Article: Promelocrinus from the Wenlock at Dudley
J. C. Brower
Two melocrinitids occur in the Wenlock at Dudley. Promelocrinus anglicus Jaekel is represented by many specimens, whereas Promelocrinus sp. is only known from one partial fragment of the arms. Growth and variation have been studied in P. anglicus. Variation of number of plates in the interbrachial areas and in the ray trunks increases distally. The number of plates in a range of interbrachials is positively linked with the number of plates in adjacent ranges. However, the arm-branching parameters are mainly independent of one another. The earliest crinoids probably bear equal-sized arms in each ray with pinnules being present in the two outer arms but lacking in the two inner ones. Throughout subsequent development, the outer arms are unbranched although new brachials form and the length of the arms is augmented. The inner arms become hypertrophied into highly ramified ray trunks. The length of the ray trunk increases by the formation of new brachials and height increase of old plates. New ramules appear as new brachials are added to the ray trunk. Once the ramules are initiated, new brachials form at the distal tips of the growing ramules. Pinnules develop on the ramules and outer arms throughout growth. Lengths of old pinnules are augmented by the same mechanisms seen in the ramules and outer arms. The ontogeny of P. anglicus suggests that the acquisition of complexly ramified ray trunks results in unusually rapid rates of growth of new brachials and length of the arms. The rate of development is approximately squared relative to idealized crinoids with simple arm configurations. Unlike most crinoids, P. anglicus is characterized by isometry or positive allometry of the food-gathering system relative to tissue volume, so that the food-gathering ratios are either stabilized or increase throughout ontogeny. In P. anglicus and other melocrinitids, a complete filtration network is present which covers the entire perimeter of the arms, and almost all food particles flowing through the arms and pinnules would be filtered. Promelocrinids and melocrinitids were full mucus net feeders like most Recent adult crinoids.