POST-METAMORPHIC THECAL DEVELOPMENT IN BLASTOIDS

Understanding early ontogeny of individuals from extinct classes depends on using appropriate modern analogues for comparison with well-preserved specimens that retain larval and early post-larval morphologies. Here we describe the early ontogeny of post-larval blastoids from the upper Lower Carboniferous Luocheng Formation (Xinxu, Guangxi Province, Peoples Republic of China) and compare early blastoid ontogeny to early modern comatulid crinoid development. Because blastoids are extinct, modern crinoids have been used for inferring blastoid functional morphology and ontogeny for over a century. Here we use the example of comatulid crinoid development, because it is easier to discern when the juvenile stage is achieved (when the juvenile detaches from the stem), as compared to articulate crinoids in which the juvenile remains fixed to the stem. Modern comatulid ontogeny is characterized by three-stages of post-embryonic development: 1) the doliolaria stage, which lasts 2-3 days; 2) the cystidian stage, which lasts about a week; and 3) the pentacrinoid stage, which lasts about 15 weeks. The end of the pentecrinoid stage in comatulids is marked by the juvenile detaching from the stem and becoming free-living. Pioneering studies of the early ontogeny of blastoids described material that likely would be classified as the cystidian stage of a modern crinoid. We have a specimen with very fine growth lines that is approximately 3 mm long, and compares to the pentecrinoid stage. The specimen has three well-developed basals and five well-developed radials, but no deltoid or oral plates. The initial plates are characterized by microperforate stereom and are surrounded by 44 to 50 distinct growth lines that are superficially characterized by imperforate stereom. Although speculative, the initial basal and radial plates appear similar to cystidian plates in Antedon, a modern crinoid. The transition to pentecrinoid stage in this blastoid would have been marked by beginning the accretion of growth lines, making the specimen seven to eight weeks old at time of death.