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Ontogeny and phylogeny in primitive calceocrinid crinoids
Published online by Cambridge University Press: 14 July 2015
Abstract
The evolution of primitive calceocrinids with four arm-bearing rays, here termed cremacrinids, has been examined for 13 species of Cremacrinus, Paracremacrinus, and Anulocrinus using numerical cladistics. The lineage was reconstructed from a Wagner tree of a Manhattan distance matrix. The characters were polarized by outgroup comparison. The Calceocrinidae originated from the Homocrinidae by development of the hinge, recumbent stem, loss of the BC interray basal and the C ray arm, and reorganization of the dorsal cup plates. The main evolutionary trends within cremacrinids consist of differentiation of the B and E ray arms from those of the lateral rays, increase or decrease in the number of branches and size of all arms, reduction of the B ray arm relative to those of the A and D rays, acquistion of additional main axils in the lateral rays, fusion of the infer- and superradials in the B and C rays, development of elongate brachials, and changes in adult body size.
Relatively complete growth sequences are known for six species of cremacrinids and these are analyzed with allometric equations for the dorsal cup, hinge, and arms. The relationships between ontogeny and phylogeny for the links in the evolutionary sequence and the number of examples of each are listed in order of decreasing frequency and importance: 1) unclassified divergences (for example, changes in the number of branches in the E ray and the formation of elongate brachials), 12 examples; 2) 10 cases of paedomorphosis; 3) no significant differences between ancestors and descendants, seven instances; 4) five links where the dorsal cup plates are rearranged; and 5) a single change by recapitulation. Some phyletic links are characterized by several patterns whereas others only exhibit one or two kinds of transitions.
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- Copyright © The Paleontological Society
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