar to early crinoid holdfasts.  Problems in assigning Echmatocrinus to the echinoderms include: (1) lack of convincing pentameral symmetry (5 or 10 arms), (2) lack of convincing stereom in the body plates, (3) lack of pentameres or organized columnals in the stalk, (3) lack of large, organized, plate circlets in the cup, especially radials beneath the arms, (4) the large, alternating, tube feet instead of smaller tube-foot triads, and (5) no moldic specimens that preserve convincing polygonal plating or sutures. 

Morphologic features of Echmatocrinus indicating likely octocoral affinities include: (1) overall similarity of Echmatocrinus cup and arms to the large, soft-bodied, living octocoral Alcyonium, (2) thin imbricate scales covering many deep-sea octocorals, and (3) average number of tentacles is about 8 in known Echmatocrinus specimens.  Problems in assigning Echmatocrinus to the octocorals include: (1) the fact that Alcyonium is colonial, with many individuals attached to a common base, (2) solitary octocorals are rare today and have not been described as fossils, (3) heavily plated octocorals have large fanlike colonies with hundreds of tiny polyps, unlike Echmatocrinus that is not fan-shaped and 25-30 times larger, (4) all living octocorals have eight tentacles, with almost no variation, vs.  6-9 arms in different specimens of Echmatocrinus, (5) no octocoral has ligaments in the tentacles, (6) no octocoral has tiny cover plates on the tentacles, and (7) Echmatocrinus doesn’t resemble sea pens, the only octocorals that are reputed to occur in the Late Proterozoic and Cambrian.  Although not conclusive, more features favor an echinoderm and basal crinoid assignment for Echmatocrinus than an unspecified octocoral assignment.