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A group of fossil sea lilies (crinoids; Iocrinus subcrassus) from Ordovician sediments in Indiana.
Guensburg, TE, J Sprinkle, R Mooi, B Lefebvre, B David, M Roux, K Derstler. 2019. Athenacrinus n. gen. and other early echinoderm taxa inform crinoid origin and arm evolution. Journal of Paleontology doi:10.1017/jpa.2019.87
Summary. Crinoids, or sea lilies, have a rich fossil record, beginning with Ordovician rocks. For more than 150 years, scientists have thought that crinoids evolved from another group of echinoderms called cystoids. This idea has been overthrown by discovery of a new fossil crinoid from Utah. The new fossil, named Athenacrinus, has an arm structure that is incompatible with cystoid ancestry, and shows that the similar structure of crinoids and some cystoids is not due to common ancestry.
Comment. Crinoids show a very interesting pattern of abrupt appearances in the fossil record that can be considered an explosion of different types (“disparity”). Crinoid classification is still a work in process, but a recent revision lists about seven major groups and about 27 large subgroups (Orders).[1] In a striking pattern, all seven of the major crinoid groups and at least 13 of the 27 large subgroups have been found preserved at or near the beginning of the fossil record of crinoids, in Ordovician sediments. This is another example of a pattern called “disparity before diversity”.[2] The abrupt appearance of so many different types of crinoids within a relatively short stratigraphic span seems consistent with a catastrophic process of burial of pre-existing diversity such as a global flood might produce, but does not fit the prediction of evolutionary theory that disparity should result from gradual accumulation of small changes (increasing species diversity) over long ages of time.
[1] Wright, DF, WI Ausich, SR Cole, ME Peter, EC Rhenberg. 2017. Phylogenetic taxonomy and classification of the Crinoidea (Echinodermata). Journal of Paleontology 91(4):829-846.
[2] “Disparity” refers to the large morphological differences that distinguish major groups, while “diversity” refers to the numbers of species, whether similar or different. (See Gould, SJ. 1989. Wonderful Life. New York, NY: Norton, p. 49.)